How to quickly detect that a rotating centrifugal pump has reversed direction
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Some time ago, I had a multi-stage centrifugal pump here; due to a fault in the control cabinet, the motor stopped operating. At that moment, the outlet check valve failed, and high-pressure water flowed back, causing the pump and its motor to rotate in reverse at high speed. The staff on duty noticed that the motor current had dropped to zero, but they failed to detect the reverse rotation in time and close the outlet valve. If this happens, how can one quickly detect the reversal of the motor? Is it reasonable to determine that the motor has stopped operating solely based on the motor current dropping to zero? What other parameters need to be monitored?
How fast is fast? I can only check the pressure gauge
Data such as current, outlet pressure, and flow rate are all provided; several pieces of data are compared with each other
The pressure and liquid level of the feeding container change, and the pressure gauge of the pump also changes
We have previously experienced situations where the check valve failed, causing the pump to rotate in reverse; the pump drove the gearbox, which in turn caused the engine to rotate in reverse, resulting in severe losses. The measure taken later was to monitor both the pump’s rotation speed and outlet pressure simultaneously; for example, if the pump’s rotation speed was 1650 revolutions per minute but the outlet pressure remained below 0.7 MPag, it was considered that the pump had reversed direction, and an alarm was issued via the DCS. For your system, the key is to determine whether your pump and motor have a speed measurement system; if so, you can use the method mentioned above and make adjustments by referring to the performance curve of the pump. If not, it may be necessary to look at the process aspects; for example, under normal shutdown conditions (zero current), what is the pressure range at the pump’s inlet and outlet? Did the pressure exceed this range during inversion? Alternatively, if there is a flow meter at the pump outlet, then when the current is zero while the flow rate is not, it is likely that reversal has occurred.
A zero current level only indicates that the motor has stopped running; whether reversal has occurred can only be determined on-site. To enable automatic start-up via DCS control, many pumps have their outlet valves left open while in standby mode. If the outlet check valve fails, it can cause the pump to reverse, and this is something that requires attention.
On-site, it is possible to: check the pump outlet pressure (which can be compared with that of the backup pump)； Steering, motor current, and the sound of the motor can be monitored via the central control system; pump outlet flow rate and feed tank level are also available for monitoring
The performance of the pump when operated in reverse is similar to that in turbine mode; its curves differ from those in normal operation. However, almost no pump manufacturers conduct tests on reverse operation performance, and some of them even advise users not to operate the pump in reverse. Recommended monitoring: flow rate, head, speed, and current, to be compared with the normal curves.
Do not rely too much on check valves; it is recommended to close the manual shut-off valve at the pump outlet after each shutdown~~
It is possible to view the pump outlet flow rate and the inlet tank level