The conducting system: Critical and often misunderstood. (Hydraulic Systems Trends).
Conductors are used to transfer fluid throughout the hydraulic system. Pipe was the earliest form of metallic fluid conductor and is still found in some hydraulic systems. People in the field still refer to metallic conductors as pipe or piping even though virtually all modern systems use metal tubing and hydraulic hose.
One of the first precepts a designer must remember is that once he has settled on a family of conductors, he must stick with them. For instance, if at all possible, do not mix pipe with tubing; metric and CU sizes; ANSI, British, DIN, or other standards. The problems that could arise from their incompatibility may not be resolvable.
American standard pipe and pipe fittings are defined by ANSI Standard B36.10, 1970. Note that nominal sizes defined in this standard do not factually exist. The outside diameter is held constant for a given nominal size because the threads cut into the o.d. must always fit those tapped into a mating port or fitting (Fig. 1). The i.d. for a given size pipe is a function of the standard o.d. and wall thickness, which in turn, is a function of "pipe schedule," which refers to wall thickness.
There are four schedules: 40, standard pipe; 80, extra heavy pipe; 160; and double extra heavy pipe (Fig. 2).
Designers must select the appropriate schedule to handle the operating pressures anticipated in the system (Table 1). Note that two pressures are given, a working pressure which is the one the designer uses to match system pressures obtained from the cycle profile and a burst pressure -- the pressure at which the pipe will theoretically rupture.
A method is also given for accommodating anticipated shock pressures. Note the column headed, "Water Hammer Factor." Multiply flow rate by this factor, P = Q x WHF = Shock Pressure. Deduct P from working pressure rating in the table, to get allowable pressure for design. Water hammer effect is used to accommodate shock pressure conditions. Multiply flow rate by this factor: P = Q x WHE Deduct P from pressure working rating in table to get allowable pressure for design.
Because many conducting systems are combinations of pipe, tubing, and hoses, a method was devised to identify all of them by a system of dash numbers.
Nominal pipe thread sizes are identified by dash sizes expressed in 16th of an inch. A certain pipe thread size is standard for each o.d. tube dash size, as shown in Table II. These combinations apply to SAB, AN, ANSI and PIT standards. Table II also shows that pipe thread dash numbers smaller than 1 in. do not correspond with the same o.d. tube dash size. For sizes 1 in. and larger, pipe and tubing dash sizes are the same.
For example, o.d. tube size -4 is equal to pipe size -2; and o.d. tube size -16 corresponds to pipe size -16. Also, refer to Table II for standard pipe and o.d. tube dash size combinations.
American standard pipe can be connected with American standard fittings which correspond to the pipe sizes listed. When selecting fittings, a designer must make absolutely certain that the fittings will withstand all the pressures (including peak) frequently encountered in hydraulic systems. A common error is to specify a pipe schedule which can sustain the design pressures, then use standard fittings from stock. The pressure mismatch can be disastrous. The pressure rating of pipe fittings must match the working pressure of the specified pipe schedule.
There are two basic types of threaded joints. One has tapered threads which produce a metal-to-metal seal by wedging surfaces together as the pipe threads are tightened. The other has straight threads and no wedging action, but has an elastomeric element to do the sealing.
Tapered threads have the advantage that an additional fraction of a turn may, in systems operating at moderate pressures, cure a slight leak. Their sealing ability depends on how perfectly the threads are formed. In practice, threads may be machined carelessly and not seal regardless of how much they are tightened. In such cases, excessive torque often results in cracked component bodies. Because of the frequency of such leaks there is a trend to stop using tapered pipe threads. Some companies limit their use to pressures below 500 psi.
USA Standard Dry/seal-NPTF are very similar to NPT pipe threads, but are shaped to first make contact at their roots and crests (Fig. 3b). When the joint is tightened with a wrench, the thread crests are crushed until the thread flanks make full contact (Fig. 3c). They do not have the built-in leakage path of NPT threads, but can still leak because of machining imperfections. Sealant should be used on these threads for lubricating, but it cannot seal a poorly cut thread.
The USA Standard pipe thread is tapered and shaped to engage mating threads on their flanks (Fig. 3a). This design leaves a small spiral groove along the thread tips which must be filled with sealant. The sealing material may also lubricate the threads and prevent galling.
The SAE straight thread fitting does not depend on thread surfaces for sealing because an O-ring in the fitting becomes compressed and does the sealing. Straight thread fittings may be classified as fixed-hex, as the adapter in Fig. 4a, or adjustable as the elbow in Fig. 4b. Used almost universally in mobile machinery, straight thread O-ring fittings are now also becoming more popular in industrial applications.
ISO has .adopted straight thread, O-ring fittings as the international standard. However, the ISO thread form and the O-Ring groove are not interchangeable with SAEs.
Tubing used in hydraulic systems, particularly pressure lines, must conform to standards such as those shown in Table III or equivalent. Tubing can be seamless carbon steel, aluminum or copper, welded steel; brazed copper, or plastic. The anticipated operating pressure in a system will control the type of tubing material used.
Seamless steel tubing is normally annealed for convenience in bending and flaring. Tubing referred to as hydraulic tubing is most widely used for fluid power applications. In contrast to American Standard Pipe, tubing is sized by its outside diameter; wall thickness varies to match pressure requirements (Fig. 5).
Tubing is usually measured by its outside diameter and has a thin wall compared to pipe. Two exceptions to this rule are Naval piping, where tubing is based on standard o.d., and in refrigeration systems, where tubing is based on standard i.d. dimensions. Though similar to pipe, tubing is used differently. Since the wall sections of tubing are relatively thin, methods other than threading must be used to connect tubing into systems.
Tubing is flared and attached with swivel nuts, brazed or welded, or connected with flareless fittings. To use all these connecting methods, the tube must have a standard o.d., or it will not fit the various types and combinations of fittings.
Rigid tubing varies in size from 3/16th to 3 in. o.d. Tubing size is given by dash numbers and is also expressed in 1/16th of an inch. For instance a 3/8th in. size would be 6/16th or -6 (dash 6). Table III shows the standard rigid tube sizes now used by industry, compared with the i.d. of medium-pressure hoses with identical dash numbers.
Metal tubing is available in steel, aluminum, copper and stainless steel. While aluminum and copper tubing are generally used only in low-pressure systems, they are ideal for applications where resistance to corrosion is mandatory; stainless steel tubing resists corrosion best. Stainless steel and carbon steel tubing have the mechanical properties required to withstand high-pressure system operations. Plastic tubing most often is used in low-pressure systems where mechanical properties are not stringent.
After the tubing material has been selected, the i.d. and wall thickness must be determined. The calculated i.d. is based on flow rate, wall thickness, working pressure and mechanical loads to which the tubing would be subjected. Tubing which meets working pressure and flow requirements can be selected with graphs, such as Fig. 6. However, in addition to working pressure, wall thickness selection must also consider mechanical stresses and strains and service abuse.
Mechanical stresses and strains are caused by vibration, relative motion of connected parts, and unsupported elements in the line which tend to stretch, bend, shorten or twist the tubing.
Service abuse can result from careless practices, such as walking on or dropping tools or other heavy and sharp objects on the tubing. Inherent rough wear that can be expected in heavy machinery where production parts, turnings, chips and other materials may strike or otherwise damage fluid lines, and on mobile equipment where rocks and gravel may strike the fluid lines.
Much can be done to minimize these problems and improve reliability through proper system design. The designer should insure that all components are anchored properly and fluid lines supported and restrained from relative motion.
The severity of service or abuse to which tubing in a system is subjected can be categorized into one of three arbitrary classifications based on design factors.
* Mechanical and hydraulic shocks are not excessive. The equipment is stationary and is not subjected to hydraulic shock. Valve shifting and cylinder actuation speeds are relatively slow.
* Considerable hydraulic shock and mechanical strain. The equipment may be mobile, with high-velocity fluid flows provided by accumulators or large displacement pumps so valves and cylinders move rapidly.
* Hazardous application with severe service conditions. Lines are exposed to potential damage, high shocks, etc.
The graphs in Fig. 6 help compute wall thickness quickly, based on relative severity of service, working pressure, and tubing o.d. When pressures are low and tubing lines small, a design (burst pressure to peak operating pressure) service factor of even 7:1 or 10:1 can be obtained easily, even with thin-wall tubing. For example, 1/4th in. o.d. x 0.035 in. wall thickness low carbon steel tubing could be rated for 2000 psi at 7:1 design service factor, or at 1500 psi at 10:1.
Where pressures and severity of service are not excessive, a design service factor of 4:1 is generally acceptable. For demanding service, 6:1 and 8:1 design service factors are recommended. However, where flow rates and pressures are high, requiring larger diameter tubing, design service factors dictate the use of very heavy tubing walls.
For example, the wall thickness for 2 in. o.d. tubing for 4000 psi service at 4:1 design service factor is seen to be beyond the graph, Fig. 6. The wall thickness would be calculated at nearly 0.3 in.
Note that this method for determining design service factors is subjective and lends itself to widely varying interpretations. Thus as system pressures have increased, designers have been forced to seek more practical selection criteria. Many now use a percentage of tubing material nominal yield strength as an allowable stress value. Standard practice in ASME Boiler Codes is to use between 50 percent and 60 percent of nominal yield strength, depending on the type of material.
Carbon steel tubing that meets SAE specifications J524 and J525 has a minimum ultimate tensile strength of 45,000 psi and a minimum yield strength of 25,000 psi. In actual practice these minimums are exceeded.
The minimum yield strength allowed by SAE J525 is 56 percent of the minimum ultimate strength, or 45,000 x 56 percent = 25,200, or 25,000 psi. Laboratory test results show that in practice the composite yield points average 70 percent (36,100/51,300) of ultimate strength, ranging between 65 and 82 percent.
In a later discussion, we will cover flexible hoses, hose connectors and hose selection criteria.
[FIGURE 6 OMITTED]
Fig. 5 Standard tubing o.d. sizes are measured in 1/16th in. increments through 3/8th in. OD; 1/8th in. increments from 1/2th in. to 1 in. and 1/4th increments above 1 in. 1/15" increments 1/8" 3/15" 1/4" 5/18" 3/8" 1/8" increments 1/2" 5/8" 3/4" 7/8" 1" 1/4" increments 1 1/4" 1 1/2" 2" Table 1 Pressure rating of steel pipe ASTM A53 Grade B or A106 Grade B Seamless. Pipe Pressure-psi Water Noni. size Sch. hammer inches no. Working Burst factor 1/8 40 3500 20,200 1/8 80 4800 28,000 1/4 40 2100 19,500 1/4 80 4350 26,400 3/8 40 1700 16,200 3/8 80 3800 22,500 1/2 40 2300 15,600 63.4 1/2 80 4100 21,000 1/2 160 7300 26,700 1/2 XXS 12,300 42,100 3/4 40 2000 12,900 36.1 3/4 80 3500 17,600 44.5 3/4 160 8500 25,000 3/4 XXS 10,000 35,000 1 40 2100 12,100 22.3 1 80 3500 15,900 26.8 1 160 5700 22,300 36.9 1 XXS 9500 32,700 68.3 1 1/4 40 1800 10,100 12.9 1 1/4 80 3000 13,900 15.0 1 1/4 160 4400 18,100 18.2 1 1/4 XXS 7900 27,700 30.5 1 1/2 40 1700 9100 9.46 1 1/2 80 2800 12,600 10.9 1 1/2 160 4500 17,700 13.7 1 1/2 XXS 7200 25,300 20.3 2 40 1500 7800 5.74 2 80 2500 11,000 6.52 2 160 4600 17,500 8.60 2 XXS 6300 22,100 10.9 2 1/2 40 1900 8500 4.02 2 1/2 80 2800 11,500 4.54 2 1/2 160 4200 15,700 5.43 2 1/2 XXS 6900 23,000 7.82 3 40 1600 7400 2.60 3 80 2600 10,300 2.92 3 160 4100 15,000 3.56 3 XXS 6100 20,500 4.64 3 1/2 40 1500 6800 1.94 3 1/2 80 2400 9500 2.17 4 40 1400 6300 1.51 4 80 2300 7500 1.67 4 160 4000 14,200 2.08 4 XXS 5300 18,000 2.47 Water hammer effect is used to accommodate shock pressure conditions. Multiply flow rate by water hammer factor: P = QXWHF. Deduct P from pressure working rating in table to get allowable pressure for design. Table II A comparison of dash-numbers for tubing, pipe, and hose. Rigid tube Pipe thread Pipe thread dash size dash size size -4 -2 1/8" -5 -4 1/4" -6 -4 1/4" -8 -6 3/8" -10 -8 1/2" -14 -12 3/4" -16 -16 1" -20 -20 1 1/4" -24 -24 1 1/2" -32 -32 2" -40 -40 2 1/2" -48 -48 3" Table III The relation of dash-numbers to tubing i.d. and o.d. and hose i.d. Dash size tube & hose Tube Tube Hose I.D. in 1/16ths (in.) O.D. I.D. medium pressure -4 1/4 .180 .188 -5 5/16 .242 .250 -6 3/8 .305 .313 -8 1/2 .402 .406 -10 5/8 .495 .500 -12 3/4 .620 .625 -16 1 .870 .875 -20 1 1/4 1.120 1.125 -24 1 1/2 1.370 1.375 -32 2 1.810 1.813 all dimensions in inches The tubing I.D. will depend on the wall thickness, selection of which is determined by the operating pressure. Wall thickness as a decimal inch or gage number.
|Printer friendly Cite/link Email Feedback|
|Comment:||The conducting system: Critical and often misunderstood. (Hydraulic Systems Trends).|
|Publication:||Diesel Progress North American Edition|
|Date:||Apr 1, 2002|
|Previous Article:||Filter targets high flow applications. (Filtration).|
|Next Article:||Enhancements for oil spectrometer package. (Fuels & Lubes).|