8 replies to this topic

[marthin_was]

Hi all ,
I have project to install new vent stack to release steam from steam PSV's and to release hydrocarbon from production line PSV's. Of course the stack will working intermittently, only when the PSV's popping. The vent line for each fluid (steam and HC's) is coming into different nozzle on the vent stack. My question is .. how to determine required vent stack height and diameter, because in this case we have two different fluid to be released, and the probability to released it at the same time nearly zero. As we know together, that steam is harmless and not flammable, what is design criteria to design vent stack for this case? I've read API 521, and my conclusion is to size diameter of the stack is to meet max Ma number = 0.5, but I've no clue about the required height calculation, because in my opinion we don't need to dispersed the fluid with air. But how about the HC's? to be honest, I still not clear yet how to use fig 3 (the figure X = velocity ratio, Y = vertical plume distance factor) in API 521 RP for calculating vent stack height..? on code it said, to get well dispersed vent gas, the height needed is around 120 Diameter vent stack? Is the reference for 120 D is from the ground? or for calculating vent stack height we need to simulate using Gauss equation?

And last question, what is the criteria to choose whether we have to use flare stack rather than vent stack, if the case is about LHV, what is minimum LHV to determine we have to use flare stack to dispose the vent gas..?

thanks a lot for your input

[JoeWong]

Hi marthin_was,
Just some inputs for you...

For a vent stack, at least 3 main criteria shall be met :
i) Toxic / flammable component well dispersed during Normal operation (Nor. Op.) and emergency situation (Emer. Op.)

ii) Radiation due to ignition of lightning (Emer. Op.)

iii) Noise level at receptor meeting noise criteria during Normal operation (Nor. Op.) and emergency situation (Emer. Op.)

If some spill over control PCV tie to vent header and release via vent stack, fluid exit temperature may be a concern in some environment sensitive area.

In your case, probably, only the emergency operation criteria shall be met.

Item (i), it is very much subject to components / species in vent gas. Local regulations will probably indicate the limits of various components. In additional, Material Safety Datasheet (MSDS) will probably indicate the limits. PHAST, FRED, etc can be used for dispersion study.

Item (ii). In the event, vent gas is HC and potentially ignited by lightning, radiation study shall be conducted to ensure radiation at the receptor (human) point is meeting criteria e.g. 6.31 kW/m2. In addition to the radiation level, the temperature of receptor (equipment) shall not damage the equipment. FLARESIM, FRED, etc can be used for radiation study.

Item (iii). Noise level at receptor (human) is meeting criteria e.g. 115 dBA with background noise. Keeping in mind, some plant owner may impose more stringent requirement i.e. 90 dBA @ receptor (human). FLARESIM, FRED, etc can be used for noise study.

The recommendation of Mach = 0.5 probably a good start for vent diameter. Vent stack height will subject to above criteria, location of vent stack, plant layout, wind speed, wind direction, etc.

Flare stack is probably much better than vent stack if the release fluid contain HC from environmental point of view e.g. impact to greenhouse effect for 1 mole of C1 is about 20 times of 1 mole of CO2. However, flare stack may be costly.

LHV requirement probably only applicable to flare stack. API521 has stated some value i.e. 300 Btu/ft3. My experiences informed me that the value subject to type of tips. Flare tip vendor probably the best candidate to advise you. The API figure is a good start for you.


Hope this help.

JoeWong

[marthin_was]

thanks Joe,

What I mean, on fig 3 API 521 RP, I dont know how to determine the minimum elevation needed, because in my understanding, the figure shown plume height, not the stack height..

thanks,
Marthin

[JoeWong]

I think you may work out the locus (e.g. 30% LEL), place you stack exit point in your plant layout (preferabaly front view) so that the locus will not reach the electrical device.

Wish other experience Che Jedi can elabrate further.

HTH

JoeWong

[chemsri]

USE flaresim

[Root]

thanks Joe,

What I mean, on fig 3 API 521 RP, I dont know how to determine the minimum elevation needed, because in my understanding, the figure shown plume height, not the stack height..

thanks,
Marthin

Hi,

In addition to above points,
1-Check Back ground limits.
2-Calculate the Stack hight based on pollution index and near by building hight(locality or near by residential area back ground limit which is set by local authority).
Hope this will help you little more.
Cheer

[AZIZ_MN]

hi,
i am curios about this kind of vent stack,

1).ur plant is running with full load and u need maintanance for vent stack how u will manage, do u have any diversion of vent header
2).if steam to vent flare is not available then what? hydrocarbon will released without steam dilution, ofcourse u will have
knockout drum and sealing pot to avoid liquid carryover but still vapour will release to atmosphere,
as per me stack hight shoud be 100mt to desperse vented hydrocarbon far away .

[breizh]

Dear Martin ,
Let you check the regulation . For example in China , The GB standard is mandatory .
Hope it helps
Breizh

[S.AHMAD]

thanks Joe,

What I mean, on fig 3 API 521 RP, I dont know how to determine the minimum elevation needed, because in my understanding, the figure shown plume height, not the stack height..

thanks,
Marthin

Hi Marthin
You are right that Fig 3 is for determining the plum rise/height. In order to determine the stack height you need to choose the worst case scenario between dispersion and flare radiation assuming the vent stack ignited by lightning or static.For dispersion use 0.5LEL as the boundary limit. For radiation API 521 gives the radiation flux that is required. After you have determined the distance from the center of the plum only then you deduct the plum height from the total height to get the stack height.

I hope this short explanation helps.

Best regards

S.Ahmad