Internal leakage in a pressure-compensated piston pump is directly related to piston and control leakage at system pressure. Lost mechanical and hydraulic power is converted to heat and returned to the reservoir through the pump’s case drain. Depending on the compensation pressure, heat generation can raise the temperature of the pump case substantially.

For example, a large pressure-compensated piston pump leaking 0.5 gpm internally at a system pressure of 2250 psi would undergo a temperature rise of 9° or 10° F. The temperature would be even higher by taking mechanical losses and fluid compression into account. The same pump operating at 4500 psi would see a temperature increase of 42° F.

As a pump wears, leakage, naturally, increases. If the leakage in this same pump increased to 1 gpm, then the resultant temperature rise would be more than 20° F at 2250 psi and 80° F at 4500 psi. These figures can all be calculated by plugging in the appropriate values into the equations appearing below and on page 50.


Power loss in pressure-control valves

A relief valve in a pump’s pressure compensation circuit is a static device that passes no flow except for minimal spool leakage. However, the valve does react to higher pressures that may approach or equal its opening, or cracking, pressure. It is extremely important to set the opening pressure of a relief valve at least 10 to 15% higher than system pressure. The main reason is because any relief valve, depending on its manufacturer, has a total non-bypass reset pressure that is a percentage of cracking pressure.

If a pressure compensated pump is set for 2250 psi, then the system relief should be set for at least 2600 psi. If the valve has a 10% reset value, or 90% of cracking pressure, then the valve will open at 2600 psi and fully close at 2340 psi when pressure drops. If not set properly, the valve will generate substantial heat. A vented relief valve normally loses power only when operating in its vented mode. The power loss, in this case, is directly related to a relatively small pilot flow across the valve vent section.

Combining a pressure reducing with a pressure relief valve in a function also can cause substantial power loss if the reducing valve is not set properly. If possible, a pressure reducing-relieving valve should be used. Technicians must be extremely careful when changing the pressure reducing valve setting. The pressure relief valve must always be set to a value 15% above that of the pressure-reducing valve. Improper settings will lose a great deal of power and, consequently, generate substantial heat.


Heat test point positions

Temperature may be measured on the surfaces of pipe, tubing, and other components by many methods, but the most convenient is to use a heat-detecting gun. The heat gun is fairly accurate if used consistently in reference to distance from the object being tested. When making measurements, be sure to position the gun a consistent distance and angle relative to the target area.

Assume the target is a valve manifold. It will have at least one inlet pipe, tube, or hose (pressure line), one return line and, often, one drain line. Select a heat test point on each line approximately two feet from the manifold, and mark the inlet, drain, and tank lines with paint to establish three test points.

Examine the drain test point for heat (flow), which would serve as an indication of internal leakage in the directional valve’s pilot sections. The inlet and tank test points will indicate the internal and external leakage flow for the entire manifold assembly and all combined functions.

Next, select a heat test point approximately two feet from the manifold and mark both lines leading to an actuator with paint lines to establish two test points per function — two for the A line, and two for the B line. Select one heat test point in each line as close as possible to the actuator, and mark a paint line on the A and B port lines. Ensure that the test point only indicates pipe or tubing temperature, not heat radiating from nearby sources. A wide temperature difference between these two points serves as a possible indication of internal or external leakage.

Two additional test points should be established; one on the pump discharge line and one on the main return line to each power unit. These two points will monitor heat into the system valve stands and heat out of the system valve stands. The reservoir temperature can be checked, but it is not an accurate method of determining leakage because heat exchangers offset temperature increases. When the temperature in the reservoir rises above recommendations, a serious fault already exists, such as internal leakage in one or more components or a clogged heat exchanger.

Initially, it is advisable to make a temperature map of a system at startup. This will serve as a reference for subsequent test readings. To avoid any confusion, two maps should be created, one with equipment running and one with equipment down or after having run in standby more for two hours. The date of the original mapping should be recorded along with the ambient temperatures near the test points.

Note that a number of conditions may alter readings from the infrared heat gun. First, because most dirt is a poor conductor of heat, buildup on component surfaces tends to act as insulation. This causes the exterior temperature of the dirt buildup to be lower than the actual temperature of the pipe or tube OD. The same is true for scale and other contaminants that accumulate on the inner surface of the pipe or tube. Not only does this scale cause the outside of the pipe or tube to be cooler than the fluid inside, but if thick enough, a layer or scale can restrict flow enough to raise fluid temperature.

In both cases, this insulating layer tends to make the fluid retain heat. The same is true for hose, which also tends to provide insulation. Therefore, pipe, tubing, and hose runs may carry heat over a longer distance than they would if pipe or tubing were clean inside and out. Because the elevated temperature occurs over a longer distance, this condition may give the impression that internal leakage is occurring when it actually isn’t.

This further reinforces the practice of keeping hydraulic systems clean. Keeping both the interior and exterior surfaces clean promotes cooling by allowing heat to transfer through the pipe or tube to the surrounding area. Of course, keeping the hydraulic fluid as clean as is practical is always a good idea to maximize performance, reliability, and service life.

James R. Mollo, deceased, had been president of Hydraulic Design Consultants, McMurray, Pa., at the time the manuscript for this article was submitted.