Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/730290
www.machinerylubrication.com | September - October 2016 | 17 www.machinerylubrication.com | September - October 2016 | 17 case the compensator spool sticks in the closed position. The relief valve should be set 250 PSI above the pressure compensa- tor's setting. If the relief valve's setting is above the compensator's setting, no oil should flow through the relief valve spool. Therefore, the valve's tank line should be at ambient temperature. If the compensator were to stick in the position shown in Figure 3, the pump would deliver maximum volume at all times. The excess oil not used by the system would return to the tank through the relief valve. A significant amount of heat would be gener- ated if this occurred. Of ten the pressure in the system is randomly adjusted in an attempt to make the machine run better. If the local knob- turner sets the compensator pressure above the relief valve setting, excess oil will return to the tank through the relief, causing the oil temperature to rise 30 or 40 degrees. If the compensator fails to shif t or is set above the relief valve setting, a tremendous amount of heat will be generated. Assuming the maximum pump volume is 30 GPM and the relief valve is set to 1,450 PSI, the heat generation can be determined. If a 30-horsepower electric motor is used to drive this system (HP = 30 x 1,450 x 0.000583 or 25), then 25 horsepower will be converted to heat when in the idle mode. Since 746 watts equals 1 horsepower, 18,650 watts (746 x 25) or 18.65 kilowatts of electrical energy will be wasted. Other valves used in the system such as accumulator dump valves and air bleed valves could also fail open and permit oil to bypass to the reser voir at high pressure. The tank lines of these valves should be at ambient temperature. Bypassing of the cylinder piston seals is another common cause of heat. Heat Removal The heat exchanger or cooler should be maintained to ensure excess heat is removed. If an air-type heat exchanger is used, the cooler fins should be cleaned on a regularly scheduled basis. A degreaser may be neces- sar y to clean the fins. The temperature switch that turns on the cooler fan should be set at 115 degrees F. If a water cooler is used, a water-modulating valve should be installed in the water line to regulate flow through the cooler tubes to 25 percent of the oil flow. The reser voir should be cleaned at least once per year. Other wise, sludge and other contaminants not only can coat the bottom of the reser voir but also the sides as well. This would allow the reser voir to act as an incubator instead of dissipating the heat to the atmosphere. I recently was at a plant where the oil temperature on a stacker was 35 0 degrees. It was discovered that the pressures were out of adjustment, the manual accumulator dump valve was partially open and oil was continually ported through a flow control that drove a hydraulic motor. The motor drove outfeed chains that only operated five to 10 times during an eight-hour shif t. The pump compensator and relief valve were properly set, the manual valve was closed and the electrician de-energized the motor's directional valve, blocking flow through the flow control. When the unit was checked 24 hours later, the oil temperature had dropped to 132 degrees F. Of course, the oil had broken down, and the system had to be flushed to remove sludge and varnish. New oil also had to be added to the unit. All of these issues were man-induced. The local knob-turner had set the compensator above the relief valve, enabling the pump volume to return to the tank at high pres- sure when nothing on the stacker was operating. Someone had also failed to fully close the manual valve, thereby permitting oil to bypass back to the tank at high pres- sure. In addition, the system had been improperly programmed to allow the chains to run continuously when they should have only been driven if a load was to be removed from the stacker. The next time a heat problem occurs in one of your systems, look for oil that is f lowing from a higher pressure to a lower pressure in the system. That's where you'll likely f ind your problem.