PIPING, TUBING AND HOSES
Piping is defined as "a series of components, such as pipe, valves, fittings, and gauges, used together to transport fluids." Whether a piping system is used to move fluids from one point to another or to process and condition a fluid, the various piping components are the building blocks in the construction of the system.
A system that only moves fluid may consist of just a few very simple components, while a chemical conditioning system may have many complex components.
This section focuses on the purpose and functions of the various piping components used in industry today.
Pipe and tube products are classified according to their application and not their manufacturing method. Most tubular products are placed in four general classifications:
2. Structural tube
3. Mechanical tube
4. Pressure tube
Although they are all similar in appearance, the four groups are used in very different applications and should be understood thoroughly before using. Pressure and temperature ratings vary drastically for different classifications of tubular products. This section discusses the different tubular products available and the selection process used to ensure you are using the proper size and type.
A pipe is defined as "a tubular product sized by a nominal dimension (nominal pipe size) and a wall thickness schedule designation." Pipe can be manufactured in a variety of methods; however, the sizing criteria will always remain the same. Pipe manufacturing methods are discussed later.
Pipe is produced in three weights, or general wall thickness classifications:
1. Standard (Std)
2. Extra Strong (XS), or Extra Heavy (XH)
3. Double Extra Strong (XXS), or Double Extra Heavy (XXH)
Light-wall, lightweight, or light-gage pipe, as it may be referred to, is another weight classification sometimes given to steel pipe. This pipe classification is used extensively in many sprinkler installations and other applications where a thinner wall pipe may be preferred. The light-wall pipe designation corresponds to schedule number 10 for steel pipe in most sizes.
Pipe dies and fittings, therefore, remain the same for specific sizes of pipe, no matter what the weight. Because of the variation in inside diameter, pipe sizes from 1/8 inch (6 mm) to 12 inches (300 mm) are designated by nominal inside diameter (ID), not by the actual inside diameter. Nominal sizes are referred to as nominal pipe size (NPS) or, less commonly, iron pipe size (IPS). Pipe sizes over 12 inches (300 mm) are classified by actual outside diameter (OD).
To further broaden the range of wall thickness for specific applications and various pressures, steel pipe is manufactured in assorted schedule numbers. These schedule numbers range from 10 to 160 and are commercially available in schedules 10, 20, 30, 40, 60, 80, 100, 120, 140, and 160.
Some schedule numbers and weight classifications of steel pipe have the same wall thickness. Wall thickness for standard-weight pipe and schedule 40 are the same for sizes 1/8 inch (6 mm) through to 10 inches (250 mm). All standard-weight pipe sizes over 10 inches (250 mm) have a constant wall thickness of 3/8 inches (9.53 mm), whereas schedule 40 pipe has a wall thickness that varies depending upon the particular size over 10 inches (250 mm).
Pipe sizes up to 8 inches (200 mm) in extra heavy and schedule 80 have identical dimensions. Over 8 inches (200 mm), extra-heavy pipe has a constant wall thickness of 1/2 inch (12.7 mm), whereas the wall thickness for schedule 80 pipe varies depending on size.
There is no exact corresponding schedule number for double extra heavy pipe. Generally, double extra heavy pipe up to 6 inches (150 mm) has a thicker wall than schedule 160 pipe. However, in sizes over 6 inches (150 mm) NPS, schedule 160 becomes the thicker-walled pipe. Figure 1 shows the relationship between wall thickness, schedule number, and pipe weight.
Figure 1: Wall Thickness and Schedule Numbers
Standard fluid line systems, whether for simple household use or for the more exacting requirements of industry, were, for many years, constructed from threaded pipe of assorted materials and assembled with various standard pipe fitting shapes, unions, and nipples.
Besides being cumbersome, inefficient, and costly to assemble and maintain, under high pressures, such systems were plagued with leakage problems. Therefore, pipes in these systems have largely been replaced with tubing because of the many advantages it offers.
Using the old method, each connection would be threaded, requiring numerous fittings. This type of system was not flexible or easy to install, and service connections were not smooth inside, producing pockets that tended to obstruct flow.
Now, bendable tubing is used which needs fewer fittings, requires no threading, and is lighter and more compact. Additionally, using tubing makes the system easy to install and service, with no internal pockets or obstructions to free flow.
Tube differs from pipe in that it does not have the more liberal tolerances for inside diameter, outside diameter, wall thickness, and nominal sizes given to pipe. Pipe sizes up to 12 inches (300 mm) are designated by nominal sizes, which are smaller than the outside diameter of the pipe, whereas, in most cases, tube sizes are identical to the outside diameter of the tube. Tubing is classified into three major types: structural tube, mechanical tube, and pressure tube.
Structural tube is used in the construction of such things as: building frameworks, roadway median barriers, bridge structures, and other general structural applications. It is available not only in round tubing shapes, but also in rectangular, square, and other special structural shapes as needed. Figure 2 shows some of the more common tube shapes available.
Figure 2: Structural Tube Shapes
A company known as ASTM has provided standards covering ferrous and non-ferrous structural tube in both welded and seamless forms. Sizing of structural tube is specified by actual outside diameter and wall thickness. Maximum sizes normally extend up to 24 inches (609.4 mm) for round tubing, with wall thickness up to 1 inch (25.4 mm). Structural tubing with other dimensions may be furnished, providing they meet ASTM specifications or their equivalent.
Mechanical tube is used in a variety of mechanical and structural applications; like structural tube, it is not intended to carry fluids or gases under pressure. Because mechanical tubing usually is manufactured for specific applications needing particular mechanical and chemical properties, only limited standards are covered by ASTM or other agencies. Sizes and dimensions usually are determined by the established end usage or customer needs.
Pressure tube is the type of tubing used most often in the piping industry. It is designed to carry fluids under pressure. Sizing is customarily designated by the tubes actual outside diameter and wall thickness, or tube gage, given in either fractions of an inch, decimals of an inch, millimeters, or wire gage (usually in the Birmingham Iron Wire Gage/BWG). Table 1 gives wall thickness equivalents for BWG, decimals of an inch, and millimeters.
Tubing is available in several material classifications, and then in sub-groupings under each material heading. The following are the general material classifications most often given to pressure tubing:
Aluminum tubing is available in both welded and seamless forms in various alloys, tempers, and wall thicknesses. Pure aluminum is seldom used for manufacturing tube, but rather is alloyed with other metals to improve its physical properties.
Most tube is produced to general-purpose ASTM B-210 standards for drawn seamless aluminum alloy tube. This tube is supplied in sizes ranging from 1/8 inch (3.175 mm) to 12 inches (304.8 mm) outside diameter and is available in straight lengths and coils. Coils are supplied in annealed temper only, with their maximum wall thickness not exceeding 0.083 inches (2.11 mm). Coiled rolls usually are supplied in 50-foot (15 m) and 100-foot (30.5 m) lengths, but lengths up to and exceeding 500 feet (153.5 m) are available.
Straight tube lengths usually are available in 12-foot (3.66 m) lengths, but they may be special-ordered up to 50 feet (15 m) or more. Wall thicknesses for straight aluminum alloy tube can range between 0.01 inches (0.25 mm) to 0.50 inches (12.50 mm).
Numerous types of copper and copper-alloy tubing are manufactured. However, the majority of copper tube used in the piping industry is manufactured from 99.90% pure copper (minimum). This copper tube can be classified into two general groupings.
One type of tubing designation has tubing measured by its approximate inside diameter and is often referred to as plumbing tube. This designation consists of tube types K, L, M, pressure tube, and DWV, a non-pressure type of tubing.
The other type of designation has tubing that is designated by the outside diameter and wall thickness measurements. It is comprised of "ACR" (air conditioning and refrigeration) tubing and "GP" (general-purpose) tubing.
Nominal ID sizes are used to designate tubing sizes for the imperial, or inch, system, with actual outside diameters of the tube being 1/8 (0.125) inch (3mm) larger than nominal tube size.
Copper tube types K, L, and M are available in either annealed (soft) temper copper or drawn (hard) temper copper.
Soft copper tube is commercially available in types K and L; however, type M soft can be supplied. Soft copper is normally supplied in coils ranging from 40-foot (12.2 m) to 100-foot (30.5 m) lengths. Coils are manufactured in nominal sizes ranging from 1/4 inch (8 mm) through to 2 inches (50 mm).
Hard copper (types K, L, and M ) is supplied in straight lengths of 12 feet (3.66 m) or 20 feet (6.10 m). Nominal sizes of copper water tube range from 1/4 inch (6 mm) through to 12 inches (308 mm), with wall thickness varying per type classification.
Copper tube that is used for oxygen lines and other medical gases is often referred to as oxygen tube. This tube basically is type K or L tube that has been specially cleaned and capped. The cleaning is a safety measure to prevent contamination and possible spontaneous combustion with organic oils or impurities in the tube.
Color-coding is applied to copper tubing to help distinguish between the various types of tube.
It should be noted that annealed copper is not color-coded in any types.
Type DWV copper drainage tube is another type of copper tube that is classified by its approximate inside diameter. Specifications for this tube are covered in the ASTM B 306 standard for copper drainage tube. DWV tube is intended for use in drainage, waste, and vent applications, which is what the "DWV" abbreviated letters stand for. The tube has a color code of yellow and is manufactured in 12-foot (3.66 m) and 20-foot (6.10 m) straight, hard temper lengths only. The wall thickness of DWV is thinner than in type K, L, or M tubing.
ACR (air conditioning and refrigeration) and GP (general-purpose) copper tubing are two types of tube that are measured and designated by outside diameters and wall thickness.
Seamless copper tube designed for air conditioning and refrigeration field service is called ACR tube. ACR tube is covered under ASTM specification B 280. It is good for carrying most commercial refrigerants (except ammonia).
ACR tube is available in 12-foot (3.66 m) or 20-foot (6.10 m) straight, hard temper lengths and standard 50-foot (15.2 m) soft temper coils.
Hard temper lengths of tube are color-coded blue and identified with the ACR inscription. This tube differs from other copper tube in that it is thoroughly cleaned, degreased, dehydrated, and capped prior to delivery (tube may be supplied nitrogen charged). Another point of deviation from other copper tube is that annealed temper tube dimensions differ in size designations.
GP tubing is the classification of seamless copper tube designed for general engineering purposes. ASTM specification B 75 covers specific requirements, while ASTM B 251 and B 251 M (Metric) standards cover common requirements, such as lengths, wall thicknesses, and other general dimensions and specifications (see Table 2).
Table 2: General-Purpose Copper Tubing (Fractional) General Dimensions
Carbon-steel and stainless-steel pressure tubing are used for various applications that can be classified into several general groupings. The following are the major grouping classifications for both carbon-steel and stainless-steel pressure tubing:
Seamless and welded boiler and superheater tubes are available in both hot-rolled and cold-drawn tube for various pressure and temperature applications. The tube is sized by outside diameter and minimum wall thickness. Boiler and superheater tubes are made to various ASTM specifications, depending on the type of operation and their designated use.
Tubing under this classification is used for heat exchangers, condensers, and similar units where the tube is used to transfer heat from one medium to another. Tubing is designated by outside diameter, minimum wall thickness, and, frequently, the exact tubing length.
Still tubing is designed for process refinery-type heater applications where the tube is subject to external furnace temperatures higher than those of the fluid or vapor contained inside the tube. It usually is supplied in seamless, hot-finished, or cold-drawn types for various ranges of temperature and pressure. Like other types of tube, it is sized by outside diameter and minimum wall thickness.
Tube classified under this heading is used for applications that require general service types of tubing. ASTM A179 and SAE J524b (Society of Automotive Engineers) standards usually are the two major carbon-steel tube specifications used under this classification. The tubes are low-carbon seamless tube, cold-drawn, and, in the case of the SAE tube, annealed for bending and flaring. Stainless-steel tubing for general service is covered under either ASTM A268 for ferritic stainless steel or ASTM A269 for austenitic stainless steel. These standards cover both welded and seamless tube, which are sized by their outside diameter and wall thickness. Refer to Table 3 through Table 6.
Table 3: Carbon-Steel Tubing (Fractional) General Dimensions
Table 4: Carbon-Steel Tubing (Metric) General Dimensions
Table 5: Stainless-Steel Tubing (Metric) General Dimensions
Table 6: Carbon-Steel Tubing (Fractional) General Dimensions
Piping is obviously one of the key elements of a pneumatic system. It has to transport the compressed air to the various components, as well as keep the system clean. The piping must also be strong enough to prevent leaks, which can lead to personnel injuries and equipment damage.
Another feature that must be incorporated is flexibility. This is to prevent vibration from damaging the lines and to allow for expansion and contraction. While hoses in the system mainly are used to supply portable equipment, they also provide flexible joints, or elbows, between moving and stationary equipment. Hoses that supply portable pneumatic equipment normally are fitted with quick-disconnect couplings. These are more suitable than threaded fittings for equipment that is frequently coupled and uncoupled.
The air distribution, or piping, system includes all of the pipes, hoses, valves, and fittings needed to connect all of the other components of the system together. From the intake filter to the compressor, and on through the coolers, separators, and equipment driven by the air, piping is required. Numerous control lines are used to connect the various control components of the air supply system.
When a compressed air system initially is installed, a provision must be made for connecting all of the system components (compressors, receivers, separators, and lubricators) wherever they are needed. Outlet lines, complete with drain legs and provisions for installing additional equipment and outlets, should also be included. Figure 3 shows a simple piping system. These pipelines must be properly designed, installed, and maintained if the equipment is to be kept operating properly.
Figure 3: Simple Piping System
In a typical installation, the piping system contains a combination of pipes, tubing, hoses, and fittings. These lines must be free of air leaks and any excessive pressure drops. The system must also have sufficient mechanical strength to withstand excessive air pressure, pulsation or hammer, vibration, and other shocks.
To provide maximum airflow efficiency in a pipe, a smooth flow pattern should exist. This is referred to as streamlined or laminar flow. A streamlined flow of air occurs when the line is large enough to handle the air flowing through it at a low enough velocity to avoid turbulence. The inside of the air line should be smooth, and the piping should not change direction more than absolutely necessary. The number of fittings used in the line should also be kept to a minimum. Any fittings used should not restrict the flow. Turbulent flows waste energy by creating heat.
When selecting the pipe for an air line, the sizes must be large enough that the pressure drop between the air receiver and the farthest point of use does not exceed 10% (or 5 to 10 psi) of the compressor cutout pressure. Inlet and discharge lines and fittings that connect the compressor to all components at the beginning of the system should also be large enough to deliver air efficiently to the air receiver.
The air line size required for each individual application depends on several things:
The size of the plant and the location of the individual compressor(s) determine how far the air has to be transmitted. Some installations use a unit, or localized, system, while others use centralized, or loop, systems that service a large area of the facility. Figure 4 shows a centralized distribution system, and Figure 5 shows a typical unit distribution system. In a loop system, auxiliary air receivers may be installed away from the compressor to reduce the distance that the air must travel during peak flow conditions. That avoids the excessive pressure drops that might occur if the peak flow had to be supplied throughout the entire system. Additional compressors can also be installed to assist during peak flow.
Figure 4: Centralized Distribution System
Figure 5: Typical Unit Distribution System
Due to the fact that air is compressible, the size of the individual lines normally is not as critical as the size of hydraulic lines. Problems can still occur, but the majority can be avoided if a reasonable amount of care is taken when selecting the pipe size. The approximate pipe sizes required to transmit compressed air at about 100 psi are given in Table 7. Note the volume range that can be handled by a single pipe diameter. More specific data on the sizing of pipes for a compressed air system can be obtained from design reference books.
When selecting a pipe size that will ensure a minimum pressure drop, factors other than diameter alone must be considered. The actual pressure losses that will occur in an operating compressed air system are shown in Figure 6. In the figure, each individual line contains various fittings, valves, tools, and components. When air is not being used, all of the pressure gauges shown should indicate the same pressure. However, as soon as there is airflow in the system, the pressure gauges will indicate various pressure drops in the system, as shown. Pressure drops and losses can never be completely eliminated, but they can be kept to a minimum by proper system design.
Figure 6: Operating System Pressure Losses
Different pipe sizes will produce different pressure losses due to the friction between the inside diameter of the pipe and the compressed air. Table 8 shows the approximate pressure losses in psi for air in 1,000 feet of piping of different sizes. The losses shown are for an operating pressure of 100 psi.
The effect that airflow volume and pipe diameter have on pressure losses has already been shown. The final two factors that must be considered are the operating air pressure and the number of fittings in the line.
As the operating pressure of the system is decreased, the pressure losses throughout the system increase. This occurs because of the fact that the forces of friction more easily affect lower air pressures.
Fittings affect pressure losses due to the change they make in the inside area. The change in the inside area of the fittings produces a larger pressure drop than an equivalent length of straight pipe. Due to the fact that pressure drops are calculated based on lengths of straight pipe, the pressure drop through a fitting is given as a length of straight pipe rather than in psi.
Table 9 gives the pressure drop for various fittings in feet of pipe length. To find out the actual pressure drop across these components, the value given in the table must be converted to a pressure drop using a piping pressure loss table. From looking at the table, it is easy to see that one or two fittings in a long line of piping will have little effect on the overall pressure drop. However, several valves and fittings in a short run of piping can have a great effect on the overall pressure drop.
The main header compressed air lines in most installations are constructed of black steel pipe. Secondary and low-pressure applications use pressure-rated hoses, metal tubing, and plastic tubing. Black steel pipe is used because of its availability, strength, availability of multiple fittings, and ease of installation. Pipe does not have the variety of sizes that tubing has, but a large variety is not really required for a pneumatic system. All rigid piping used in a pneumatic system should have a minimum burst pressure of at least eight times the operating pressure of the circuit.
The nominal dimensions of schedule 40 (standard) pipe, including working pressure, bursting pressure, and area, for some of the common sizes are shown in Table 10. For stronger pipes, the wall thickness and the inside diameter will change, but the outside diameter for each nominal pipe size remains the same. This allows one size of pipe threads to be used for all pipes having the same nominal size.
The bursting pressure for schedule 40 black steel pipe is much higher than the actual working pressure of most pneumatic systems. This provides a safety margin for system operation. For example, if a 3-inch pipe is used to carry 250-psi air, the safety margin (SM) is:
Although pneumatic pressures are much lower than hydraulic pressures, a high safety factor is still desirable. Compressed air can still be very dangerous if allowed to uncontrollably expand. An example of uncontrollable expansion is when an air line bursts.
Schedule 40 pipe has a safe working pressure for most air lines of up to 175 psi. The piping must also be properly mounted and supported to prevent mechanical shock and vibration. When shocks, vibration, higher pressures, corrosion, and other abuses are expected, a higher safe working pressure can be obtained by using schedule 80 (extra strong) pipe. In addition, the threaded connections are less likely to break with schedule 80 pipe due to the thicker wall section.
There are three methods of connecting rigid pipes in a pneumatic system:
Threaded 125-pound cast iron fittings are actually rated for 125-psi saturated steam applications, but they can be used for 125-psi air lines that are not subjected to shock, vibration, or bumping. The more durable 150-pound malleable iron fittings are better suited for air line use. These fittings are stronger and more corrosion-resistant. 300-pound malleable iron fittings are the most suitable for rough service. They are approximately twice as strong as the 150-pound fittings. The letters MI or the number 150 or 300 on them can identify malleable iron fittings.
Air lines with welded fittings have the least leakage and pressure drops. However, welded lines are not as easy to install as lines with threaded fittings, and it is harder to install additional outlets when required. Large air lines are more often welded than small air lines because they usually are not changed after installation. Threaded adapters are available for changing from welded piping to threaded piping at outlet points.
Flanged connections are used most often for medium and large connections between piping and equipment. Large compressors, aftercoolers, separators, and air receivers are often equipped with flanges. Even though both iron and steel flanges are manufactured, steel flanges are the only ones recommended for use in pneumatic power systems. Because they are stronger than cast iron, they can take more abuse and do not crack as easily. Even though there are several different classes (pressure ratings) of steel flanges, 150-pound and 300-pound flanges are the ones most commonly used.
Piping for a compressor inlet should be large enough that the air velocity in the pipe does not exceed 2,500 fpm. Short intake lines can be the same size as the compressor intake opening provided that the intake flow velocity is not more than 2,500 fpm. If the compressor intake line is more than seven feet long, the intake pipe size should be one size larger than the pipe that would have a velocity of 2,500 fpm. When determining the intake pipe length, include the equivalent lengths of fittings that are used.
The actual velocity of intake airflow can be determined by dividing the compressor's free air capacity (cfm) by the area of the intake pipe in square feet. For example, if a 100 cfm compressor has a 2 -inch inlet connection for a filter, the air velocity through a 2 -inch intake line is:
This velocity exceeds the maximum velocity recommendation of 2,500 fpm. If the size of the intake pipe is increased to 3 inches, the air velocity now becomes:
The airflow has now decreased to an acceptable value for this application.
Rigid piping is also used for compressor discharge, distribution, and workstation lines. Piping is recommended over tubing for these applications because it is more economical and requires fewer supports.
When rigid pipe is installed in compressor discharge and distribution lines, the horizontal pipe runs should slope downward approximately 1 inch for every 10 feet of pipe. The slope should also be in the direction of airflow. An example of this is shown in Figure 7. This allows any water in the lines to flow into drain legs that collect it. Drain legs normally are fitted with a manually operated valve so that the water can be removed periodically.
Figure 7: Example Drainage Slope Installation
To eliminate as much water as possible from the distribution lines, the outlet points should always be taken off the top of the distribution line. Additional water and other contamination can be removed from the outlet line by the outlet drain legs. Outlet lines should also be sized large enough to handle the amount of air required by the tools they are to operate.
Piping used in pneumatic systems has the National Pipe Thread (NPT) label. An NPT has a spiral clearance along the crest of the thread; it seals against air leaks with flank (face or side) contact and the help of pipe compound (pipe dope). The pipe compound also tends to reduce friction, prevent galling, and permit uniform tightening of threaded connections.
When connecting pipes, follow these simple guidelines:
Metal tubing differs from rigid pipe in several important ways. First, it can be manufactured by a drawing process, by welding, by extrusion, or by rolling. Second, tubing has more accurate inside and outside diameters and a better surface finish. Third, tubing is made in many different diameters and wall thickness. Finally, metal tubing is made of stronger and more flexible materials than rigid pipe. Tubing materials include the following:
All metallic tubing is recommended for use in pneumatic lines with pressures of up to 250 psi.
Metal tubing that is larger than 2 inches in diameter seldom is used in pneumatic power systems. Sizes of less than 5/8 inch outside diameter usually are preferred. Smaller sizes of tubing can be used for control lines and to make neat, close fitting connections between different pieces of equipment. Larger tubing is used for finned, air-cooled compressor intercoolers and aftercoolers. Table 11 shows some of the sizes of metallic tubing that are available. Note that the tube size is the same as its OD (outside diameter).
The size of the tubing used to connect pneumatic components is determined by the size of the tapped holes in the equipment for line connections. These usually are large enough to provide sufficient airflow. Whenever smaller or larger tubes are connected to the threaded openings, bushings are used in the pipe's threaded hole to accommodate the change in size. Although most sizes of tubing have high safe working pressures, it is a good idea to check the strength of any tube before it is used for something other than a normal application.
Metal tubing (Figure 8) generally is easier to install than pipe because it requires fewer connections. Most required changes in direction can be made by bending. Care must be used when bending tubing though because once it is bent, it cannot be straightened or changed without deforming the tube.
Figure 8: Proper Tube Bending
Bending tools are used to bend the tubing to the proper radius. The proper bend radii of several different sizes of tubing are shown in Table 12. A bend radius that is smaller than the ones shown is undesirable because it causes increased frictional pressure losses in the line. Tubing should be formed into a smooth bend that maintains a round cross-section. The total number of bends in the line should also be kept to a minimum.
Figure 9: Pipe Bend Measured from Radius
In addition to care in bending or forming a tube, the following precautions should be followed to achieve tight tubing connections:
There are a number of different types of pneumatic tube fittings available from various manufacturers. The fittings are designed to connect the tubing to equipment ports and to connect pipe and hose fittings firmly and without leakage. Most fittings have recommended safe working pressures for various types of service. The published maximum working pressures recommended for each type of service should be used in selecting, installing, servicing, and replacing tube fittings. Some of the different fittings used with pneumatic systems are shown in Figure 10.
Figure 10: Common Metallic Tube Fittings
Fittings are designed to prevent leaks at connections and protect the tube against damage. Flared fittings use a long shouldered nut or support sleeve for increased stability when vibration is likely to be encountered. Both regular and inverted flared fittings, as well as permanent fittings, are available in most tube sizes up to 2 inches in diameter. Compression and threaded sleeve fittings are available for tubing up to 1 inch in diameter. These fittings do not use flared ends; therefore, their pressure ratings are lower.
A proper tube installation requires that the bends made and fittings selected minimize shock loads and temperature changes. Too many, too few, or poorly located bends will also cause an increase in flow losses and leakage. The following general guidelines should be used when installing metallic tubing:
Nonmetallic, or plastic, tubing is available in a number of different materials. The most common materials available include:
Most plastic tubing is limited to working pressures below 100 psi and temperatures below 190F. However, some kinds of tubing can be used for pressures up to 145 psi at 375F. The advantage of plastic tubing is that it is resistant to chemical attack and will not corrode. In addition, polyethylene is available in colors, which can be helpful in troubleshooting complicated systems. Nonreinforced nylon is also used for extreme temperature applications (between 100F and 225F and at pressures up to 250 psi). Plastic or nylon tubing is excellent for pilot control lines and low-pressure lines that do not flex much.
Some of the fittings available for plastic tubing are shown in Figure 11. These include brass compression, compression, O-ring, and barbed fittings. Compression fittings support the plastic tube and prevent it from collapsing when the tube nut is tightened. Barbed fittings may be used with or without some type of hose clamp, depending on the line pressure. Always observe the manufacturer's pressure and temperature recommendations for plastic tubing and fittings.
Figure 11: Nonmetallic Tube Fittings
There are many types of tubing joints used to connect pipe or tubing. This section concentrates several types of tubing connections, both flared and flareless.
Either flared or flareless joints can connect tubing. Each application has its advantages, and both are commonly used. Each type is discussed separately in the following paragraphs.
Flared connectors provide safe, strong, dependable connections without the necessity of threading, welding, or soldering the tubing. The connector consists of a fitting, a sleeve, and a nut (Figure 12).
Figure 12: Flared Tube Connector
Flaring is done by evenly spreading the end of the tube outward, as shown in Figure 13. The angle of the flare must be accurate to match the angle of the fitting being used. A flaring tool is inserted into the squared end of the tubing and then hammered into the tube a short distance, spreading the tube end as required. This is called the impact method. An alternate method uses a screw-type flaring tool as described earlier.
Figure 13: Flared Tube Ends
Figure 14 illustrates how a flare is used to form a leak-proof joint between a tube and a fitting. It also shows some incorrectly made flares.
(A) Correctly Made Flare, (B) Flare Too Small, (C) Flare Too Large, (D) Flare Is UnevenFigure 14: Flared Fittings
Figure 15 shows the resulting flared joint. Note that the flared section is inserted into the fitting in such a way that the flared edge of the tube rests against the angled face of the male connector body. In this fitting, a sleeve supports the tubing. The nut outside the sleeve is tightened firmly on the male connector body, making a firm joint that will not leak even if the tubing ruptures because of excess pressure.
Figure 15: Flared Fitting
Some flares use a single thickness of the tube. Other flares are made with a double thickness of metal in the flare surface. Called double flares, they are stronger and usually cause less trouble if properly made.
Most flares are made at a 45o angle to the tube. Flares on steel tubing, however, usually are made at a 37o angle. This is because steel tubing is not as easily flared as copper tubing.
To make a flare of the correct size using a flaring block, you would perform the following steps:
1. Carefully prepare the end of the tube for flaring. The end must be straight and square with the tube, and the burr from the cutting operation removed by reaming. Figure 16 shows the steps necessary to prepare a tube for flaring.
Figure 16: Tube Prepped for Flaring
2. First, use a smooth mill file to square the end of the tube. Use care that no fillings enter the tubing. Next, use a burring reamer to remove the slight burr remaining after the cutoff operation.
3. Place the flare nut on the tubing with the open end toward the end of the tubing. Insert the tube in the flaring tool so that it extends above the surface of the block, as shown in Figure 17(a). This allows enough metal to form a full flare.
4. If the tube extends above the block more than the amount shown, the flare will be too large in diameter and the flare nut will not fit over it. If the tube does not extend above the block, the flare will be too small, and it may be squeezed out of the fitting as the flare nut is tightened. Figure 17(b) shows the appearance of a completed flare.
Figure 17: Tube Flaring
5. To form the flare, first put a drop of oil on the flaring tool spinner where it will contact the tubing. Tighten the spinner against the tube end one-half turn and back it off one-quarter turn. Advance it three-quarters a turn and again back it off one-quarter turn. Repeat the forward and backing off movement until the flare is formed.
Some facilities make the flare using one continuous motion of the flaring tool; that is, without a back-and-forth motion. Others believe that constantly turning the tool without back-turning may harden the tubing and make it more likely to split. Other facilities like to use a flare that is not completely formed and is instead about seven-eighths complete. They depend on the tightening of the flare nut on the flare to complete it.
Double-thickness flares are formed with special tools. Figure 18 illustrates a cross-section through a simple block-and-punch type of tool that makes a double flare. The correct shape of the double flare is shown in the final section in this figure. Some flaring tools are fitted with adapters that make it possible to form either single or double flares with the same tool.
Figure 18: Block-and-Punch Tube Flaring
In a simple block-and-punch tool for forming double flares on copper tubing, the tube is clamped in the body of the flaring block (A). The female punch bends the end of the tube inward (B). The male punch is inserted partially in the flared tube (C). The male punch folds the end of the tube downward to make a layer that is twice as think and expand the flare into its final form.
Figure 19 shows the steps for making a double flare.
Correct procedure for forming double flares using adapters with a combination single and double flaring tool. (A) Tubing, (B) Block, (C) Adapter, and (D) Flaring Cone.
Figure 19: Double Flare Tooling
Double-thickness flares are recommended for only the larger size tubing 5/16 inch and over. Such flares are not easily formed on smaller tubing. The double flare makes a stronger joint than a single flare.
Figure 20 shows examples of flareless fittings. The plain tube end is inserted into the body of the fitting. As shown, there are two threaded outer sections, but in this case, a ferrule or bushing is located between them. As the threaded members are tightened, the ferrule bites into the tubing, making a tight joint. This is also known as a compression fitting.
Figure 20: Flareless Fittings
Another type of flareless tube connector is called a bite-type connector. Bite-type connectors eliminate all tube flaring, yet provide a safe, strong, and dependable tube connection. This connector consists of a fitting, a sleeve or ferrule, and a nut. A flareless tube connector is shown in Figure 21.
Figure 21: Flareless Tube Connector
Flareless tube fittings are available in many of the same shapes and thread combinations as flared tube fittings. The fitting has a counter-bore shoulder for the end of the tubing to rest against. The angle of the counterbore causes the cutting edge of the sleeve or ferrule to cut into the outside surface of the tube when the two are assembled together. The nut presses on the bevel of the sleeve and causes it to clamp tightly to the tube.
Before installing a new flareless tube connector, the end of the tubing must be square, concentric, and free of burrs. For the connection to be effective, the cutting edge of the sleeve or ferrule must bite into the periphery of the tube. This is accomplished by presetting the sleeve or ferrule on the tube using a presetting tool with the same dimensions as the fitting body; this can be obtained from the fitting manufacturer. If a presetting tool is not available, a suitable male-thread fitting may be used. If a fitting must be used, a steel fitting is preferred for this operation. If an aluminum fitting is used as a preset tool, it should not be reused in the system.
After presetting, the connector is disassembled for inspection. If the sleeve or ferrule is satisfactorily installed, the connector is ready for final assembly in the system.
This section describes several types of tubing joints normally used in piping systems, including:
Expansion joints are used in piping systems to absorb thermal expansion and contraction due to fluid temperature changes.
The most common types of expansion joints available are the slip type and the metal bellows type.
Slip-type expansion joints (Figure 22) have a sleeve that extends into the body of the expansion joint. Packing located between the sleeve and the body controls leakage. Leakage in this type of expansion joint is very small and typically can be completely stopped. Although leakage is very small with this type of joint, a leak-free seal is not guaranteed; therefore, it cannot be used in a system where there is a zero leakage criterion. This type of joint requires periodic maintenance, either to compress the packing by tightening a packing gland or completely replace the packing. Slip-type expansion joints are well-suited for lines having straight-line (axial) movements of large magnitudes. However, slip joints cannot tolerate lateral offset or angular rotation (cocking), since this would cause binding, galling, and possibly leakage due to packing distortion.
Figure 22: Slip-Type Expansion Joint
Bellows-type expansion joints (Figure 23) do not have packing. This allows this type of joint to be used where zero leakage is mandatory. Likewise, they do not require the periodic maintenance (lubrication and re-packing) that is associated with slip joints.
Figure 23: Bellows-Type Expansion Joint
Bellows joints absorb expansion and contraction by means of a flexible bellows that is compressed or extended. They can also accommodate direction changes by various combinations of compression on one side and extension on an opposing side. Thus, they can adjust to lateral offset and angular rotation of the connected piping. However, they are not capable of absorbing torsional movement. Typically, the bellows is corrugated metal and is welded to the end pieces. To provide the requisite flexibility, the metal bellows is considerably thinner than the associated piping. Thus, these expansion joints are especially susceptible to rupture by overpressure.
This type of expansion joint is also particularly susceptible to cyclic stress, and the bellows can fail due to metal fatigue if the accumulated flexing cycles exceed the designed fatigue life.
When assembling a bolted joint, proper preload control must be maintained on the threaded fasteners to achieve a safe, fail-proof joint. If a threaded fastener is torqued too high, there is danger of failure on installation by stripping the threads, breaking the bolt, or making the fastener yield excessively. If the bolt is torqued too low, a low preload will be induced in the joint. This leads to fatigue, vibration failure, and joint leakage if sealing is required. For every bolted joint, there is an optimum preload recommendation. This is achieved by properly torquing the bolt/nut assembly.
To achieve a fail-safe design of mechanically fastened joints, the following factors should be considered:
Figure 24 shows a typical rigid bolted joint. In this section, the term bolt is used to describe all types of externally threaded fasteners. The term nut is used to describe internally threaded fasteners.
Figure 24: Rigid Bolted Joint
NOTE: The same properties and principles apply equally to fasteners and joints of all metals.
Whenever a fastener, or joint part, is deformed by the action of external forces, the internal molecular forces of the fastener or part attempt to resist changing shape. In general, a "bolt" can easily be tightened to the point of failure.
When a bolt is tensile-loaded, it elongates elastically until stressed beyond its elastic limit. It then yields and begins to demonstrate plastic behavior. As the tensile load increases, the elongation process continues, but at a much faster rate, until the bolt fractures. Figure 25 shows typical load elongation characteristics of three popular strength grades of carbon-steel inch and metric bolts.
Figure 25: Typical Load Elongation
When a bolt is tightened in a joint, as in Figure 25, load elongation behavior occurs. Because of torsional stress introduced by wrenching the bolt head or nut, the bolt yields and fractures at lower tensile stresses.
The reduction of tensile stress capacity caused by twisting stresses varies depending on friction, but generally the value is 15%.
When a bolt/nut assembly is tightened, the bolt elongates and the joined material compresses. The tensile load developed in the bolt is known as preload. The bolt squeezes the joined material together as though it had been placed in a vise.
Those using fasteners must be aware and have a basic understanding of the types of stresses a bolt/nut assembly is expected to resist. There are five types of stress; these types are described below and shown in Figure 26.
Figure 26: Stress Areas
1. Tension: Tension is a result of forces applied axially to the bolt. A tension force tends to elongate the bolt.
2. Compression: Compression is the squeezing together of a part that may cause flattening out of the part, such as a washer that is squeezed under the head of a bolt or against a nut.
3. Bending: Bending is the deflection of an object subjected to forces that act either at right angles to the object or in the axial direction, but not along the axis of the object.
4. Twisting (Torsional): Twisting is the result of turning forces on the bolts axis while under load or being torqued.
5. Shear: Shear is exhibited when layers of molecules in the material of a bolt slip on one another. Pull exerted 90 to the bolts axis is a shear stress applied to the bolt and joint.
Figure 27 shows both a point of shear on a shaft and keyed hub arrangement and a torque load being applied through the hub.
Figure 27: Shaft Stress Areas
Mechanically fastened joints can be classified as being either rigid or gasketed. Both types of joints have many variations and design features. Each joint may be used for the purposes of:
A rigid joint does not have any gasket or other soft material at the joint interface. The joint is a solid connection and usually made of metallic material. The material acts elastically and is compressed under full bolt/nut load. Many structural joints are designed as rigid connections where full metal-to-metal contact is made between the joint halves.
A typical example of a gasketed joint is a pipe flange connection. Usually, a soft, flexible material is located at the joint interface. The gasket material has sufficient "flowing" or pliable characteristics when it is squeezed to conform to any irregularities and seal off potential leak points.
Analyzing gasketed joints is difficult and complex. A gasket is installed at joint interfaces with the sole purpose of deforming to provide a positive static seal that prohibits leakage. Most gaskets display considerable plastic and elastic properties under compression. A factor that further complicates an accurate analysis of a gasketed joint is that the gasket material has a different coefficient of thermal expansion from those of joint flanges and the connecting fasteners.
Gaskets are integral components in bolted joints. The gasket is subjected to compression incurred from the bolt preload. The gasket prevents metal-to-metal contact and, because of the gaskets deformability characteristics, it controls the stiffness of the bolted joint.
Basic flange joints, as shown in Figure 28A, are suitable for all types of flat gaskets, whether plain or jacketed. For moderate pressures up to 200 psi, the simple flange joint at the top is applicable. For higher pressures, some variation of this joint is required.
Figure 28: Basic Flange Joints
Figure 28B displays a reduced section joint (ring joint). The gasket wall section has been reduced to give a high gasket stress without any change in bolt stress.
The tongue and groove joint (Figure 28C) is useful for applying extremely high stress to gaskets and for holding high fluid pressures. Since it completely confines the gasket, it limits the flow that might otherwise occur in many materials, especially neoprene.
Metal-to-metal flange joints contain a gasket also. These joints are particularly suitable for truly compressible materials, such as cork, neoprene, and soft metal gasket materials. These joints, identified in Figure 29A, B, and C, are similar in design to bolted joints in which O-rings are used for sealing purposes.
Figure 29: Metal-to-Metal Flange Joints
Rectangular-, round-, or oval-shaped gasket materials can be located in a machined groove in one or both flange faces. The initial gasket volume (size) must not exceed that of the machined cavity. The initial shape should be such that less than 20 to 25% deflection of the gasket occurs as the joint is compressed by the bolted preload. These bolted joints are excellent for machinery applications such as gear location and bearing preload, where it is absolutely essential to maintain accurate clearances or alignment. They are also useful for limiting the amount of stress on a gasket.
The major problem in gasketed joints that are bolted together is determining how much preload should be developed in the connecting fasteners. If the preload at the gasketed joint is too low, the joint will leak or be weak. Conversely, if the preload value is too high, there is a strong risk that the gasket will be excessively crushed or warped. This leads to leakage conditions or, worse, the joint flanges becoming distorted, misaligned, or broken.
One recommendation is to keep the combined tensile load (preload plus service load) below the bolts yield strength. If a bolts yield strength is exceeded, the bolt will be permanently deformed.
Bolt preload should be kept as high as necessary to prevent leakage without causing damage to the bolted joint; generally, the higher the better. Frequently, however, the best level of preload can only be determined through field tightening and re-tightening.
In pipe sizes larger than 2 inches, it is common practice to use flanged connections or joints where pipelines, piping components, valves, or equipment must be frequently disassembled for maintenance. A flange may be cast or forged as part of the valve, fitting, or machine on which it is used, but the mating flange that connects the pipe to the part may be attached to the pipe by welding, use of a lap joint, or by threads.
Flanged fittings normally are made of cast iron or steel. Cast iron fittings should not be used where they may be subjected to shock. Cast iron flanged fittings are made in three classes: 25, 125, and 250 pound.
Cast steel fittings and forged steel flanges are available in ratings from 150 pounds to 2,500 pounds.
Figure 30: Slip-On Flange
Figure 31: Welded-Neck Flange
Figure 32: Lap-Joint Flange
Figure 33: Screwed Flange
Figure 34: Reducing Flange
Figure 35: Blind Flange
Equal in importance with the type of flange used is the facing machined on the flanges of the bolted joint. Two types of contact surface finishes are used in mechanical and piping bolted joint designs: "smooth" tool finish and "serrated."
The surface finish of a flange or joint is an important factor in determining the extent to which a gasket must flow to secure an impervious seal. The finish most frequently provided on cast iron and steel pipe flanges and on equipment joints is the smooth tool finish. Bolting that results in adequate gasket flow to form a complete seal is best accomplished with a smooth finish. It may be very difficult to secure a tight joint with flanges having rough surfaces.
The serrated finish consists of spiral or concentric grooves, usually about 1/64 inch (.40 mm) deep with 32 serrations per inch (25.4mm).
NOTE: Where metal gaskets are used, a smooth surface produced by grinding or lapping usually is provided. Some facings in bolted joints mate metal-to-metal without a gasket. An example of this is a steam turbine's upper and lower casing. A mirror-like, smooth, hand-scraped finish is necessary to retain the high steam pressures. Grinding, lapping, or hand scraping can produce this surface finish. It is evident that the surface finish varies with the type of contact face and gasket used and, therefore, should be specified accordingly.
Figure 36: Raised-Face Flange
Figure 37: Male and Female Flange (Large)
Figure 38: Male and Female Flange (Small)
Figure 39: Tongue and Groove Flange (Large)
Figure 40: Tongue and Groove Flange (Small)
Figure 41: Flat-Face Flange
Figure 42: Ring-Joint Flange
Figure 43: Male-Female Screwed Flange
Two methods used to secure flanges are commonly available. One method is to use a bolt/nut arrangement, and the other method is to use a studbolt with one or two nuts to secure it.
Studbolts have largely displaced regular bolts for bolting flanged piping joints used for applications such as:
Studbolts have four major advantages over regular bolts for joint securing:
Figure 44: Permanently Fixed Studbolt
A studbolt is a cylindrical bar threaded at both its ends. It can be one of two types. Figure 45 shows a fully threaded stud bolt. Additionally, figure 46 identifies a studbolt threaded at both ends with its center portion unthreaded.
A studbolt has no head; it cannot be turned with a wrench on its own. If it is necessary to turn a studbolt, one method used is to double-nut one end of the stud (see Figure 45). Both nuts are first screwed well down on the studbolt. The lower nut is then held in position with a wrench, while a second wrench is used to tighten the upper nut down onto the lower nut. This practice locks both nuts together, allowing one to tighten the studbolt into the tapped hole. To release the nuts, the upper nut is held firmly while the lower nut is loosened off, down the stud bolt.
Figure 45: Double-Nut Method
Another method is to use a fabricated stud driver, as in Figure 46. It works well in awkward positions and on short studs.
Figure 46: Studbolt Driver
Figure 47 shows how a studbolt is installed correctly in a blind tapped hole. One end of the stud is screwed into a tapped hole in the body of the main component.
Figure 47: Studbolt in a Blind Tapped Hole
This hole must be of sufficient depth to allow the studbolt to be tightened down to the unthreaded center section of the stud. The projecting end of the stud is then passed through a clearance hole in the mating component and tightened with a nut.
NOTE: When bolts or studbolts are installed into blind tapped holes, caution must be taken to prevent "hydraulic lock." If lubrication is used on fastener threads, be careful not to apply too much. Excess lubricant may accumulate in the bottom of a blind hole. If the bottom of the bolt or stud presses on teh liquid, a "hydraulic lock" is created. The action, as shown in Figure 48 may cause the casting or part to crack or burst.
Figure 48: Studbolt Causing Hydraulic Lock
Studs should be removed from clearance holes or tapped holes carefully to prevent thread damage. Double-nutting the stud is one common method used to remove a stud from a part when the studs threads are in good condition.
Frequently though, a stud may have stripped or damaged threads; therefore, double nuts will not work.
In this case, a cam- or wedge-type stud puller, as shown in Figure 49, can be used to remove a damaged studbolt. This specialized puller grips the stud with a knurled cam or wedges as turning pressure is applied with a socket wrench drive.
Figure 49: Wedge-Type Stud Puller
It is important to use high-quality, annealed tubing and quality cutting tools. Proper deburring of both the inside diameter (ID) and the outside diameter (OD) is required to remove all metal chips and burrs.
Prior to making bends, it is necessary to mark the tubing. First, make a reference mark on the end of the tubing to indicate where layout measurements begin. Next, make a measurement mark to indicate where the tube should be aligned in the bender. Always make this mark a full 360 around the tubing.
This section describes the process of installing tubing and the testing and inspection necessary after installation is complete.
Pipes and fittings, with their necessary seals, make up a circulatory system for liquid-powered equipment. Properly selecting and installing these components is very important. If it is improperly selected or installed, the result would be serious power losses or harmful liquid contamination.
The following is a list of some of the basic requirements of a circulatory system:
The three common types of lines in liquid-powered systems are pipes, tubes, and flexible hose, which are also referred to as rigid, semi-rigid, and flexible line.
You can use piping that is threaded with screwed fittings with diameters up to 1 inches and pressures of up to 1,000 psi. Where pressures will exceed 1,000 psi and for required diameters are over 1 inches, piping with welded flanged connections and socket-welded size are specified by nominal inside diameter (ID) dimensions. The thread remains the same for any given pipe size regardless of wall thickness. Piping is used economically in larger-sized hydraulic systems where large flow is carried. It is particularly suited for long, permanent straight lines. Piping is taper-threaded on its OD into a tapped hole or fitting. However, it cannot be bent. Instead, fittings are used wherever a joint is required. This results in additional costs and an increased chance of leakage.
The two types of tubing used for hydraulic lines are seamless and electric-welded. Both are suitable for hydraulic systems. Seamless tubing is made in larger sizes than tubing that is electric-welded. Seamless tubing is flared and fitted with threaded compression fittings. Tubing bends easily, so fewer pieces and fittings are required. Unlike pipe, tubing can be cut and flared and fitted in the field. Generally, tubing makes a neater, less costly, lower-maintenance system with fewer flow restrictions and less chances of leakage. Figure 50 shows the proper method of installing tubing.
Figure 50: Method of Installing Tubing
Knowing the flow, type of fluid, fluid velocity, and system pressure will help determine the type of tubing to use. (Nominal dimensions of tubing are given as fractions in inches or as dash numbers. A dash number represents a tubes outside diameter in sixteenths of an inch.) A systems pressure determines the thickness of the various tubing walls. Tubing above 1/2 inch OD usually is installed with either flange fittings with metal or pressure seals or with welded joints. If joints are welded, they should be stress-relieved.
When flexibility is necessary in liquid-powered systems, use hose. Examples would be connections to units that move while in operation, units that are attached to a hinged portion of the equipment, or units that are in locations that are subjected to severe vibration. Flexible hose usually is used to connect a pump to a system. The vibration that is set up by an operating pump would ultimately cause rigid tubing to fail.
Rubber hose is a type of flexible hose that consists of a seamless, synthetic rubber tube covered with layers of cotton braid and wire braid. Figure 51 shows cutaway views of typical rubber hose. An inner tube is designed to withstand the material passing through it. A braid, which may consist of several layers, is the determining factor in the strength of a hose. A cover is designed to withstand external abuse.
Figure 51: Flexible Rubber Hose
When installing the flexible hose, do not twist it. Doing so reduces its lift and may cause its fittings to loosen. An identification stripe that runs along the hose length should not spiral, which would indicate twisting (Figure 52). Protect flexible hose from chafing by wrapping it lightly with tape, when necessary.
Figure 52: Installing Flexible Hose
The minimum bend radius for flexible hose varies according to its size and construction and the pressure under which the system will operate. Consult the applicable publications that contain the tables and graphs showing the minimum bend radii for the different types of installations. Bends that are too sharp will reduce the bursting pressure of flexible hose considerably below its rated value.
Install flexible hose so that it will be subjected to a minimum of flexing during operation. Never stretch hose tightly between two fittings; when under pressure, flexible hose contracts in length and expands in diameter.
Teflon TM-type hose is a flexible hose that is designed to meet the requirements of the higher operating pressures and temperatures in todays fluid-powered systems. It consists of a chemical resin that is processed and pulled into a tube shape of the desired size. It is covered with stainless-steel wire that is braided over the tube for strength and protection. Teflon-type hose will not absorb moisture and is unaffected by all fluids used in todays fluid-powered systems. It also is nonflammable; however, use an asbestos fire sleeve where the possibility of an open flame exists.
Carefully handle all Teflon-type hose during removal or installation. Sharp or excessive bending will kink or damage the hose. Also, the flexible-type hose tends to form itself to the installed position in a circulatory system.
Flaring and brazing are the most common methods of connecting tubing. Preparing a tube for installation usually involves cutting, flaring, and bending. After cutting a tube to the correct length, cut it squarely and carefully remove any internal or external burrs.
If you use flare-type fittings, you must flare the tube. A flare angle should extend 37 on each side of the centerline. The areas outer edge should extend beyond the maximum sleeves ID but not its OD. Flares that are too short are likely to be squeezed thin, which could result in leaks or breaks. Flares that are too long will stick or jam during assembly.
Keep the lines as short and free of bends as possible. However, bends are preferred to elbows or sharp turns. Try not to assemble the tubing in a straight line because a bend tends to eliminate strain by absorbing vibration and compensating for temperature expansion and contraction.
Install all the lines so that you can remove them without dismantling a circuits components or without bending or springing them to a bad angle. Add supports to the lines at frequent intervals to minimize vibration or movement; never weld the lines to the supports. Since flexible hose has a tendency to shorten when subjected to pressure, allow enough slack to compensate for this.
Keep all the pipes, tubes, or fittings clean and free from scale and other foreign matter. Clean iron or steel pipes, tubes, and fittings with a boiler-tube wire brush or with commercial pipe-cleaning equipment. You can remove rust and scale from short, straight pieces by sandblasting them as long as no sand particles will remain lodged in blind holes or pockets after you flush a piece. In the case of long pieces or pieces bent to complex shapes, remove rust and scale by pickling (cleaning metal in a chemical bath). Cap and plug the open ends of any pipes, tubes, or fittings that will be stored for a long period. Do not use rags or waste for this purpose because they may deposit harmful lint that can cause severe damage in a hydraulic system.
Properly selected tubing, combined with quality tube fittings, can provide leak-tight systems.
When installing fittings near tube bends, there must be a sufficient length of straight tubing to allow the tube to be bottomed in the tube fitting (see Figure 53).
Figure 53: Installing Tube Fittings
Figure 54, Figure 55, and Figure 56 show sizing tables, inch/metric conversion tables, and decimal dimensions for correctly sizing tubing to be installed.
Figure 54: Tube Installation Sizing Tables
Figure 55: Tube Installation Conversion Tables
Figure 56: Tube Installation Decimal Equivalents
The hazards presented to personnel, equipment, facilities, the public, or the environment because of inadequately designed or improperly operated pressure systems include blast effects, shrapnel, fluid jets, release of toxic or asphyxiant materials, contamination, equipment damage, personnel injury, and death. Therefore, it is necessary to perform testing on a tubing system after it has been put together. The testing may be as simple as a visual inspection or some type of pressure testing, or it may be more complex. The type is dictated by plant procedures.
Whenever practical, send pressure vessels and systems to a test facility for pressure testing. If this is not practical, test the equipment in accordance with the requirements of your company. All pressure tests shall be conducted remotely and be observed (or conducted) and certified by an inspector. See Figure 57 for the relationships of test and retest pressures to the maximum allowable working pressure (MAWP) and maximum operating pressure (MOP).
The inspector observing or conducting the test will verify that required documentation is signed and the successful pressure test has been completed. The inspector will complete all forms based on observations and test results.
Figure 57: Relationships Between Test Pressures, MAWP, and MOP
Test pressure vessels in accordance with the requirements used by your company using an inert fluid. Initially test manned-area vessels at 150% of their MAWP or at the test pressure specified. Take appropriate diameter measurements accurate to within 0.001 in. (0.025 mm) both before and after testing to show that detectable plastic yielding has not occurred during pressurization.
Remote-operation vessels should be tested at a pressure that is consistent with the functional reliability required (usually 125% of the MAWP). If it is determined that a pressure test is not practical, then inspect the vessel ultrasonically. In addition, check the vessel for surface cracks using either the magnetic particle test or (for non-magnetic vessels) the fluorescent penetrant test.
Test non-hazardous liquid, inert gas, and compressed air systems at 125% of their MAWP using an inert fluid.
Test toxic, oxygen, radioactive, and flammable fluid systems at 150% of their MAWP using an inert fluid.
Leak-check pressure vessels and systems at their MAWP, as required, after successful pressure testing. Gross leakage can be detected by observing the drop in pressure on the test gauge during pressure testing and can be pinpointed with leak detection fluid. Small leaks can be located with commercial leak detectors.
Inspection intervals for pressure vessels will be determined using the in-service inspection criteria used by your company. Depending on the type of vessel service, the intervals may range from 2 years to a maximum of 10 years. Relief devices on pressure vessels will be inspected every 3 years. In addition, pressure systems and vessels will be re-inspected before reuse whenever they are disassembled, moved, or redesigned, or when the application changes, even if the working pressure is reduced.
If the vessel or system has been damaged or modified, it will be retested. This determination shall be made by a responsible designer and a inspector.
DOT compressed gas cylinders normally are retested every five years. The gas supply vendor is responsible for retesting and restamping the cylinders before returning them to the user. Portable trailer banks and stationary DOT cylinders that have previously been filled in place will be sent back to the vendor for retest if the last test date exceeds five years. The industrial gas section of Procurement and Materials will be the custodian of these cylinders.
If it is impractical to pressure-test a vessel or system at a high-pressure test facility, pressure-test it in place using either gas or liquid.
The responsible designer shall prepare the required test procedure, direct testing personnel, and witness in-place pressure testing of the vessels and systems for which he/she is responsible.
The responsible individual is similarly responsible for in-place retesting of pressure equipment for which he/she is responsible.
Although others may be designated to observe and direct testing or retesting, responsibility for safe conduct of the test and safe functioning of tested pressure equipment cannot be delegated.
A test procedure normally required for every pressure test conducted in the field is to use the applicable standard procedure for pressure testing in place. Because the requirement that testing be conducted in place usually is apparent to the responsible designer, the test procedure normally should be included in a work order.
Procedures for in-place testing of vessels and systems shall be signed by:
If oxygen or toxic, flammable, or radioactive material is involved, the building coordinator or area supervisor shall be advised of pressure tests planned for his/her facility.
The pressure test shall be observed (if not conducted) by a pressure inspector. The responsible user shall have a pressure installer (or pressure operator) set up the test.
This template may be used as guidance for preparing an actual procedure for conducting low- and intermediate-pressure in-place gas tests.
The following actions shall be taken before actual pressure testing is started:
Persons not directly involved in the test shall leave the area. The responsible designer or responsible individual (or a designated alternate) and a pressure inspector shall witness the test, which shall consist of the following steps:
The following template may be used as guidance for preparing an actual procedure.
The procedure below may be used for conducting in-place hydrostatic pressure tests.
Persons not directly involved in the test shall leave the area. The responsible designer or responsible individual (or a designated alternate) and a pressure inspector shall witness the test, which shall consist of the following steps:
In most installations, air hoses are used for connecting portable equipment to line-mounted air stations. These lines normally range from 5 to 50 feet in length. Hoses are also used for flexible connections between moving and nonmoving pieces of equipment. When they are used to connect moving to non-moving equipment, hoses provide insulation from shock, noise, and vibration. Hose may also be used for temporary connections and for connections that are connected and disconnected frequently. However, hoses should not be used to connect plant air lines permanently because they are not as durable as pipe.
Figure 58 shows an air hose cross-section, which consists of three major sections:
Figure 58: Typical Air Hose Cross-Section
The inner tube usually is made of a synthetic rubber that resists oil, water, and heat. The reinforcing material can be natural or synthetic yarns or fibers, metal wire braiding, or combinations of these. The reinforcing material determines the strength of the hose. A plastic, rubber, or synthetic cover usually is placed over the reinforcing material to protect it from abrasion.
Most hoses are classified as low-pressure hoses, which will withstand pressures of up to 175 psi. Flexible conductors should have a minimum burst resistance of at least five times the operating pressure of the circuit in which they are installed. Most of these hoses range in size from inch to 5/8 inch inside diameter and have one or two reinforcing plies. The recommended maximum operating temperatures can be as high as 200F, depending on the material in the inner tube.
A hose should only be used to connect one piece of equipment to a plant air line. If it is necessary to connect more than one piece of equipment, use a Y-connector and two separate hoses. At the same time, select the proper size of air hose. Selection factors must include the amount of air required for each tool and the frequency of operation. If more than one tool requires air at the same time, larger-diameter hoses might be needed. Because air hose IDs are smaller than those for the same size of pipe, the proper size air hose for an application usually is one size larger than the size of the connecting pipe thread. If the same size pipe and hose are used, the pressure drop could deprive the air tool of sufficient air pressure.
There are a number of different types of permanent and reusable hose fittings available. Figure 59 shows several examples. A hose fitting consists of two parts. One part grips the hose, and the other part attaches the hose assembly to the fitting. The part of the fitting that grips the hose must be installed using sufficient clamping force on the hose to prevent it from blowing off. However, it must not be so tight as to separate or cut the reinforcement between the inner tube and the outer cover.
Figure 59: Typical Hose Fittings
Reusable fittings are screwed together to attach them onto the hose, whereas the permanent fittings are assembled at the factory using special presses. Methods of assembling reusable fittings onto hoses vary with the manufacturer. To ensure the highest possible safe working pressure for the finished hose assembly, always follow the manufacturer's instructions carefully.
Quick-disconnect couplings are used in applications that require quick connection and disconnection of pneumatic equipment. This type of coupling is very convenient because it does not require a shutoff valve. One part of the coupling contains a leak-proof, spring-loaded poppet or seal, while the other part contains a device to open the poppet when the coupling is connected.
Special care should be used when disconnecting this type of air line. This is especially true if long or high-pressure air lines are involved. A blast of escaping air can blow dirt or chips into the eyes or face and cause severe injury. Safe working habits recommend that air hoses be depressurized before they are disconnected, even though self-sealing couplings are used.
Because hoses are more flexible than pipe or tubing, their installation procedures are easier. Never stretch hoses between connections or bend hoses sharply. In addition, route hoses properly and clamp them to prevent accidental damage from rubbing and mishandling. Several good and bad examples of hose installations are shown in Figure 60.
Figure 60: Example Hose Installations