PSV sizing for chiller

[Bourbon103]

I have to size a relief valve for a water chiller. The chiller consists of a compressor feeding R-123 to a water cooled condeser which then feeds and cools water in an evaporator finally returning back to the compressor.

As far as the water goes, tube rupture is being considered. My question is in regards to the refrigerant. Is there a need to consider the refrigerant in relief valve sizing and/or is it covered by codes (as it is not mentioned in ASME XIII)?

Thanks.

 

[Bourbon103]

ASHRAE STANDARD 15 possibly?

 

[quark]

R123 chillers are generally low pressure machines and operate at about 85PSIg discharge. All the chillers I saw and used come with a built in pressure relieving devices on the discharge. Do you want to go for an additional one?

 

[Bourbon103]

I have a manufacturer's maintenance/operating manual on the unit and it shows the safety valves in the diagrams so I assume they want a check on the valves that came with the unit (the unit is circa 1960's).

 

[CHD01]

ASHRAE 15 is the correct standard for refrigeration systems. The formula differs from ASME. Based on the refrigerant there is a factor f that you will use. Then based on the overall length and diameter the required relief rate will be determined in lbs air per minute. See Section 9.7.5.
The formula is C = fDL,

C = lbs air /min
f = refrigerant factor
D = diameter
L = overall length (not just tangent length)

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

 

[EGT01]

Bourbon103,

If you are in the US, I think you will find that the vessels that make up your refrigeration unit will have been constructed according to ASME Section VIII so they need to be protected accordingly. ASHRAE 15 would be an acceptable standard to follow since it was developed specifically for refrigeration systems and meets the requirements of ASME VIII. Usually with refrigeration systems there are not many overpressure scenarios that need to be addressed. The nice thing about ASHRAE 15 is it covers the typical cases and gives a cookbook method of how to design the relief system. ASHRAE 15 only address the minimum requirements for overpressure protection so you need to satisfy yourself there are no others.

Generally there are only two cases that need to be considered for the refrigerant side, one is as noted by CHD01 which covers a heat input case. The other case that should be addressed is a blocked compressor discharge as noted by Quark. Calculations for determining the required capacity for these cases are given in ASHRAE 15 and have been developed to be used with refrigeration systems.

My experience is that the refrigeration units are generally purchased as a package unit with the manufacturer sizing the relief devices on the refrigerant side and including any of those on accessory equipment like oil coolers for the compressor. In the past, the relief system documentation for those type systems usually didn't get the attention that others on process equipment received so you probably have nothing to start with. All refrigeration manufacturer's I've dealt with followed ASHRAE 15 for sizing the relief devices so you would probably be better off following the minimum requirements in ASHRAE 15 to be consistent with the approach that was likely used by the manufacturer.

I would add that ASHRAE 15 goes into some detail about designing the relief device outlet piping but doesn't say much about the inlet piping. There has been some changes in the requirements for the outlet piping so if you follow the current version of ASHRAE 15 you may find the outlet piping inadequate if your installation was designed following an older version. Be sure you follow your relief device manufacturer recommendations for the allowable builtup backpressure. As for the inlet piping, I would follow the ASME guidelines for inlet pressure drop less than 3% of valve set pressure.

 

[CHD01]

EGT01 is right on; the only thing I would add is that the components that refrigeration manufacturer's like to use - like 3-way valves usually result in greater than a 3% inlet loss. It is recommended that the inlet loss be accepted if less than the relief valve blowdown, ah but what is the limit that is the question. This must be resolved with the site. I feel 25-50% of blowdown is acceptable. By the way, I usually see Henry valves in refrigeration service which have a rather large blowdown.

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