Restriction orifice upstream of control valve

[ProcessRookie]

Hello,

I want to install an RO upstream of a control valve to limit the max flow to relief valves downstream for the relief case which is control valve fails open. Does anyone know of any issues or problems with installing the RO upstream instead of downstream of the control valve?

Thanks,

PR

 

[BlackSmoke]

Of course, it depends on your fluid and the selected valve.

You'd want to consider if your fluid is near saturation. If your restriction takes too much pressure drop, by the time the fluid reaches the control valve you might cause cavitation at the valve's restriction, leading to controllability issues and lifetime reduction.

Is there a reason you can't use a smaller valve instead?

 

[LittleInch]

I would echo black smoke. Installing an RO upstream of a control valve to restrict flow means that at some point the RO is going to be taking a significant proportion of the pressure drop you want your valve to take in normal (max) flow which will cause the valve and the control system real problems. What it tells me is either your control valve is over sized or your relief system under sized. Most control valves should be about 70 -75% open at your maximum flow case, so if yours is lower you might have the wrong valve or trim.

You could always consider a mechanical stop on the control valve to prevent opening beyond a certain percent to limit fail open flow instead as an RO is really a bit brutal and can have unintended consequences. They can also be accidentally removed by operators later on not realising that they are there for that purpose if they find it is restricting flow or causing control problems. Most ROs I see are fitted in front of manual isolation valves to limit venting rates, not control valves.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[ProcessRookie]

Thanks for the replies.

Its gas I'm dealing with so cavitation is not an issue.

Little Inch, of course the control valve is oversized, hence why I'm looking for solutions to get round this as the replacement valve we have ordered is not arriving in time for the shut down. I've looked at altering the valve trim but this can't be done in time either. 'Most control valves should be about 70 -75% open at your maximum flow case'- I don't know what you mean here, the max flow case for relief occurs when the control valve fails fully open (100%). I want the orifice to be a device to limit the maximum flow for the relief case only, during normal operation the control valve should dictate the flow, at levels well below what the orifice is sized for.

We size relief valves based on mechanical stops failing so that is not an option, and as for operators removing this RO, its a 30 barg flammable gas line so I don't consider that plausable.

Thank you for your advice on operational difficulties.

And as always, thanks for your time.

 

[LittleInch]

What I meant was that for your maximum normal operating flow case, i.e. one of your cases identified on the control valve data sheet, the max opening percent of your valve should be about 65 to 75% open. If at your process maximum flow the valve % open is less than that then you run into the problem you describe, i.e. at 100% open you have too much flow. Just because it's gas flow doesn't mean that the valve will not be affected. High swirl and flow upset from an RO immeadiately in front of the valve may well seriosly affect the valve operation in normal flow.

Of course operators won't remove the RO in service, but during a shutdown??... This is why you need to actually stamp / mark on the actual RO itself , DO NOT REMOVE - Safety Critical ITEM or paint red or whatever your plant does to indicate that the RO is not removable. The fact this is a shutdown action does prompth the question what happens at the moment - Is your relief system undersized for max flow?? Anyway that's just being curious, but a potential over pressure scenario seems to have been identifed.

Of course the real solution is to order up a new valve as you seem to have done for the next shutdown so that you have time to do it properly. Excuses of time pressure may work in the short term but are not really the way to do it long term. I happen to agree with you on the mechanical stop scenario - I was just offering options but they are not 100% reliable.



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[dcasto]

Why not just size the control valve properly?