CENTRIFUGAL PUMP CASING WARMS UP DURING OPERATION

[davincigee]

Hello guys,
I'd be glad if you can help me with this. For some time now, I have been observing that our mechanical pumps casing have been heating up whenever they are in operation. These pumps are 3 in number and are always operated in parallel. They can either be operated using direct-on-line or variable frequency drives. These pumps supply gasoil (diesel) from the tanks to the loading gantry where there are automated valves that close and shut depending on when a truck is ready to be loaded with gasoil or not. They all take their feed from a 12" suction manifold via tee'd off 4" pipelines and discharge into 12" discharge manifold via tee'd off 3" pipelines. Usually, when the casings get warm, it is due to low head in the tank from which the liquid is being pumped from or, the suction strainers are clogged. We've checked all these and yet the problem still persists. Another thing I have observed is that when these pumps are run "manually" (i.e. direct-on-line), they tend to have this problem yet when we switch to the VFDs, the casings stop getting warm, indicating that cavitation has ceased. My question is, can cavitation occur in the pump casing when these pumps are deadheading against a closed automated valve at the gantry? Is there any remedy to this aside ensuring that the operation of the automated valves are synchronised to that of the pumps so that when the automated valves are closed, the pumps also stop?

Thanks guys

 

[Artisi]

Nothing to do with cavitation, if you are running a pump "deadheaded" all the energy (power) input into the pump is being dissapated into the product within the pump case, input sufficient power over sufficient time and you can possibly bring the water to the boil.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[DubMac]

Right, no cavitation to see here, move along....

To calculate the heat buildup in a pump running close to deadhead:

Temperature rise in centrifugal at reduced flow:
T = H/778 * (1/e - 1)
T = Temperature rise in Deg.F
H = total head in feet
e = pump efficiency at capacity corresponding to H

 

[LittleInch]

davincegee,

You have a few options.

1) link the requirement for flow ( your loading valves) to pump stop. I have seen pump start buttons on the loading system so that the operator presses that, waits 10-15 seconds for everything to get pressurised, then opens the loading valve. When all the loading valves are closed, the pump stops. A simple bit of logic in the control system.
2) create a minimum flow loop back to the storage tank, either using an ARV or some sort of instrumented control system
3) If the gap between loading usage is small (< 5 minutes), use the VFD to ramp down to minimum speed then ramp up once the loading starts

Low head in the tank and increase in pressure drop due to strainers (located these on the suction side is a bad move) shouldn't normally lead to increased temperature. If you're cavitating I don't think it normally results in this sort of thing, but I'll be corrected by others

Of course with 3 pumps in parallel, the chance of getting them all to flow at similar rates is low. Even with "equal" pipework routing, there is often an imbalance which could affect things as well.

a layout sketch of your system would be good to see

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

As the others have noted, the pumps are deadheading at low or zero flow conditions. This is indicated by the fact that the heating does not occur when the VFD's are operating. The heating from deadheading will be greater with a fixed speed pump.

An inexpensive fix is to modify the control system to shut down the pump when flow stops. The pressure can be monitored with a simple pressure switch. Another fix is to install a recirculation loop.

 

[davincigee]

Guys,
Great contributions. Very much enlightened by these submissions.

Mighty grateful