Pump Sizing Before Pipe Design 2

[JD P.E.]

My question is related to industrial process plant design and engineering.

I understand that early on in the engineering process pumps are roughly estimated in order to provide cost estimates and to beat lead times. My question is how are they sized before system pressure drop is known? What's the typical practice on this? Are there rules of thumb to go by such as Req'd head + X% for piping/fittings, etc? Or do you just handle it via an oversized pump/control valves or VFD?

 

[TugboatEng]

This is how you end up with improperly sized pumps. You don't want to do your design, find out the pump is too expensive, buy your initial pump case anyways, and then fail. If the initial design requires too costly a pump you need to lower your expectations of performance until your budget tolerates it.

 

[mk3223]

I understand that early on in the engineering process pumps are roughly estimated in order to provide cost estimates and to beat lead times.

The Engineer may estimate the pump sizing by preparing a preliminary hydraulic profile for the pump sizing based on the requirement of the potential process component, such as heat exchanger, orifice, control valve, etc., and piping conditions with proper extra hydraulic head. It's important that the Engineer is to review the hydraulic for the required components and piping conditions with the definitive project details and to finalize the pump sizing prior to purchasing the pump.

To the Engineer, why is the reason to oversize the pump or control valve or VFD?

 

[FacEngrPE]

Often forgotten by management is that preliminary pricing before project definition has a very wide margin of uncertainty.
As the design progresses the cost estimate will change, and the uncertainty will be less.

The project budget must be adjusted toward the estimate before bid time.

Picture from Sewershed Investigation Guidance Manual - City of Norfolk, Virginia

 

[dvd]

VFD control reduces flowrate AND available pump head - if you need a specific pressure and adjustment on the flowrate, there may be difficulties in attaining this with VFD.

 

[georgeverghese]

A short list of things that can mess up pretty bad your expectation of beating lead time and avoiding cost blowouts during preliminary design, which have little to do with pump developed head:
a) Requirement for speed control, speed turndown and reducing startup current
b) Electric motor hazardous area service requirements
c) Materials of construction for pump changes due to trace quantities of corrosive elements in feed stream
d) API shaft seal flush plan for pump and how this is affected by (a)
e) Change in plans for pump sparing

The more you shake the rattle on these early to pin down these details, the lower the risk of unwelcome surprises further down the road.