View Full Version : PRV Pipe sizing calculator! Help!

06-01-2016, 05:38 PM
We currently have a ht/lt pack that we look after. We have carried out a survey and noted that the prv's have not been piped to outside and have suggested that this be done asap. The pack is in a busy basement area, runs on r404a and holds approximately 140kg. It has two Henry valves rated at 24.8 bar. I need a simple calculation to work out the required pipe size to take the refrigerant away in the event of the valves venting. The run would be around 50-60 metres long. Any help/advice would be welcomed as all I've found is some pretty complicated equations!

06-01-2016, 05:42 PM
try this one, will give you the idea.

and this one,


06-01-2016, 06:04 PM
Thanks for this. So do you think sizing it as a discharge line or liquid line would be sufficient? From the Tecumseh link.

06-01-2016, 06:24 PM
In your case, I would use a bigger size then the suction, Just have it secured very well along its length.
What size are your vents and how do you think to keep it clear on the outside?

06-01-2016, 06:50 PM
They are standard dual port Henry valves either 5/8 or 3/4 outlet. I've assumed that although it has two outlets per valve, the volume can only be based on the connection onto the receiver size? One connection going to two PRV's. Not too sure about keeping it clear. It may well face downwards. Also the only place I've been shown is an occasionally used sideway!

06-01-2016, 09:19 PM
On the dual valves only one valve should be used at a time on each manifold set, so manifold would be set for this.
It's only for servicing the valves every 5 years so you don't have to empty everything each time.

Relief line out let must not be reduced in size & dual reliefs can be piped into small header just after outlets, instead of 2 separate lines running to a header.

How many reliefs do you have & what is each outlet size (dual manifold is one relief valve).

07-01-2016, 08:49 AM
We have two dual port manifolds with 5/8 outlets, so two 5/8 outlets in total. R404a, pipe run 50-60m. Valves are Henry valves and vent at 24 bar. We were looking at joining the two together into a header and then running out one main. Just don't know how big to run the main pipe out? Also I believe it could be smaller gauge pipe used as it's not system side?

07-01-2016, 07:53 PM
Hi Guys!
What's all this nonsense about pipe resistances etc.
As far a I know! (and I am willing to be corrected) provided you do not reduce the vent line diameter the vent line will vent!
Yes you could have a bit of back pressure but if a PRV lifts and then closes. The internal pressures are going to rise and wain.
But ultimately you have vented away safely, to many Insurance assessors are steam trained and try to apply over complicated formulas.
The pressures and gas temperatures make venting Steam quite critical.

However what is a PRV on a fridge system there for?

It is to release high pressures to maintain vessel integrity. Which can be easily achieved with an open bore pipe.
If in doubt because of the long runs and bends go up a pipe size.
We use threaded steel pipe, copper pipe or Stainless whatever the customer or ourselves deem relevant.
Its still an open pipe with multiple lift pressures dependant upon plant design.

I find it difficult to understand why we are quoting refrigerant line parameters, for a Vent line.
The pipe surfaces are never going to be subjected to the pressures that pipes at MWP are!

07-01-2016, 08:18 PM
Australian & New Zealand code says use pipe for relief vents that you would normally use in the refrigeration system itself.
I work with ammonia, so probably a bit more critical, but rules the same.
Have to be thinking where it is to be vented as well so as not to affect anybody if it vents.

UK code may be different, so can't help you there.

If connecting more than 1 relief into a common header, the header must be sized to allow for all reliefs venting at same time (if building on fire).

So cross sectional area of pipe has to be calculated allowing for all reliefs to go off, in this case 2 only.
Relief vent to header cannot be reduced before entering header.

2 off 5/8" tubes could equate to 1 to 11/8" common header size(see chart).

To me pipe resistance etc is important on relief header so relief valve can vent correctly keeping pressure vessel within design pressure at all times.
End of header needs to be weather & vermin proof & checked for obstruction annually.

The pressure relief valve also has to be the first thing calculated, so end user would expect manufacturer got that right.
If a supplier or manufacturer of the equipment, then that is also done to a code, which hopefully covers your codes in your part of the world, if not start from scratch.