9 replies to this topic

[vaporiser]

Hi All,

I have following questions.
a) How do we decide whether an equipment should have a Thermal Relief Valve or a Pressure Relief Valve?
What are the advantages & disadvantages of both types?

Please cite examples.

Thanks

[Erwin APRIANDI]

Dear vaporiser

a. A thermal relief valve is designed to relieve hydraulic pressure (pressure exerted by an expanding, supercooled liquid) and not generated vapor, while a pressure relief valve is a safety device that relieves overpressure

b. I think the application of both relief valve is depend on the requirement, so you can not compare both

[ankur2061]

Vaporiser,

I had prepared a standard on Vent & Relief systems for a middle east O&G company where I had provided some guidelines for where to and where not to provide "Thermal Expansion Relief Valves" (TRV's). I am reproducing here what I had written in the standard:

TRV’s are normally provided for:

- Piping in storage areas or transport pipelines which will be regularly blocked in during normal operation and can have a pressure rise due to solar heating or heat tracing.

- The cold side of heat exchangers which can be blocked in with flow on the hot side.

As a general guide, TRV’s are not needed for:

- Process plant piping;

- Storage or transport piping sections which are not normally shut in for operational or emergency purposes;

- Lines in which there is normally two-phase flow;

- Systems which are not totally liquid filled, i.e. less than 95% liquid.

Hope this helps.

Regards,
Ankur.

[AdrianaBarbu]

HELLO !
MY NAME IS ADRIANA AND I AM A PROCESS ENGINEER.
I HAVE A QUESTION REGARDING THE SET PRESSURE OF THE TRV (THERMAL RELIEF VALVE)
I MADE A DATA SHEET FOR A FEED PROJECT IN WHICH I SPECIFIED AS SET PRESSURE OF THE TRV TO BE THE VALUE OF THE SHUT OFF PRESSURE OF THE PUMP
AFTER THIS THEY SENT ME BACK A COMMENT IN WHICH THEY ASKED ME WHY DIDN'T I TOOK AS SET PRESSURE THE SHUT OFF PRESSURE MULTIPLIED BY THE 10% OVERPRESSURE?
FOR MY POINT OF VIEW THE SHUT OFF PRESSURE OF THE PUMP IS THE CORRECT VALUE
IF IT IS CORRECT OR NOT, I AM ASKING FOR AN ADVICE!
THANK YOU VERY MUCH!
HAVE A NICE DAY!

[ankur2061]

Adriana,

Where is your TRV located? Is it located on the pump suction or discharge piping?

While mentioning that in a pump circuit the set pressure of a TRV can be provided as the pump shut-off pressure it can also correspond to the design pressure of the pipe at the design temperature, which would be equal to or greater than the pump shut-off pressure.

As an example if your pipe material is of Material Class 1.1 (Carbon Steel) having a rating of 150# rating and a design temperature of say 75°C than the design temperature of your pipe would be 18.5 barg as per "Table 2-1.1 - Pressure-Temperature Ratings for Group 1.1 Materials" as per the standard "ASME B16.5 - Pipe Flanges & Flanged Fittings".

You can use the design pressure of your pipe in the pump circuit as a set pressure for your TRV based on the above example, instead of the pump shut-off pressure which gives you another option for the TRV set pressure. However, there is nothing wrong in providing the TRV set pressure as the pump shut-off pressure which would somewhat be conservative unless your piping design pressure and the pump shut-off pressure are the same.

Hope this helps.

Regards,
Ankur.

PS: Adriana, writing in bold letters is considered to be shouting. Please don't do that again.

[Propacket]

Vaporiser,

Excellent reply from Ankur should suffice. This is actually the extract of what API RP 521 states about TRVs. I would like to add some more from the API RP521.

Generally, 3/4" x 1" valve is sufficient for a TRV.
If there is a reason to believe that above size is not adequate and a higher TRV size is required, following equation can be used to determine the valve capacity.

GPM= BH/(500GC)
Where B=Coefficient of Volume Expansion (per deg F)
H=Heat Transfer Rate (Btu/hr)
G=Sp Gravity
C=Sp Heat of Trapped Fluid (Btu/lb. deg F)

Regards,
Haseeb Ali

Edited by P.Engr, 10 May 2012 - 07:27 AM.

[fallah]

I HAVE A QUESTION REGARDING THE SET PRESSURE OF THE TRV (THERMAL RELIEF VALVE)
I MADE A DATA SHEET FOR A FEED PROJECT IN WHICH I SPECIFIED AS SET PRESSURE OF THE TRV TO BE THE VALUE OF THE SHUT OFF PRESSURE OF THE PUMP
AFTER THIS THEY SENT ME BACK A COMMENT IN WHICH THEY ASKED ME WHY DIDN'T I TOOK AS SET PRESSURE THE SHUT OFF PRESSURE MULTIPLIED BY THE 10% OVERPRESSURE?
FOR MY POINT OF VIEW THE SHUT OFF PRESSURE OF THE PUMP IS THE CORRECT VALUE
IF IT IS CORRECT OR NOT, I AM ASKING FOR AN ADVICE!

Adriana,

Such relief device for liquid relief is normally called PRV. The comment of your client isn't so clear and set pressure multiplied by 1.1 (10% overpressure) is normally titled relieving pressure of the PRV. Please ask your client for further clarification.

In FEED stage when the pump curve isn't available, the design pressure of the pump discharge (can be utilized as PRV set pressure) is normally estimated as follows:

Pd=Ps max.+(1.2*head*d max)/10.2

Where:

Pd _{= design pressure at pump discharge (bar g)
Ps max. = design pressure of suction drum + static head at dmax. and at HLA
head = head of the pump at design point (m)
d max= maximum specific gravity of pumped liquid under normal operating conditions}

Fallah

[S.AHMAD]

Dear Adriana
1. Set pressure shall be the maximum allowable working pressure (MAWP) of the system that we want to protect.
2. The 10% allowable overpressure is the maximum allowable for the relieving pressure (MARP) which is not the MAWP. MARP is allowed pressure that is during popping of PRV only whereas MAWP is continuous basis.
3. Therefore, your understanding is correct, your clients are confused.

Edited by S.AHMAD, 10 May 2012 - 07:47 PM.

[AdrianaBarbu]

Good morning!
First of all thank you very much for the fast answers and for the interest.
Regarding my Bold writing I didn't know the rules, so from now on I will consider the advice.

And back to my question...
My TRV is mounted in a depot on the discharge line of a pump which loads the rail tank cars/storage tanks.
Considering that all the pumps in this tank farm have 7 barg as shut-off pressure I was thinking about taking this as set pressure of the TRV.
I discussed also with the piping engineer which told me that the piping design pressure according to the material class is 19 bar.
As I said first I put the set pressure in the TRV data sheet 7 barg.
Which value would be more correct? 7 barg or the design pressure of the pipe?

Thank you very much again!
Have a nice day and all the best!

[S.AHMAD]

1. I have been using the line class limit as the set pressure for TRV.
2. However, there is nothing wrong to set at lower pressure except the TRV may popped unnecessarily resulted in loss of valuable products.

Edited by S.AHMAD, 11 May 2012 - 02:06 AM.