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Often the only tests and actions performed on a hydraulic system involve changing the filters, sampling the oil and checking the oil level. As long as the system is operating, the mentality of “if it ain’t broke don’t fix it” frequently prevails. However, on any given system in your plant, you should perform 15 to 20 regular reliability tests while the system is operating. There are also several checks and procedures that should be completed during shutdowns or down days. The following preventive maintenance and reliability tests can help improve the efficiency and operation of your hydraulic systems and machines.
An example of an oil reservoir in a hydraulic system
A good time to check the oil reservoir is when your plant or system is down. Maintenance mechanics and electricians usually chuckle when I tell them that the reservoir should be cleaned at least once a year. One mechanic at a large wood products plant told me that the reservoir on one system hadn’t been cleaned since the mill started up 17 years ago.
Other than oil storage, the two main purposes of the reservoir are to dissipate heat and to allow contaminants to settle. If the reservoir is not cleaned, not only will its ability to dissipate heat be diminished, but it will act as a heat sink. Temperatures can easily soar well above the maximum recommended level of 140 degrees F. Oil will then start breaking down, leading to 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 the system components.
Many reservoirs contain a suction strainer to keep large particles from entering the pump. Most suction strainers have a 74-micron rating, whereas the tolerances inside pumps and valves are typically 3 to 8 microns.
Always use a lint-free cloth when cleaning a reservoir. If a solvent is employed, be sure it is recommended for hydraulic systems. Even small amounts of the wrong solvent can impair certain additives.
When oil is removed from the reservoir, it should be filtered going into a storage tank with a flushing and filtering unit, which can remove solid contaminants and water. Use a quality, high-capture-efficiency filter (ISO 16889) that matches the target cleanliness level of the system. Unless the oil is severely degraded, it is not necessary or even desirable to change it.
After the reservoir is cleaned, run the oil through the filters while refilling. The entire system should then be flushed to clean the oil in the lines to the valves and actuators.
System flushing is done by connecting the inlet and outlet lines of the cylinders and motors. If possible, electrically or manually actuate the directional valves to allow the fluid to recirculate through the piping. If this is not possible, bypass the directional valves by connecting the pressure and tank lines to the outlet lines of the actuators. Utilize the machine’s existing pump to recirculate oil through the lines. Connect a high-velocity flushing unit so it recirculates oil in the reservoir through the filters during the flushing process. Allow the system to run for as long as possible.
Many times the heater is disconnected during the summertime or may have been omitted from the reservoir when it was initially built. Check the heater thermostat on the reservoir to confirm that it will turn on at a minimum of 70 degrees F. If the pump is mounted on top of the reservoir and the oil temperature drops below approximately 60 degrees F, then some cavitation of the pump may occur.
Most reservoirs utilize two switch settings - warning and shutdown. The problem is that the difference between these 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 if a hose ruptures.
The breather cap is usually the most neglected component on the reservoir. Verify that the breather cap filter has a capture efficiency that matches the target fluid cleanliness. This is the first line of defense for contaminants entering the tank. Depending on the 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 of breather. Remember that money spent upgrading your breather cap is never wasted.
A flushing unit can be used to remove
solid contaminants and water from the oil.
Mineral oil will begin breaking down at 140 degrees F, but many systems will not shut down the unit until the oil temperature reaches 160 degrees F. Hydraulic systems are designed to operate below 140 degrees F. For every 15 degrees F that the oil increases above 140 degrees F, the life of the oil will be cut in half. If the oil temperature rises above that level, then a problem exists in the system. This could be caused by a cooler malfunction or excessive bypassing at the pump. Set the high-temperature switch at 140 degrees F to shut off the pump, preventing oil breakdown.
In a shell-and-tube type of heat exchanger, oil flows over the tubes. 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, the water flow should be 25 percent of the oil flow. The water flow can be controlled by manual valves, a water-modulating valve or an electrical solenoid valve. Circulating hot wash oil or light distillate through the tube or shell side can effectively remove sludge or similar soft deposits. Soft salt deposits may be washed out by circulating hot, fresh water. A mild alkaline solution such as Oakite or a 1.5-percent solution of sodium hydroxide or nitric acid can be used. The tubes should be flushed in the opposite direction that the oil normally flows.
If an air cooler is employed, verify that the cooler fan is turned on at approximately 120 degrees F and turned off at about 105 degrees F. Keep the fins clean so daylight can be seen through them. If necessary, combs should be utilized to straighten the fins on the unit. When cleaning the fins with an air hose, care should be taken so as not to damage them.
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 the maximum level. 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 percent of the maximum pump volume. Vane pumps usually bypass more than piston-type pumps. If 10 percent of the maximum volume flows out of the case drain line, then 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.
Fixed-displacement pumps can be tested by checking the flow through the relief valve. Turn on the pump and record the flow out of the relief valve tank line for one minute. Next, reduce the setting of the relief valve to its minimum setting. There should be less than a 10-percent difference in flow rates between the two tests. If a pump is badly worn, the flow will be considerably less at the highest pressure.
Oil purity is shown for a system before
it was flushed (left) and then after one,
four and 16 hours. This particular
system had high water content prior
An accumulator that is used for volume should be pre-charged with dry nitrogen to one-half to two-thirds the pump’s compensator setting. When the hydraulic system is turned off, a charging rig with a gauge can be utilized to check the pre-charge level.
To confirm an accumulator is operating properly, check the side of the shell with a temperature gun or infrared camera. The bottom half should be hotter than the top half. If heat is only indicated at the bottom, the accumulator may be overcharged. If there is no heat, the bladder may have ruptured, the piston seals may be bad, the pre-charge may be above the compensator setting or all the nitrogen may have leaked out. If heat is felt all the way to the top, the accumulator is undercharged.
Another check that can be made is to watch the system pressure gauge while the system is operating. The pressure should not normally drop more than 100-500 pounds per square inch (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 out of the bleed valve. Care should be taken so all personnel are away from the bleed valve prior to turning on the pump. If there is continuous flow, the piston seals or barrel may be worn. If no flow exists, recharge the accumulator to the proper dry nitrogen level.
Test a fixed-displacement pump by turning on the pump and
recording the flow out of the relief valve tank line for one minute.
Check all system hoses for the proper length and wear. Hoses rarely burst due to the rated working pressure being exceeded but rather because of a poor crimp or rubbing on a beam, another hose, etc. Hose sleeves are available from a variety of manufacturers if rubbing cannot be avoided. Hoses generally should not exceed 4 feet in length unless they move with the machine.
Also, examine the system piping to verify that a hose is installed prior to connecting to a valve bank or cylinder. The hose will absorb the hydraulic shock generated when the oil is rapidly deadheaded. One exception to this rule is that hard piping should be used when connecting to a vertical or suspended type of load. Pilot-operated check valves and counterbalance valves can be employed to hold the load in the raised position.
Continuous flow out of a bleed valve
may indicate worn piston seals.
Inspect system clamps to confirm they are the correct type for hydraulic lines. Beam and conduit clamps are not acceptable, as they will not absorb the shock generated in the piping or tubing. Clamps should be spaced approximately 5 feet apart and installed within 6 inches of the pipe or tubing termination point.
On any system, one or more valves will be closed while the system is operating. These include relief valves used with pressure-compensating pumps, air bleed valves and accumulator dump valves. The tank lines of these valves should be checked regularly with a temperature gun or infrared camera to verify that the valves are closed and no oil is being lost back to the reservoir.
A reliability and preventive maintenance schedule should be developed for each of the hydraulic systems in your plant. By performing these tests, your systems will operate safer and at maximum efficiency while reducing unexpected downtime.