Allowable dP across control valve 3

[flinana]

Hi,

I am told by our feedwater valve supplier that the maximum allowable dp across this particular valve is 3,4 bar. During operation we actually need to have a dp above 3,4 bar in order to maintain level in the boiler. In fact we operate at around with a dp of around 7 bar.
What would imply changing the valve to allow a higher dp?

thanks

 

[ione]

Having a higher dP across a control valve for a given fluid and flowrate means reducing the valve flow coefficient Cv, generally being:

Cv = Q * (SG / dp)1/2

The flow coefficient depends mainly on the mechanical design (geometry).

Now if your supplier told you that specific valve is rated for a max dP of 3.4 bard, you can ask for another type of control valve capable to handle the required dP or fit the line with more than a control valve.

 

[gerhardl]

Hello flinana, ione is right.

You could in addition ask why this valve has the given limitation, and in addittion give all necessary data of what you actually have at maximum, minimum and normal flow, regarding requirements for the valve : eg. flow and presuure before (P1) and after (P2) the valve in addition to dP. (Plus type of fluid, temperature and condition)

The reason for the given limitation could be given by several reasons, and a more complete picture could give a better choice of suitable valve. Perhaps even a valve at higher price, but one that would do the job properly for a longer time.

For (boiler?) feedwater there should be no problem finding the proper solutions, as there is a large number of suppliers of high-quality valves for this application.

One reason for the limitation could be (purely guess) that your valve is a direct operated spring controlled control valve (or temperature controlled valve), and perhaps limited by valve construction and spring capacity.

A motor operated special control valve or perhaps better a special pilot operated control valve for the purpose might do the job better and avoid damage and/or cavitation if this is also a problem with the original.

 

[sheiko]

yes, each control valve has a minimum controllable flow (ie a minimum Cv) corresponding to a maximum available pressure drop. This minimum controllable flow sets rangeability and turndown of the valve.

"We don't believe things because they are true, things are true because we believe them."