As the weather turns cooler, you may not worry too much about the increase in oil temperature, but the truth is that any industrial hydraulic system operating above 140 degrees is too hot. Considering that every 18 degree increase in temperature above 140 degrees, the life of the oil will be reduced by half. A system operating at high temperatures will produce sludge and paint film, which can cause the valve core to jam.
Pumps and hydraulic motors bypass more oil at high temperatures, causing the machine to run at a slower speed. In some cases, high oil temperature will cause the pump drive motor to draw more current to run the system, thus wasting electrical energy. O-rings will also harden at higher temperatures, causing more leakage in the system. So what inspections and tests should be carried out if the oil temperature is higher than 140 degrees?
Every hydraulic system generates a certain amount of heat. Approximately 25% of the input electrical horsepower will be used to overcome the heat loss in the system. Whenever oil is transported back to the tank without doing useful work, heat is generated.
The internal tolerances of pumps and valves are usually a few thousandths of an inch. These tolerances allow small amounts of oil to constantly bypass internal components, causing the fluid to rise in temperature. When oil flows through the pipeline, it encounters a series of resistance. For example, flow control, proportional valves and servo valves control the flow of oil by restricting the flow. When oil flows through the valve, a "pressure drop" occurs. This means that the pressure at the inlet of the valve will be higher than at the outlet. Whenever oil flows from a higher pressure to a lower pressure, it will generate heat and absorb heat in the oil.
When the system was originally designed, the size of the reservoir and heat exchanger were adjusted to remove the heat generated. The cistern allows some heat to be dissipated into the atmosphere through the walls. If the size is right, the heat exchanger should eliminate the heat balance so that the system can operate at approximately 120 degrees Fahrenheit.
Figure 1. The tolerance between the piston and the barrel of a pressure compensated piston pump is about 0.0004 inches.
The most common type of pump is a pressure compensated piston pump. The tolerance between the piston and the barrel is approximately 0.0004 inches (Figure 1). A small amount of oil at the pump outlet will bypass these tolerances and flow into the pump housing. Then, the oil returns to the tank through the case drain pipe. This shell drain flow does not do any useful work, so it is converted into heat.
The normal flow from the case drain pipe is 1% to 3% of the maximum pump volume. For example, a 30 gallons per minute (GPM) pump should have approximately 0.3 to 0.9 GPM of oil returning to the tank through the case drain hole. This severe increase in flow rate will cause the oil temperature to rise significantly.
In order to check the flow, the pipeline can be transplanted to a container of known size and timing (Figure 2). Unless you have confirmed that the pressure in the hose is close to 0 pounds per square inch (PSI), do not hold the line during this test. Instead, fix it to the container.
The flow meter can also be permanently installed in the case drain pipe to monitor the flow rate. This visual inspection can be performed periodically to determine the amount of bypass. When the oil flow reaches 10% of the pump volume, the pump should be replaced.
A typical variable displacement pressure compensation pump is shown in Figure 3. During normal operation, when the system pressure is lower than the compensator setting (1,200 PSI), the internal swash plate is held at the maximum angle by the spring. This allows the piston to be fully in and out, allowing the pump to deliver maximum volume. The flow from the pump outlet is blocked by the compensator spool.
Once the pressure rises to 1,200 PSI (Figure 4), the compensator spool moves, directing oil to the internal cylinder. As the cylinder extends, the angle of the swash plate moves to a position close to vertical. The pump can only deliver enough oil to maintain the 1,200 PSI spring setting. The only heat generated by the pump at this time is the oil flowing through the piston and the casing drain pipe.
To determine the heat generated by the pump during compensation, the following formula can be used: Horsepower (HP) = GPM x PSI x 0.000583. Assuming that the pump bypasses 0.9 GPM and the compensator is set to 1,200 PSI, the heat generated is: HP = 0.9 x 1,200 x 0.000583 or 0.6296.
As long as the system cooler and fuel tank can remove at least 0.6296 horsepower, the oil temperature will not rise. If the bypass increases to 5 GPM, the heat load increases to 3.5 horsepower (HP = 5 x 1,200 x 0.000583 or 3.5). If the cooler and fuel tank cannot remove at least 3.5 horsepower, the oil temperature will rise.
Figure 2. Check the oil flow by connecting the case drain pipe to a container of known size and adjusting the flow rate.
Many pressure-compensated pumps use a relief valve as a safety backup to prevent the compensator spool from getting stuck in the closed position. The relief valve should be set 250 PSI higher than the setting of the pressure compensator. If the setting of the relief valve is higher than that of the compensator, no oil will flow through the relief valve spool. Therefore, the valve's tank line should be at ambient temperature.
If the compensator is stuck in the position shown in Figure 3, the pump will always provide the maximum capacity. The excess oil not used by the system will be returned to the oil tank through the safety valve. If this happens, a lot of heat will be generated.
Usually the pressure in the system is adjusted randomly to make the machine run better. If the local knob regulator sets the compensator pressure higher than the safety valve setting, the excess oil will return to the tank through the safety valve, causing the oil temperature to rise by 30 or 40 degrees. If the compensator cannot be switched or set above the safety valve setting, it will generate a lot of heat.
Assuming that the maximum pump capacity is 30 GPM and the safety valve is set to 1,450 PSI, the amount of heat generated can be determined. If a 30-horsepower electric motor is used to drive the system (HP = 30 x 1,450 x 0.000583 or 25), 25 horsepower will be converted into heat in idle mode. Since 746 watts is equal to 1 horsepower, 18,650 watts (746 x 25) or 18.65 kilowatts of electrical energy will be wasted.
Other valves used in the system, such as the accumulator discharge valve and bleed valve, may also fail to open and allow oil to bypass the tank at high pressure. The tank lines of these valves should be at ambient temperature. Bypassing the cylinder piston seal is another common cause of heat generation.
Figure 3. This figure shows a variable displacement pressure compensated pump in normal operation.
Figure 4. When the pressure increases to 1,200 PSI, please pay attention to the changes in the compensator slide valve, internal cylinder and swash plate of the pump.
The heat exchanger or cooler should be maintained to ensure that excess heat is removed. If an air-type heat exchanger is used, the cooling fins should be cleaned regularly. It may be necessary to use a degreaser to clean the heat sink. The temperature switch that turns on the cooler fan should be set to 115 degrees Fahrenheit. If a water cooler is used, a water regulating valve should be installed in the water pipe to adjust the flow through the cooler pipe to 25% of the oil flow.
The water tank should be cleaned at least once a year. Otherwise, sludge and other pollutants will not only cover the bottom of the reservoir, but also the sides. This will allow the reservoir to act as an incubator instead of dissipating heat into the atmosphere.
I was in a factory recently and the oil temperature on the stacker was 350 degrees. It was found that the pressure was out of adjustment, the relief valve of the manual accumulator was partially opened, and the oil was continuously delivered through the flow control of the driving hydraulic motor. The motor drives the discharge chain, which runs only 5 to 10 times during an 8-hour shift.
The pump compensator and safety valve are set correctly, the manual valve is closed, and the electrician cuts off the power of the motor directional valve to prevent the flow from passing through the flow control. When the device was inspected 24 hours later, the oil temperature had dropped to 132 degrees Fahrenheit. Of course, the oil has broken down and the system must be flushed to remove sludge and varnish. New oil must also be added to the device.
All these problems are caused by man. The local rotary knob sets the compensator above the safety valve, which enables the pump volume to return to the tank under high pressure when nothing is running on the stacker. There were also people who failed to fully close the manual valve, causing the oil to bypass the tank under high pressure. In addition, the system was incorrectly programmed to allow the chain to run continuously when it should only be driven when the load should be unloaded from the stacker.
Next time there is a thermal problem in your system, look for oil that flows from a higher pressure to a lower pressure in the system. This is where you might find the problem.
Read more about best practices for hydraulic systems:
10 hydraulic reliability checks you might not perform
Seven most common hydraulic equipment errors
How do you know if you are using the correct hydraulic fluid?
5 major hydraulic errors and the best solutions
Al Smiley is the President of GPM Hydraulics Consulting in Monroe, Georgia. Since 1994, GPM has been providing hydraulic training, consulting, and reliability evaluation for companies in the following fields.