6 replies to this topic

[gboyega01]

Hi,

Someone Assist,

I am considering the inadverent opening of a control valve upstream of the vessel i am trying to protect with a relief valve, but i dont know the formula i can use for the Cv calculation of the valve and even what is going to be the significant of the Cv of the valve on my PSV sizing?

[pleckner]

Please gat a copy of this extremely valuable book (free from Fisher).

http://www.documenta.../book/cvh99.pdf

You will want to read Chapter 5. It will show you step-by-step how to work the equations for a control valve using the ISA standards, for either liquid or vapor/gas. What you want for your problem is to RATE (rather than sizie your control valve. You need the maximum Cv of your particular control valve and some other parameters defined in the handbook. You should have no trouble obtaining the data you need. You then assume the worse inlet pressure you can expect at the time of your scenario. The downstream pressure will the relief pressure. Using the maximum Cv and this pressure difference, you will calculate the flow rate using the equations from the handbook. This will be your relieving flow.

Cv is basically an indication of the capacity of the control valve.

Once you download this book, and read the material, post any specific questions you may have.

[jprocess]

Dear Phil,

You then assume the worse inlet pressure you can expect at the time of your scenario.

Can the worst inlet pressure be the relieving pressure of upstream vessel or this scenario is considered to be double jeopardy?

[pleckner]

That's exactly what you should consider as the worst case upstream pressure. It is not double joepardy because the two events are not simultaneous but sequencial in nature and may be related. The upstream vessel goes into relief for some reason and the operator tries to relieve the pressure by putting a control valve into manual and stroking it open.

[peeyar]

Dear Phil,

You then assume the worse inlet pressure you can expect at the time of your scenario.

Can the worst inlet pressure be the relieving pressure of upstream vessel or this scenario is considered to be double jeopardy?

Phil
What about by pass valve do we consider that because in operation I have seen many by passses being manuplated and left un attended
PEEYAR

[jprocess]

To Phil:
The relieving condition of upstream vessel may not be due to fire case because F&G system will close any ESDV downstream of mentioned vessel. For other case it can be possible as you stated. For examle for blocked outlet due to control valve closure the operator try to open the valve and stroking it open.
To Peeyar:
As I know you should consider both the control valve and related by pass fully open.

[pleckner]

@peeyar:

Yes, you need to take into account the by-pass valve but think this through. Let's assume I have a 3" line with a control valve in it and a by-pass around the control valve with a ball valve for isolation. The control valve will most likely be 2" (typiclly one size less than the line) and the by-pass and valve will most likely be 3" (typically the same size as the primaly line). The control valve has a Cv and the by-pass valve will also have a Cv and it will most likely be greater than the Cv of the control valve. People want to assume that the fluid has two-open flow paths but in reality they have two restricted flow paths and the fluid will tend to go to the path of least resistance. If the Cv of the by-pass valve is significantly greater than the Cv of the control valve, I would think the control valve side will see little of the flow. You do a fluids pressure balance to get the relieving flow.

@jp:

You asked if the worst inlet pressure to the control valve can be the relieving pressure of upstream vessel or would this constitute double jeopardy?

In my response I didn't say anything specific about what that upstream relieving scenario would be, fire or otherwise. You take the worst upstream pressure that you can realistically determine to be and if this is a fire relief case then so be it.