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Centrifugal Pump operating with bypass line

Centrifugal Pump operating with bypass line

(OP)
Hello,

I have the following situation:

Process Data Sheet indicates:

-Qrated: 10 m³/h

-Head 50 meters

- General Note: "Pump to operate with minimum flow bypass line with restriction orifice. Pump Vendor to indicate Pump minimum flow, and this minimum flow shall be added to rated flow (Qrated)."

After selecting a pump according to Process Data Sheet (operating point: 10 m³/h and 50 meters), pump vendor informed minimum flow for the rated impeller 3 m³/h.

So, I understand that the pump shall operate with 13 m³/h (10 m³/h to process line + 3 m³/h always recirculating in the bypass line with RO). But when operating with 13 m³/h, the developed Head will be 40 meters with the rated impeller.

My question is: Operating with this smaller Head of 40 meters instead of 50 meters will be ok? Because Process required 50 meters.

Thanks.

Peter











RE: Centrifugal Pump operating with bypass line

Short answer: No,

you should select you OP so that 50 m corresponds to intended flow+min flow. However, you could allow for the pressure being lower at the OP so that the flow will be less than 3 m³/hr. Or install a device that will only let flow pass when the pressure is higher than expected such as a spring loaded check valve.

Best regards, Morten

RE: Centrifugal Pump operating with bypass line

Pedro - Did you really think this was going to be ok? - Need 50m head, got 40m??

Your choices are:
Go back and select a pump with parameters 13m3/hr @ 50m
Install an automatic recirculation valve (ARV) or similar as Morten says which only activates the min flow bypass when it is actually needed (i.e. forward flow through the pump is <3m3/hr)
Look at the actual proposed usage of this pump and work out whether it will ever operate in a minimum / no flow scenario and whether a min flow bypass is actually required. Some process engineers just do this as a standard option regardless of whether it is needed or not and don't care about the waste of energy involved.



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

RE: Centrifugal Pump operating with bypass line

You need to rethink you question, to me it seems very disjointed.
Setout exactly what you want to achieve, ignore the data sheet at the moment.
You need to establish - X flow @ Y head,the minimum flow advised by the pump vendor has nothing to do with the pump application duty point.

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.)

RE: Centrifugal Pump operating with bypass line

(OP)
I will try explain a little better.

1) Pump Data Sheet:

-Qrated= 10 m³/h
-Head= 50 meters
-Note: After selecting the pump, pump manufacturer to inform pump minimum stable flow. This pump minimum flow shall be added to rated flow. It will be installed a by-pass line at pump discharge with a restriction orifice to ensure, if someone closes process line, by-pass line will continue operating at pump minimum flow avoiding damage to the pump since it will be pumping the minimum continuum stable flow.

2) Selecting the pump with Qrated 10 m³/h and H= 50 meters, pump manufacturer informed me the pump minimum flow (3 m³/h for the rated impeller Ø 200 mm. Note: I cant' start selecting with 13m³/h because each pump has it own characteristic minimum flow.

3) This is the pump curve for the rated impeller (Ø200 mm).

My doubt is: Literature only tell us what to do with the flow when operating with a continuous recirculating line with restriction orifice. "The pump and prime mover must be sized to simultaneously supply both the bypass flow and the process flow", but don't tell us about the Head.

I only have two options:

1) Operating with 13m³/h and Head= 40 meters for the rated impeller (Ø200 mm) when both lines (process and recirculation)opened. Is that ok? operating with 40 meters instead of 50 meters indicated at Data Sheet? Consequences? (Point 1 at the image).

2) Furnish a Ø220 mm new impeller , that is capable of providing 13 m³/h and Head= 50 meters. But why Literature don't tell us about this option? (Point 2 at the image).





RE: Centrifugal Pump operating with bypass line

Ok,

you don't seem to want to do anything other chose the pump, but for the future, please note than min flow lines are valid in a few occasions, but in many others they are nothing more than a complete wasteof energy....

the choice of min continuous flow is always a bit hit and miss depending on the vendor, but 30% of rated flow is a good place to start. It might be 25 or 35 at the most so you won't be far away.

If your duty is 50m at 10 for the process then that's what you need, not 40m head.

I assume by "literature" you mean the process data sheet? They don't tell you because that isn't their job ( silo engineering at its best), but yours.

There are instrumentation methods available which will trip the pump on low flow / no flow but I guess that isn't your job to investigate that either (silo engineering strikes again) neither apparently installing devices to only send flow down the bypass when it's actually needed. To be fair this is a fairly small pump so it might not be worth it, but larger units the energy loss over a years operation can be enormous, continuously wasting a minimum of 25% of the energy you're putting into the motor.

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

RE: Centrifugal Pump operating with bypass line

"After selecting a pump according to Process Data Sheet (operating point: 10 m³/h and 50 meters), pump vendor informed minimum flow for the rated impeller 3 m³/h."

That's when the pump vendor should have read the spec, which, as you have quoted it, clearly states that the operating point of 10m3/h at 50m must be adjusted to include minimum continuous flow.


If the recirc line orifice is sized for 3m3/h continuous flow, then you need a pump selected for 13 m3/h at 50m head.

End of story, and very straightforward, in my opinion.

RE: Centrifugal Pump operating with bypass line

What did the PUMP data sheet say though???

"Process data sheets" are strange things which get used to transmit "process" data to the mechanical engineer who then writes the information into a Pump data sheet.....

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

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