19 replies to this topic

[Ali Z]

Situation: I have a brown field project in which a RV is installed downstream of a compressor. There is a bit of distance between the compressor and the piping location on which this RV is installed. The design case of the RV is blocked flow (of a downstream control valve). Our goal is to re-evaluate the RV for new flow rates.

 

My Assumption: I suppose this RV is there to protect the piping downstream of the control valve and also to protect the compressor in case the discharge pressure of the compressor exceeds the set pressure of the RV.

 

Question:  I think, anyway, the losses from the compressor to the RV inlet should be calculated to make sure the pressure in the discharge of the compressor doesn't exceed its design pressure, but my question is that should I start the calculation of the 3% inlet losses rule (for RV) from the nearest piping header tee (on which this RV is installed)(Point 2 in the attached file) to the RV inlet OR from the compressor discharge (Point 1 in the attached file)  to the RV inlet? My guess is that the nearest piping header tee should be the start point for this calculation, but wanted to double check that.

 

Attached is a scheme to make it clear.

Attached Files

•  Untitled.jpg   29.44KB   26 downloads

[Bobby Strain]

Why not check both? Then let us know your result.

 

Bobby

[latexman]

You are not certain whether the RV is protecting the compressor, piping downstream of CV, or both.  When you answer that, the starting point of the dP becomes obvious, right?  Of course, point 1 does both.  It's the safest solution.  If everything checks out using point 1 with no money needed, game over, everyone wins, time to celebrate!   But, if not, you need to noodle on it some.

Edited by latexman, 11 May 2017 - 10:51 PM.

[cea]

Your compressor downstream piping should have been designed for maximum discharge pressure of compressor. RV is required only if it is not so; or there is spec change in downstream piping due to system reason.

 

If discharge piping is not designed for maximum discharge pressure, it is advisable to shift RV close to compressor discharge or else considering 3% inlet loss rule right from compressor discharge, becomes inevitable.

 

In case RV is installed to take care for piping segment after spec break, then spec break location will be your starting point to calculate inlet losses.

[Ali Z]

Thank you so much everyone. FYI, this is an existing super old plant. The inlet losses to the compressor is more than 3% and to the tee is less than 3%, and that is why I asked the question in the first place. 

The spec changes in the downstream control valve from 300# to 150#, so the RV is for piping protection. But isn't this 3% rule there to avoid chattering and lower flow relieving too? I mean, yes, if the starting point is the spec break it will only make sure that, taking losses into account, the pressure in the 150# piping doesn't exceed the requirement, but shouldn't this 3% rule also be calculated from the source of the pressure (compressor) to avoid chattering and lower flow relieving? because my understanding is that if the inlet losses from the source of the pressure is more than 3%, it reaches the re-setting pressure of the RV (blow-down pressure range), which is usually 5 to 10%, and chattering might happen. What do you think?

Edited by Ali ZirGach, 12 May 2017 - 08:11 AM.

[latexman]

You are right.  I use 7% (93%) as the reseat pressure in gas/vapor service.  Does the RV capacity exceed the required flow significantly?  If so, some RV vendors, not all, can install a mechanical stop to reduce the RV capacity to the required flow or slightly above.  Then, if you are lucky, the inlet dP may be <= 3%.  Also, some RV has an adjustable blow down ring that lets the reseat pressure be increased, which gives a larger stable operation window if the dP exceeds 3%, but this is a last resort, IMO.  It used to be allowed, but I'm not sure about this with today's Code.  Anyone?

[Ali Z]

You are right.  I use 7% (93%) as the reseat pressure in gas/vapor service.  Does the RV capacity exceed the required flow significantly?  If so, some RV vendors, not all, can install a mechanical stop to reduce the RV capacity to the required flow or slightly above.  Then, if you are lucky, the inlet dP may be <= 3%.  Also, some RV has an adjustable blow down ring that lets the reseat pressure be increased, which gives a larger stable operation window if the dP exceeds 3%, but this is a last resort, IMO.  It used to be allowed, but I'm not sure about this with today's Code.  Anyone?

So you think the 3% should also be calculated from source of pressure to avoid chattering?

[Bobby Strain]

Much of this conversation makes no sense. You introduced another element with a piping spec break. But your sketch shows the PSV to be located on piping from the compressor with no valve to the PSV. You really don't care about the compressor discharge pressure as long as the PSV set pressure is set at the compressor design pressure. And, no, you don't need to have 3% loss to the compressor. No matter what you do, it is likely that the valve will chatter anyway. But the 3% requirement is to be sure that it is not a high frequency chatter. The riser to the valve is likely much smaller than the compressor piping. And, you might give us a sketch that shows your system, along with the pertinent calculations. We are commenting without benefit of a good basis.

 

Bobby

[latexman]

Yes, to eliminate chattering.  But we do not have all the details, and there may be less expensive options.  That was my intent.  One was mentioned before.  Another is a non-modulating, pilot operated RV with a remote pressure tap.  By chance, can the existing RV be modified to that?  But hard piping to < 3% dP, is the most simple, conventional, and robust way.  It just may be the most expensive way, and it may not. We just don't have the details.

[Ali Z]

Yes, to eliminate chattering.  But we do not have all the details, and there may be less expensive options.  That was my intent.  One was mentioned before.  Another is a non-modulating, pilot operated RV with a remote pressure tap.  By chance, can the existing RV be modified to that?  But hard piping to < 3% dP, is the most simple, conventional, and robust way.  It just may be the most expensive way, and it may not. We just don't have the details.

 

 

You are right.  I use 7% (93%) as the reseat pressure in gas/vapor service.  Does the RV capacity exceed the required flow significantly?  If so, some RV vendors, not all, can install a mechanical stop to reduce the RV capacity to the required flow or slightly above.  Then, if you are lucky, the inlet dP may be <= 3%.  Also, some RV has an adjustable blow down ring that lets the reseat pressure be increased, which gives a larger stable operation window if the dP exceeds 3%, but this is a last resort, IMO.  It used to be allowed, but I'm not sure about this with today's Code.  Anyone?

Thank you, it makes sense. We are going to order a new RV and will request from vendor to have an adjustable blow down ring, and will let them know about the inlet losses. It is less than 5%, so it should be OK, thank you very much for the information.

[Bobby Strain]

Most designers calculate inlet loss for the rated capacity, not the required capacity. So, be sure you get it right before ordering a new valve.

 

Bobby

[Ali Z]

Most designers calculate inlet loss for the rated capacity, not the required capacity. So, be sure you get it right before ordering a new valve.

 

Bobby

Of course, that's in the code. Basic stuff, but thanks.

[latexman]

Personally, I'd rather have a RV with a mechanical stop by the vendor than an adjustable blow down ring, but I don't know if that will get the rated capacity down to where you need it.  Adjustable things tend to get played with by people that don't know what they are doing.

[fallah]

 

Question:  I think, anyway, the losses from the compressor to the RV inlet should be calculated to make sure the pressure in the discharge of the compressor doesn't exceed its design pressure, but my question is that should I start the calculation of the 3% inlet losses rule (for RV) from the nearest piping header tee (on which this RV is installed)(Point 2 in the attached file) to the RV inlet OR from the compressor discharge (Point 1 in the attached file)  to the RV inlet? My guess is that the nearest piping header tee should be the start point for this calculation, but wanted to double check that.

 

 

Ali,

 

The matter is clearly dictated in API 520 Part II as "...when a PRV is installed on a process line, the 3 percent limit should be applied to the sum of the loss in the normally non-flowing PRV inlet pipe and the incremental pressure loss in the process line caused by flow through the PRV...".

 

Hence, you should start from point 1 but with taking above mentioned point into consideration...
 

[latexman]

Excellent advice, fallah, for the CV fails open/protect downstream piping scenario (Post #5), but for the CV fails closed/blocked flow design case (Post #1) it does not apply.

[fallah]

Excellent advice, fallah, for the CV fails open/protect downstream piping scenario (Post #5), but for the CV fails closed/blocked flow design case (Post #1) it does not apply.

 

I think mentioned rule is general and is irrelevant to the different relieving conditions might be created in two scenarios...then can be applied for both scenarios...
 

[latexman]

I will have to noodle on that.

[Bobby Strain]

In Texas and Louisiana, to noodle is to catch a catfish with one's bare hands. While wading in the swamp.

 

Bobby

[latexman]

I'm familiar with that.  I've seen it on TV.  I don't think I'd do that willingly.  Kinda scary.  The variant I was using was learned in Michigan in the early 2000's frommy kids and fellow engineers with small kids that watched PB&J Otter on Playhouse Disney.  They had the "Noodle Dance" and song:

 

Noodle, use your noodle

Noodle, do the noodle dance

 

Solve a problem, it's no strain

Use your noodle, that's your brain

 

Noodle, use your noodle

Noodle, do the noodle dance

 

There's an answer you can find

Use your noodle, that's your mind

 

In a bind, just use your mind

Use your noodle

[latexman]

fallah,

 

You are correct.  My bad.  Thanks!