Despite their workhorse reputation, the hydraulic systems in your plant may need more attention than they’ve been getting.
By Jane Alexander, Managing Editor
In many facilities, hydraulic-system maintenance involves little more than changing filters, sampling oil and checking oil levels. Be advised, however: “If it ain’t broke, don’t fix it,” is not a best practice.
According to fluid power specialist Al Smiley, every hydraulic system in a plant should regularly undergo 15-20 reliability tests—while the equipment is running. Several checks and procedures can be performed during down days or shutdowns. Smiley offers these preventive maintenance tips to help improve the reliability of your system.
The best time to check a reservoir is when the plant or hydraulic equipment is down. Note, too, that reservoirs on these systems should be cleaned at least once a year.
Other than oil storage, the two main purposes of the reservoir are to allow contaminants to settle and to dissipate heat. If the reservoir is not cleaned, not only will its ability to dissipate heat be diminished, it will act as a heat sink. Temperatures can easily soar well above the maximum recommended temperature of 140 F. At that point, oil will begin to break down, creating sludge and varnish in the system. If the contaminants are not removed from the reservoir, they will be drawn into the pump, causing premature failure of system components.
To prevent large contaminant particles from entering the pump, many reservoirs contain a suction strainer (#6 in diagram above). The suction strainer usually has a 74-micron rating, whereas the tolerances inside pumps and valves are 3- to 8-micron. Be sure to use a lint-free cloth when cleaning out the reservoir. If using a solvent for this task, make sure it’s recommended for hydraulic systems. Even very small amounts of the wrong solvent can impair certain additives.
System cleaning and flushing
When oil is removed from the reservoir it should be filtered to 1 micron into a storage tank with a flushing and filtering unit that will remove solid contaminants and water. A number of commercially available units can be used for this. Unless the oil is severely degraded, it is not necessary (or desirable) to change it. Run the oil through the 1-micron filters when refilling the unit, after the reservoir has been cleaned. The entire system should then be flushed to clean oil in the lines to the valves and actuators.
Flush the system by connecting the inlet and outlet lines of the cylinders and motors together. If possible, electrically or manually actuate the directional valves to allow the fluid to recirculate through the piping. If this isn’t possible, bypass the directional valves by connecting the pressure and tank lines to the outlet lines to the actuators. Use the existing pump on the machine to recirculate the oil through the lines. Connect the flushing unit so that it recirculates the oil in the reservoir through the 1-micron filters during the flushing process. Allow the system to run as long as possible.
Reservoir heater setting
Often, a heater is disconnected in summer months or omitted from the reservoir when built. Check the heater thermostat on the reservoir (#2 in diagram) to verify that it will turn on at a minimum of 70 F. If the pump is mounted on top of the reservoir and the oil temperature drops below approximately 60 F, cavitation can occur.
Most reservoirs use two switch settings (#7 in the diagram): a warning-level setting and a shutdown-level setting. The problem with this arrangement is that the difference between the two levels may be several hundred gallons of oil. By eliminating the warning switch and setting the shutdown at a higher level, oil loss will be minimal when a hose ruptures.
The breather cap (#8 in diagram) is usually the most neglected component on the reservoir. Verify that the breather cap has a minimum rating of approximately 10 microns. This is the first line of defense for contaminants entering the tank. Depending on system location, the breather cap may need to be changed a couple of times a year. Many breathers have a mechanical indicator that will provide a visual indication when the element is dirty. Other options include pressurizing the reservoir with an internal bladder or using a moisture-removal-type breather. Remember that money spent upgrading your breather cap is never wasted!
While mineral-based oil will start breaking down at 140 F, many systems will not shut down the unit until the oil temperature reaches 160 F. Hydraulic systems are designed to operate below 140 F. For every 15-degree increase in oil temperature above 140 F, the oil’s life will be cut in half. An oil temperature above 140 F indicates a problem in the system. This could be caused by a cooler malfunction or excessive bypassing at the pump, valves, cylinders and hydraulic motors. Set the high-temperature switch (#10 in diagram) at 140 F to shut off the pump and prevent oil breakdown.
Heat exchanger flushing and cleaning
In hydraulic systems, oil flows over the tubes in a shell-and-tube-type heat exchanger (#12 in diagram). Water flow is ported through the tubes in the opposite direction. The heat in the oil is transferred from the oil to the water. To achieve the most efficient heat transfer, water flow should be 25% of that of the oil flow. (The flow rate can be controlled with manual valves, a water-modulating valve or an electrical-solenoid.)
Circulating hot wash oil or light distillate through the tube side or shell side will usually effectively remove sludge or similar soft deposits. Soft salt deposits can be washed out by circulating fresh, hot water. A mild alkaline solution, such as Oakite, or a 1.5% solution of sodium hydroxide or nitric acid can also be used. The tubes should be flushed in the opposite direction of the normal oil flow. If an air cooler is used, verify that the cooler fan is turned on at approximately 120 F and turned off at about 105 F.
Keep the fins so clean that daylight can be seen through them. If necessary, use combs to straighten the fins on the unit. Take special care when cleaning the fins with an air hose so as to not damage the fins.
On variable-volume pumps, check the flow out of the case drain line by porting the line into a container and timing it. This test should be made with the outlet pressure at maximum. It is not recommended that the line be held during this test.
Secure the line to the container prior to starting the pump. The normal case flow is 1-5% of the maximum pump volume. Vane pumps usually bypass more than piston-type pumps. If 10% of the maximum volume flows out of the case drain line, the pump should be changed. An excellent method of monitoring the case-drain flow while operating is to permanently install a flow meter in the case-drain line.
Test fixed-displacement pumps by checking the amount of flow through the relief valve. Turn the pump on and record the flow out of the relief-valve tank line for one minute. Then reduce the setting of the relief valve to its minimum setting. There should be less than 10% difference in flow rates between the two tests. If a pump is badly worn, the flow will be considerably less at the higher pressure.
An accumulator that’s used for volume should be pre-charged with dry nitrogen to from one-half to two-thirds of the pump-compensator setting. When the hydraulic system is turned off, a charging rig with a gauge can be used to check the pre-charge level.
To verify that the accumulator is operating properly, the side of the shell can be checked with a temperature gun or infrared camera. The bottom half or two-thirds should be hotter than the top half.
If heat is only felt at the bottom, the accumulator may be overcharged. If no heat is felt, the bladder may be ruptured, the piston seals may be bad, the pre-charge may be above the compensator setting or all of the nitrogen has leaked out. If heat is felt all the way to the top, the accumulator is undercharged.
Another check involves watching the system pressure gauge while the equipment is operating. Normally, the pressure shouldn’t drop more than 100-500 PSI when the accumulator is properly pre-charged.
If piston accumulators are used, the charging rig should be installed when the system is down and the oil bled off the top of the piston. With the pump on and the bleed-valve open, there should be little or no flow exhausting out of the valve. Make sure all personnel are away from the bleed-valve prior to turning the pump on. A continuous flow from the bleed valve would indicate that the piston seals or barrel were badly worn. If no flow exists, recharge the accumulator to the proper dry nitrogen level.
Check system hoses for proper length and wear. Hoses infrequently burst because the rated working pressure is exceeded. They normally rupture because of a poor crimp or the hose is rubbing on a beam or another hose. If rubbing cannot be avoided, hose sleeves are available on a reel from various manufacturers. Hoses should usually not exceed approximately 4 ft. in length unless they move with a machine.
Also inspect system piping to verify that prior to connecting to a valve bank or cylinder, a hose is installed. The hose will absorb the hydraulic shock that is generated when the oil is rapidly deadheaded. An exception to this rule is that hard piping should always be used when connecting to a vertical or suspended type load. Pilot-operated check valves and counterbalance valves are used to hold the load in the raised position.
Inspect system clamps to verify that that the proper hydraulic clamps are used to clamp hydraulic lines. Beam and conduit clamps are not acceptable, as they will not absorb the shock generated in the piping or tubing. The clamps should be spaced approximately 5 ft. apart. A clamp should also be installed within 6 in. of the pipe or tubing termination point.
On any hydraulic system, one or more valves could be closed while the system runs. These include relief valves used with pressure-compensating pumps, air-bleed valves and accumulator-dump valves. The tank lines of all valves should be checked regularly with a temperature gun or infrared camera to verify that they are closed and no oil is being lost back into the reservoir.
The big picture
To improve the reliability of your equipment, Smiley says it’s important to develop a preventive maintenance schedule for every hydraulic system in your plant, and stick to them. Heeding this advice can help your operations boost the efficiency, safety and uptime of these critical plant systems. MT
Al Smiley is President of GPM Hydraulic Consulting (gpmhydraulic.com) of Monroe, GA.