Pump or valve first?

Tom Jenkins said:

A couple of additions:

Many centrifugal pumps can be dead-headed, but for most of the time must be kept short to avoid overheating and physical damage to the pump.

(1)

Ifyou want to prevent backflow from the system through the pump I'd suggest using a check valve. That will allow you to open the discharge isolation valve before starting the pump.

(2)

I would only use a valve on the pump suction for isolation during maintenance. During normal circumstances it should be open. Starting a pump with a closed suction valve is a good way to induce cavitation and early pump failure.

(3)

Click to expand...

(1)

I once worked on a High Temperature Hot Water (HTHW) system that ran 24/7. 3 out of 4 pumps recirculated the water, and 2 out of 3 just kept the system pressurised to 3 bar +. The water was heated to about 130 deg.C, so the pressure had to be maintained. The only time there was any flow through the pressurisation pumps was when HTHW was drawn from the recirculating main.

(2)

Check-valves, or non-return valves, are generally frowned upon in process industries. They are sometimes difficult to CIP (Clean In Place), leading to longer cycle times, and introduce a flow restriction which has to be accounted for by increasing the size of pumps.

(3)

Pumps are very often used for multiple source vessels, a CIP set for example, will have (at the very least) a Pre-Rinse Tank, a Detergent Tank, a neutralising Acid Tank, and a Final Rinse Tank. Valving is then used to deliver to one pump.

Many pumps don't like to be run "dry", so opening the route and waiting to detect flow ensures the pump is flooded before starting it. CIP "Scavenge" pumps, on the other hand, which are used for keeping the vessel you are cleaning as empty as possible, are therefore designed to run with minimum "flooding" of the pump cavity.


Summarising -

You must always check with the pump manufacturers, let them know and understand your applications. If you just go ahead and use a pump without verification from them, any warranties will almost certainly be null