Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/756816
22 | November - December 2016 | www.machinerylubrication.com 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. Accumulator Testing An accumulator that is used for volume should be pre-charged with dry nitrogen to one-half to two-thirds the pump's compen- sator 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. Check Hoses 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 manufac- turers 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 HYDRAULICS Test a fixed-displacement pump by turning on the pump and record- ing the flow out of the relief valve tank line for one minute.