12 replies to this topic

[Nirav]

I have one simple suggestion. You can implement it after evaluating it.

You must be knowing pressure drop of RO (restriction orifice). It must be available on its datasheet (if it is not available, you can find out by calculations). Now, deduct this value from actual vessel operating pressure for your calculation on your own program. Put this reduced operating pressure of vessel on your program.
i.e.
Vessel Pressure to be put on program = Actual vessel op. pressure - pressure drop of "RO".

I hope it helps.
Thank you,

[latexman]

helena,

If this is a Code application, I hope the flow area of the RO is at least as big as the PSV’s nominal inlet flange size.

[latexman]

helena,

Excellent, you comply with that aspect of the Code.

[latexman]

It depends on what Code your vessel was built to? Which depends on what country it was built in and in what country the vessel sits.

In the U.S., pressure vessels are built to the ASME Code and low pressure tanks are built to API standards. As such, there are parts of the ASME Code and API standards that address sizing relief devices.

Being from the U.S., I asked about a part of the ASME Code.

[pleckner]

Helena, I have to ask: What in the world is a restriction orifice doing in a PSV inlet line????

If you have no idea, then find out. If no one knows, have it removed immediately! Since the bore size is the same as the PSV inlet flange, I can't see what this item is doing other than adding pressure drop and this is not a good thing for the inlet to a PSV. You should be aware of the 3% rule and this RO may very well kill it.

You can look up orifices in Perry's Handbook and it will tell you how to go about performing the calculation. There are also many articles out there. You can Google "Restriction Orifices". You know the flow rate from the rated capacity of your PSV. You know your inlet pressure as this is the relieving pressure. You then solve for the downstream pressure.

As far as doing the PSV inlet line pressure drop calculation, you can use the Darcy equation for non-compressible flow if you wish since your pressure drop should not be exceeding about 3% of the PSV set pressure. You do not need fancy software to perform this calculation. Do you have a copy of CRANE Technical Paper 410? This will tell you everything you need to perform the calculation.

If you insist on using your software, then do the orifice calcuation off-line and add the pressure drop to the line calculation by converting the orifice pressure drop into a pseudo line lenghth; sort of an equivalent length.

Your goal is to see if you are violating the 3% rule.

[Nirav]

hello,

I will have to make the correction for vessel pressure.
Yes, you are right. While calculating, you must deduct the pressure drop of RO from "relieving pressure". Since your vessel pressure during relief would be higher than that during normal operation.

Further, whatever Mr.Phil said is correct. In general, there should not be any restriction to flow between PSV and the vessel being protected. But I have observed RO in following kind of application.

In reciprocating compressor circuits, sometimes if required, RO is provided on outlet nozzle of suction knock out drum based on pulsation analysis. This helps to reduce pulses being generated due to compressor operation.


Thank you,

[Nirav]

Since the RO is upstream of the PSV, isn't the relief pressure the downstream pressure and I need to solve for the upstream pressure? The pressure indicator is between the RO and PSV, hence the pressure downstream of the RO must be the relief pressure?

Refer to attached sketch. I have shown 4 nozzles of discharge volume bottles. It can be two also.
Now, i'm not wrong, RO would be there on nozzles of dicharge volume bottle. But i'm not sure whether at nozzle C&D or A&B.

If these RO are just to supress pulsation flow, it won't have high dP compared to other RO used in process applications. Because the diameter of such orifice for pulsation application would not be very small compared to nozzle ID. We have had such application in one of projects. I had been told at that time that the main purpose of such RO is to alter the flowing pattern. Unlike other process applications, the purpose of these ROs is not to restrict the flow by providing high dP. Since many times pulsation analysis is carried out by compressor vendors, these ROs are supplied along with compressor.

Further, since these ROs are mounted on nozzles, I think we can consider to apply relieving pressure at the downstream of RO and then calculate the inlet pressure drop of line upto PSV. You can discuss this with other experienced persons in your organization. I'm also looking for other's point of view over here.

Thank you,

[pleckner]

What are we trying to protect here, a vessel, pipe, the compressor, the dampeners, what? The line loss calculation starts at the item being protected. For instance, for a vessel you start the calculation at the inlet to the exit nozzle, taking into account the entrance losses into the nozzle as well as all fittings, etc. from that point up to the inlet of the PSV. If the RO pressure drop is small, OK then there may be no problem. Just check the overall pressure drop against the 3% Rule.

Again, the RO imparts a pressure drop, no different than a valve. The upstream pressure into the RO will be the relieving pressure minus all the losses between the item being protected and the RO. It isn't more complicated than this.