Summer's Here and Hydraulic Oil Temperatures are Rising. Here's what YOU need to know before the system shuts your plant down.

Remember that hydraulic system that was running at 145° last September that you didn’t worry about because cooler weather was on the way? That same system is most likely now operating at 160° and may have shut down on high temperature. The fact is, any industrial hydraulic system that is running above 140° is running too hot. For every 15° increase in temperature above 140°, the life of the oil is cut in half. Systems that operate at high temperature can cause sludge and varnish which result in the sticking of valve spools. Pumps and hydraulic motors bypass more oil at high temperature causing the machine to operate at a slower speed. In some cases, high oil temperature can waste electrical energy by causing the pump drive motor to pull more current to operate the system. O’rings harden at higher temperatures causing more leaks in the system. So what checks and tests should you make if the oil temperature is above 140°?

First of all, any hydraulic system generates a certain amount of heat. Approximately 25% of the input electrical horsepower will be used to overcome heat losses in the system. Whenever oil is ported back to the reservoir and no useful work is done, heat will be generated.

The tolerances inside pumps and valves are normally are in the ten thousandths of an inch. These tolerances permit a small amount of oil to continuously bypass the internal components causing the fluid temperature to rise. When oil is flowing through the lines a series of resistances will be encountered. For example, flow controls, proportional valves and servo valves control the flow rate of oil by restricting the flow. When oil flows through the valves a “pressure drop” occurs. This means that a higher pressure will exist at the inlet port of the valve than the outlet port. Anytime oil flows from a higher to a lower pressure heat is generated and is absorbed in the oil.

When a system is initially designed the reservoir and heat exchangers are sized to remove the heat that is generated. The reservoir allows some of the heat to dissipate through the walls to the atmosphere. Heat exchangers, if properly sized, should remove the balance of the heat allowing the system to operate at approximately 120? F. 

The most common type of pump is the pressure compensating, piston type pump. The tolerances between the pistons and barrel are approximately .0004”. A small amount of the oil at the pump outlet port will bypass through these tolerances and flow into the pump case.  The oil is then ported back to the reservoir through the case drain line. This case drain flow does no useful work so is therefore converted into heat. 

The normal flow rate out of the case drain line is 1 – 3% of the maximum pump volume. For example, a 30 GPM pump should have approximately .3 -.9 GPM of oil returning to the tank through the case drain. A severe increase in this flow rate will cause the oil temperature to rise considerably.  

To check the flow, a flow meter can be connected in the case drain line. Make sure the pressure in all lines is at 0 PSI before disconnecting any lines. Secure the line to the container prior to turning the pump on. Turn the pump on and observe the flow on the meter. If the oil flow reaches 10% of the pump volume, the pump should be changed. A flow meter can also be permanently installed in the case drain line to monitor the flow rate. This visual check can be made regularly to determine the amount of bypassing. 

A typical variable displacement, pressure compensating pump is shown in the picture. During normal operation when the system pressure is below the compensator setting (1200 PSI) the internal swashplate is held at maximum angle by the spring. This allows the pistons to fully stroke in and out permitting the pump to deliver maximum volume. Flow from the outlet port of the pump is blocked through the compensator spool.

Once the pressure builds to 1200 PSI, the compensator spool shifts directing oil to the internal cylinder. As the cylinder extends the angle of the swashplate moves to the near vertical position. The pump will only deliver enough oil to maintain the 1200 PSI spring setting. The only heat generated by the pump at this time is the oil that flows past the pistons and through the case drain line. 

To find the amount of heat the pump is generating when compensating, the following formula can be used. 

           HP = GPM X PSI X .000583

Assuming the pump is bypassing .9 GPM and the compensator is set to 1200 PSI, the amount of heat generation is:

           HP = .9 X 1200 X .000583

           HP = .6296

As long as the system cooler and reservoir can remove at least .6296 horsepower of heat, the oil temperature should not increase. If the bypassing increases to 5 GPM, the heat load increases to 3.50 horsepower. If the cooler and reservoir are not removing at least 3.50 horsepower, the oil temperature will increase.

           HP= 5 X 1200 X .000583

           HP = 3.50

Many pressure compensating pumps utilize a relief valve as a safety back up in case the compensator spool sticks in the closed position. The relief valve should be set 250 PSI above the setting of the pressure compensator. Since the relief valve setting is above the compensator setting, no oil should flow through the relief valve spool. Therefore, the tank line of the valve should be at ambient temperature.

If the compensator were to stick in the position shown, the pump would deliver maximum volume at all times. The excess oil not used by the system would return to tank through the relief valve. A significant amount of heat will be generated if this occurs.  

Many times pressures in the system are randomly adjusted in an attempt to make the machine run better. If the local knob turner turns the compensator pressure above the setting of the relief valve, then the excess oil would return to tank through the relief, causing the oil temperature to rise 30 or 40 degrees. If the compensator fails to shift or is set above the relief valve setting, a tremendous amount of heat will be generated. Assuming the maximum pump volume is 30 GPM and the relief valve is set to 1450 PSI, the heat generation can be found. 

           HP = 30 X 1450 X .000583

           HP = 25 HP

If a 30 HP electric motor is used to drive this system, then 25 HP will be converted to heat when in the idle mode. Since 746 watts = 1 HP, then 18,650 watts (746 X 25) or 18.65 KW of electrical energy will be wasted!

There are other valves that may be used in the system such as accumulator dump valves and air bleed valves that could fail open and allow oil to bypass to the reservoir at high pressure. The tank lines of these valves should be at ambient temperature. Another common cause of heat is bypassing of the cylinder piston seals.

The heat exchanger or cooler should be maintained to insure that the excess heat is removed. If an air type heat exchanger is used, the cooler fins should be cleaned on a regularly scheduled basis. A degreaser may be necessary to clean the fins. The temperature switch that turns the cooler fan on should be set at 115? F.  If a water cooler is used, a water modulating valve should be installed in the water line to regulate the flow through the cooler tubes to 25% of the oil flow.

The reservoir should be cleaned at least once per year. If not cleaned, sludge and other contaminants can not only coat the bottom of the reservoir, but the sides as well. This will allow the reservoir to act as an incubator instead of dissipating the heat to the atmosphere. 

I recently was at a plant where the oil temperature on a stacker was 350 degrees! It was discovered that the pressures were out of adjustment, the manual accumulator dump valve was partially open and oil was continually ported through a flow control that drove a hydraulic motor. The motor drove outfeed chains that only operated 5 -10 times during an 8 hour shift. The pump compensator and the relief valve were properly set, the manual valve was closed and the electrician de-energized the motor's directional valve, blocking flow through the flow control. When the unit was checked 24 hours later the oil temperature had dropped to 132 degrees F! The oil, of course, had broken down and the system had to be flushed to remove the sludge and varnish and new oil added to the unit. All three of these issues were man induced! The local knob turner had turned the compensator above the relief valve allowing the pump volume to return to tank at high pressure when nothing on the stacker was operating. Someone had failed to fully close the manual valve thereby allowing oil to bypass back to tank at high pressure. The system had also been improperly programmed to allow the chains to run continuously when they should have only been driven when a load was to be removed from the stacker. The next time a heat problem occurs in one of your systems look for oil that is flowing from a higher to a lower pressure in the system. That's where you'll find your problem!