Restricted Orifice- After control valve 1

[chemical08]

Hi all,

I am confused after going through one P&ID of gas processing facility I have been working. What are the reasons behind placing RO-restricted orifice after control valve/valve? To my knowledge RO use in measuring flow rate and creating pressure drop. But I need more reasons for this case putting after valve with explanation/calculation.

thanks in advance
chemical08

 

[Latexman]

Maybe it was put there to shift where the control valve controls closer to it's centerpoint.

Good luck,
Latexman

 

[tickle]

RO means restriction orifice. Therefore it restricts flow. An orice used for flow measurement requires pressure measurement either side.

Where is the RO? Is it on a vent line or a liquid drain line? Is it a control valve or an actuated valve. What is the valve controlling?

If on a liquid drain line it will be to limit gas break thorough if the valve fails open so to prevent gas over pressuing of receiving vessel / pipework.

If on a vent - it is to control the venting.

 

[dcasto]

an RO down stream of a control valve is used either to reduce sound, reduce cavitation or allow for a larger trim in the valve. Somethimes they are called chokes. Fisher even will custom make chock tubes with a set Cv.

 

[maddocks]

It was likely put there to restrict the failure case for sizing a downstream PSV. We do it typically on large dump valves feeding LP vessels downstream where we don't want to install a massive PSV.

 

[chemical08]

thanks Maddocks, dcasto for explanation.

maddocks can you please elaborate more with example/articles/website link? Most RO we have are after PSV or after control valve-mainly controlling flow.

 

[Maximpro]

The RO (restriction orifice) purpose is to limit the flow:
- for example in flare system lines to provide sufficient purge gas capacity, not to allow any increasment of it. PCV are always sized for higher rates than application requires and RO is the way how to keep your purge rate at min.
- sometimes RO are used at drain lines from compresors' scrubbers. If gas somehow escapes through this line, there will be a sound that may warn operators about the upsets

 

[davsy]

chemical08

In your last post you said

Most RO we have are after PSV or after control valve-mainly controlling flow

if the RO is downstream of a PSV then the RO has not been installed to keep the size of the Relief valve down. It would need to be between the control valve and the relief valve to achieve this.

I have used ROs for two of the reasons mentioned previously, to adjust the point at which the valve operates and also to restrict the size of downstream relief valves. Both are in common use.

 

[Ninja1980]

It well could be the control valves gives a constant pressure upstream of the RO. Thus, a constant flow will be maintained though the orifice assuming the downstream pressure is fixed.

I’ve seen this arrangement before on purge gas supply lines, not that I think it’s a particularly good way of doing it.

 

[anandm104]

I am not sure but it is done in this way when cavitation is predicated.

 

[sheiko]

Hi,

chemical08, the following link may help you:

http://webwormcpt.blogspot.com/2008/07/flow-element-fe-upstream-or-downstream.html

"We don't believe things because they are true, things are true because we believe them."

 

[Glenfiddich]

after a control valve where there is gas or liquid flashing to gas, you typically swage up to the required line size. having the RO on the bigger line size puts backpressure on the smaller line size-keeping the flow from going sonic in the smaller line size.

 

[andy70]

Chemical08, maddocks (Petroleum)'s comment is in line with mine. I think you should try reading more about gas blowby. This will give you a better understanding.

 

[rocketscientist]

People,

Following the discussion I noted a few issues. First, never, ever, put an RO downstream of a PSV! Never, got it. There should be nothing downstream of a PSV except a nozzle leading into a vent tank.

I've heard all the arguements about an RO downstream of a control valve to act as a silencer. Obviously, this is only true in dry gas service. You would not want an orifice plate where it accumulate condensed liquid quickly, even if you had a weep hole at the bottom.

The only practical reason for having an RO is to restrict the flow of the CV; as a prevention to full throttle or to keep the flow within a certain range. It does not make much sense to throttle the valve Cv (Cv vs % open) since you want the valve at the controllable range (25-50% for a ball or butterfly and 15-30% for a globe). By adding an RO you might force the valve to operate out of effective control maybe like 75% or above. So, having an RO in-line complicates control significantly. Flow restriction seems the only practical reason.

I've attached a useful program for ROs and FEs. Remember, some of the pressure is recovered. There's a handy relationship in the ASME report on orifices for calculating loss against Beta (Do/Dpipe ID).

Also, Cunningham revealed back in the 1950's that supersonic flow is possible through an orifice if the plate is thin. If it is thick, based on the ratio of the plate to the orifice diameter, then the flow is restricted to a mach number of 1. If you dig into a Mechanical Engineering textbook on flow you will find more discussion sonic and supersonic flow.

Good luck,

Dirk Willard

 

[maddocks]

Can anyone advise on the method of calculating the RO thickness? Given a 4" flange mounted RO with a 0.52"hole, I need to come up with a plate thickness other than my semi-educated guess of 1/8" thick.

 

[rocketscientist]

To know the thickness, you must know the required pressure drop and the corrosion allowance. Remember that any stress you put on a plate will increase corrosion. My rule of thumb is to use more than one orifice for a dP > 50 psig. If pressure is variable, consider an orifice and a throttling valve with block valves for isolation. Look at the hidden folders in the spreadsheet I provided and you will find recommendations for the thicknesses. Believe me, thinner is not better.

Where there is a chance of fouling or scale you will have to take some precautions against it. These include: heat-trace/insulate; increase the pipe velocity, if that helps --- for a slurry; decrease the pipe velocity, if erosion is a factor; or replacing an orifice with a valve (A diaphragm valve may work.)if the solution is particularly prone to be sticky. Also, with fouling, my usual rule is: tees for elbows; crosses for tees. Or, you can cut a 1-in. nozzle into an elbow, as an afterthought, to allow for rodding. This approach doesn't work for the blind spaces in a tee, hence the need for a cross.

If you are not sure of the thickness and materials make sure you plan a monitoring program.

Dirk Willard

 

[TD2K]

Heck, I'm trying to remember where I've seen that paper on the Web talking about 'choking' across an orifice. If my memory serves me right, the orifice thickness had to be several times the orifice bore.

I'm not having any luck finding it but you might have success with some attempts with google.

I usually see these installed to set the maximum flow rate to a downstream lower pressure rated piece of equipment. I've also seen them used to reduce the pressure drop for a control valve by taking part of the pressure drop. The problem is that turndown is poor. Try to increase flow and you run out of dP quickly. Reduce flow and most of the pressure drop is taken across the control valve.