Frequent Pressure Fluctuation in Proportional Relief Valve


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In coal pulveriser hydraulic loading system the proportional relief valve is make ATOS, model: AGMZO-TERS, getting malfunctioned leading frequent pressure fluctuation and pressure drop in the system. The command to PRV operation is going from DCS (4-20mA) and supply voltage is 24VDC. The problem is that its working fine for some half to one hour after that pressure gets fluctuated and dropping. The supply was checked it was 24vDC only and properly grounded. Hydraulic oil quality was as per NAS 1638 grade 9. But unable to find why there pressure fluctuation. If I replace with new valve then there is no pressure fluctuation.