Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/934530
18 | January - February 2018 | www . machinerylubrication.com HYDRAULICS pressure does not build up, the pump or relief valve is bad. If the system is operating at a slower speed, a volume problem exists. Pumps wear over time, which results in less oil being delivered. While a flow meter can be installed in the pump's outlet line, this is not always practical, as the proper fittings and adapters may not be available. To determine if the pump is badly worn and bypassing, first check the current to the electric motor. If possible, this test should be made when the pump is new to establish a reference. Electric motor horsepower is relative to the hydraulic horsepower required by the system. is is shown in the following formula: electric motor horsepower (hp) = gallons per minute (GPM) x pounds per square inch (psi) x 0.00067. For example, if a 50-GPM pump is used and the maximum pressure is 1,500 psi, a 50-hp motor will be required. If the pump is delivering less oil than when it was new, the current to drive the pump will drop. A 230-volt, 50-hp motor has an average full load rating of 130 amps. If the amperage is considerably lower, the pump is most likely bypassing and should be changed. e temperature of the pump housing and suction line should also be checked. A severe increase in tempera- ture indicates a badly worn pump. Testing a Variable Displacement Pump e most common type of variable displacement pump is the pressure-compensating design. e compensator setting limits the maximum pressure at the pump's outlet port. e pump should be isolated as described for the fixed displacement pump. If pressure does not build up, the relief valve or pump compensator may be bad. Prior to checking either component, perform the necessary lockout procedures and verify that the pressure at the outlet port is zero psi. e relief valve and compensator can then be taken apart and checked for contamination, wear and broken springs. If a volume problem exists in the system, perform the following tests: 1. Check the tank line temperature of the relief valve with a temperature gun or infrared camera. e tank line should be near ambient tempera- ture. If the line is hot, the relief valve is either stuck partially open or is set too low. 2. Install a flow meter in the case drain line and check the flow rate. Most variable displacement pumps bypass 1-3 percent of the maximum pump volume through the case drain line. If the flow rate reaches 10 percent, the pump should be changed. Permanently installing a flow meter in the case drain line is an excellent reliability and troubleshooting tool. 3. Check the current on the drive motor. 4. Ensure the compensator is 200 psi above the maximum load pressure. If set too low, the compensator spool will shift and start reducing the pump volume when the system is calling for maximum volume. Performing these recommended tests should help you make good decisions about the condition of your pumps or the cause of pump failures. If you change a pump, have a reason for changing it. Don't just do it because you have a spare one in stock. Conduct a reliability assessment on each of your hydraulic systems so when an issue occurs, you will have current pressure and temperature readings to consult. ML About the Author A l Smiley is the president of GPM Hydraulic Consulting Inc., located in Monroe, Georgia. Since 1994, GPM has provided hydraulic training, consulting and reliability assessments to companies in the United States, Canada, the United K ingdom and South America. Contact Al at gpm@gpmhydraulic.com. To isolate a fixed displacement pump and relief valve from the system, close a valve or plug the line downstream (left). If pressure builds, a component downstream of the isolation point is bypassing (right). Hand Valve Closed 1,200 PSI 1,200 PSI 1,200 PSI 1,200 PSI