7 replies to this topic

[processengineer1998]

I am going to size a centrifugal pump to transfer liquid from one plant to another plant through 2 km pipeline.
maximum flowrate which could be transfered by pipeline is limited by downstream plant and is around 40 m3/hr.
since maximum flowrate is fixed , i am not sure that is it still required to consider some margin(e.g. 10%-15%) over this maximum flowrate(40m3/hr) for calculating rated flowrate(44-46m3/hr) of pump?
please advice.
thanks in advance.

[Pilesar]

What are the consequences if the pump is not large enough? A too large pump does not annoy nearly so much as a too small pump. If your chosen impeller size the maximum that will fit, then choose a larger pump case. Impellers erode over time and you cannot count on the pump performance to stay exactly as if it were new. Will the pump operate at or near-below its best efficiency point? Will the downstream plant limit flow by control valve? How certain are your system curve calcs? Will there be a density change with temperature or composition which affects the flow? If I were sizing this, I would want enough safety factor in the pump and motor to cushion for unknowns. There may be more than one right answer to sizing your pump, but including a bit of margin in the sizing is definitely not 'wrong'. A robust design is usually a better choice in the long run than the theoretically optimum design.

[Bobby Strain]

Are you providing a new pipeline also?

 

Bobby

[processengineer1998]

What are the consequences if the pump is not large enough? A too large pump does not annoy nearly so much as a too small pump. If your chosen impeller size the maximum that will fit, then choose a larger pump case. Impellers erode over time and you cannot count on the pump performance to stay exactly as if it were new. Will the pump operate at or near-below its best efficiency point? Will the downstream plant limit flow by control valve? How certain are your system curve calcs? Will there be a density change with temperature or composition which affects the flow? If I were sizing this, I would want enough safety factor in the pump and motor to cushion for unknowns. There may be more than one right answer to sizing your pump, but including a bit of margin in the sizing is definitely not 'wrong'. A robust design is usually a better choice in the long run than the theoretically optimum design.

Dear Pilesar,
Thanks a lot for your good response to my question, as a conclusion i understood from your explanation and reasons that I should considered some margin for pump as rated flow even if i now the maximum flowrate transfering via pipe line is fixed due to downstream unit limitation(40m3/hr is design of downstream unit).
Now there is another question which i face with.
Downstream of pump we need control valve and some instrumentations, do all of them need to be sized for rated flow (44_46m3/hr) like pump or they could be sized based on maximum flowrate(40m3/h)?what about hydraulic calculation,is it necessary to do hydraulic for both rated and maximum cases?
Thanks
Thanks in advance.

[processengineer1998]

Are you providing a new pipeline also?

Bobby

Dear Bobby,
Actually it is an expansion project between two existing plants, so all piping,instrumentation and pump are new.downstream unit will be common for two plants, some time unit will be utilised by one plant and other time by another plant.
Thanks

Edited by processengineer1998, 14 October 2016 - 02:09 AM.

[processengineer1998]

thank you very much Pilesar and Bobby

[J_Leo]

When you do the pump calculation, the pump is in the circuit from source (for example, a vessel), passing through flow element, the control valve, piping, fittings.. to the destination (s). Once you choose one flow rate for hydraulic calculation, it applies to all the items in the circuit. Generally speaking, the pump hydraulic calculations should include all cases, max, normal, even turndown. The hydraulic caculation can also provide infomation for the control valve pressure drop for all cases, which can be used for control valve specification.

 

Regards,

Leo

Edited by J_Leo, 30 October 2016 - 09:36 PM.

[ankur2061]

Hi,

 

Is this a continuous service or intermittent?

 

If continuous, what about minimum safe continuous flow for the pump? Have you considered it? If not, as a rule of thumb I would consider 30% of the maximum flow as that value, which becomes around 12 m³/h in your case. The final value for minimum continuous flow will be given by the pump vendor.

 

In the aforementioned case you will require to add the minimum continuous flow to the maximum transfer rate, if you decide to have a minimum recirculation line with only a restriction orifice in it.

 

If it is an intermittent planned pump transfer, you may ignore the minimum continuous flow requirements.

 

Regards,

Ankur.

Edited by ankur2061, 31 October 2016 - 02:41 AM.