Centrifugal Pump Sizing Considering MCSF

[CuriousMecEng]

As per my understanding pump vendors size centrifugal pumps (consider an API 610, OH2) for the rated flow(Rf). However, if the MCSF flow is through an orifice, this flow needs to be added to the rated flow, and the new flow (Initial rated flow + MCSF) becomes the rated flow (New rated flow: Rf_N). However, MCSF also depends on rated flow. Minor changes observed. (Consider the same pump selection at both flows).

Can this be ignored in sizing both the pump and min recirculation line? Since this goes into a loop while selecting.

Please do correct if my understanding is wrong.

MCSF:Minimum Continuous Stable Flow

Thanks in advance.
Stay Curious!!

 

[LittleInch]

I think your understanding is correct.

A continuous min flow recirc line though is not great. Consider either an automatic recirculation valve (ARV) or a controlled valve to only flow the min flow when you need it and not when you don't.

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

 

[CuriousMecEng]

@LittleInch: Thank you for the response.

However for low kW pumps (<20kW), as per my understanding, it is economical to go with the orifice.

Also, my query was regarding pump selection, in vendors' software. Since MCSF changes with Rated flow, how do finalize a pump model? (Since it can lead to a loop).

Thanks in advance.

 

[pierreick]

Hi,
Better not to use acronym !
Vendors are working based on a specification sheet you are supplying with min , normal ,max flow rate . In other words it's your responsibility to supply a sketch with information on how you want to operate your pump.
Probably good to read : links underneath

https://www.pumpsandsystems.com/follow-these-steps-more-reliable-pump

http://www.liquiflo.com/v2/files/pd...-4-MinFlowRateforCentrifugalPumps-Feb2016.pdf

my opinion only.
Pierre

 

[CuriousMecEng]

@pierreick: Thank you for the response. Acronym reference updated in the original query.

I understand it is the client's responsibility on deciding the operational philosophy.
My query is regarding, how the vendor is selecting the pump. ie we can have access to the selection program of M/s Flowserve, M/s ITT etc.

 

[LittleInch]

Yes it could lead to a loop but it would be a decreasing one. Mcsf is a bit vague for many pumps with a band. Around 25 to 30% commonly.

So if you wanted a pump to do 100m3/hr forward flow, they know this is 70%, so would size it for 142. Then 30% mcsf is taken off to get back to 100.

Now if this pump is used infrequently then maybe a constant drain is OK, but I just hate wasting energy and with the cost of it escalating the impact of an ARV or similar would pay for itself faster than before.

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

 

[CuriousMecEng]

@LittleInch: Thank you for the response.

 

[CuriousMecEng]

@pierreick: Thank you for the additional references shared.

 

[SB1976]

Hi CuriousMecEng,

Please note :
1. MCSF is dictated not from percentage over Best Efficiency Flow but rather from the vibration criteria. As per API 610, vibration limits have been specified for pumps. MCSF correspond to that flow wherein the vibration limit is reached on the left side of the H-Q curve. For the pumps that you are handling, vibration limit =3.0 microm RMS. So, after adding MCSF flow to required process duty flow/net flow, you only need to check whether the new MCSF is meeting the vibration limit or not.

2. For recirculation line , MCSF as the sizing basis is not quite enough. You should size it for full pump rated flow to avoid choked spillback line during start-up / shutdown.

Trust this helps.

 

[Jungpao]

Hello CuriousMecEng,

Just to answer on how pump is sized usually. This is coming from a very limited experience using the selecting program from one of the OEM (used to be). Quick reminder, the MSCF is calculated by the program.

For rated with MSCF recirculation line, usually we select with the original target rated first. After that, by using the same pump size, we add the recirculation which increase the impeller diameter. Therefore, it causes change (you guess it) on the MSCF.

In my understanding, the MSCF is changed in concurrent with the size of impeller diameter (using the same pump size). However, this may be not true and based from my experience using OH2 pump. I'm not sure about other type.

For simplification allow me to use as below :
Iteration 0. X= rated target, A=1st MSCF
Selection 1. X+A= new rated, B=2nd MSCF
Selection 2. X+B= new rated again, C=3rd MSCF
etc.

Keep in mind, all the conditions (Head, NPSHa, suction pressure, etc.) and liquid properties (Temperature, Specific Gravity, Viscosity, etc.) are being kept the same. Until finally the latest new rated flow - MSCF = original rated flow. After the first iteration, the change is not too significant / minor usually.

This may be not true for others. Hope this helps a little bit. If you need more clarification, please do let me know. Thank you