×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Isolation valves for PSV
5

Isolation valves for PSV

Isolation valves for PSV

(OP)
Should I put isolation valves before and after PSV for maintenance proposes or during PSV calibration?

I don't find this kind of rule in the API RP 520 part I and II. Maybe you guys have this experiences please share.
Thanks

RE: Isolation valves for PSV

ASME code doesnot allow to operate any equipment without PSV on line.If you have two PSVS then you can have CSO isolation valve.Isolating PSV when plant operation is unsafe and risky.

RE: Isolation valves for PSV

No safety codes would allow the operation of equipment without a full compliment of statutory relief valves, however if the capacity and pressure settings have been selected to exceed statutory requirments then in theory the excess valve capacity could be isolated during operation. In which case the isolation valve would be on the inlet to the relief valve.

The idea of this is to allow maintenance while the plant is in service but whether this is acceptable depends on the pressure. In my experience no one would be allowed to maintain a valve in service above 40bar with just a single isolating valve.

RE: Isolation valves for PSV

(OP)
Dear friends,
I learn the PSV connection on the vessels from several P&IDs and it shows that some of them use isolation valves in the outlet connection. May be to protect the back pressure from headers during shutdown time.
Pranit:What is CSO isolation valve?
athomas: What kind of code for the statutory relief valve
I still don't get the point. It impacts on the cost very much. Thanks

RE: Isolation valves for PSV

CSO valves are Car Seal open valves, Any new construction we are placing gate valves under relief valves to simplify lockout/tagout for inspection. This way the entire process/vessel does not need to be locked out, just shutdown and isolated.When a car seal is cut/removed forms need to be filled out managers called, and the form needs to be posted. When work is complete, the valve must be car sealed open again before putting back in service.
 We do a car seal inspection monthly to ensure the valves have not been tampered with.    

RE: Isolation valves for PSV

2
Isolation valves upstream and downstream of PSVs are usually only required if there are two PSVs in parallel.  This then allows one PSV to be removed for maintenance / recalibration / certification while the other remains in service.  Of course it is necessary to interlock the isolation valves in such a manner that the vessel is always connected to a PSV.  We use this system extensively on 125 bar sour service. Occasionally we provide double block valves upstream of the PSV where the duty is dirty in order to ensure tight shutoff should the PSV need to be swapped out.  Be careful that the downstream valve is specified for the correct duty or ensure that both the upstream and downstream valves around the PSV are either both open or both closed.  The off line PSV should never sit there with it's inlet open ond outlet closed while the other PSV is in line.

RE: Isolation valves for PSV

My comments apply to relief valves discharging to atmosphere

RE: Isolation valves for PSV

In case of PSv discharge is to atmosphere isolation valve at outlet is not required. If discharge is connected to flare / relief  header yes there is need to have isolation valve at both inlet & outlet sides. However there is need to interlock the isolation valves such that at least one of the PSV is always on line.

RE: Isolation valves for PSV

wartono,

sorry for sloppy use of English by statutory I meant valves that are required by the design code.

athomas236

RE: Isolation valves for PSV

2
Just remember that our 1st Priority is to NOT install isolation valves on the inlet to a relief valve.  Only if it cannot be avoided are ISO valves installed.  If installed, they are only used for ISO purposes after removing the vessel or equipment from service.  Even then, procedures must clearly cover this and in-area inspection provided for the duration of the isolation; as alluded to by others in this thread this can be covered in alternate ways using iso valves that are mechanically linked or key-lock systems used to guarantee adequate relief.

If a relief valve needs to be isolated with the vessel in service, then a dual relief valve installation with a 3-way valve that ALWAYS maintains 100% flow area as the valve postion is changed is required.  Also remember that when 3-way valves are used that resistance values are different based on the position of the valve for most valves.  This difference can be substantial.
 

The more you learn, the less you are certain of.

RE: Isolation valves for PSV

I may be missing something, but I always thought that we were allowed to place isolation valves upstream and downstream of a single PSV if:

1.  Both valves are CSO
2.  A pressure gauge is available in clear view of the isolation valves
3.  An operator is stationed at the PSV location for the entire time the valves are closed (i.e., maintenance).

I would really like to hear if this isn't correct, because I've seen this done on several projects by one of the major Engineering companies...

Regards,


Bob

RE: Isolation valves for PSV

kbander

It sounds as if what you are describing will work although I would prefer a postive lock on the valve.  All I am saying is that we do try to avoid isolation valves before and after relief valves.  In a chemical plant environment this has been our policy; when ISO vales are needed we try to mechically link valves together or use key-lock valves to be sure there is always a relief valve in service.  The best option is a 3-way valve with dual relief valves; we have even used 3-way valves on both the inlet and discharge lines where we went to a common discharge manifold.  What you describe as a CSO valve sounds like a conventional gate valve with the ability to seal it so that a seal is broken if it is opened.  We would probably opt for a postive lock versus just a seal.
For those who have multiple relief valves on line with ISO valves where the required relief capacity still exists with one relief valve out of service (usually ofr piping only); I might consider what you suggest - but I would still prefer a positive locking procedure.  Your other cautions I agree with.

The more you learn, the less you are certain of.

RE: Isolation valves for PSV

kbander:

What the Code actually says regarding isolation valves for relief valves is described in UG135e; and in Appendix M.  How routinely we apply these procedures to each RV installation is what I think we need to be alert to.

UG135e basically says there should be no ISO valves between a vessel and its relief valve or in its discharge line except "when these stop valves are so constructed or positively controlled that the closing of the maximum number of block valves possible at one time will not reduce the pressure relieving capacity provided by the unaffected relieving devices below the required capacity; or under the conditions set forth in Appendix M"

APpendix M says in essence that:

A stop valve may be installed between a vessel and its relief valve for inspection repair purposes ONLY.  When done, it should be arranged to be locked or sealed open; and it shall not be closed except by an authorized person who shall REMAIN STATIONED THERE during that period of the vessel's operation within which the valve remains closed; and who shall AGAIN LOCK OR SEAL the valve in the open position BEFORE LEAVING THE STATION. (M-5a)

M-5b allows a block valve in the discharge line of a relief valve subject to pretty much the same comditions as for the inlet line specified in M-5a.


 

The more you learn, the less you are certain of.

RE: Isolation valves for PSV

(OP)
Friends:
I thanks you very much for your helpfull comments especially to CHD01 that give me a clear picture of installing ISO valves on PSV. Thanks.

RE: Isolation valves for PSV

One word of caution, "Beware of gremlins" no matter how good the check and inspection procedure is, positive lock or CSO block valves in a PSV line is a dangerous-risky business and a severe liability. Just be aware that at some time or another the plant and personnel in the area will get bit. Even if the standards approve the design in some situations. Before installing, at least run a Risk Analysis involving equipment failue and subsequent damage due to overpressure.   

RE: Isolation valves for PSV

saxon - I agree 100% this is why I try to emphasize ISO valves should be a last resort and are the exception rather than the rule!  As a motel chain use to say in a TV add - thanks for your support!

The more you learn, the less you are certain of.

RE: Isolation valves for PSV

CHD01-your comments give me very good instruction. Now,we are working on a process plant(EO/EG).The relief valves designed by basic design suppliers can not satisfy the requirement of " at least one inspection per year" of the Codes.Because all the relief valves in PIDs are single and no isolation valves are installed both in inlet and outlet of the relief valves,and no bypass lines and valves are in place for inspection purpose when the relief valves are out of service. So we want to do something to satisfy the Code's requirement. Would you and anybody please give me some instructions? Thanks!

SCL

RE: Isolation valves for PSV

I'll think about your situation; but believe you will be forced to modify the installations.  There is one possibility.  You are only required to inspect annually if you have no prior experience with the relief valve in its installed service; or have not a record of inspection results for a valve.  With supporting inspection data or previous experience relief valve inspections can be extended to a maximum of 5 years.  Thus depending on the size of your plant and the number of relief valves you have and how critical the service is you may be able to either take extended major shutdowns for relief valves over a period of several years OR possibly stagger your relief valve inspections to reduce your workload IF an annula plant shutdown is required.

The more you learn, the less you are certain of.

RE: Isolation valves for PSV

I need to add that the inspection record for a relief valve needs to indicate no relief valve problems were observed on inspection in order to justify a longer inspection period; and that you do not just jump from 1 year to 5 years - ideally you should go step by step.

The more you learn, the less you are certain of.

RE: Isolation valves for PSV

Thanks.
I forgot to tell you that we are working on the detail design of one EO/EG process plant. The current setting of the relief valves in P&IDs are as I described above.

RE: Isolation valves for PSV

Sir CHD01 (I do apologize if you are a Lady),

Anfortunately your motto is very true and I do like it.  That's why ignorance is a bliss.

RE: Isolation valves for PSV

No lady am I, just an engineer who's getting easier to live with as he ages.  Glad you like the motto; I'll be honest with you - I don't know if I made it up or I heard it some time ago.  But I do think it has a lot of truth.  The other motto I would use is to not trust everything you read!

The more you learn, the less you are certain of.

RE: Isolation valves for PSV

API-RP 520 part II does address the installation of a "CSO" type valve between the PSV and the vessel.

ASME Sec VIII Div I Appendix M covers the matter in detail, Please refer.

Mohammad Ali Ansari

RE: Isolation valves for PSV

ALIANSARI:

I STAND BY MY 10/9 MEMO IN THIS THREAD!

The more you learn, the less you are certain of.

RE: Isolation valves for PSV

As an addendum to the above - for those who must be able to maintain and inspect a relief valve while the process is online - relief valve manufacturers (eg Crosby {Tyco}) can supply bespoke units that overcome some of the problems indicated above by using linked 3-way, 100% open valves either side of two relief valves.  Crosby have a brochure on what they call 'safety selector valves', which may be worth a look.  I am not sure how the costs compare with a conventional set-up.  

I have not personally used one of these systems before, but am considering doing so for a future project, so would be interested in any feedback from anyone who has used this system before...  

RE: Isolation valves for PSV

Isolation valves are permitted, but only in very special circumstances.  

The isolation valves must be mechanically linked to prevent simultaneous shut-off.

The old rules simply relied on administrative control of lockable valves, but this was determined to be un-safe.

There are special valves available from safety valve manufactuers (described above). They are not cheap, but less expensive than separately mounted isolation valves, and certainly much safer.


Good luck,


RE: Isolation valves for PSV

Dear,
you can have 2 PSVs with a 3 way diverter valve before the PSVs, this way ,you can allways have the needed protection while calibrating one of the PSVs.
Regards

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login



News


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close