Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1080534
16 | January - February 2019 | www . machinerylubrication.com HYDRAULICS several pumps on one particular system during the past year. When the reservoir was drained and the suction strainer inspected, a large split was found down the center of the screen. When I asked how long it had been since the strainer was changed or cleaned, the mainte- nance mechanic said, "never in the 17 years I've been at the plant." Suction strainers should be removed from the reservoir at least twice per year and cleaned or changed. Often there will be a suction filter access flange located where the pump suction line enters the reservoir. is permits removal of the strainer without draining all the oil from the tank. A better method of cleaning the fluid to the pump's suction port is to mount an external filter in the line. is filter should also have a visual indicator to monitor the condition of the element. Pressure Line Filters As pumps operate, metal breakdown occurs. When the pressure exceeds 2,200 psi in a system with a fixed displacement pump, a pressure filter should be mounted in the pump outlet line. is will filter the metal particles prior to being directed to the system. When a variable displacement pump is used at pres- sures higher than 1,500 psi, a filter should be installed in the pressure line. e best method of monitoring the element condition is through the use of a pressure switch. Once the element becomes partially contaminated and the pressure drop across the element reaches the switch setting, an electrical signal will be sent indicating the condition. e switch may be used to provide an alarm on the operator's screen that the filter is nearly contaminated. Most pressure filters contain an internal bypass check valve to allow the oil to continually flow to the system once the element is contaminated. e rating of the check valve spring is usually 7-10 psi higher than the setting of the pressure switch. e best method of filtering is to install a dual filter system where one element is active and the other is a standby. When the pressure switch indicates the online element is contaminated, the clean filter can be selected without shutting down the system. Pressure filters are also commonly used immediately upstream of proportional valves and servo valves. is is due to the extremely tight clearances inside the valves. e majority of these filters are of the non-bypassing type. e filter should be mounted as close as possible to the valve. It is imperative to change these filters on a regular basis to prevent collapsing of the element, resulting in a catastrophic failure of the valve. Return Line Filters Filters are often connected in the return lines of a system's directional valves. is allows the oil that exhausts out of the cylinders and motors to be filtered before returning to the reservoir. However, this type of filtration is only effective if at least 20 percent of the system volume is ported through the element in one minute. For example, with a pump volume of 100 gallons per minute (GPM), a minimum of 20 GPM should flow through the return filter. Many systems, such as reject kickers, only operate sporadically, rendering the return line filter ineffective. Visual indicators and pressure switches can be used to monitor the element's condition. e issue with visual indicators is that they can fail and therefore should not be relied upon. A filter maintenance schedule can be established by initially sampling the oil for several weeks or months. I once consulted with a plant in Oregon that changed the large return filter on its press every month. After the oil was sampled monthly, it was discovered that the ISO cleanliness code was only exceeded after eight months. e plant then scheduled the element to be changed every six months. Case Drain Filters Any oil that bypasses a variable displace- ment pump or an externally drained hydraulic motor will flow through the case drain line and into the tank. e oil that bypasses will contain contaminants generated by the metal breakdown in the pump and motor. A small filter can be installed in the case drain line to remove the contaminants. Prior to installing these filters, the rating of the pump or motor shaft seal should be checked. For most variable displacement pumps, the shaft seal rating is 10-15 psi. Hydraulic motor shaft seals usually have a higher rating (approximately 50 psi). An internal or external check valve should also be used to allow the oil to bypass when the element becomes contaminated. Kidney-loop System A kidney-loop system consists of a separate pump and filter. Frequently, a heat exchanger is also connected in the loop. e pump constantly recirculates the oil in the reservoir through the filter and cooler. ese filters can be quite large. e reservoir volume should be turned over five to seven times in one minute by the recirculating pump. For example, if the reservoir holds 3,000 gallons, to filter the oil five times (15,000 gallons) in one minute, a 250 GPM pump will be required. Gear-type Filters often are connected in the return lines of a system's directional valves. Visual indicators can fail and should not be relied upon. Kidney-loop system filters may be quite large.