Does the inlet pressure gauge gradually drop to zero after the centrifugal pump starts? ?

The situation isn’t clearly explained. When a centrifugal pump is in operation, the inlet is generally under negative pressure (suctioning air into the pump’s inlet), and this pressure varies depending on the outlet flow rate and pressure. It’s not clear what changes have taken place under the corresponding operating conditions

This pump is for positive-pressure water supply! ! What I mean is that the pressure gauge reading gradually drops to zero, while the actual pressure remains normal! ! At the same time, the pressure gauge was undamaged (checked after shutting down the pump) ；

Has the water level dropped? Or are there impurities in the water that are gradually clogging the inlet filter?

It can’t be the situation described on the fourth floor! ! The water level is high, and the filter screen has just been cleaned! !

If the pump is fine, it must be the cut-off valve at the inlet side of the pressure gauge that is faulty, preventing the pressure gauge from accurately measuring pressure.

The actual pressure is normal! The pressure gauge is normal! Is the pressure gauge slowly dropping to zero? This statement is contradictory! How far from the pump’s inlet is this inlet pressure gauge installed? If it’s very close, I think it’s normal; although water is supplied under positive pressure, there will still be pipe resistance in the pipeline from the front storage tank to the pump inlet! Especially after the pump starts, the flow rate of water in the pipes increases, and so does the pipe resistance; as a result, the pump’s inlet pressure gradually drops to zero. After startup, it is the actual inlet pressure that is shown on the pressure gauge! It won’t be a positive pressure of 30 KPa at this time. If it was installed newly, the solution is to increase the diameter of the inlet pipe and clean the filter screen. If it’s not a new pump and it was working fine before, it’s likely that the inlet pipe or the inlet filter is blocked, or that the inlet pipe is scaled.

Back to floor 7. This phenomenon is quite contradictory, but it does exist! ! I wonder if that expert could explain the working principle and structure of ordinary stainless steel pressure gauges! ! This helps me understand this problem

Support the claim from the 7th floor. :victory:

It should be a Bourdon tube pressure gauge: when the medium enters the Bourdon tube, the tube is compressed and deformed, which causes the pin at its end to move within the slits of the sector gear. The sector gear then drives the small gear equipped with the pointer, and the pointer indicates the pressure on the dial. If the pressure gauge is not broken. Could it be that the impeller has come loose from the shaft? In our company, we have had cases where the nuts used to secure the impeller came loose and the keys were worn out

If the pressure is normal, then there must be a problem with the pressure gauge. . . .

I have always wondered about this issue: could there be a leak at the connection point of the pressure gauge, allowing air to seep in gradually and affecting the force applied to the fixed end of the spring tube? ! In that case, theoretically the pressure reading will gradually decrease until it reaches zero! ! These are just my humble thoughts! ! Hehe

Check whether the impeller is clogged.

This is a normal phenomenon; it’s not a device malfunction.

Estimate changes in the inlet process conditions, such as increased pipeline resistance or a decrease in liquid level. At this point, it is necessary to check whether NPSHA is lower than NPSHR; if so, the inlet pressure should be adjusted, otherwise cavitation may occur.

The original poster is quite interesting. If there’s a suspicion of a leak, just disassemble the pressure gauge, replace the sealing material, and then try running the pump again – that way you’ll know for sure. There’s no need to worry about such things when simple solutions exist.

We have encountered this situation as well; my analysis is that the pressure at the pump outlet is too low, and the pump outlet valve opens too quickly, causing the pump to stop working. Based on our experience, it’s sufficient to stop the pump and restart it (by slowly opening the outlet valve).