6 replies to this topic

[muddu4]

Dear All,

 

I am doing some calcuations for depressurizing of Stablizer ovhd system. Please refer attached PFD for reference. Depressurizing valve is located on stabilizer ovh column along with PSV which is designed for fire case. Based on my understanding during fire  stabilzer reboiler would be affected causing rise in temperature further vaporizing liquid in bottom which will result in high pressure in column and to protect this we already have PSV in overhead line but as fire continues and PSV is pop-out we need to depressure the system to 100psi in 15 mins as per API 521.

 

In Stablizer ovhd condenser around 97% vapor is getting condensed (normal operation).

 

My concern is tube side being sea water and since it doesnt have any automatic valve can we take credit of sea water not being blocked in case of fire contingency and most of the vapors are getting condense.

 

Can we just consider only stablizer ovh receiver liquid volume for depressurization calculations  and size our valve based on receiver volume alone?

 

Hoping a positive guidance on this problem from our experts.

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[Bobby Strain]

Well,

   You postulate a scenario that was never intended for design of relief and depressuring systems. The PSV is sized based on blocked in equipment with liquid inventory that is subject to fire exposure. Depressuring is on the same basis sans relief valve flowing.

   And I note that you have not properly designed the column pressure control system. If you consider the response of the DP control valve on an increasing tower pressure, you quickly realize that the valve opens, when, in fact, the proper response is to close, directing more flow through the condenser. But, alas, it will work because the operator will raise the DP set point so that the valve never opens.

 

Bobby

Edited by Bobby Strain, 17 July 2014 - 10:23 PM.

[muddu4]

Bobby Strain - Thanks for the reply

 

i agree with you about PSV and despressurizing valve sized on same basis. But the idea over here to relieve more pressure with the help of this depressurization valve and correct me if i am wrong during depressurizing valve set pressure wil be less than PSV and it will react before the PSV opens & i guess we dont take credit of PSV.

 

DP control valve on by-pass line is for maintaining operating pressure of overhead receiver. Incase of low pressure in overhead line this DP control valve will open in order to maintain vessel operating pressure. Incase of high pressure in overhead system this DP control valve will remain closed directing more flow through condenser.

[fallah]

muddu4,

 

It cannot say that PSV and BDV on the same system like as you described have been sized on the same basis. Actually the PSV is to be designed to maintain the system pressure at the specified set pressure and the BDV will release the pressure to 50% of the design pressure within , let say, 15 min; then they have different functions. On the other hand, the PSV will be opened automatically when the pressure reaches to the set pressure while in most cases the BDV will be actuated manually by the operator or automatically by F&G system. One more thing; the PSV will never open before the BDV and in most fire cases the PSV will never open regardless of the BDV has already activated or not, because the PSV in fire case mostly will be installed for code compliance and cannot protect the vessel from failure...

 

DP control valve on by pass line has been installed there to maintain a specified differential pressure between overhead and condensate receiver. If the pressure in the overhead decreases, the DP valve will tend to be more closed to increase the pressure at upstream to compensate the decrease in differential pressure. On the other hand, if the pressure in the overhead increases, the DP valve will tend to be more open to decrease the pressure at upstream to compensate the increase in differential pressure...

[muddu4]

fallah thanks for prompt reply.

I totally agree with your point of view.

Can you please put some more light on design/relieving flowrate consideration  of this depressurization valve.

Currently the system is designed considering liquid inventory of fractionation column as normal bottom level,draw off tray capacity, plus holdup per tray plus 50mm

additionally liquid volume on shell side of condenser (1/3 tube space subracted) and inventory in overhead receiver.

 

Can we not consider overhead vessel alone assuming Sea water flow is not blocked incase of fire contingency.

[fallah]

muddu4,

 

Depressurizing study through which the peak flowrate and Cv of the BDV are to be specified, as a dynamic operation is normally being done with proper software like HYSYS. Then you will input the system info to the software and it will give you at least the peak flowrate and Cv of the depressurizing valve by which you can size the depressurizing line and relevant valve. Therefore you can read API 521 to get the concept and then using the HYSYS to get the required data bases on your system characteristics and parameters.

 

Regarding possibility of taking credit for sea water cooling in fire case, you should refer to "Depressurizing Sequence" of the plant (and fire zone) in which the Stabilizer is included. If, for example, after fire occurence in relevant fire zone the total power off will be happened then one cannot take credit for such cooling because sea water pumps have been tripped. Of course, in general being or not being such cooling services available, the initial pressure for depressurizing will be considered as vessel design pressure means effect of such cooling even if being available would had already been ignored in relevant study and calculations...

[Jiten_process]

Just a small point to add. Even if you want to take credit of condenser, see that, the condenser performance is not same at relieving pressure as it was during normal operating pressure. Relieving pressure is higher than set pressure that means higher temprature and less LMTD. So is true with other properties contibuting to heat transfer phenomena,  and you might not even have column in equilibrium during relieving scenario. That means, condenser will not condense gases at the rate it was condensing during normal operaiton even if you have cooling medium flow persistent. But when we talk about fire, its actually much worse scenario rather than just a PSV pop up with rated flow. Normally for PSVs on column, relieving rate calculation during other abnormal scenarios like reflux failor or cooling failure etc are studied on simulators running column simulation. This is also mentioned in API-521.

Edited by Jiten_process, 07 August 2014 - 07:12 AM.