What is the reason for the pressure in the balance tube of a multi-stage centrifugal pump to be zero?

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Previously, whether the pump was stopped or running, as long as the pump inlet valve was open, the pressure gauge on the balance pipe would show 0.3 MPa. But now, whether the pump is stopped or running, the pressure gauge on the balance pipe shows 0 MPa. The filter was checked and found to be clean, and the issue persists even after replacing the pressure gauge. Strangely enough, the two pressure gauges on the pipelines for the mechanical seal flushing fluid (PLAN32) at the front and rear ends also showed 0 MPa, and replacing the gauges did not help either. When this pump is operated now, the outlet pressure, flow rate, and vibration levels are all within normal ranges. I consulted the manufacturer, but they were unable to provide a satisfactory answer. Fortunately, now that the backup pump is running, all parameters are normal as before. This pump has been shut down for the past two months. It was working normally before it stopped operating; the pipelines were purged before shutdown, and at that time all three pressure gauges showed incorrect readings. The pressure gauges were replaced, but once the pump was restarted, this problem arose. Has anyone encountered such a problem before?