Single RV's or duals RV's 1

[jamesbanda]

Dear sirs,

I am seeking advice here to understand the pratices used else where in the industry.


Summary information
---------------------
I am working on a small capital project for a new plant installation. We are considering cutting costs to install only single RV's. Not dual. We think savings of 1 to
0.25% capital costs per plant by avoiding duals.

Shutdown information
--------------------
Typically, we plan to shutdown every year but in reality a breakdown shuts us down for a significant period every year and at best every 2 years.

Proposals/discussion:

All our relief cases are for fire. All operational cases are prevented by SIL1 protection. Therefore, can we put a value underneath the RV's and service while the plant is running with a valve under the RV's - fire engine on standby ? or is this totally un-heard off (which is what i think is the case) and legally not allowed and we need to consider these options.

1. install duals if we must service < 1year
2. install singles and get reliability of testing such that we can go 2/3 years without shutdown for the valves (once we fix plant relability)..

 

[dcasto]

For ASME code vessels the maximum inspection interval is 5 years for RV's. If the fluid in the vessels dictate a lesser inspection period, you can place rupture discs under the RV to protect the RV from the material.

 

[BigInch]

I never would have thought about putting a Rupture Disc under a relief valve. Is there a reason for me never having thought about doing that?

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Ashereng]

I believe that you can have a car-sealed valve ahead of a pressure relief valve. But, you need to check up on this.


I have done projects where there were rupture discs in front of a PSV (in this case, every PSV). The primary justification was savings due to material. The PSV can be carbon steel, and the rupture disc a more exotic material. The cost of the rupture disc and pressure switch justifies the additional cost of the PSV in exotic material (along with availability and delivery). This project was some years ago (maybe 7 or 8). I haven't seen many though.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[pleckner]

In the first part of my series on Rupture Disks I present a cost comparison on this issue. You can review the information at:

http://www.cheresources.com/asiseeit3.shtml

I also talk about reasons why you might want a Rupture Disk before and after a PSV.

 

[MortenA]

James

It boils down to this: How would the operator like a shut-down due to a RV having to replaced? If dont even have a warehouse spare then add the time it will take to service/get a new valve. If the valve has to be replaced the lead time is usually around 6 months.

The regular maintenance seems to be covered.

Best regards

Morten

 

[BigInch]

You'd have to shut down the line to replace a rupture disc too.

If its critical service, you should have the spare in the warehouse.



BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[MortenA]

And if you have the spare in the warehouse how much extra cost is it to have two valves installed?

Best regards

Morten

 

[sshep]

Dear Pleckner,

Your rupture disk article was a nice read, but I noticed that there was no discussion of the gage and vent (usually equiped with excess flow tubing check valve) that is often found between the rupture disk and PSV to detect if the disk is burst. This is a common recommended installation detail of rupture disk manufactures, and fittings for these accessories are often included on the disk holder.

The gage is to check if the disk has popped, but this depends on an inspection program. The use of a pressure transmitter addresses the human factor, but can get more expensive than the original problem being addressed. There are even fancier methods using continuity checks (thin wire between a film sheet) for which I have no experience.

The fear being addressed by the vent is that the rupture disk (being thin metal) can get a small leak such that no differental is available to pop the disk. These vents often drip hydrocarbons (slipping and fire hazard) if the disk is loses integrity. In other words, the decision to install a rupture disk is not trivial. Hopefully you will address these considerations in future papers.

best wishes,
sshep

 

[pleckner]

sshep:

The article I referenced above is only one part of a six part series posted on the website. The issue you address is discussed in Part 5. However, I do not get into specific details on the tell-tale other than what is required by Code. Details on specific types of tell-tales is not discussed because each installation may have different requirements.

 

[sjrfc2]

dcasto: I'm interested in the 5 year max inspection interval for ASME vessel RVs. Do you know which Code spells that out?

Thanks

 

[JAlton]

A couple of comments:
Safety-Relief Valve (SRV) Inspection Intervals are recommended in NBIC. I have been a member of ASME Sub-COmmittee on Safety Relief Valves for over 15 years. To my knowledge, ASME does not discuss inspection intervals except in Sec. VII, Recommended Care of Power Boilers. Para. C4.110 Recommends 1 yr for Boiler Safety Valves & C4.220 Recommends 6 months to 2 years for auxiliary equipment such as feedwater preheat & evaporators.
For a discussion of recommended frequency, see National Board Inspection Code, Para. RB-8410, which has 1 year for Steam, 3 years for Air & clean, dry gases, 5 years for SRV in combination with a rupture disc, 5 years for propane & refrigerants and per inspection history for all other fluids.
For a discussion of SRVs in Combination with Rupture Discs, see ASME Sec. VIII, Para. UG-132.
From the context, it seems that the phrase dual valve means a redundant valve. I say this because the ASME Code requires multiple valves for boilers over a specified heat area. However, redundant valves with an isolation valve between the SRV and the Vessel are permitted by ASME Sec. VIII, Non-Mandatory Appendix M, See ASME Sec. VIII, UG-135(d).
A rupture disc downstream of an SRV is typically an indication of a very corrosive service with backpressure. More typical, is the rupture disc upstream or "under", i.e. in combination with the SRV. This has serves several functions. (1) It is a tighter seal than a direct spring SRV alone, (2) It protects the SRV from corrosive or viscous fluids which might impede the moving parts in the SRV over time, and (3) It permits a sterile seal on the vessel contents since the SRV sealing area (seat contact point) is not necessarily air tight. A leaking SRV (API 527 permits up to 40 BPM Leakage) may allow atmospheric contamination back into a system during a shutdown. Whereas, a Rupture DIsc is a solid membrane.
I hope these comments are helpful.


JAC