View Full Version : Relief Valve on Suction

21-10-2003, 05:40 PM
Just come from a supermarket in Luton which has just has a new coldroom installed with an artic circle remote condensing unit.
Looking at the pipework i saw that a relief valve has been fitted to the SUCTION ???? side just before the compressor.
Can anybody out there tell me why, It this some new EEC legislation that nobodys heard of. I cannot believe it's been put there purly to protect the compressor, surely a pressure switch to shut the system down would be better than blow 20kg of R404 into the atmosphere. Apart from that the unit as far as i can see is standard, Autoreset LP/LP, Relief valve on Reciever.

ANY IDEA's anyone cause i Havn't a Clue (NO CHANGE THERE THEN)

21-10-2003, 09:31 PM
hello raymond
what type is the relief valve is it a standard PRV or a rupture disc type.The PRV is spring loaded and designed to open when the operating pressure exceeds the preset pressure.And to reseal after the operating pressure has decreased.The rupture disc remains intact but bursts at a set pressure.It is good practice to fit one to protect the compressors against abnormal operating conditions,
see ya :)
regards mark

21-10-2003, 09:46 PM
AH Greetings Mr Fiddy,
The valve i think is a Henry Type 523, if this doesn't ring a bell look at the HRP catalogue Pg 54. I was only asking because Simon and i were at a site today and had never seen one on the suction before. We wern't sure as it's a new installation wether it was now a legallity to fit it. Personally it seems a weird idea, why vent refrigerent to the air when a simple HP switch wired into the control circuit would suffice and not vent until an engineer was on site to rectify.

22-10-2003, 10:14 PM
hi raymond
The PRV described is a standard pressure relief valve,usually used with a halocarbon gauge to indicate on larger systems how much refrigerant has been lost,I guess the one your describing vents to the atmosphere via a copper tube.As far as im aware there is no mandatory requirement to have one fitted.It is more a statutory requirement or guide line of good practice by the manufactures ,(on condensing units over a certain hp)To hopefully save the compressors from premature failure.ha ha
regards mark

22-10-2003, 11:48 PM
Hi rdocwra
The fitting of a suction prv is now very common on multi compressor packs. It is fitted where the the suction pressure, say in the event of a fire on the shopfloor, exceeds the max working pressure of the pipe. Based on R404A at 32degC the max design pressure will be about 14.1 barg, the prv(s) on the receiver will be about 24.8 barg @ 55degC. This is so that where the pipe mwp is below 24.8 barg, theoretically the pipe doesn't burst or explode and hurt somebody. It might have been more cost effective to install thinner 18swg pipe with a prv rather than to install thicker walled pipe say 16swg or 14swg without, the thicker pipe being able to withstand 24.8 bar and hence only high side prv(s) being required. It came into play with BS 4434 and now superceded with EN 378. The system may have also had to be inspected by a third party notified body due to the size of the receiver. By the way was it fitted to a vent line to atmosphere.

22-10-2003, 11:58 PM
Hello All,
Yes the prv was conected to a copper pipe that faced outwards towards the outside of the store so if it blows the gas SHOULD !!! go straight outside. The unit is only a faily small single pot unit but never having seen this before so i thought i would ask.

23-10-2003, 12:08 AM
Sounds like it wasn't necessary and Arctic are fitting them as course to save themselves from grief, although it is the design engineer/contractors responsibility as the system designer to determine if necessary and not a manufacturer.

23-10-2003, 12:44 AM
Hello biffdecki,
I don't honestly think it is from artic circle as it's not actually part ot the condensing unit, i work on AC units most days and this is the first i have seen. But more strangly from everything else that looks to have been done on this system (ie ON THE CHEAP !!!!!!) i am surprised the contractor whoever it was acually put it on.

23-10-2003, 07:45 PM
Does the store where the condensing unit is fitted have any statutory guide lines.Ie installation guide.I know some stores have strict installation practices .Which as i said in my first post is" "good practice" for all the above reasons .
Its primarily a secondary safety device if the reason was due to gas type im sure it would have been made a mandatory requirement for units running on R22,years ago.
regards mark

23-10-2003, 08:24 PM
Hi Mark,
I would assume a man in your'e elevated position within the company (Only joking) would know about store installation practices. Lets just say, its one of our PRIMARY contracts and the company begins with "S".
The gas type for the record is R404a



23-10-2003, 08:53 PM
[QUOTE]Originally posted by rdocwra
[B]Hi Mark,
I would assume a man in your'e elevated position within the company (Only joking) would know about that.

yes its a tough job but somebodys got to do it
any way the stores we look after dont have mere condensing units he he:D

01-11-2003, 07:39 PM
Back when HP80/402a became a retrofit gas for racks, there was a mandate from Copeland, I believe, that reliefs be installed on each crankcase.

The folks a Copeland were concerned about the high pressures possible if a valve plate failed and the compressor began to bypass.

We did two changeovers with these valves. hen we smartened up and went to 404a...

04-11-2003, 11:27 PM
Hi rdocwra:)
Both the low and the high side have their design Allowable Pressure. PRV's on the high side are common, even with a H.P switch, you would never think of relying on the H.P only as a protection on the high side, so why do we find a PRV strange on the lowside. Perception I would say:)
PRV's on the lowside are necessary if the low side AP is lower than the highside AP, in most cases this is so, hence most systems should have lowside PRV's.
Regards. Andy:)

06-11-2003, 09:22 PM
hello raymond
The PRV in a suction line,only applies to equiptment over a certain size ,and a volume of refrigerant.This is a requirement bought in by insurance companys. Firstly a strengh test with OFN then followed by a pressure test with OFN,this has to be signed by a witness .The prvs then need to be changed every 2 years.once the primary tests are complete and pass they can then issue all the cover notes required.
Its a secondary back up against incompetance and to coinside with pressure vessel regulations.
I think the need to fit one on a single CU was unnecessary.
hope this answers your question raymond.
regards mark.

06-11-2003, 11:49 PM
Hello all,
Thanks for the replies, it answers most of my thoughts.
This one was the first and only one i have ever seen with a suction PRV.
However, as you all know we live in a world where venting refigerent is a serious offence. We are even taught to pump down to remove gauges if possible. fitting more PRV especially to small units seem risky (Not sure if that's the right word but it will do). I certainly take on the point they might come into effect if there is a fire in a large system to stop explosions. However for run of the mill systems surely a pressure switch on the lowside, or two if you want to be sure would suffice ti just shut the system down.
any thoughts anyone ???

07-11-2003, 07:12 PM
ok raymond
say the pressure controls dont work and or are linked out,
say the engineer etc?? shove discharge gas up the suctions rear,
It is merely a back up to cover a number of different scenarios,
And our ,meaning us .backsides.If the s*it hit the fan at least weve took all the right precautions.
regards mark

07-11-2003, 11:59 PM
Hello All

Try this link


Handy info for this stuff, check out fact finder 5, 7 & 8

25-11-2003, 07:13 PM
Hello Chaps,
Just had a Thought (Don't knock it it doesn't often happen)
EN378, Strength & leak testing. How do you do this with a suction PRV. Are you meant to check the high side by valving off the reciever outlet test that then blow the OFN down to acceptable levels, open up the system and do the lowside.
If so what happens if the compressor is letin by or th Reciever valve let's by.
I know i am just being pickey, but interested to hear.


25-11-2003, 11:14 PM
Hi rdocwra:)
I test the Low side and the high side separately.
Test the low side then allow this OFN into the high side. Blow off the low side and open to atmosphere, bring the high side up to strenght test, blow off to leak test pressure, then to atmospheric.
The shut off valves should hold, if in an old system they don,t hold you go back to valves that will hold and repair the defective valves on the way.
Regards. Andy:)

21-01-2004, 12:57 AM
RDOCWRA... I'm in agreement with you... a PRV on a 404a system?... especially on the low side? I never would have thought either. LOL! Maybe at some time someone wired a 3 phase screw wrong? Still don't know if that could do it would it? LOL!