Machinery Lubrication

Machinery Lubrication May - June 2018

Machinery Lubrication magazine published by Noria Corporation

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www . machinerylubrication.com | May - June 2018 | 25 2. e oil entering the pump is 126 degrees F, while the oil in the case drain is 135 degrees F. As the pump wears, the bypassing will increase, causing an increase in temperature of the case drain line. If the pump is checked a month later and the temperature in the case drain is 145 degrees F, then the pump has worn considerably. e flow rate should then be checked by installing a flow meter in the line. If the flow rate reaches or exceeds 10 percent of the maximum pump volume, the pump should be replaced. In the previous example, a flow rate of 3 gallons per minute (GPM) would indicate a badly worn pump. Since the amount of case drain flow can vary from one pump manufac- turer to another, the key is to make initial temperature checks when the pump is relatively new to establish a reference. Permanent installation of a flow meter in the case drain line is also recommended. 2. Check the Tank Line Temperature of the Air Bleed Valve Check the tank line temperature of the air bleed valve at point "C" in Figure 1. e purpose of the air bleed valve is to automatically bleed air out of the line when the pump is first started. ese valves are most commonly found on systems where the pump is mounted above the oil level. Once the air is bled out and the hydraulic pressure builds to the spring setting (approximately 12 pounds per square inch), the valve will shift closed. e valve is rela- tively small and can only handle a flow rate of 2 GPM. In most systems, the pump volume is higher than 2 GPM. Although the full pump volume usually cannot flow through the valve, heat will be generated if the valve fails open. is can increase the oil temperature as well as cause the actuators to move slower, particularly if a low-volume pump is used. 3. Check the Tank Line Temperature of the Relief Valve Check the tank line temperature of the relief valve (RV ) at point "D" in Figure 1. In a system where a pressure-compensating pump is used, the relief valve spring should be set 250-300 pounds per square inch (PSI) above the compensator setting. e purpose of the relief valve is to provide a flow path in the event the compensator spool fails to shift and reduce the pump volume to a near 0 GPM output. e tank line of the relief valve should be at ambient temperature. In Figure 4, the tank line is 99.7 degrees F, well below the oil temperature in the reservoir. If this line is hot, then the pump compen- sator spool has failed to shift, the relief valve is stuck partially open or the plant knob-turner has increased the compensator setting above the relief valve setting. 4. Check the Tank Line Temperature of the Check Valve A check valve (CV) is commonly used in a filtering and cooling loop, as shown on the schematic in Figure 1. e purpose of the check valve is to protect the heat exchanger from high pressure. e rating of the check valve spring is usually 65-100 PSI. If the cooler is of the air design, the internal tubes can become plugged with contamina- tion. A lso, if the unit is initially started while the oil is cold, higher resistance will be developed in the system. In either case, oil will flow through the check valve when the spring setting is reached. W hen operating normally, the tank line (E) should be at ambient tempera- ture. In Figure 5, the check valve's tank line is 142 degrees F, indi- cating that the cooler tubes are most likely contaminated. Figure 3. An air bleed valve Figure 4. Image showing the tank line temperature of a relief valve Figure 5. Image showing the tank line temperature of a check valve ML

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