7 replies to this topic

[BWang]

Hi,

I was confused on this topic described as in the subject when I was doing the depressuing calculation via Flarenet for blow down valves in a closed flare system.

Normally I picked up the peak flow of depressuring calculation from either Hysys or certain spreadsheet to input the blowdown scenario in Flarenet. However that peak flow is obtained from initial pressure depressuring to atmosperic pressure, which there is no back pressure issue. What I really concerned is in the closed flare system, if there is much higher downstream pressure for blowdown, sometimes even higher than the depressuring pressure, can I still use that peak flow as a correct one or not?

What I had done for those cases is to assume a backpressure, then calculate the peak flow in Hysys and input it into flarenet, then get a new backpressure from flarenet to calculate the new peak flow in Hysys. Using Trial and Error method (iteration) until both of backpressure and peak flow are match in Hysys and Flarenet.

I don't know if I am in the correct path regarding to this matter. Hope to get your feedback or advices.

Thanks for your time in advance.

Regards,
BWang

[ashetty]

Hello,

I'm not sure what a closed flare system is but I'm assuming it is a closed drain system. Normally, all systems depressure to atmosperic pressure, so I'm a little confused. If I am wrong, please let me know. But back pressure exists even when flaring is to atm pressure, based on relieving rate, length of piping etc. So your flareheader will have to be Sized accordingly. Just make sure backpressure at any PSV oulet is within the norm (10% for conventional PSVs etc).

Thanks

[ashetty]

Sorry...last sentence in previous post should read as 10% of PSV set pressure for conventional type.

[shan]

Hi BWang,

In the critical flow conditions, back pressure has no impact on the released flow rate. The close flare system should be designed so that all relief valves and blowdown valves release in the critical conditions in all the cases. If not, you may have to install a restrict orifice to limit blowdown flow or increase flare header sizes.

It is impossible for a lower pressure release to flow through higher pressure piping section. If it is true, when you open the blowdown valve, gas would back flow into your container through the opened valve, because flow direction is always the same as pressure gradient.


Regards

Shan

[BWang]

Sorry...last sentence in previous post should read as 10% of PSV set pressure for conventional type.

Thank Ashetty for your comments.

But what I referred to in my case is blowdown valve rather than PSV.

Regards,
BWang

[BWang]

Sorry...last sentence in previous post should read as 10% of PSV set pressure for conventional type.

Shan,

Thanks for your good feedback. I think I got your thoughts.

Thanks again.

Regards,
BWang

[JoeWong]

...In the critical flow conditions, back pressure has no impact on the released flow rate...

I agree with Shan in general.

(Infact under critical flow via restriction orifice...back pressure do have influence on the flow rate... Ignore this statement if it confuse you...)

...The close flare system should be designed so that all relief valves and blowdown valves release in the critical conditions in all the cases...

This is the ideal solution. However, in reality, many blowdown line (valve with restriction orifice) would see critical flow. However, there are still some in the system (those with lower pressure) would see subscritical flow especially during total plant or zone simultaneous blowdown.

You may consider the following...

i) Consider ATM in HYSYS for depressuring
ii) Let Flarenet calculate the back pressure at each BDV/RO
iii) Check each BDV/RO critical pressure is higher than back pressure. If yes, no problem.
iv) If not, you need to adjust the back pressure in HYSYS and rerun Flarenet again.
This significant reduce you iteration.

Backpressure higher than system pressure would result back flow. You may have tie to low backpressure point which is further downstream of the flare header...

HTH.

[BWang]

JoeWong

What I did for the blowdown scenario in the flare system is very close to the situation you commented.

In my case there are 7 blowdown valves depressuring at the same time under the different initial pressure individually. Meanwhile the RO is not permitted in the downstream of BDV based on our client's DEP. So I could not ignore the impact of the backpressure on the peak flow at the beginning and did the iteration between Hysys and Flarenet. I was not confident in what I did at that time.

Now I am feeling better. Thanks for your feedback.

Regards,

BWang