Feed Water Control

[Mech2612]

Hi Everyone,

I am trying to diagnose the source of a control valve modulation issue which I believe is coming from the feed water pump.

The following issues are present -

Hot well temperature is around 95 Deg C.
Audible cavitation from feed water pump - caused from the high inlet temp? There is a spill back line with orifice. No VSD on the motor, On at all times with control valve doing the modulation.
Issues with condensate return elsewhere in the system. Failed open steam traps and undersized return pipework contributing to the high tank temp.
Feed water control valve is modulating between 0-20%. Span is okay and it maintains boiler water level at full steam demand, but never opens above 20%.

Am I correct in thinking the issue lies with the pump/spill back? Or has the control valve not been sized correctly for the pump output? A Kv of 16 and DN40 pipework appears normal for the application.

Any help would be much appreciated.

Thanks

 

[goutam_freelance]

Feed water control valve is modulating between 0-20%

This is not in the ideal control range. It should be about 20-80%.

There is a spill back line with orifice

What is the capacity of the spill back line. It should ensure that the pump flow is more than pump minimum flow, otherwise there may be cavitation and noise.

Failed open steam traps and undersized return pipework contributing to the high tank temp.

The feed temperature will affect NPSHA. Please check temperature and NPSHA/NPSHR.

 

[pierreick]

Hi,
Consider these links to support your work, in particular valve sizing.

https://www.emerson.com/documents/automation/article-properly-size-control-valves-fisher-en-38152.pdf

Cavitation in Control Valves | Emerson SG

Fisher anti-cavitation trims for control valves help eliminate or control cavitation damage in clean and dirty service.

www.emerson.com

Pierre

 

[LittleInch]

Sounds like a mixture of not enough NPSHA and the pump being oversized if your Control valve is down at 20% open.

Does control valve logic have a fixed max percent in it somewhere?

Or is level control actually ok like this?