Pressure Safety Valve Location

[lana]

Hi everybody,

I would appreciate any explanation about the safety valve location on the high side of the system.

In EN378-2000 standard is explained about this valves on the pressure vessels, i.e. receiver in DX system.
I was wondering if there is a good reference for the best location and why?

1- Compressor discharge line.
2- Condenser outlet line.
3- Liquid line.
4- On the top of the receiver.

Many thanks for your help.
Cheers

[Josip]

Hi, Lana

Hi everybody,

I would appreciate any explanation about the safety valve location on the high side of the system.

In EN378-2000 standard is explained about this valves on the pressure vessels, i.e. receiver in DX system.
I was wondering if there is a good reference for the best location and why?

1- Compressor discharge line.
2- Condenser outlet line.
3- Liquid line.
4- On the top of the receiver.

Many thanks for your help.
Cheers

Good question..

1. We use to install SV always in position before discharge stop valve on compressors (if someone forget to open it) to have oil separator with SV.

2. Condenser outlet line is connected to receiver and there should be SV on top of it. Maybe to have installed another one on inlet side of condenser (in gas area), possible and depends on safety rules..

3. On liquid lines is better to use by-pass valves around stop valves and not SV to not release liquid into atmosphere. Sometimes is inevitable to install one in liquid area i.e. thermosyphon refrigerant oil coolers.

One rule is there: whenever is possible to trap liquid install by-pass valve or safety valve. It cost less then damage which can be done

What to say: following some rules, having some experience and common sense you cannot fail

From my experience: from design to installation and start up, sometimes even running the plant teaching operators, I was always on plant and I prefer to have as much as possible safety installations/plant.

Best regards, Josip

[lana]

Thank you Josip for your reply.

I would like to know if there is a written reference to the proper location of Safety Valve. What you kindly explained is totally correct and acceptable.

Theoretically the pressure is constant from the compressor discharge to the TEV inlet but in reality there is always pressure drop in the lines and the condenser. So the pressure after the condenser is less than the inlet (by the amount of the pressure drop).

Installing the safety valve on the liquid line is not a good option as you mentioned but what about the other three places? As you explained we can have the valve installed on all three places. Which is the best option? or is there a "best option".

Of course if we install in all three locations then there will be no problem at all.
But as the standard indicated, for small units only one would be enough.

Thanks again.
Regards
LANA

[US Iceman]

Hi Lana,

For smaller systems you are likely to only see a relief valve on the high pressure receivers. However, when you start to get into much larger systems, the designer will usually install these valves ANY place there is a potential for refrigerant over-pressurization.

One thing you have to keep in mind. There are two types of pressure relief valves; gas & liquid. The liquid type valves are sometimes called a hydrostatic valve. And, as Josip mentioned you do not want a liquid type valve to relieve to the ambient. It always better to find a way to have these valves relieve back into the system (oil or liquid refrigerant).

Gas relief valves are always placed in the gas area of the vessel and required when the vessel volume exceeds a specific value. I don't know what the exact value is for that standard but it should be listed. All pressure vessels generally require some type of over-pressure protection.

Any other possible locations where relief valves are/or should be used is left up to the discretion of the system designer. These other locations are generally found to be areas where refrigerant can become isolated by the closing of valves also.

[lana]

Thanks a lot US Iceman,

Very useful info.

Cheers

[Josip]

Hi, Lana

Sorry I'm not at home with small systems, but as I told you and US Iceman confirmed:

Any other possible locations where relief valves are/or should be used is left up to the discretion of the system designer.

and

...following some rules, having some experience and common sense you cannot fail

You are a clever guy...

Best regards, Josip

[lana]

Thanks Josip,

I always install safety valves on the top of the receiver for small systems.
Just wanted to confirm, is there any better place or a technical reason for that, now I know with the help of your explanations (& the US Iceman's) that it could be anywhere on the high side.

Thanks a bunch for your kind replies.
Best regards
LANA

[US Iceman]

Lana,

Do not make the assumption that pressure relief valves are only installed on the high-side of a system. You will find them on the low-side also.

The purpose of a relief valve is to prevent over-pressurization from ANY source or reason.

If the system is shut down and is allowed to reach ambient temperature the low-side could also achieve this.

There are also some instances where higher pressures are created by other sources than temperature such as transients.

When designing a system the problem is to look for any possible reason a system may over pressurize, at ANY location in the system. This is a safe assumption to draw upon.

[lana]

As always right on point.

Appreciate your time and effort.
Thanks

[Brian_UK]

Been following this thread as it is not a normal subject for me, mainly A/C.

Having done some reading of information on the web there were a couple of references as the the possible cause of an over pressure situation which may have been forgotton by some - me especially.

The reference to pipelines reaching ambient can be dismissed when you consider the worst case - the building is on fire

When you think of that possibility then you start to see all sorts of places that could perhaps need a SPRV.

We keep learning don't we?

[US Iceman]

hey Brian,

Believe or not, your comment about fires is actually very good. There are two actual cases where you have to size relief valves differently.

One is if vessel or component is within a specific distance to combustible material (a fire condition) and the other case is where no combustibles exist within a specified distance.

In the example of combustible materials within a certain distance the required relief valve capacity increases by a 2.5 factor over the other lower case.

This is based on ANSI/ASHRAE Standard 15 for Refrigeration Safety Code, which we use here. I suspect other codes or standards are similar.