Pipeline depressurization time calculation 1

[cloud19]

Greetings. My team is working on a project to make a branch connection on a 16" natural gas pipeline. For the execution, we need to depressurize 15 kilometers pipeline. This will be done with a 2" pipeline that is connected to flare. I need to roughly estimate the time required for this task. It does not need to be highly accurate, a good approximation will be enough. I have never done this kind of depressurization calculation before and honestly I am not finding many references. Please guide me on which co-relations to use, or which reference to use. Thank you for your time.

 

[pierreick]

Hi,
Consider the doc attached to support your work, the storage capacity will be the capacity of the 5 km pipe DN 16", and the leak size will the depressurization pipe DN 2"
EDIT: Be careful with the temperature @ the outlet and the resistance of the pipe to low temperature, 2 inches seems too big to me, you need to have a way to control the release (valve +orifice)

You should be able to perform the calculation using excel.


Good Luck
Pierre

 

[cloud19]

pierreick, thank you for your response. Pipeline initial pressure is around 800 PSIG. We need to totally depressurize so the final pressure is 0 PSIG. The composition can be considered as 96% Methane and 3% Ethane and rest Propane+Butane.

 

[pierreick]

Hi,
You may want to use this excel sheet for your product (mixture)
Molecular weight: 16.81 g/mol; cp/cv mixture :1.3011
Volume of the pipe to be released: 1710m3

Increase the time as necessary Add cells and copy and paste the calculation), control the temperature at the outlet by decreasing the diameter of the release as explained in my 1rst reply.

Note: Why don't you sell the product to downstream customer, better than to flare it,this is the best way to drop the pressure in the main header (16") before depressurization to minimize the risks and save time?

Good luck
Pierre

 

[MortenA]

@OP free consultancy service is always nice, but what will you say if its wrong? I would recommend that if this is of any importance to you you fork out the dough and get a consult to do the calculation. If you just plan to wait anyway then go ahead...

Best regards, Morten

--- Best regards, Morten Andersen

 

[LittleInch]

To do anything you need to first find out the maximum capacity of your vent / flare and how the pressure is let down. Also how they deal with the low temperatures you will get initially when you open up the vent / flare line valve

Without this information no calculation will work.

Best practice now is to use a portable piston compressor and pump the gas around the block valve to eliminate waste and gas emissions.

Part of your problem will be getting to zero. The time taken to get to say 2 or 3 bar will double to get to 0 barg. Even then gas will continue to emerge. Then you need bags and local vents?

Why are you not just hot tapping this line?

But a 50-60 bar 16km 16" through a vent - allow 3-5 days is my initial estimate to absolutely no flow.

There are many programs which will do blowdown calcs. I suggest you find one or employ a FA engineer to do it for you.

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

 

[georgeverghese]

There are many operational and safety issues here - best to farm this exercise out to you local engineering company and assign an Owner's Operations representative to this project also. Watch out for low temps on this 2inch depressure line and on the plant vent collection headers. If the flaring volume is small, and pressure drop through the 16inch line is minimal, then you could consider this 16inch line as a single lumped volume, and then you could use the standard batch depressure routine in Hysis or Simsci-ProII. Else specialist pipeline depressurisation software will be required.
Thats a lot of gas to flare - your Company may be obliged to report this flared volume to the Govt.

 

[pmover]

an option to consider and depends upon length of downtime and pipeline configuration.
in this section of pipeline, if there are take-offs for serving customers, consider isolating the pipeline and letting the pressure gradually decrease while consumers withdraw the gas. upon which the pipeline pressure reaches min pressure, change supply to take-offs, and then bleed remaining gas to flare. objective is to minimize gas loss.

LI is correct. additionally, venting to flare means having a large clear area and other analysis.

 

[ReonArd]

I had a similar situation a while ago. It requires an iterative calculation, because the pressure profile is continuously changing in time. Consider the following:
- To control the flow, you will need a valve; for the calculation I assumed a 2" globe valve with Cv = 48
- As Littleinch has said, if you aim to reach zero pressure, the depressurizing time will effectively be infinity. For practical reasons set it to something that is safe a practical (i.e. you can open the system). I assumed final system pressure of 15 psig. If you choose almost zero, the time required can go to days.
- Temperature - depressurizing will create a delta t.

Given all that, I ran some initial estimate for 1,767 m3 of methane at 25C depressurizing from 800 psig to 15 psig through a 2" globe valve and get 6.8 hrs. See the calculation here:

https://sheet.zohopublic.eu/sheet/p...a8b6ce0892576a847f321f64b4085fb99f0?type=grid

 

[chicopee]

Shouldn't there be a purging cycle once the pipeline is depressurized of flammable vapor. I have used some element of pierreick's equation to pressurize a large air tank using the spread sheet to calculate the time required to achieve the required pressure.

 

[1503-44]

You could hottap the connection without purging, as just a reduction of pressure is all that may be necessary for a hot tap. If cutout is required, two hottap stopped placed on either side of the cutout will save all that purging expense.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."