6 replies to this topic

[shahidulislam48]

Hello,

 

What are the criteria that have to be considered during "under flow" or "over flow" selection for glove type control valve?

 

Many thanks

Shahidul

[Chemitofreak]

Hi,

 

I guess by overflow and underflow you mean maximum flow and minimum flow:

 

a) Max Flow = Design Margin x Normal Flow (Design Margin is generally 10% for process and 15-20% for utilities)

 

Min Flow = Based on Turndown requirement

[Zauberberg]

Or you refer to "flow-to-open" and "flow-to-close" terminology? If yes, read more at https://blog.craneen...ow-to-close-ftc

[shahidulislam48]

Hi,

 

Thanks for your answers.

 

Although I don't clear about the answer from chemitofreak.

 

According to the reply of Zauberberg, yes you are right.  I was asking about that.

Although I had visited that site before it doesn't clearly convey the message of selecting right flow direction for different process criteria.

 

I would like to have some information that would lead me to choose right flow directions from different considerations.

 

Many thanks.

[Zauberberg]

See attached file. This should answer your query.

 fail-safe.pdf   177.43KB   44 downloads

[shahidulislam48]

Thanks Zauberberg for your co-operation by sharing that file.

It really helps.

[Zauberberg]

The last few paragraphs of the attached text provide direct answers on your question (see also below).

 

Following the incremental safety logic, you would choose a flow-to-open arrangement for e.g. pressure control valve, and a flow-to-close arrangement for e.g. hot oil flow to reboiler.

 

If there are dedicated safety instrumented system (SIS) valves for isolation/venting, following this philosophy may be an unnecessary overkill - it all depends on the actual facility and the associated risks. I personally see these arrangements as important safety features only in cases when there are no dedicated isolation/blowdown valves, operated by the SIS.

 

 open-and-close.png   29.46KB   5 downloads