Thermal Relief Valves 1

[pe1969]

How do you specify the set pressures and back pressures for a number of thermal relief valves in a cascading relief system where a long process line is blocked in many sections? What is the best type of TRV to be used? Balanced bellow or pilot-operated? What is the benefit to use pilot-operated TRVs? Do you have to specify spring setting (differential set pressure)? Is there any reason to drain the last section of line near the storage tank? I have read a document which recommends to drain the last section and set the TRV set pressure at 110% of its MAWP in order to take the advantage of 133% max. pressure allowed for piping.

 

[MJCronin]

I suggest that you post this question in the "Chemical Plant Design" forum of this website

MJC

"There comes a time in the affairs of man when he must take the bull by the tail and face the situation." W.C. Fields

 

[MortenA]

Re. pilot operated valves you could take a look at my FAQ re. this PSV type. Its general though.

http://www.eng-tips.com/faqs.cfm?spid=408&sfid=744

Re capacity: I would generally assumed that your PSV's would be small e.g. D size and still too big. Then it wouldnt matter much 10% or 33% overpressure. If its only pipe (no vessels etc) then i guess you could use the 33% overpressure allthough some company stadnards may prohibit this (in order not to get a "confusing" design).

Best regards

Morten

 

[CHD01]

1. While there are exceptions for pipe that allow up to 33% overpressure for a certain period of time during a year of ervice, you should try to follow the rule that the relief valve set pressure should not exceed the pipe design pressure or MAWP.

2. In general each of your thermal relief valves should be set for the same set pressure IF the pipe codes is the same (i.e. same design pressure or MAWP).

3. The "D" orifice is the smallest STANDARD relief valve with an orifice of 0.11 in2; but smaller and cheaper valves are available and preferred for this service. Consult your vendor; just get an approved and ASME vertified valve. The valve should be a conventional valve; not a balanced bellows. I cannot think of a reason right now to use balanced bellows.

4. A pilot operated valve makes no sense to me at all, this is a case with minimum storewd energy for release; the valve will be open briefly and then will close.




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

 

[Montemayor]

pe1969:

Firstly, the term used for the application you're describing is "Thermal Expansion" relief valve. The difference in terminology is why I believe you may have read some literature that doesn't apply to this simple, and direct relief application.

As MortenA and CHD01 both point out, any thermal expansion relief application is going to involve a relief valve that is at the bottom of the capacity ratings and is, quite honestly, a very insignificant instrument. Don't read me wrong....what I'm trying to point out is that although it is an IMPORTANT instrument and one that should be evaluated in any potentially blocked liquid scenario, it is probably the smallest PRV you will ever install.

Inherently, all this type of PRV does is relieve the incremental liquid expansion volume created by a thermal increase in the parent liquid - a quantity that, if you take the time to calculate it, is so small that it usually is measured in a few cubic centimeters. That is why these type of valves are usually 3/4" to 1" in size and even OSHA concedes that NO FORMAL RELIEF CALCULATIONS ARE REQUIRED FOR THEIR SIZING AND APPLICATION. This tells you that these valves don't carry any "bells and whistles" and are pretty simple and routine.

Again, don't get me wrong. It's important that you recognize this application scenario; but it is just as important to not waste your time calculating back pressure and other effects when you are dealing with a quantity of discharge liquid that probably won't fill the immediate outlet PRV nozzle and, therefore, won't register any recognizable back pressure in the discharge header. This is just one of those good ole' common sense engineering applications that help bring us back to the reality that not all in this world is dependent on partial differential equations and the theory of relativity.

What is important in this application is identifying and justifying the relief of the discharge liquid. For example, if you are dealing with Methanol, Acrylonitrile or Ethylene Oxide it is imperative that you evaluate the SAFE manner and location target of the discharge fluid. Many times this exercise has forced me to employ Expansion Chambers in the blocked line rather than thermal expansion PRVs. This method avoids the discharging of toxic or undesirable emissions.

I hope these comments help to ease and justify your application.

Art Montemayor
Spring, TX

 

[CRG]

Caution:

Do not set all TRVs to the MAWP in a cascading relief system unless all TRVs are of a balanced design (TRVs that reference atmospheric pressure are balanced vs. unbalanced if they reference downstream pressure).


CHD01 why would you recommend unbalanced TRVs in a cascading relief system? Not all balanced TRVs have bellows.