New to the PRV world 1

[badal]

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?

 

[Latexman]

Why go to all that trouble for one PRV? There's no profit in it as far as I can see. I'd tell the plant who can do it so they can contract with them, or let you subcontreact it out to them.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[LittleInch]

I think you're expecting to do too much here. That sort of information should be provided by the process engineer or determined by a team of people looking at all potential scenarios, but with someone there with process knowledge of the particular system.

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

 

[badal]

So your saying it is not up to me to determine the worst case? What tells me where to use a rupture disk? And you are right there will not be much if any profit in this one. The profit is in the next 2,000 if i can pull this off.

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?

 

[don1980]

badal - He likely sized the relief design using ASHRAE Standard 15. That's the pressure relief standard for refrigeration systems. It's a relatively straight-forward procedure.

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.

 

[badal]

Im sizing for a 30% propylene glycol mixture. Any idea where i can find the MW, specific heats, viscosity of the mixture?

 

[badal]

Also, I've seen a discussion on this talking about getting those values from hysys. Anyone know what that is and if it has the info for glycol solutions?

 

[Latexman]

Google "A Guide to Glycols".

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[badal]

THANKYOUTHANKYOUTHANKYOU!!!! I've been looking for something like that for 3 days.

 

[Latexman]

There's a button for that.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[badal]

All right. I've come full circle on this. I have read API 520 and 521-2008 in their entirety and it appears only two equations matter for this scenario. Before you read on, please note that I am not interested in your comment if it is "your in over your head" or "give up. you don't know what your doing". I admit I do not know what I'm doing that is why I'm asking questions. I am looking to the only place I have available to me for expert help because I do not know anyone personally that can help. I've turned to you. The all knowing interwebz.

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?

 

[badal]

The comment above was not directed at anyone that has responded so far. You have been more than helpful. I was just trying to cut any trolls off at the pass.

 

[Duwe6]

Rupture Disk part is easy: You use them by themselves IF you need a huge orifice, and IF not reclosing is not a problem, plus the cost of a PRV with that huge orifice is too much to bear.

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.

 

[LittleInch]

badal,

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

 

[badal]

The owner has called it a surge tank, but I would have called it a buffer tank. There is no gas volume that I know of. Only chilled water in and chilled water out. I have had the same thoughts about the operating and maximum tank p and t. All that is connected to this system is a chiller, pumps and air handler. So if fire or sun is not the case (as I've been told), I don't know how the system is supposed to get up to max p and t unless magic is involved.

Thanks for the great responses.

 

[LittleInch]

Hmmm, One wonders what it's purpose is then??

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

 

[badal]

I deal in HVAC design every day and never have to worry about PRV's. An expansion tank is usually all we call for. I would expect an expansion tank in addition to this surge/buffer tank. The tank is there to increase the volume of chilled water in what is a very small system. It keeps the chiller from short cycling. I've asked for drawings, but have not received any to date. I think what we have here is that a very large companies has a policy requiring documentation on something that they don't really even need.

 

[LittleInch]

sorry,

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

 

[badal]

No offense at all. I actually tried to go back and edit my post because I sound like a jerk.

 

[badal]

Assuming that I am going down the right road, how do I get the heat flow rate (heat transfer rate) in btu/hr? I have found charts with the densities, thermal conductivity and various other characteristics of glycol, but I'm striking out getting the phi for equ 2 in 521.