What do those pressure gauges on a pump tell us?

I was speaking with a good friend this week who was trying to quiet down the cavitation in a couple of centrifugal pumps that sounded like rock tumblers.  He had already taken a look at the pressure differential across the pumps and noted that it was minimal, but didn’t know what the numbers should be.  So we needed to start with the designed flow rate of 57GPM (gallons per minute).   The pump curve (fig. 1) showed that the pump would flow 57GPM at 65TDH (feet of head).

    Figure 1 

   

To convert the feet of head to a pressure reading we can divide the head by the pressure caused by the height of water above any given point.  Approximately 2.3 feet of water height equals 1 PSI, so in this case:

          65 TDH / 2.3 = 28.26

Now all we had to do was add the 28.26 PSI to the reading at the suction side and we could get the pump in the ballpark.  Since our suction side pressure was 30 PSI we needed to get to 58 PSI on the discharge, which was done by adjusting the Bell & Gossett Triple Duty Valve.  Once the adjustment was made the pump quieted down and the building occupants were happy.  Although not an exact method of balancing it will keep everyone happy till the balancer gets there with his pressure differential readout kit to set it up. 


So the gauges actually tell us how many feet of head the pump is generating when operating and by using the pump curve chart we can figure out the flow rate through the pump at that head pressure.

Something to keep in mind is the pressure readings work opposite of flow as the pressure goes up the feet of head increases and the flow goes down and vice versa. Also if the pressure is to low and the flow is too high you can get a pressure drop in the eye of the impeller that could cause the liquid to flash to a vapor creating air bubbles, as these bubbles move out along the impeller vane they will collapse and cause pump cavitation.  As these bubbles collapse they create tiny pits on the impeller in as little as a few hours these pits can add up and cause serious damage to an impeller.   


Next time you are called to a noisy pump take a look at the pressure differential and make sure the pump is operating along its curve, if not a little balancing might save a lot of problems later.