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

 

[CHD01]

Keef:

The MANUFACTURER does not guarantee your installation meets the 3% rule; they only tell you what the Cv is so YOU can calculate the pressure drop. YOU are responsible for the calculations. The two companies you mention or not going to provide 3-way valves with Cv's that result in low pressure drops; you will probably need a larger valve. Remember that you need to calculate pressure drop at the relief valve CAPACITY - not the required flow.
The more you learn, the less you are certain of.

 

[pleckner]

Keef:

Sorry if it looks like CHD01 and I are ganging up on you but you must be careful what you write in these forums. There are many very inexperienced people that will pick up on advice coming from these forums and all of us have the responsibility to be as tecnically correct as possible and not just drop suggestions/fixes without carefully thinking them through.

As a note to all. No vendor on the planet will quarantee that the inlet lines and discharge lines/tail pipes of pressure relief devices are adequate (and if they do, they are crazy); this includes relief device vendors. The owner's engineer is fully responsible for their own system.

 

[genericmech03]

Everyone,

Thanks for all of your responses. Greatly appreciated. This has generated far more debate than I imagined.

Seeing as the three-way changeover valves and the full port ball valves are doing the same thing my dilema now comes down to a matter of sizing.

As I will never be able to get a manufacturer to do the calc's and tell what's needed for my situation, how do I go about this myself? Can anybody suggests some references on this or perhaps give a brief rundown on what it is we are calculating?

Thank you


Generic

 

[pleckner]

Assuming we are talking about two installed PSVs; one as a spare for maintenance.

It's all a matter of determining the pressure drop from the vessel to the inlet of the relief valve. This is a basic exercise in fluid flow calculations which every chemical engineer should be able to accomplish no matter how much experience they have.

1. Use the RATED (stamped capacity) flow of the relief valve, NOT THE RELIEVING FLOW!

2. Using the Darcy equation for fluid flow, determine the pressure drop. If the pressure drop is within about 3% of the PSV set pressure, everyting is OK. If not, it will be time to re-think some things.
a. You must determine a 'K' value for all the included fittings and valves in the line. This includes the entrance loss into the pipe from the vessle (nozzle), any elbows or tee connections and of course the isolation valve; whether it is a ball valve or the 3-way valve that was discussed. To get the pressure drop of the valves, you should obtain the fully opened Cv of the isolation valve you intend to use. Then you can use equations in CRANE Technical Bulletin 410 to get the total pressure drop. CRANE also gives some K values for typical ball and 3-way valves but you would be better to get the Cv of the actual valve you are using.

In summary, these are the frictional losses you are working with or should be aware of:

COMMON WITH ALL CONFIGURATIONS:
1. inlet pipe
2. entrance loss into the pipe from the vessel
3. isolation valve

For 3-way valve configuration:
1. One or two 90 long radius elbows.

For two, ball valves and depending on the piping configuration:
1. None, one or two 90 long radius elbows
2. None or one branch flow tee connection

 

[PVRV]

pleckner, (good to see you threads)

Came across three-way ball valves. They seem to be made of bits and pieces

I agree with CHD01 there are some good ones; but definetly not from China.

CHD01, can you please pass along your email I need to send you those pictures (Floating Roof Tanks) I promised.