Distance between blowdown stacks in gas pressure boosting facilities 5

[abdolmaleki1975]

Hello dear friends and experts;
In a natural gas pressure boosting station, it is necessary to add a blowdown stack for emergency evacuation of the compressor station gases.
While in the area there are two stacks of incoming pipelines (56 and 40 inches), so in order to locate the third stack, we need to know the safe distance between the stacks.
Of course, I referred to the API 521 standard of the 2014 version, but the distances seemed very strange, and because I did not know the safety standards, I need help.
Thank you for your guidance.

 

[pmover]

think about the quantity (mass flow) being vented and the pipe size need to vent the gas within the station piping as rapidly/safely as possible to a non-hazardous area.

What pipe code is being used?

see attached image for location of blowdown stacks - there are 3 16" diameter blowdown stacks for these plants in this compressor station. 30 & 36" pipelines.

 

[1503-44]

Locating them away from property lines may be advantageous.
You can do a dispersion analysis to predict where the explosive limit boundaries are located under various wind conditions.

 

[abdolmaleki1975]

Hello dear pmover,
As I said, with a 16-inch line we want to vent the gas in the area of the gas station (between the inlet and outlet valves to the shed where the compressors are located)While in the common area with the inlet and outlet pipeline of the station complex, two other stacks are already available and I want to know how far I should be from these two stacks to locate the third stack. It should be noted that all three stacks are 16 inches
The pipeline code is Asme B 31.8, but as I said, for safety distances as well as considering the prevailing wind, I referred to the api 521 standard, which I thought was very strict.
Sorry I did not notice that your three 16-inch lines are vented at one point or from three points (in this case, what is the distance between those three points and how is this distance calculated?)

 

[1503-44]

What distance did you determine from API 521?

https://r5.ieee.org/houston/wp-content/uploads/sites/32/2017/10/EAC-101-October-3-2017-rev1.pdf

 

[abdolmaleki1975]

 

[abdolmaleki1975]

 

[abdolmaleki1975]

Dear 1503-44,
I want to specify the location of the new stack according to the existence of two existing stacks
I mean, I want to know what the safe distance should be from these two dear stacks
And also about the height of the new stack, how high it can be.
It should be noted that the existing stacks are about 3 meters above the ground

 

[1503-44]

There is no guidenence on stack location that I know about.
They must be placed in a safe location outside electrical classified areas. See my link above.
Blowdowns will require their own classified area, within which ignition sources shall not be permitted.

The Two important things to consider are

1) You must not create fire hazard. Locate them away from ignition sources. Safe horizontal distances and adequate heights are determined by dispersion modeling. For continuous and emergency flaring, the flammable gas concentration should be below 20% LEL at the various points, assuming the most conservative wind speed (of all the locales). I include all points on the nearest property line.

Stack design, blowdowns and flares. Blowdowns should meet the 20% of LEL requirement.

https://www.engstack.com/kb/design-philosophy-flare-vent-relief-and-blowdown-systems/

2) Max Noise levels must be within OSHA or whatever your country's regulator determines is permitted.

Noise and silencers

http://sandv.com/downloads/1607pras.pdf

 

[LittleInch]

I'm the same - no specific distance or spacing.

Normally you space them based on knowing whether they are all capable of discharging or not at the same time or if you can guarantee only one at a time.

Also most nowadays work on the potential for ignition when venting. So even if they are a vent, assume that the gas could ignite and locate it accordingly. Those vents at the moment don't look like they conform to that possibility.

Then you have dispersion analysis and usually for a blowdown type the worst case is at low flow where the cloud can get down to ground level. Full flow - no problem.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[abdolmaleki1975]

Thanks to my good friends LittleInch and 1503-44,
Unfortunately, I have not yet found the answer to the problem, but I am trying
It should be noted that the flow in the existing stacks that come from the pipelines is planned and is done in the time of pigging is a full flow. And the flow in the [highlight #EF2929]new line[/highlight], which is designed in the case of compressor shot down, which will not be programmed, which is also of the full flow type.
What do you think about the Api 521 standard?
Is there an effective approach?
Anyway, thank you very much for your help, dear friends and experts

 

[1503-44]

You can use 521 for the valve sizing, if you want to. It is a "voluntary standard".
I have always used the Emerson sizing procedure.

API 537 may have some answers to your stack questions. I do not know the details about what that API contains.

HC Engineering article about the 7th release of 521.

https://www.hydrocarbonengineering....pi-publishes-7th-edition-of-api-standard-521/

 

[GD2]

abdolmaleki1975,
There is no any known standard for flare stack separation distance.
Make sure that it is outside the electrical classification area, the stack is high enough to meet the radiation level per API 521, there is no toxic hazards from H2S and So2 dispersions reaching out to on-site and off-site personnel, and no likelihood of flammable gas ignition from cold vent or flare flame out.


GDD
Canada

 

[georgeverghese]

Why do you say

"It should be noted that the flow in the existing stacks that come from the pipelines is planned and is done in the time of pigging is a full flow."

Normally, there should be no full flow to vent / flare stacks during planned pipeline pigging. We do this by control of the incoming pipeline flow to match the downstream process by means of a special DCS operated choke valve on the pipeline pig receiver bypass ( which is brought into operation by operator at DCS a few hours before the pipeline liquid slugs and high pressure arrives at the onshore gas terminal), so there is no excess flow to stack due to high pressure or high level in slug catcher (and/or any other constraint process parameter in downstream process units).

 

[abdolmaleki1975]

Dear georgeverghese,
This plant is not a gas refinery but a gas booster facility
During time Pigging,I think that the gas flows out of the existing stacks with full flow and the flow is not dead or low.
By the way, this explanation is in response to my dear friend LittleInch
I thank and appreciate all the loved ones, especially 1503-44, for their answers and attention

 

[1503-44]

You probably won't want full flow during actual pigging, as controlling pig velocity to reasonable speeds may mean using flow rates much lower than full capacity of the pipeline, but in any case your PSV needs to be designed for that full flow possibility. Overpressure from any source or reason is most likely during times of high flow rates, which also means highest pressures. High flow rates at a stack can also be experienced due to blowdowns required to quickly reduce pressures when evacuating a pipeline for imperative safety emergencies, or maintenance reasons. Dispersion modeling should include all possible stack flow rates and pressures, from 0 to max.

A black swan to a turkey is a white swan to the butcher.

 

[1503-44]

And now I am not totally clear about the purpose of this blowdown.
Is this for compressor station blowdown alone, or a pipeline blowdown, or a combination of both?

If it is for compressor station blowdown, the valve should not be a PSV. Use a fully opened ball valve, set to open fully when the ESD is triggered. Size the valve for the the blowdown of the compressor station when at the compressor station's design pressure.

If this is for pipeline blowdown, you probably won't want full flow during actual pigging, as controlling pig velocity to reasonable speeds may mean using flow rates much lower than full capacity of the pipeline, but in any case your valve needs to be designed for that full flow possibility. High flow rates at a stack can be experienced due to blowdowns required to quickly reduce pipeline pressures when evacuating a pipeline for imperative safety emergencies, or maintenance reasons. For a pipeline blowdown, usually a manually operated ball valve will do, if you want either full open, or full closed positions. To control pig velocity, it may be better to use a plug valve, as you will get better flow control at partially open positions.

If you want continuous overpressure protection, a modulating pressure relief valve usually does that job. It is set to the pipeline's or the compressor station's design pressure. If the design pressures of the pipeline and compressor station are different, then you will need two valves, but it is probably possible to connected to the same blowdown stack.

So, what exactly do you require?
Pipeline blowdown? = Ball or plug valve, manual may suffice.
Emergency Compressor station blowdown? = Ball valve connected to auto open via ESD.
Non-emergency Compressor station blowdown? = Ball or plug valve,, manual may suffice.
Pipeline or compressor station overpressure protection = Modulating pressure relief valve.


A black swan to a turkey is a white swan to the butcher.

 

[abdolmaleki1975]

Dear 1503-44,
Thank you very much for your attention
But our problem is how far the new blowdown stack is from the two existing blowdown stacks. And that the new blowdown stack is only designed to connect to the compressor station ESD system.
So forget the process design of valves and pipeline problems because this has been done and the current issue is the location of the new stack.
With best regards

 

[LittleInch]

abdol,

We keep telling you that there is no set distance and you keep asking us the same question. Not everything is allowed for in the design codes - they expect design Engineers to design things, not just be able to read a code.

If you're that concerned about it just put the new vent stack on the other side of the facility in an area where if it catches fire when it vents it won't burn your plant down. Then do a dispersion study on your new vent to make sure you don't create an explosive atmosphere at ground level, especially towards the end of the blowdown or if the wind blows the "wrong" way.

Also someone is telling your a version of the truth about the use of those vents during pigging. At best they simply blow down the pig traps, but there is no way anyone releases gas at full flow to allow pigging operations to be undertaken. apart from the cost and environmental damage you're creating a hazard, especially with them being so close to the pipework where people will be working.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[abdolmaleki1975]

Hello dear LittleInch,
The problem is that we are in an urban area and it is not possible to buy extra land. Also, the problem of the prevailing wind is another reason.
The only safe area I can use is the area inside the attached photo, which unfortunately already has two more stacks in place.
Anyway, thank you for your attention, friends, I think I should involve my safety and electrical engineering colleagues in my organization in this issue. Or refer this post(thread)to a safety engineering association in this forum.
Best wishes