What you need to know about high-pressure equipment
High-pressure abrasive and waterjet cutting systems have unique properties that must be understood to maximize performance and ensure safety. This article discusses the principles of water compressibility and pressurization, metal fatigue, high-pressure plumbing, seals, valves, and making and installing ultrahigh pressure fittings.
Abrasive and waterjet cutting equipment operates at pressures up to 60,000 pounds per square inch (PSI). Most shops have no other equipment operating at such high pressures and little experience working with high pressure. This article explains the unusual physical phenomena that affect high-pressure equipment and then shows how these phenomena affect design and maintenance of high-pressure systems.
The most unusual aspects about ultrahigh-pressure (UHP) systems are:
- The water is compressible.
- A very large amount of energy is put into a small amount of water.
- The containment vessel operates at its fatigue limits.
Water is a liquid that usually is regarded as being incompressible, which is a good approximation at most ordinary pressures. However, at the high pressures needed to drive an abrasive jet, water compresses up to 11 percent. Figure 1shows the percentage compression of water at pressures up to 100,000 PSI.
Compressibility of Water
This compressibility has an important safety implication. A sealed pressure system can contain a substantial amount of stored energy, just like the energy stored in a compressed-air system. A pump that is shut off and not vented may surprise the mechanic who opens a fitting. Be sure that the pressure is drained away before opening high-pressure fittings.
Heat and Energy Conversion
Pumping water up to 60,000 PSI requires a great amount of energy. In fact, it takes about the same energy as it does to heat the water to 166 degrees F. So pumping a gallon per minute (GPM) of water takes about the same amount of energy as it would to heat 1 GPM from an icy 46 degrees to the boiling point. Where does this energy ultimately go?
First, if the water is shot out of a nozzle, the energy becomes the kinetic energy of the high-speed jet. Some of the jet energy then is used for cutting, and the remainder becomes heat when the jet is stopped by the catcher. This is why the catcher tank becomes warm with use.
A high-pressure leak allows the pumping energy to dissipate directly as heat. For this reason hot or even boiling water issues from leaking fittings. In fact, anywhere the pressure drops by more than 5,000 PSI, the temperature difference can be felt by hand. The hand test can be used as a diagnostic tool in troubleshooting high-pressure equipment.
Bend a paper clip back and forth until it breaks. It probably will take a dozen or so bends, which indicates low-cycle metal fatigue. By sufficiently
reducing the bending stresses you could extend the moment of breakage to, say 1 million cycles. This falls in the range of high-cycle fatigue in which the metal doesn't need to deform plastically to fail. Springs, aircraft structures, cannon barrels, and high-pressure plumbing parts all fail by high-cycle fatigue, because of the combination of exceptionally high operating stresses and cyclic loading.
Fatigue Crack in High-pressure Cylinder
Typical pressure vessels for operating at 60,000 PSI have a wall thickness equal to the bore diameter. For example, 1/8-in.-bore tubing usually has a 3/8-in. OD. Yet even with this thick wall, the stresses at the bore are very large—so large that if the pressure cycles up and down from 60,000 PSI to zero at a high cyclic rate, cracks will form at the bore and extend outward a little with each pressure cycle. Finally, the crack reaches the outside wall and a leak occurs. Figure 2depicts the outside of a fatigued high-pressure pump cylinder.
A corrosive environment greatly accelerates fatigue crack growth. Cylinders that will last thousands of hours in normal operation will fail in tens of hours in an acidic environment. Monitoring water quality is one strategy for combating fatigue failure. Autofrettage is another.
In autofrettage, the pressure vessel is pressurized once to a very high pressure that yields the bore but not the outside. Then, when the pressure is removed, the outer layers spring back and compress the inner ones. The built-in compressive stress then helps extend the fatigue life of the component. High-pressure pump parts always are treated in this manner, and high-pressure tubing and other high-pressure plumbing parts often are.
High-pressure components almost always are connected together with tubing. Properly preparing and connecting UHP tubing is critical in maintaining a safe and leak-free high-pressure system. Following a few dos and don'ts when working with UHP fittings will help you achieve a properly installed and maintained tubing system.
High-pressure Tubing Fitting
Shown in Figure 3, the components of a typical high-pressure fitting are:
- High-pressure Tubing — Normally high-strength stainless steel with a bore one-third the OD. Standard sizes for service to 60,000 PSI are 1/4 in., 3/8 in., and 9/16 in. OD.
- Tube Cone — Coned at a 59-degree angle.
- Tube Thread — Threaded with a left-hand thread to mate with the collar during assembly.
- Body — The female receptacle for the tubing. Examples are elbows, T's, swivels, and valves.
- Body Cone — Coned with a 60-degree angle.
- Body Thread - Threaded with a right-hand thread.
- Slotted Collet — Provides the appropriate spacing between the collar and the gland nut. It also carries bending loads to prevent strain on the threads.
- Collar — Has left-handed threads. It screws onto the end of the tubing and carries the load that forces the sealing points together.
- Weep Hole — A designed leak point provided for safety purposes that allows fluid to escape in case the pressure in the fitting exceeds what the sealing point can tolerate. Never try to plug a weep hole.
Making the High-pressure Seal
Proper cone angles in the body and tube components are critical for creating the high-pressure seal. The seal is created when the very tip of the tube cone metal edges, angled at 59 degrees, seats on the metal edges inside the body cone, which is angled slightly larger at 60 degrees (see the seal surface highlighted in Figure 3).
The slight angle difference allows the end of the tube cone to seat inside and make contact with the body cone angle, which creates a metal-to-metal joint between the two components. The high-pressure seal is created at this metal-to-metal joint.
Properly Assembling a UHP Fitting
After the tubing and body have been properly coned and threaded:
- Slip the gland nut and collet onto the tubing.
- Screw the collar onto the threaded end of tubing following the "3-Thread Rule": Allow three threads to be exposed on the end of the tubing or between the collar and coned end of the tubing. This allows the tube to seat fully inside the coned body and create the seal.
- Apply a small amount of antiseize compound to the gland nut threads.
- Insert the tubing into the cone of the body.
- Screw the gland nut into the connection until fingertight.
- Tighten the gland nut to the specified torque value.
Do the following when making and installing UHP fittings:
- Do make sure fitting components are clean before assembly. Any dirt or contamination can compromise the metal-on-metal seal and create a path for the pressurized water to escape and erode the joint.
- Do apply antiseize to lubricate the gland nut threads before assembling the fitting. This will prevent the metals from galling together and ensure that the fitting can be disassembled later.
- Do follow the 3-Thread Rule when assembling the collar and the tubing to allow for proper seating.
- Do tighten the gland nut to the proper torque specification, which is specified based on connection size. If the fitting is not tight enough, it will leak.
- Do periodically inspect the UHP tubing and fittings for leaks. Fatigue cracks in the tubing will cause a fine mist to spray from the tube.
- Do replace or repair any damaged or leaking components in the UHP fitting.
- Do not use damaged or leaking components in your UHP fittings or assembly. Any leakage at high pressure will erode and irreparably damage the component.
- Do not overtighten the gland nut. If the nut is too tight, it may swage the end of the tube shut and cause a high flow restriction.
- Do not use a thread-sealer such as Loctite® or Teflon® tape in the UHP fitting. It is not the threads that cause the seal in a high-pressure fitting, but rather the metal-to-metal seal between the coned tubing tip and the coned seat.
Coning and threading tools are available from various manufacturers, and they can be used for both making new assemblies and repairing damage to existing ones. Straight tubes also can be coned and threaded in a lathe, and the 60-degree countersink often can be resurfaced with a center drill. The tubing also can be bent with a high-strength tubing bender, and long lengths of tubing that are bent into either long arcs or tight coils can be used to provide a flexible joint for connecting to a moving device.
Shutoff valves and metering valves are the two basic types. A metering valve has a long, slender stem that fits into a long, tapered seat (Figure 4). When the pressure drops over a long distance, erosion is avoided, but the valve can't be shut off completely without the stem wedging into the seat.
The shutoff valve has a rather steep angle (Figure 5) and can be shut off completely, but it will erode quickly if used for metering. The valves can be used only for their intended purpose, and if both functions are required, two valves must be used.
High-pressure equipment has a few unusual features, but if the principles are understood and proven techniques are used, you should encounter no difficulty working with it.