What is in this article?:
Next to contamination, cavitation causes more pump damage than anything else. Cavitation occurs when a pump needs 10.8 gpm at its inlet, but only gets 10.5 gpm. The missing 3 gpm winds up as voids or vacuum bubbles that implode when they go from suction to pressure. The implosions are rapid and damaging to adjacent surfaces when outlet pressure is high. They can take a pump out of service in hours. When outlet pressure is low -- under 200 psi -- there is still some noise and damage but it is minimal.
Some mobile equipment shuts the inlet to their pumps when the equipment travels, only allowing 1or 2 % of pump flow. This small volume goes through an open-center circuit at less that 15 psi, so implosions are not a problem. Another advantage is fuel savings. Because pump flow is so low, horsepower drain is much less.
Cavitation comes from several situations that are easy to rectify:
- Long suction lines with many turns.
- Undersize suction lines.
- The pump mounted too far above the fluid.
- Fluid viscosity too high (either wrong viscosity or low temperature).
- A collapsed suction hose.
- Turning the pump faster than the manufacturer recommends.
- A clogged inlet strainer
- A blocked air breather (especially in circuits with oversize rods or single-acting cylinders).
Any of the above could be eliminated immediately with a supercharging pump. This is a separate pump operating at low pressure (usually under 30 psi), that forces fluid to the system pump inlet.
Most of these conditions also can be eliminated by good design practices:
- Locate the pump close to the tank -- preferably alongside or under it.
- Never use a suction line smaller than the pump inlet port.
- Use the fluid recommended by the pump supplier, and install tank heaters if the system will be exposed to temperatures below 65°F.
- Never use pressure hose for suction lines. The lining of a pressure hose is not firmly attached to its body and can collapse under vacuum. Use hose specifically designed for suction service.
- When a pump must turn faster than recommended, install a supercharging pump or elevate the tank to provide head pressure. Make sure a vacuum gauge at the pump inlet never goes above 1.5 to 3 psi.
- Use a good filtering system – rated at least 10 µ -- so the suction strainer cannot block flow. Consider the suction strainer as insurance against startup contamination large enough to wreck a pump instantaneously.
Another situation that occurs in suction lines is air leaks. Air leaks are not cavitation but make the same noises and damage as vacuum cavitation. The only way to tell the difference in these situations is to look at the oil in the tank. If the oil is foamy from aeration, there is an air leak in the circuit. If the fluid is clear or almost clear of bubbles, there is a vacuum cavitation problem.
Air leak problems can come from poor piping practices. It is best to never use a standard pipe union in the inlet line. It is practically impossible to seal a standard union against an air inlet leak. Plumb the inlet line with as few fittings as possible and make sure any joints are sealed. If a plumbing connection is suspect, apply some of the system fluid to each joint to see if the noise stops. This type air leak problem usually shows up at system start. It seldom happens to a running circuit.
On systems that have been running for some time, a good place to look for air leaks is at the pump shaft seal. Fixed-volume pumps have their drive shaft sticking out of the housing and inside the housing is suction vacuum. When a shaft seal wears or is damaged from heat, it may let atmospheric air in before it lets oil leak out. The oil application test works here also, but can be messy because of shaft rotation speed.
The suction line is the most important line on the hydraulic circuit. Fluid can be pushed through pressure lines but a suction line only has one atmosphere (approximately 14.7 psi at sea level) with which to work. Most pumps are slightly damaged above 3 psi. At 4 psi and higher, cavitation noise is evident and pump damage escalates. Higher vacuum accelerates the damage.
What causes cavitation damage?
Erosion is the result of cavitation implosions as fluid passes from the inlet side of a pump to the outlet side. Figure 8-34 shows how the change from vacuum to pressure makes the vacuum or air voids collapse or implode. At low pressure, these voids merely close up and no damage is done. At high pressure, the fluid does not stop when the void is full but continues at high velocity through the void and impinges metal surfaces to the point of getting into the metal’s pores. Pressure drops as the next pulse of fluid approaches and high-pressure fluid in the metal pores rushes back out. During this part of the cycle, some very small particles of metal are dislodged and a cavity starts to form. Because this high- to low-pressure cycle can happen more than 200 times per second on a 12-vane pump at 1200 rpm, it is easy to understand how a pump can be physically damaged so quickly. These implosions are in the area where metal against metal is the only sealing action between vacuum and system pressure, so once the metal erodes, pump efficiency decreases because fluid bypasses through the damaged area.