Tight shutoff in control valves 5

[reactor22]

Hi Gentlemen,

Can we know whether tight shutoff is required before sizing a control valve?

thanks

 

[ash9144]

That is for you to determine based on process. If process can handle some unreacted material passing downstream or material downstream passing into reactor then it isn't needed. If the process is negatively impacted by material going either way across the valve then tight shut off is needed. Best practice would be to have tight shut off.

 

[JLSeagull]

In addition to the reactor example use tight shutoff for control valves that vent gas to a flare or liquid to a drain system, etc. Tight shutoff is a relative description. All throttling control valves leak. IEC60534-4 / FCI 70-2 provide a testing basis for control valve leakage based upon leakage classes. Class II a balanced cage style control valve like a Fisher ED leaks like a sieve (0.5% rated capacity tested with air or at 50 psig). Class V leakage is 0.0005 ml per mm orifice diameter bar dp using water. Class VI could be more or less than Class V as it is stated in ml and bubbles per minute depending upon body size. The leakage for a valve at NPS 6 based upon Class VI leakage is 4 ml or 27 bubbles per minute. In contrast to API STD 598, resilient seat leakage at NPS 6 is zero; and metal seated valves are similar to Class VI at 24 bubbles per minute at about 80 psig.

 

[reactor22]

Thank you very much gentlemen. I clarified the issue.

 

[JimCasey]

Everything JLSeagull said is correct. FCI70.2 uses 50 psi as a test pressure for most classes. The measured leakrate confirms accuracy of fit and assembly.
However, Shutoff in service is seldom 50 psi. It is not correct to directly extrapolate the "test" leakage to the installed condition. Prudent practice is to provide an on-off isolation valve and a modulating control valve.

 

[djack77494]

I like JimCasey's suggestion, "provide an on-off isolation valve and a modulating control valve," since TSO control valves tend to be expensive and high maintenance items. The separate TSO isolation valve should be much less so.

 

[GrimesFrank]

An on-off isolation valve and a modulating control valve is the exact company standard we have.
Philosophy around here is a control valve is for 20-80% and its tandem iso valve is for 0%.
In my naivete I thought that was the only way to go.

As an aside what if you find the iso downstream of the CV, what are the practical adversities?

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