6 replies to this topic

[mangesh]

Need Help,

i am doing calculation for thermal relief valve
i got the formaula from API for calculating the relieving rate but got stuck with relieving temperature

i need help to calculate relieving temperature in thermal relief case for tube side TSV (shell and tube heat exchanger)

i came across some thumb rules
like 1) 10 to 15 degC + operating temperature degC = relieving temperature degC
2) hot side inlet temperature can be relieving temperature of TSV

but no one confirming wheather this kind of information is part of any STD

more information regarding problem see the details

shell side

fluid name blowdown condensate
flow 5.09 kg/s
inlet/outlet 213.7/45.0 degC
pressure inlet/delta P 20.5/0.7 barA

tube side

fluid name demin water
flow 58.9 kg/s
inlet/outlet 40/55 degC
pressure inlet/delta P 7/0.5 barA

TSV on tube side

Regards,
Mangesh

[fallah]

Mangesh,

In the event of tube side blocked-in and blowdown condensate continue flowing through shell side, demin water trapped in tube side will continuously heated-up by liquid in shell side. Then the pressure in the tube side will rise until TSV set pressure and demin water is to be relieved due to thermal expansion. Accurate relieving temperature might be obtained via a complex calculation and may be done by a trial and error method.
Actually, blocked-in volume liquid in tube side would be heated-up from operating pressure/temperature up to set pressure then you should find the corresponding temperature at the relief point as relieving temperature.

Edited by fallah, 18 December 2012 - 07:49 AM.

[S.AHMAD]

1. Why is that you want to know the fluid relieving temperature?
2. In sizing the TSV, we do not have to know the relieving temperature since it is changing over time. It does not remains constant over the relieving period.
3. Normally we use the exchanger duty to determine the relieving rate and by using this approach, the TSV is over-sized and considered as the worst case scenario.
4. For determining properties of fluid (density, specific heat), take the average temperature between inlet and outlet. Or if you want to be more conservative, use the average temperature between minimum and maximum possible temperature.
5. In most cases, 3/4" TSV size is sufficient.

Edited by S.AHMAD, 19 December 2012 - 04:15 AM.

[Anup87]

Hi Mangesh,

Normally we check the applicability of scenario based on,

does hot side temperature is capable of vaporising cold side liquid blocked in. If it is not exceeding the bubble point of cold side fluid then your scenario will be not applicable. Since, liquid is incompressible and it will not result in large expansion of liquid.

For case, where hot side is capable of vaporising cold side fluid. We shall take relief temperature as bubble point of cold side fluid at relief pressure.

Thanks,
Anup B

[S.AHMAD]

Anup

Please check your understanding with regard to liquid thermal expansion and the requirement to have TSV due to block-in scenario.

[Anup87]

Dear all, I am extremely sorry to quote wrong. Thanks Ahemad for correcting me.

Yes API-521 talks about having a PSV for liquid expansion and says having PSV of 3/4 X 1 for cases where minimal relief is expected. And for other cases where relief is expected to be large we can consider relief temperature conservatively as worst as possible, which will be hot side inlet temperature. Please correct me if this is not the case.

However, I am not sure why we don't give PSV for liquid expansion at our organisation. I will try to get the answer here.

Edited by Anup87, 21 December 2012 - 12:01 AM.

[S.AHMAD]

Anup

Congratulations!.
Not many people are willing to admit for making mistakes. You are one of those good guys.