6 replies to this topic

[Rahul Maurya]

Dear Sir,

 

I have a question.

 

When we draw the P&ID of the control valve or any flow meter, why we are giving the reducer after and before it??

 

Is there any particular name of the control valve arrangement shown in to the figure???

 

Please let me know...

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[fallah]

Rahul,

 

Your query isn't so clear...Please clarify...

 

the reducers are added before and after CV because the size of CV is usually smaller than the relevant line size...

[shantanuk100]

Dear Rahul,

1. The purpose of a control valve is to control the flow or pressure drop of fluid across the valve by presenting some sort of resistance to flow based on throttling the fluid, and for this, some minimum throttling is always needed before we are actually controlling the fluid and making a difference in the value.

2. If you select the valve trim to be the same as the line size, then there is a considerable amount of throttling required before you can actually make an observable difference to the fluid flow or pressure. So, you are essentially wasting the top end range of your control valve and only able to effectively control after you reach much lesser control valve opening %. And Control Valves are quite expensive, so this wastage of the top end is not preferred.

3. So, to avoid this, the valve opening is typically kept to a size less than the line size. This leads to you being able to control the fluid quite soon as your valve opening is changed, and thus you don't waste any operational section (% opening) of the control valve without being able to control the fluid.
A desirable valve control range would be to operate it at 60-80% of valve opening for the maximum flow case.

4. The valve size is typically never greater than the line size, because that leads to very poor control, for the very same reason as explained above. Also, once operational (once the valve opening reaches a value where it affects fluid flow), such a valve will have increased sensitivity, so even a small change in valve opening causes a very large change in the fluid which is undesirable.

5. Now, as discussed in point 3, since the valve opening is kept to a lesser size than the line size, we require to add a reducer and expander before and after the valve in order to connect the valve to the line.

Regards,
Shantanu

Edited by shantanuk100, 17 March 2016 - 08:58 PM.

[Rahul Maurya]

Shantanu:

 

One more thing.

I have given an interview.  That guy told me that there is particular name for this control valve arrangement.

 

Do u know about this??

[shantanuk100]

The arrangement is called a double block (double isolation) control valve arrangement/manifold/station with two drain valves for upstream and downstream draining.
Usually, if the control valve is of a fail open type, a single isolation valve upstream of the control valve is sufficient, and if the control valve is of fail closed type, then two drain valves are required (one to drain fluid between upstream isolation valve and control valve) and one downstream of the valve (to drain fluid between downstream isolation valve and the control valve).
Your arrangement thus is a double blocked (double isolated), normally closed control valve manifold with two drain valves.

Regards,
Shantanu

Edited by shantanuk100, 21 March 2016 - 12:16 AM.

[fallah]

 

Rahul,

 

The CV arrangement you shown is the "most complete control valve set/station" or "Full control valve station" including by pass line with relevant isolation valve, two isolation valves around the CV to redirect the flow through the by pass in the case of maintenance, two drains with relevant isolation valves after and before CV to drain the trapped liquid between CV and isolation valves (because the CV will be in FC position after any failure).

[AZIZ_MN]

Rahul,

 

The CV arrangement you shown is the "most complete control valve set/station" or "Full control valve station" including by pass line with relevant isolation valve, two isolation valves around the CV to redirect the flow through the by pass in the case of maintenance, two drains with relevant isolation valves after and before CV to drain the trapped liquid between CV and isolation valves (because the CV will be in FC position after any failure).

and Fallah,

Bypass line size followed as reducer size? may be not in all case but most of the arrangement with bypass line dia lesser than main line