May 9, 2006
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:
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.
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:
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.
After the tubing and body have been properly coned and threaded:
Do the following when making and installing UHP fittings:
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.