New to the PRV world
New to the PRV world
(OP)
I have been thrown into the world of ASME pressure vessels and PRV sizing. I am a PE, but my experience is in HVAC and plumbing consulting. Not process piping or pressure vessels. I've been asked by a local large manufacturing plant to size a PRV for a small chilled water system surge tank. I have zero experience doing this. I have read Tyco's and Crosby's guides and several other publications on how to do this over the past few days. I also have access to documentation that a previous engineer provided for sizing some valves. I at least know a few of the things that I don't know now, but I realize it is very involved and more then can be explained in a single post. I have not yet purchased ASME section VIII, API 520 or API 521 and realize those may answer all of my questions. I have my tank and system information. What I don't know is how to determine the worst case cause of failure or overpressure or when a rupture disk is required. I realize these are elementary questions and I may be in way over my head, but some guidance would be appreciated. What books/codes/references do I need to buy? Is there a course I can take to get my feet wet? Should I throw in the towel before I get too deep into this?





RE: New to the PRV world
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: New to the PRV world
Rupture discs are typically when a huge difference in shell and internal pressure is possible where the pressure rise on a tube failure is so fast that a valve can't lift fast enough before the shell splits into several fragments. However this a process safety issue, not a mechanical design issue. I've only seen them on heat exchangers, but can be on other equipment also.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: New to the PRV world
I have a file from the previous engineer that Im trying to use to learn the process. He died early last year so i cant go to him for help. He was sizing a PRV and rupture disk for a propane system. He used an external fire and came up with 1576lb/hr as a calcd relief rate, but used 2760btu/hr to size the valve. Why did he do that?
RE: New to the PRV world
Note that you definately don't want to install just a rupture disk by itself on this vessel. It's OK to install a disk under a PRV to prevent leakage - that's commonly done on refrigeration systems - but a disk by itself would risk losing the entire inventory. That's not good for safety, environment, or health.
RE: New to the PRV world
RE: New to the PRV world
RE: New to the PRV world
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: New to the PRV world
RE: New to the PRV world
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: New to the PRV world
What I know:
surge tank in chilled water loop to be protected: max p and t 125psi and 210F - MAWP = 125psi
Controlling scenario: thermal expansion of liquids or as 521 says, Hydraulic expansion. Info provided by owner.
system p and t: 22psi and 45F
Fluid is 30% glycol/water
equation 28 in 520 should be used to find A
equation 2 in 521 will give the q required to solve A above.
I believe I'm relieving to atmosphere so backpressure will be built up only. I will determine this using the flow rate from equation 2 in 521.
Rupture disk not required per the internwebz and owners instruction. I read the section on rupture disks in 520 and am still not satisfied as to when one is required. It says they "can" be used in certain situation but I cant find where they "shall" be used.
What I don't know or am unsure of:
I've assumed this is a liquid full system
equ 28 of 520: I assume P2 is the builtup backpressure gauge only?
to Solve Kv I need the reyonolds number. how do I get that?
equ 2 of 521: it wants phi, heat transfer rate. How do I get that?
After all this is said and done, 521 says an NPS 3/4 x NPS 1 is common. Do you experts find that to be true?
RE: New to the PRV world
RE: New to the PRV world
R/D's are used below PRV's to keep a dirty and/or corrosive fluid from contacting the PRV; Inconel R/D protecting a PRV with carbon steel body and stainless trim. When used in conjunction, there will be a reduction factor that has to be used. ASME gives 0.9 if the flow reduction is 'unknown'. All the major R/D mfr's have flow tested their stuff with the common PRV's; the actual factors tend to be in the .97 to .985 range, almost negligible. Be sure to show it in your sizing calc's.
RE: New to the PRV world
Don't take this the wrong way, but I still think you haven't really worked out what your relieving situation is. the things that come to mind for me are:
You refer to this as a "surge tank". Now that to me implies pressure spikes in the system that this tank is designed to smooth out and hence it also implies that it is sealed, but has a gas volume to absorb sudden pressure changes.
However your post above refers to a potentially very significant temperature and pressure variation from normal ( 45F/22 psi to 210F/125psi). However it is not clear what purpose this tank covers if it is wholly liquid filled. The pressure rise for water due to temperature is quite significant per degree and therefore some expansion volume or pressure relief would be required for the limits you provide. Unless this was a very large system or conversely a very small system where the temperature rise was very rapid, a simple small Thermal Relief Valve (TRV) is all that is required. Hence the flow or sizing criteria is actually the temperature rise rate and hence the volumetric flow due to that temperature rise. This is likely to be really quite a small flow.
There again I might have this completely wrong. Only you know what your system is and how it works or can go wrong, but from what I can see in front of me I think you're going down the wrong path. Too big a valve is as bad as too small.
In terms of bursting disc, you need to look at rate of pressure rise versus flow and capacity of the relief system plus probability of this actually occurring. Hence in this case how likely is it that the temperature rise or other action would cause the pressure to exceed the design (125psig). If this is very low / almost negligible, then a bursting disc might be the best option as it would protect the system in the unlikely event of it going wrong, but not require a valve which needs maintenance, could rust or stick open. All needs information which we don't have.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: New to the PRV world
Thanks for the great responses.
RE: New to the PRV world
in terms of pressure you can work out what temperature rise would be required to achieve the pressure - see this FAQ http://www.eng-tips.com/faqs.cfm?fid=1339
I think you could be quite surprised about how little temperature it would take - basically a warm day when the chiller isn't working is more than enough to rise 100psi. This is for a completely liquid filled system - any air volume able to be compressed will of course reduce this pressure rise considerably, but you know your system better than me... I do though find it difficult to believe you have a system like this without some sort of gas filled expansion tank.
Of course what goes out needs to go back so if you actually relieve fluid then it needs some means of putting it back once you start the chillers again - a real pain. Hence why most systems use an expansion tank. Maybe this tank is supposed to be gas filled and some idiot has filled it to the brim?? Do you have any P & IDs, original drawings or manuals for this system? or anyone that was there when it was first installed?
Only if there is a substantial expansion tank is it difficult to see where the excess pressure is coming from.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: New to the PRV world
RE: New to the PRV world
I wasn't trying to be smart - if there is an expansion tank then a relief valve system would seem to be superfluous and may well be where a bursting disc would satisfy the requirement to protect the system in a once in a million chance of being required is there. Unless your tank can be isolated from the expansion tank then it's all a bit of a mystery to me... Maybe just do the sums and show with the expansion tank and an air temperature of 50C you can't get close to the design pressure?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: New to the PRV world
RE: New to the PRV world
RE: New to the PRV world
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: New to the PRV world
The intent of this site is so an engineer, knowledgeable in the subject, can ask peers questions, and so engineers, knowledgeable in the subject, can answer those questions so the "askee" can become a better engineer. If the "askee" is not knowledgeable in the subject, then what is the purpose. It just takes too long to teach the subject. I hope you don't intend to put your PE stamp on this relief design.
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: New to the PRV world
RE: New to the PRV world
This sort of thing is not what the calculation is for.
You probably need to just assume some sort of temperature rise over time and run a few cases which look reasonable. If this thing is insulated (as you know most chilled systems will be) then temperature rise of 5C per hour?, maybe 10 at a push.
whatever it is I can't see the volumes being enough to go bigger than the 3/4 x 1" TRV you quoted at the start.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: New to the PRV world
If you read through API 520/521 you will find the calculation to find the heat input rate due to external fire.
You probably can fill the vessel with your pumps, you might have a blocked outlet while the pumps are running. Then you can over pressure the vessel that way.
After reading through the thread I do see that you probably should have contracted this out to an experienced PSV designer. They probably could do the design for you fairly quickly.
Aside from that, I would consider going through the 16 API scenarios again and double check that you've gotten everything correct. After you solve all the possible scenarios for flow rate and area you can select the correct orifice. Then you could find the appropriate inlet and outlet size, and complete your pressure drop calculations to ensure you meet the 3% inlet and 10% outlet rule.
But I would seriously consider getting an experienced designer to do this work in liu of you putting your stamp on a report that may not be done correctly as you've not had much experience in this subject.