Machinery Lubrication

Machinery Lubrication March April 2018

Machinery Lubrication magazine published by Noria Corporation

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lines. In many cases, the pump is mounted below the level of the system valves, cylinders and motors. e check valve downstream of the pump will prevent the lines from draining once the electric motor is turned off. If the oil in the lines drains through the pump and into the reservoir, a vacuum will occur. Air will be pulled into the lines through the O-rings and seals of the valves and actuators. is can create issues when restarting the system, as the air will need to be bled out. Block Oil Flow from the Accumulator Some systems have a hydraulic accumulator installed downstream of the pump and check valve. When the system is turned off, there is pressurized fluid inside the accumulator. e check valve will block flow from the accumulator, preventing the reverse rotation of the pump. You can observe the pump shaft or electric motor fan to verify that the check valve is good. Please note that all systems using an accumulator should have a method of bleeding the hydraulic pressure down to zero psi when the system is turned off. Prevent Oil Flow from the Online Pump to the Offline Pump On many systems, one pump is used as a backup or spare (Figure 3). Each pump will have a check valve at the pump outlet port. e check valve will block flow from the online pump to the offline pump, preventing reverse rotation. I remember being called into a papermill that kept losing one of the two pumps on its chemi-washer drives. e shaft seal of one pump continually blew out. When the mill ran out of spares, personnel had to ship their last pump by air freight to the factory in New York. e timeline was so critical due to downtime costs that the pump was still warm when they received it back from the factory. Just prior to installing the pump, we removed the check valve in the case drain line and found it stuck in the closed position. is prevented the oil in the pump case from draining, which resulted in blowing out the seal. Frequently, a check valve is used for pressure control. A common application is to employ it as a relief valve to protect a heat exchanger (as shown in Figure 4). In this case, the spring rating is usually 65-100 psi. If the oil is cold, the inlet pressure to the cooler may reach the check valve's rating. e check valve will then open and direct the pump volume around the cooler. A check valve will also provide protection for an air-type heat exchanger if the tubes become contaminated. A few years ago while teaching a class at a sawmill, I observed the students doing their hands-on exercises on the edger. Although a check valve was shown on the schematic to protect the air cooler, the lines to the check valve were plugged off. I asked one of the mechanics about it. He said the check valve was taken off years ago and that they had changed the cooler the week before because of ruptured tubes. When troubleshooting hydraulic systems, most everyone looks for something large to be the problem, such as a pump, valve or cylinder, but every component has a function. Be sure you understand the purpose of the check valves in your systems. 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 ML Figure 3. In some systems, one pump is used as a backup or spare, with each having a check valve at the outlet port. Figure 4. A check valve may also be used as a relief valve to protect a heat exchanger. www . | March - April 2018 | 15

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