Removal of Safety Relief Valves with out losing pressure 2

[genericmech03]

Hi,

It's a bit of a long shot but does anyone know of anyway of removing a safety relief without removing the contents of the vessel and losing pressure. We have an anhydrous NH3 chilling system, so in order to perform routine maintenance on our relief valves, each vessel must be emptied, which is costly and time consuming. In the case of some vessels the entire system must be drained

Cheers

 

[Keef]

genericmech03,

You might want to consider installing a changeover valve. This will allow you to connect two safety relief valves, but only having one in service at a time. If you need to remove one valve for maintenance, you just swap over to the other valve and remove the faulty one.

 

[deanc]

Please see ASME SecVIII Div1 Appendix M,M-5

 

[PVRV]

No, unless you have a 0-0 blind.

I would consider it good practice to install a pair PSRV on every pressure vessel; each able to relieve the worest upset condition.

At any rate, isolating valves (full bore ) should be installed to readily remove either PSRV for overhauling. Should a common blow down line exist a pair of isolating valves should be in place for each PSRV.

Cheers

 

[genericmech03]

Thanks all for your response,

After a bit of investigation I have found that all PSRV must be installed integrally to the pressure vessel. That is, there can be nothing between it and the pressure vessel that may possibly isolate it, and therefore negate its function.

Your comments have given me food for thought. Thank you

PVRV,

I didn't understand the comment "No, unless you have a 0-0 blind".

Also, is there any situation where two PSRV in parallel must be installed??

Cheers

GenericMech

 

[PVRV]

GenericMech,

1. I need to have two relieving devices in place as a
contingency against the following:

1.1 Mulfunction (Flutter/Chatter/Galling/Passing)
1.2 Overhauling without shutting down the train,
plant or entire facility for a "valve" not to
mention depressurizing, Purging, etc
1.3 Relieving when its needed most

To achieve the above positive isolation is required between
the pressure vessel and the PSRV. Without restricting the
nozzle size or effecting the releiving capacity I can install a full bore plug valve (1/4 Turn). At times the released gases are directed towards a flare through a common blow down line now I want to avoid another vessels PSRV released gas to backflow to me while the PSRV is being removed/replaced. Thats why two isolating valves are required in such a situation.

Note : I always overhaul all relieving devices, SV/PRV/PSRV/PVRV every two years.



0-0 Specticle blind

Cheers

 

[genericmech03]

Thank you PVRV!,

Oh me of little faith! I just found the appropriate clause in the standard,(Clause 8.8.4 AS 1210) that refers to isolating PSRV's.

I will have to purge the current system's to put these stop valves in place, but at least it will only be a one off situation, allowing easy repair and maintenance.

What controls do you have in place to ensure the stop valves can never be left in the closed position? Locking and Tagging, or some sort of mechanical interlock?


Cheers

 

[deanc]

Speak to your Inspecting Authority as listed in 1210 8.8.4a
Refer to the ASME Code for suggestions,Appendix M. Most of
the valves for this purpose are set up to accomplish what you need.

 

[PVRV]

genericmech03


Problem with locking and tagging is that you start of with 400 keys and as the years pass by the number decreases and the keys are lost.

There are inter-locks.(Three way valves or key locks with open/closed position stipulation)

I would suggest that operations (process guys) handle this aspect genericmech03 since mechanical design and inspection scope is limitedin this regard.

At any rate, i will get some specifications for you

Cheers

 

[deanc]

Ideas? Take a look at

http://www.solares.net

 

[genericmech03]

Thanks guys,

That website's product information wouldn't load up, can anyone suggest an alternative supplier

Cheers

 

[CHD01]

Instead of using locks, etc just purchase a full-port 3-Way ball valve; one for the inlet and if a common discharge exists you will also need one for the outlet. 3-Way valves can be purchased with equal flow coefficients for either branch. If you get cheaper ones where the Cv's differ between branches you sometimes have to size the 3-Way valves one size larger than the connecting pipe in order to get the pressure drop down to less than 3%

This way, only one relief valve is in service at a time; the only other thing you will have to do is to incorporate bleed piping to clear the pipe between the 3-Way valve and the relief valve that your taking off-line for maintenance. The more you learn, the less you are certain of.

 

[pleckner]

3-way valves are most certainly an option. However, I know of plants that cringe with pain when anyone suggests the use of 3-way valves. They claim they get leakage through the "closed" port. A good means of isolating redundant relief valves is to use full port ball valves with mechanical linkage. This is extremely effective and cheap.

I also must point out that any isolation valve used MUST BE A MINIMUM OF FULL PORT to the inlet line size. Whether this is in relation to the 3% rule or not is immaterial as it is required by code for whatever reason they deemed necessary (and I'm sure they are concerned about the 3% rule).

 

[CHD01]

Pleckner: I agree, but good 3-way vavles are available. By the way, if a larger 3-way valve is used than the connecting relief valve inlet flange - then a full port valve is of course not required. Its only required that the net prt ara be at least equal to the relief valve inlet. The more you learn, the less you are certain of.

 

[pleckner]

Agreed

 

[BenjyMac]

genericmech03,

Try this web-site. Not sure where you are as this is a UK company but may have outlets where you are.

http://www.castell.com/

Basically you can attach these interlocks (keys) onto existing valves by modifying the valve head. You can get the keys that remain in the valve. This system would ideally be used when you have 2 relief valves and you want to isolate one for maintenance. They are idiot proof.

Good Luck

Benjymac

 

[Keef]

It seems to me that there has been a lengthy discussion on the issue of 3 port valves for this application. As I origianlly suggested, a changeover valve, which is a type of 3 port valve specially designed for this purpose, would do the job. No need for keys, interlocks, mechanical linkages, or having to worry about the pressure drop through whatever valve arrangement you construct. Maybe I'm wrong, but to me it seems a lot simpler.

Keef

 

[pleckner]

Keef:

A 3-way valve is certainly simple but you can't ignore the inlet line pressure drop as this must be taken into account when checking for the 3% rule.

Do you have any specific valve (manufacturer) in mind? Do you know of the valve's fully opened Cv so a pressure drop can be determined? Without this information, just saying a 3-way valve is OK is not.

 

[CHD01]

Cv does vary a lot between 3-way valve manufacturers and depending on type of valve - even between branches. In my experience, MOST OF THE TIME it has taken a 3-way valve one size large than the relief valve inlet to get within the 3% rule. The more you learn, the less you are certain of.

 

[Keef]

I am not just on about any old 3 way valve. I am aware of the 3% rule.

I am aware of two companyies that offer such valves. Leser and Spirax Sarco. The manufacturers can double check that the changeover valve they offer complies with the 3% pressure drop requirement. Take a look at these manufacturers info.

Keef