Pump Sizing with Minimum Flow Return

[labolsh]

I need to select a pump which is required to deliver 21 m3/h of water from the raw water tank into the water treatment plant. The pump discharge is designed with a minimum flow return line. If the water treatment plant is not in operation, the pump will continue pumping, and water is diverting back to the raw water tank via the minimum flow return.

Has anyone come across such design? Raw water tank is 10 m height and the required pressure at the inlet of the water treatment plant is 7 bar.

My questions are:-

How to select the pump?

How to read from the centrifugal pump curve, the minimum flow of a pump if the outlet valve is close?

How to size the minimum flow return line to create a balanced pressure to suit the different conditions?

What if 2x100% pump is installed and the two pumps are required to operate together when water level in product tank is low?

 

[quark]

Why do you want to maintain minimum flow line? If you design your storage with enough capacity, you can stop the pump when not required. I operated many plants with on/off logic.

The minimum flow rate is a recommendation by the pump manufacturer. In the absence of data, you can consider it as 20%. you should take care that the return line is at a maximum possible distance from the suction point to avoid short cycling.

Maintaining correct minimum flow in all conditions requires more than correct line sizing. You have to go for automatic control. There are many threads in the past that discussed this issue.

When your plant capacity is 21m³/hr, two pumps of 21m³/hr capacity won't help you when the tank levels are low. Your water quality will be disturbed. Only advantage is that you will have 100% standby.

PS: Don't double post threads (in various forums)

 

[BigInch]

20% is not a good minimum flow for (continuous) recycle to a tank. That is the absolute minimum to prevent overheating when on recycle directly from discharge to suction and a centrifugal pump should only be at that flowrate when starting or stopping, if at all. I suggest you size the return line for 50% if recycling is going to be frequent, i.e. anything more than shutting down or starting up.

We have many transfer pumps with automatic recycling back to tank or directly to suction triggered when discharge pressures go high and use Yarway valves for that purpose.

If you use two pumps, it is preferable to have them identical if they are to operate in parallel. Therefore it is convenient if 1 pump is sized for normal flow and when the second pump is brought on, that will allow you to fill the tank at twice the normal rate.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[25362]

Try:

http://www.driedger.ca/ce1_cp/CE1_CP.html

and
thread124-130586
thread408-53164

 

[pmover]

labolsh - please do NOT duplicate postings in this site - other posting is in chem plant plant design and operations.

thanks!
-pmover