25 replies to this topic

[CheAmine]

Hello everyone,

 

My name is Amine and I am a junior process engineer working in an oil and gas field. This is my first topic in Cheresources.

 

We have a thermal relief valve located in a propane line used for refrigeration purposes set at a pressure of 29 barg. I would like to know the temperature at which the pressure will reach 29 barg in case the upstream and downstream valve gets closed.

 

I have done HYSYS simulation and I have considered the vapor phase fraction to be 0 and the pressure to be 29 barg. Based on these conditions, the temperature was found to be 77 C. However, as per the data sheet of the relieiving temperature is 18 C.

 

Could you please let me know the reason of the difference between the two results. Moreover, what is the exact meaning of the relieving temperature.

 

Thank you.

 

 

 

 

[manojkaila]

Dear,

 

Propane is light hydrocarbon so when during thermal release the upstream and down stream pressure there are pressure difference. Outlet pressure or downstream of the PSV always very less. This lower pressure create partial vaporization of propane and it will absorb the latent heat from the surrounding. This phenomena create low pressure. Hope this will help. My experience is from operation side others friends from design background will give more details about HYSIS.

 

Good luck

 

Manoj 

[fallah]

 

I have done HYSYS simulation and I have considered the vapor phase fraction to be 0 and the pressure to be 29 barg. Based on these conditions, the temperature was found to be 77 C. However, as per the data sheet of the relieiving temperature is 18 C.

 

Could you please let me know the reason of the difference between the two results. Moreover, what is the exact meaning of the relieving temperature.

 

 

CheAmine,

 

Please specify the existing back pressure...
 

[CheAmine]

Manoj,

 

Thank you for your feedback.

 

Fallah,

 

The existing back pressure is 1.01 barg.

[fallah]

 

Fallah,

 

The existing back pressure is 1.01 barg.

 

CheAmine,

 

Please upload your simulation detail and/or the TSV data sheet...Please check if the relieving temperature in data sheet is -18 C...? 
 

[CheAmine]

Fallah,

 

As mentioned in the data sheet, the relieving temperature is 18 C.

 

Please find attached the TSV data sheet and the simulation I developed.

 

Regards,

 

 

Attached Files

•  TSV data sheet.png   876.51KB   9 downloads
•  simulation.zip   44.9KB   44 downloads

[Art Montemayor]

CheAmine:

 

Why are you using HYSYS to find the saturation temperature of liquid propane at 29 barg?  This is a waste of time, effort and of HYSYS.  All you need to do is go to
http://webbook.nist....Pa*s&STUnit=N/m
and you will find the saturated liquid temperature as 77 ^oC.

I hope you don't also use HYSYS to find saturated steam values.

 

Thermal relief valves are placed on piping and some vessels in order to safeguard against the hydraulic expansion of a liquid - not against its temperature.  That is why the setting is for a maximum pressure value and not for a temperature value.  The relief valve doesn't care what the temperature is.  Your relief valve is set to protect the lowest MAWP vessel or piping in your locked-in liquid system - probably for the fire case.  I say this because if this is a typical propane refrigeration system, you would never reach the 29 barg value under normal operating conditions for the saturated liquid - even in your high pressure liquid propane reservoir which is located directly after your propane condenser.

I think you are making a mountain out of a mole hill and overworking the application of the purpose and design of a pressure relief valve.

 

The problem is a simple one of protecting the lowest rated equipment MAWP within the blocked-in system and the temperature that is related to that saturated pressure is normally an academic value.  If indeed you have a fire case, that is another problem that is far more serious and the scenario and circumstances have to be carefully studied to avert a disaster.

[CheAmine]

Art Montemayor,

 

Maybe I should have described the problem in more detail. The thermal relief valve is already overdue for inspection, and we can't take it for calibration/inspection since it requires a plant shutdown (There is no isolation valve provided upstream of the TSV). 

 

Lets assume now that there is a locked-in liquid in the system, and the pressure increased to be equal to the set point of the TSV due to high ambient temperature/heat leak. Then the TSV can either Work or Fail (since it is already overdue for inspection).  I just want to analyze if there is any possibility of the pressure inside the pipe to reach the set point of the TSV. For this I wanted to check the temperature that is required so that the pressure increase to the set point of the TSV.

[CMA010]

In case of thermal expansion of a liquid (liquid full system) you usually only need a temperature increase of a couple of degrees to reach set pressure. API 521 (§ 4.4.12.4.1) gives formulas how to calculate the pressure increase.

[fallah]

CheAmine,

 

In fact if the difference between the temperature at the time the liquid being trapped and the black body piping temperature is considerable and the thermal expansion rate of the liquid would be high; then the TSV might be opened several time during a day at the temperature intervals within mentioned overall temperature difference. So, there would be several relieving temperatures for such temperature relief valves...

[Art Montemayor]

What Fallah is asserting is the basic truth about what will happen when you trap a pipe segment filled with 100% liquid.  That is what TRVs do and what is expected of them.

 

I don't think that is what you want to tolerate in your refrigeration process - especially because the relieved fluid is combustible and flammable propane.  This brings up my question:  Why do you have a thermal relief valve installed in a propane refrigeration system?   In all my years of doing refrigeration applications, both in design and operations, I have never seen or heard of a situation where you would lock-in 100% refrigerant in a piping run.  I'm very curious to find out all the rest of this story.

 

I anxiously await your reply.

[CheAmine]

CMA010,

 

This is the equation I was looking for. I will do an exercise to find out the temperature required to increase the pressure up to the set point of the TRV. Thank you.

 

 

Art Montemayor,

 

The TRV is located between an ESD valve and a level control valve. Considering the worst case scenario, I think there is a possibility of a blocked-in liquid incase the ESD valve gets closed due to one of the interlocks and the control valve being closed by the panel operator (or closed due to instrument air system failure). In that case, the TRV might either work or fail if the blocked-in liquid is not drained.

 

I suppose if the TRV was located between isolation valves, then putting a locked-open tag for the isolation valves would be sufficient to overcome this problem.

[farid.k]

Personally, thermal expansion relief valve is needed when lines or equipment which can be left full of liquid under non-flow conditions and which can be heated (normally by means of sun radiation) while completely blocked-in.

For your case, I trust that it is not necessary to have thermal expansion relief valve in between ESD and control valve. I trust that during normal condition, control valve will regulate the flow and condition completely blocked-in/non flow condition is not happened especially in refrigeration loop system.

 

I did run the simulation though and found that in order the 100% liquid propane to be at 29 barg, required temperature is about 77oC too and when it relief to a 1.01 barg to flare system, temperature will be drop to -25 oC due to vaporization.

[Art Montemayor]

In order to stress my point, I repeat that I've never seen or heard of a TRV being installed on process piping within a refrigeration system.  My years of experience have shown me that such a device is not needed when the process is engineered and instrumented correctly.  The act of blocking-in any refrigerant liquid is to avoided and I've found that it is not required in a safe and reliable operation.

 

The key design point in any mechanical refrigeration system is to never block-in liquid within piping.  Liquid refrigerant is normally always blocked-in within pressure vessels - such as economizers, flooded vaporizers, and the usual reservoir.  Each of these blocked-in vessels are designed for saturated conditions and have an inherent vapor space which mitigates any hydraulic expansion.

 

The design and operation is that simple and it has been thus even before I got into engineering - which is 56 years ago.

My advice to all young engineers:  Always try to avoid the use of TRVs - especially those on toxic, flammable, or hazardous fluids.  Design your process and instrumentation such that you can avoid their use if possible.

[Ganesh Buddha]

TRV is very relevant to such operations...

 

please note that the value of -42 deg mentioned in the valve data sheet plays a pivotal role in determination of the relief temperature...

 

can you share the temperature considered in the simulation, as, if you have considered the process temperature in simulation which may not be same as the temperature mentioned on the valve data sheet you may as well get a new value from simulation.

 

when heated under isolation the first relief is important, as in this case there will be vapor generation more likely in subsequent reliefs and there will be considerable increase in relief temperatures thereafter...  

[shan]

C3 liquid turns into two phase flow after the pressure drop through level control valve.  The isolated piping section gas/liquid ratio varies with temperature and pressure.  No TRV is need because 77 C is not realistic ambient temperature.   

[fallah]

 No TRV is need because 77 C is not realistic ambient temperature.   

 

But the black body temperature can be as high as 85 C in some regions like some countries in middle east...

[shan]

The official highest recorded temperature is 56.7 C, which is measured on 7/10/1913 at Greenlan Ranch, Death Valley, California, USA per Guinness World Records.  C3 saturated pressure is 19 Barg at 56.7 C.  The piping section will never reach 29 Barg set pressure of TRV.

[farid.k]

Attached file tells how to determine the need of thermal expansion relief valve.

kindly refer attached picture. hope this help.

Attached Files

•  Untitled.jpg   180.07KB   28 downloads

[CheAmine]

I have used the equation provided in API 521 6th edition (provided in section 4.4.12.4.1). It is used to calculate the pressure rise due to simultaneous heating of the pipe and blocked-in liquid.

 

I have attached the excel sheet herewith. I have solved for T2 rather than P2 since the final pressure is known (29 barg). I have found that the final temperature of the blocked-in liquid that is required so that the final pressure of the blocked-in liquid reach 29 barg is -14.7 C !

 

Can any one verify my calculations.

 

 

Attached Files

•  Thermal Expansion Relief valve temperature calculations.xlsx   35.28KB   122 downloads

[shan]

How do you determine T1=-15C?

[CheAmine]

How do you determine T1=-15C?

 

Shan,

 

The normal operating temperature and pressure are -15 C and 2.9 barg.

[shan]

C3 is vapor at -15 C and 2.9 barg.

[farid.k]

C3 is vapor at -15 C and 2.9 barg.

C3 @-15 oC , 2.9 barg should be in liquid phase.

[breizh]

http://encyclopedia....or_Pressure.GIF

 

In case .

 

Breizh