8 replies to this topic

[Antonieta]

Dear All,

I am new in this chat but I have been reading your comments on Hysys dynamic Depressurisation which are really worth to read! Please Joe could you send me the procedure when you have it to gladyssoto7@hotmail.com?

I used Hysys dynamic Depressurisation to determine the minimum fluid temperature dowstream of the valve. Very low temperatures (-83C) were calculated using a vessel U value of 5W/m2K and isentropic eff of 90%. Please could you advice how I can calculate the mean metal temperatures of the flare piping dowstream of the vessel valve? I suspect that Hysys could do it, if not any of you have any information about it?

Thanks in advance,
Ann

[Zauberberg]

Antonieta,

In order not to repeat the whole discussion from Eng-Tips forum, please refer to the following page:

http://www.eng-tips....d=202829&page=1

You will find there some useful conclusions and references.
Regards,

[Antonieta]

Zauberberg,

Thanks a lot!!

[Zauberberg]

Check these articles as well, they offer some further assistance:


[attachment=631:FLARE_SY...MPORTANT.pdf]
[attachment=632:Practica...e_Design.pdf]

[Antonieta]

Zauberberg,

Thanks a lot! I've been reading the posts and I see this is a complex problem. Anyway, do you know where I could find an equation to calculate the mean metal temperature?

Thanks in advance,
Ann

[JoeWong]

Dear All,

I am new in this chat but I have been reading your comments on Hysys dynamic Depressurisation which are really worth to read! Please Joe could you send me the procedure when you have it to gladyssoto7@hotmail.com?

I used Hysys dynamic Depressurisation to determine the minimum fluid temperature dowstream of the valve. Very low temperatures (-83C) were calculated using a vessel U value of 5W/m2K and isentropic eff of 90%. Please could you advice how I can calculate the mean metal temperatures of the flare piping dowstream of the vessel valve? I suspect that Hysys could do it, if not any of you have any information about it?

Thanks in advance,
Ann

Ann,
There is no fast rule for this subject as far as i know...

Cold gas passing BDV/RO, metal may/may not experience low temperature (-83 degC). Reasoning...

May experience low temperature
- Just downstream of BDV/RO, cold fluid will get contact with flare piping and slowly pull heat from the flare piping, Piping temperature will gradually approaching fluid temperature. If fluid contain cold liquid, high heat transfer of liquid will speed-up the cooling effect by vaporization.

May not experience low temperature
- if the relieving fluid is in cold gas form, low heat transfer coefficient and high velocity gas will minimise the contact time and heat transfer efficiency...

Other factors affecting the metal temperature
- there will be heat input from ambient as well (sometime it is negligible) to increase to maintain metal temperature.
- the vessel pressure will start to drop after the depressurization is initiated, less JT is expected and fluid temperature gradually increase with time.
- flare piping will have it latent heat which tends to slow down the cooling effect
- heat conduction from flare piping support

ALL these are dynamic and inter-related.

I don't think there is any fast rule for mean metal temperature. You may check with the expect who advised you. I personally willing to learn from him/her.

Thanks for reading my posts. I am compiling some info related to depresurisation in the link suggested by Zauberberg...

JoeWong

[Antonieta]

Joe,

Thanks a lot for sharing your experience with us. I am concerned because not using metal latent heat to estimate the metal temperature, instead:
-the heat transfer coefficients,
-Tamb=2C,
-Tgas=-55.6C and
-piping dimensions=12".

As you said "relieving fluid is in cold gas form, low heat transfer coefficient" (U= 6W/m2K in free convection, U=40W/m2K in forced convection) "and high velocity gas" (Reynold number= 7E5) "will minimise the contact time and heat transfer efficiency".

These results give a Tw,internal=-51C (free convection) and Tw,external=-7.53C which means a DTw= 43C and a mean metal temperature of -29C. Could this DTw be related with the latent heat of the material to see if it is reasonable?

Thanks in advance,
Ann

[JoeWong]

Joe,

Thanks a lot for sharing your experience with us. I am concerned because not using metal latent heat to estimate the metal temperature, instead:
-the heat transfer coefficients,
-Tamb=2C,
-Tgas=-55.6C and
-piping dimensions=12".

As you said "relieving fluid is in cold gas form, low heat transfer coefficient" (U= 6W/m2K in free convection, U=40W/m2K in forced convection) "and high velocity gas" (Reynold number= 7E5) "will minimise the contact time and heat transfer efficiency".

These results give a Tw,internal=-51C (free convection) and Tw,external=-7.53C which means a DTw= 43C and a mean metal temperature of -29C. Could this DTw be related with the latent heat of the material to see if it is reasonable?

Thanks in advance,
Ann

Antonieta,

Based on what you have mentioned (i don't into detail), i have the feeling that you have considered steady state heat transfer and calculate all data accordingly. Haven't you ?

If so, i would like to highlight to you that entire process is transient and dynamic. You calculation is referring to steady state one point at a time.

Assuming you are correct, Tw,int = -51 degC, Tw, external = -7.53 degC, i wouldn't take advantage of mean / average metal temperature (Tw,avg=-29 degC) for safety reason. But you should discuss this issue with your piping material engineer to expedite if your assumption would be fit specifically the area experiencing low temperature...If you designed to ASME code, and the maximum operating pressure is not too high, your piping material engineer may consider to take advantage of low temperature low stress allowance.

Metal latent heat basically is heat content of metal pipe before it is cooled. Say for example, the metal initial temperature is 20 degC and fluid is -55.6 degC. Then the DT should be the different between these two figures.

I regret to tell you that if i were you supervisor, i wouldn't encourage and accept a manual calculation (steady state) to understand the behavior of a dynamic system. I would encourage my engineer take more conservative approach. If i opt to make some cost saving, i would take this issue up with a thermal specialist and request a detail & rigorous transient modelling for confirmation. I would encourage my engineer to take more effort on operation system to minimise the temperature drops.

Hope this little inputs help you.
JoeWong

[Antonieta]

Joe,

Thanks a lot! you are rigth, I was trying to have a manual calculation to confirm the results of the transient calculations (we are waiting for that), at least to have an idea of the range of results I should be expecting.

Regards,
Gladys