7 replies to this topic

[smuk]

Hi,

I want to know how to control minimum flow in a centrifugal pump with variable speed drive. Can anyone help.

Thanks

[JoeWong]

Smuk,
Don't quite understand your question.

VSD assist in energy saving, increase equipment lifespan, minimise noise, etc as discussed in "Variable Frequency Drive (VFD) helps in Many Aspects".

For pump minimum flow control, number of ways can be implemented :
(i) A restriction orifice on pump discharge recycle line
(ii) A flow meter on pump discharge with control valve on recycle line
(iii) Use Automatic Recirculation Valves (ARC) valve
(iv) Flow-Delta P and flow meter on pump discharge with control valve on recycle line

as discussed in "Centrifugal Pump Minimum Flow Control Strategies".

Another more complicated way is FLOW-DP method discussed in "Flow-Delta P Protection Strategy"

[Qalander (Chem)]

Smuk,
Don't quite understand your question.

VSD assist in energy saving, increase equipment lifespan, minimise noise, etc as discussed in "Variable Frequency Drive (VFD) helps in Many Aspects".

For pump minimum flow control, number of ways can be implemented :
(i) A restriction orifice on pump discharge recycle line
(ii) A flow meter on pump discharge with control valve on recycle line
(iii) Use Automatic Recirculation Valves (ARC) valve
(iv) Flow-Delta P and flow meter on pump discharge with control valve on recycle line

as discussed in "Centrifugal Pump Minimum Flow Control Strategies".

Another more complicated way is FLOW-DP method discussed in "Flow-Delta P Protection Strategy"

Dear Joe Hello/good Morning, Nice Helping elaborative reply;must appreciate.
regards
Qalander

[smuk]

Smuk,
Don't quite understand your question.

VSD assist in energy saving, increase equipment lifespan, minimise noise, etc as discussed in "Variable Frequency Drive (VFD) helps in Many Aspects".

For pump minimum flow control, number of ways can be implemented :
(i) A restriction orifice on pump discharge recycle line
(ii) A flow meter on pump discharge with control valve on recycle line
(iii) Use Automatic Recirculation Valves (ARC) valve
(iv) Flow-Delta P and flow meter on pump discharge with control valve on recycle line

as discussed in "Centrifugal Pump Minimum Flow Control Strategies".

Another more complicated way is FLOW-DP method discussed in "Flow-Delta P Protection Strategy"

[smuk]

Hello Joe,

Thanks for your elaborate reply. In some flowsheets showing VFD for discharge flow control, I did not find any control for recycle flow. I felt that for those pumps whose minimum process flow is less than pump minimum flow, recycle flow control is a must. I wanted to know the views of the group, and thank you very much for the reply.

smuk

[mrj]

Dear Smuk


"I felt that for those pumps whose minimum process flow is less than pump minimum flow, recycle flow control is a must"
Purpose of a VFD is to save power for pumps with high capacity but if there are any restrictions like this then either pump selection is wrong or use a control valve instead VFD.

mrj

[djack77494]

It could be that some (maybe small) forward flow is always expected and therefore a separate minimum flow circuit is not required. Speed control of the pump's driver will allow for very large variations in flowrates and will have a similar impact on the minimum flowrate requirements as well.

[JoeWong]

I am a process engineer but not a pump specialist...but following are my view points.

A centrifugal pump Suction specific speed (NSS) is normally maintain as 8000-10000 (just a general guideline). Some manufacturer may design higher base on their experience and knowhow. For a centrifugal pump with VFD, as the flow demand is lower, the VFD will operate pump with lower speed. These will reduce the NSS and reduce the NPSHr as well.

However, this is only one factor, there are other factor affecting the NPSHr such as pump casing and impeller design will have different degree of internal recirculation, axial thrust force, efficiency curve, etc. There will be a low limit of NPRHr (which will be defined by the pump manufacturer).

Your process design may have considered shutdown pump before this low limt is reached.