Backwash Pump Control

[ohiowater]

We are designing for backwash control of a dual media rapid sand filter (18" anthracite/12" sand). We are proposing to utilize a constant speed, 7,500 gpm vertical turbine/lineshaft pump drawing from Clearwell for backwash. Backwash header will have a rate of flow controller (integral venturi/butterfly) to control backwash rate. Also proposing a blow-off on backwash header to prevent media lifting. Do we need a pump control valve on backwash pump (such as ball or cone)? What would be the advantages or alternatives?

 

[GarySCWSVI]

I hope you meant by "prevent media lifting" that you meant media discharge from the vessel. Media lifting is supposed to happen when backwashing media. You should need a way to prevent the pump from providing more backwash flow than is required to lift the media bed and clean it of accumulated sediment. If what you call a "pump control valve" is that then yes you need it. I think the more appropriate term would be a "Flow Restrictor".

If you meant by "blow off" and air vent at the top of the vessel to vent of any air that may get into the unit prior to or at the same time as starting backwash then your proposal is a good one.

Gary Schreiber, CWS VI
The Purolite Co.

 

[bimr]

If you are talking about a Claval, OCV or similar pump control valve, then no, you do not need one. Those valves are not normally used on applications such as you are talking about because the pressures and backpressures that you are pumping against are relatively small and there should be no significant pressure surges. A standard check valve on the pump dishcarge is recommended and should suffice.

Pump control valves are used where your pump is starting/stopping against a pressure head. If flow reverses through the pump, damage to pump may occur.

It is doubtful that you need a blow-off on the backwash header either. Again, the pressures that you are talking about on a filter are normally low. The design pressure of the piping system will be higher than the discharge pressures that are normally encountered.