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# How To Size A Thermal Relief Valve

31 replies to this topic

### #1 Plumbercs

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Posted 18 September 2008 - 02:07 AM

Appreciate if you could advise on how to size a thermal relief valve for :

blocked in line of 0.3 litre volume
line content = liquid nitrogen

Will a standard 0.062 sq in orifice RV be sufficiemt ?

Thanks.

### #2 ARAZA

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Posted 18 September 2008 - 09:10 AM

Hi There,

So far I've not seen anybody doing calculations for Thermal relief valves, when there is a requirement for a TSV, usually a valve of minimum size (3/4" x 1" OR 1/2" x 3/4") available is picked up. This size of the valve is usually 10 to 50 times more than what it is required. The basic function of this valve is relieve the pressure build-up due to thermal expansion.

To perform the calculations, you need to calculate the relieving rate based on the thermal co-effcient of expansion of the fluid involved. What you have given here is the total volume of the system.

If you want to satify yourself and do the calculation, pl, refer to API 520 equation 3.2 under section sizing of the pressure safety valve.

Hope this helps.

ARAZA

QUOTE (Plumbercs @ Sep 18 2008, 03:07 AM) <{POST_SNAPBACK}>
Appreciate if you could advise on how to size a thermal relief valve for :

blocked in line of 0.3 litre volume
line content = liquid nitrogen

Will a standard 0.062 sq in orifice RV be sufficiemt ?

Thanks.

### #3 Art Montemayor

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Posted 18 September 2008 - 11:54 AM

Plumbercs/ARAZA:

I'm taking the liberty to interject myself into this thread because I believe this is a perfect example of what can go wrong in an engineering project/assignment when there is a failure to communicate correctly and exactly.

I point directly to the basic data that the OP (Plumbercs) has supplied. He/she states that there is a LIN (Liquid Nitrogen) line blocked in and that a thermal relief is required. I believe this is WRONG, and flawed information. Allow me to explain.

If the line contains LIN, I'm willing to bet that the contents are basically SATURATED LIQUID that is subject to vaporization when exposed to a heat leak through the pipe insulation. The LIN will convert from a liquid to a saturated vapor. Since the vapor is compressible, there will be NO HYDRAULIC pressure exerted by the contents; there will be compressed vapor pressure exerted.

A thermal relief valve is designed to relieve hydraulic pressure (pressure exerted by an expanding, supercooled liquid) and not generated vapor. Araza has correctly advised that no calculations are required for a thermal relief valve case; HOWEVER, he/she has mistakenly assumed that the OP's original claim or assertion of a thermal relief case is correct --- and that may be a mistake. This MAY NOT BE AN APPLICATION FOR AN EXPANDING LIQUID.

If I am correct in that this LIN is saturated, then the application calls for a PSV sized to handle the expected rate of vaporized nitrogen generated by the pipe's heat leak. This is an important point because the difference between both interpretations is a very different type (& size) of relief device. We don't want to make a fatal mistake in estimating the correct relief fluid type and rate of relief.

Plumbercs, please clarify or confirm EXACTLY what fluid conditions you have in the pipe and whether you expect to have any LIN vaporization occuring prior to - and during - the required relief.

### #4 djack77494

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Posted 18 September 2008 - 04:23 PM

I'm not sure if it's an industry wide standard, but we use the term "thermal expansion" (and to a lesser degree "thermal relief") in a very narrow sense. As Art mentioned, they really only apply to (very) subccoled liquids.

Based on our use of the "thermal expansion" scenario or case used in analysis of pressure relief needs to mean that a minimal PSV (say 3/4" x 1") is adequate, I'd argue that we have to be very careful in how we use the term thermal expansion. We use it only in liquid filled systems and only when we do not expect any vapor to be generated. Also, we do NOT call situations where substantial heat transfer is expected to fall into the thermal expansion catagory. Thus, for example, a subcooled liquid in a heat exchanger where it might absorb heat from the other fluid present does not meet our criterion for thermal expansion. An isolated vessel that does not have an external jacket or an internal coil that can supply heat probably does meet the criterion. We would never use this term in conjunction with any liquified gas or cryogenic liquid. In a broad sense, the Fire case is a type of thermal expansion, but, of course, we sharply differentiate between fire and other types of heat gain. In a similar manner, we should sharply differentiate between thermal expansion and other PSV sizing considerations.

### #5 demank

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Posted 23 June 2009 - 09:05 PM

I also confuse in sizing Thermal expansion. The 3/4" x 1" valve size really adequate? Cause 3/4" x 1" not shown in API 526.

### #6 JoeWong

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Posted 25 June 2009 - 04:00 PM

demank:

It doesn't really matter whether 3/4" x 1" is or is not shown in API 526.

Why not calculate once (may be once in lifetime)...then you will know the limit.

Find the answer by yourself...this is the spirit of pro-active engineer.

### #7 demank

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Posted 25 June 2009 - 05:37 PM

JoeWong:

That's good idea. unfortunately, I still cannot find the method to calculate orifice area for the given PSV size. how to calculate orifice area 3/4" x 1"?

For example:
orifice area of 1D2 is 0.110 sq-in.
orifice area of 1E2 is 0.196 sq-in.
Both of them are the PSV size of 1" x 2".

### #8 JoeWong

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Posted 26 June 2009 - 12:22 AM

This has been discussed in earlier post.

Besides, you may search from PRV website i.e. Crosby, Farris, etc

One the PRV series offer by Crosby is Series 60 and 80 Pressure Relief Valves. Check out...

### #9 skearse

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Posted 29 June 2009 - 09:04 AM

We happen to be very sensitive to the issue of thermal relief due to past "experiences." We may be one of the few organizations out there that actually has a rigorous standard specific to thermal relief sizing, when it should be applied, and a full "tree-diagram" to help determine thermal relief. Almost invariably, a 3/4"x1" valve is more than adequate, but as Art pointed out, ensuring that this is a hydraulic expansion rather than a vaporizing liquid scenario is one of the biggest "gottchas" in the system. Ensuring that the system will not rise above the saturation temperature of the fluid is one of the first things that we do. If the heat input will cause the system to rise above that, we size it similar to a fire case valve (to determine the vapor relieving rate=Q/H) rather than the hydraulic expansion relieving rate (liquid relieving rate=bQ/500gCp).

We will also look for methods other than a TRV to provide overpressure protection, usually due to the effluent handling problems (long runs back to storage tanks, etc.) Automated blowdowns, carsealing maintenance valves, etc., are oftentimes employed if possible. Relieving a cryogenic fluid through a TRV would seem particularly troublesome due to the possibility of freezing the valve as the nitrogen flashes across the nozzle.

### #10 thakur

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Posted 17 July 2009 - 03:07 AM

hi dear,
but how do we come to know that rise in heat will lead to the thermal expansion or vaporisation?

and in case vaporisation takes place how sholud we size the tsv for such case?

### #11 Qalander (Chem)

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Posted 17 July 2009 - 04:01 AM

Dear thakur;

I am really astonished to see your query;

does vaporisation is not always an expansion in itself thus creating highly separated molecules in vapor/gaseous phase!

### #12 thakur

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Posted 17 July 2009 - 04:55 AM

ok let me be a specific

i am refering thermal expansion with the reference of liquid and vaporisation for the case where latent heat is getting transfered for vaporisation phenomena.

### #13 Qalander (Chem)

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Posted 17 July 2009 - 06:11 AM

And dear,

the discussion pertains specifically to the 'TRV's' i.e. Thermal Reliving Valves alone!

### #14 skearse

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Posted 17 July 2009 - 10:31 AM

Use the orifice size and then determine the actual valve size, just like any other valve calculation, rather than the other way around.

### #15 skearse

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Posted 17 July 2009 - 10:37 AM

Are the conditions sufficient that the temperature and heat input would raise the fluid in the system above its vaporization/saturation temperature? If yes, then you're looking at a vapor relief, rather than a liquid expansion relief. For instance, the OP has subcooled nitrogen liquid in the system. As Art points out, even a small heat input, even at ambient conditions, would likely be enough to cause the liquid to begin vaporizing. Then, since it's a vapor relief case, your flowrate could be determined by taking your heat input rate and dividing it by the vaporization enthalpy of the fluid-similar to the relieving rate calculation in a fire case.

### #16 Qalander (Chem)

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Posted 17 July 2009 - 12:03 PM

Dear skearse Hello/Good Evening,

### #17 jerald04

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Posted 03 October 2009 - 08:45 AM

Hi all,

Thermal expansion cases are designed with 25% overpressure.

May I ask if this rule extends to pressure vessels too (eg. on heat exchanger cold liquid side to be heated up by a heatind medium) or only to long piping cases?

Thanks.

### #18 fallah

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Posted 03 October 2009 - 11:08 AM

jerald04:

Refer to API 521 FIFTH EDITION section 5.14 especially 5.14.1 to get some valuable points on your request.

### #19 jerald04

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Posted 04 October 2009 - 02:10 AM

Hi Fallah,

Appreciate your reply. Unfortunately, I only have the 4th Edition copy of API 521. I tried looking up the 4th edition on possibly similar chapter (which i guess is 3.14.1 on 4th ed.), and i believe we should also set the same 25% overpressure guideline since liquids need the higher pressure to "relief". Please correct me if I'm wrong.

I have another question:
I read through some other threads and it seemed like it is common to relief TRVs discharge to a location downstream of the block valve. In such cases, the back pressure of the relief valve would have almost the same value as the set pressure, since the operating pressure (super-imposed pressure) are typically near to the set pressure (lets assume that the discharge line is short where the build-up back pressure is very small). We know that the spring setting can be adjusted to take into account of any constant superimposed back pressure.

My question is: Is there a limit to how much we can adjust the spring setting to account for the constant superimposed back pressure? Are such adjustments allowed for conventional type TRVs, even if the superimposed back pressure is high and exceeds the 10% rule for discharge piping back pressure for conventional PRVs?

Thank you.

Edited by Art Montemayor, 26 June 2012 - 01:43 PM.

### #20 fallah

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Posted 05 October 2009 - 02:39 PM

Jerald04,

I have attached one relevant page of API 521 fifth ed. for your information.

I think the limit of adjustment of spring for constant superimposed back pressure specified by manufacturer and is around 10% of set pressure.

TRVs follow the same rules applied for PSVs in effects of back pressure.

Pleasae submit your question clearly for better assistance.

### #21 blup blup

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Posted 24 March 2010 - 09:57 PM

Sorry to interrupt the discussion, i have similar problem with thermal relief valve.

the location of this TRV is at pump discharge system to metering skid. The normal discharge pressure of pump is 5 barg & 35 degC during operation with pipe ID of 12 inch (carbon steel 150#) and service fluid gasoline.

When the gasoline transfer finish, the discharge system is blocked-in (the system pressure is @ 5 barg & 35 deg C)and can cause thermal expansion. My TRV set pressure is 6.5 barg. While no operation and blocked-in condition, TRV is always poping in a short interval period of time which causing gasoline loss since the discharge TRV pipe routed to closed drain system.

My first prediction that the TRV set pressure is too close to normal operating pressure of system (only 1.5 barg difference).

I would like to know how to estimate / calculate the pressure increase due to thermal expansion of gasoline and how fast this built-up pressure.

does anyone here know how to do the calculation?

Much appreciated for the respond.

Thank You.

### #22 ankur2061

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Posted 25 March 2010 - 08:57 AM

blup blup,

I would not bother to do any calculation. Instead of that, I would just increase the set pressure of the TRV to the design pressure of the pipe (150 #) @design temperature. After all the ultimate function of the TRV in this case is to protect the disharge piping for the blocked-in case.

Hope this helps.

Regards,
Ankur.

### #23 JoeWong

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Posted 25 March 2010 - 03:57 PM

blup blup,

You may consider method in API STD 521 2007 Addendum May 2008 section 5.14.3 to calculate the pressure rise and liquid expansion rate (hence thermal relief rate). May read "Thermal Relief of Non-Flashing Liquid in Pipe".

### #24 fallah

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Posted 27 March 2010 - 01:00 AM

Hi Joe,

The section 5.14.3 isn't included in updated sections of API STD 521 2007 Addendum May 2008.Would you please clarify?

### #25 JoeWong

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Posted 30 March 2010 - 05:40 PM

fallah:

I am not sure which edition of API 521 you are referring to...But please refer to
API STD 521 5th Edition, Jan 2007 Addendum May 2008, "Pressure Relieving and Depressuring Systems", Section 5.14.3 Special cases.