Frequent operation of the relief valve causes no loading when the compressor pressure is low

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This post was last edited by a985658 on 2011-3-3 19:12. It deals with practical issues on-site: a 10KV, 5000m3 capacity, 1500KW air compressor of Korean origin made by Seah, equipped with a Siemens S7 400 PLC and a Siemens touch screen – each set. The compressor has been in use for about 2 years. Under normal conditions, the outlet pressure is maintained between 7 and 8 bar; if it drops below 7 bar, the compressor issues a command to increase pressure, the inlet valve opens, the pressure relief valve closes, and the compressor enters a pressure-increasing mode ； If the pressure rises above 8 bar, the compressor issues a unload command; the inlet valve closes and the relief valve opens ； When the pressure is between 7 and 8, a capacity control mode may occur. Suddenly before the New Year, the outlet pressure dropped below 7 bar; the compressor kept issuing unload commands, yet no alarms were recorded in the alarm log. The supplier’s explanation was that this fault was caused by the compressor operating above its limit too many times; there is a protection mechanism inside the compressor to prevent its relief valve from activating frequently. In reality, the vent valve does operate frequently; under normal conditions, it switches between capacity control and unloading mode once every 30 seconds, meaning the valve opens and closes once per 30 seconds. What I would like to know is what causes the vent valve to operate so often? Is it due to inadequate internal procedures or too low gas consumption on our part? I’m also looking for ways to solve the issue of failed loading. I’ve tried shutting down the device, resetting it, and then restarting it; sometimes it manages to load properly, but other times it doesn’t. I’m not sure what the correct method is to overcome this protection mechanism I wonder if any fellow sailors have encountered such a problem; I would appreciate it if you could offer some advice. A few points need to be noted: 1. It seems that there was no issue with frequent valve operation during the initial phase of compressor operation ； 2. The PLC programs were created by Koreans; some of them are locked, making it impossible to understand the program logic ； 3. The internal program of the CPU was previously lost; later, it was uploaded from the CPU of another identical compressor and then downloaded to this CPU. 4. Since the compressor was put into operation, there has been an alarm indication on the touch screen labeled “Lower limit pressure alarm.” This alarm appears regardless of whether the lower limit pressure is set to 0, 5, or the highest value of 8; it occurs sometimes every few dozen seconds, and other times every few minutes or tens of minutes.