Stuck Pig/Chocked flow in liquid Butane pipeline

[Iradah]

Hello all, I need your urgent help on this.

We got a stuck pig in our 6" Butane transferring pipeline of 30 km length. The pig is stuck and blocking the line but allowing for our daily production flow to pass through the very small areas around it (the pig is not 100% sealing the diameter of the pipeline). We want to push this pig out of the pipeline by applying large delta P across it. The inlet pressure to the pipeline is coming from a centrifugal pump. The outlet pressure at the other side is controlled by a pressure control valve to maintain the liquid state of butane.
The proposal was to drop the pressure downstream the pig to atmospheric by flaring. I was trying to know how much flow we will get at this delta P assuming the upstream pressure will not change.
I can see that keeping the upstream pressure constant, Butane will total evaporate at pressures below its vapor pressure and a chocked flow will be created.

My question is what will happen once we reduce the pressure downstream to atmospheric:
1. No change will be observed at upstream conditions, the pressure and flow will remain the same as no more flow can pass through the restricted area, liquid will be evaporating at this area which will cause cavitation and damage to the pig and pipeline?
OR:
2. The upstream pressure will be the same but the flow will decrease to the quantity that can pass through the restriction without being chocked, however, still flashing will occur at the restricted area?
OR:
3. The upstream pressure will increase to the maximum the pump differential head allows for and the flow will be decreased to the chocking limit?

I appreciate your urgent help on viewing what will happen to the upstream conditions, what flow and pressure changes will happen when I drop the pressure downstream in comparison to the conditions just before reaching the maximum flow (Chocking conditions). Also, what risks are associated with this high delta P and flashing to the pipeline.

 

[LittleInch]

Please please do not do this. The low temperatures will damage the pig and could rupture your pipeline.

You have flow so use another pig with a taking device to find the blockage.

Look at the design and see if you can see where the blockage might be, usually bends or low spots where debris could gather. Plotting pressure versus flow might get you some idea where the extra pressure drip is. If you have access to the pipeline at block valves plot the head profile along the line.

You might need to depressurise and cut the bit of pipe put, but this is stool better than your proposed action. You will flare a lot of gas and then be left with a pipeline with a lot of vapour in it. The only way to get it working again is to put a pig in which people will be reluctant to do.

I repeat, your proposed action is very risky and given that you have flow, you have clearly got a large hole past the pig. Have you tried flowing backwards even if you need to bring in a pump or change some pipework? It's still a lot beret than flashing butane across it.

Let us know how you get on and what caused the blockage, usually construction debris and bends or elbows too tight.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[LittleInch]

The other thing is that I'm not sure you will actually generate the differential pressure you think as the butane will vapourise to maintain it until you've got rid of all the liquid and then the gas back pressure would be quite significant when your bust pumping it in at the max flow. You'll be flaring two or three times the liquid volume flowing in to try among get a large differential across the pig, which by that point will either frozen in place or busy disintegrating....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Iradah]

LittleInch,
We have reviewed the pressure vs. flow and we think we know where the pig has stuck. We are now excavating at the anticipated blockage location to ensure the location.
There is a strong objection to send another big. We will lose totally the flow if the new pig is providing a very good sealing and got stuck with the original pig.
I want to correct myself, butane will not totally vaporize. Flashing it will drop its temperature so not all of it will be in the vapor phase.

My question is what will be the risks of flashing butane at the big restricted area?
Also, I need to understand what will happen to the flow and pressure upstream the pig once the flow reaches the chocked conditions but I continue dropping the downstream pressure. Will the upstream pressure increases up to the maximum head of the pump? Will the flow decrease to the amount that makes it above the chock state at this high delta P? Or the upstream conditions will remain the same and simply the flow will be constant but I will get localized vapor explosions?
In the second post, are saying that it is not possible to have liquid at high pressure at one side and two phases at atmospheric pressure on the other side of the pig?

 

[zdas04]

First what kind of pig have you gotten stuck? If it is a mandrel pig then it might be too heavy to shift while leaking. If it is a sphere then you can almost certainly shift it with a poly bullet pig since spheres have so little contact area with the pipe.

How do you know you have choked flow? If the dP happens over some distance (inches) then you end up with a large friction dP and the pressure at the outlet is less than the pressure required for critical flow. If you were getting choked flow across a pig it would be really loud and you could likely hear it through a considerable amount of dirt.


David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[Iradah]

zdas04,
Attached is the photo for the pig. It is heavy and has 3 meters length.
I simulated the situation by putting a small pipeline (0.1 meter) and applying the observed pressure drop across the pig at a certain flow. This has given the diameter of the pipeline that simulates the restricted path. The simulation shows that at the calculated diameter of the pipeline, the flow velocity is sonic and the simulation does not converge.
The only way to converge the simulation is to increase the upstream pressure or reduce the total flow rate. When I did the simulation, the flow rate was increasing when I kept the upstream pressure constant and dropped the downstream until I reached a certain downstream pressure where the message of sonic velocity came.
The downstream pressure is below the vapor pressure of Butane at the inlet temperature, so I think that vapor is forming in the restricted area and causing chocking. I think there will be damage to the pig. This pig is more than 2 million $ cost. And also, I think that the flow rate will be constant at the maximum it reached just before chocking and there will be no change to the upstream pressure, do you agree on that statement?

 

[zdas04]

Don't tell us that you ran a smart pig without first running a sacrificial dummy pig. A 3 m long $2 million pig has a fairly long minimum bend radius, it used to be something like 40 pipe diameters, but I know that there are some new articulated pigs that can traverse a 6 D bend (I heard of one that would traverse a LR elbow, but I haven't seen it myself). If you have a pig stuck in a bend you have two choices: (1) reverse the flow to try to get it back to the launcher (can be difficult since some of the sensors are uni-directional and have devices to prevent backflow); or (2) find it and cut it out. Your chances of bulling it through forward are approximately zero.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[1gibson]

Cut it out, use it as an excuse to revise the features at that location so maybe it doesn't get stuck next time?

 

[LittleInch]

Iradah,

It's time to tell us the full story and not drip feed information. You didn't attach the picture.

So:
What size is your line?
How old is it?
Have you run a pig before on it?
What min radius are your bends?
What type of pig is actually stuck?
Can you pump backwards?

Stuck pigs are bad news and difficult to resolve, but what you are doing and what you plan to do is even worse. It is quit elikely that you are at sonic velocity as the gap around the pig will be small and the fact you think you're vapourising now will be doing a lot of damage to the pipe and I don't want to think about what the temperature impact is.

You need to realise that you're in a bad place now and trying to maintain flow is doing no one any good. Put in a tracker pig, find for defininite where the pig is stuck, de-pressurise the pipeline and cut the thing out. If you can't reverse flow, this is the only remidial action. If this is a long articualted pig then another pig will not be able to push it out as it will just fold up on you.

Yes you can get liquid one side and two phase the other, but my point is that once you introduce vapour into this line by lowering the pressure dowstream the pig it won't magically all convert back into liquid if you raise the pressure and the delaing with a two phase line at the receiving terminal is not going to be easy.

Give us informaiton and people here will help, but sometimes there is no good news, only least bad news.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Iradah]

LittleInch,
What size is your line?
The line is 6”. There is spec change in the pipeline ID by few millimeters.

How old is it?
The line is operating since 27 years

Have you run a pig before on it?
Two pigs: Gauge and Geometry

What min radius are your bends?
I am not sure, I will check

What type of pig is actually stuck?
See the photo

Can you pump backwards?
The company owning the pig said that the pig is not designed to flow backwards and it will damage.

I was not involved in this operation until the pig got stuck. The pigging company did not find any reason in the pipeline that makes the pig stuck. However, they think it is not stuck at the ID spec change location. The flow vs. pressure trend shows that it was stuck first before the spec change and then we increased the pressure from the upstream and push it so it moved but then stuck again and did not move. We think it is stuck now at the spec change location.
I have to write the procedure for recovering it by increasing the delta P across it. This is what was agreed on by our management and the pigging company management.
Why increasing the delta P is rejected and is too bad and worse than the current situation? Is there any risk on the pipeline? Is it the vapor formation risk on the pipeline? Is it the risk on the pig? Why the chance to move it by high delta P is zero as mentioned by zdas04?
Is there a chance to get a total blockage while trying to push it?
My initial question was trying to understand what will happen to the upstream pressure and flow once the flow reaches the chocked conditions.
Please let me know what further information you need.

 

[Iradah]

Gauging photo.

 

[Iradah]

Smart pig photo.

The gauging one was run first, then the geometry and then the smart one.
The gauging and geometry photos are for the ones after being retrieved from the pipeline. For the smart, the photo is for it before sending it inside the pipeline.

I would like to emphasize again on the risks associated with pushing the pig by high delta P and the level of these risk. Thank you.

 

[zdas04]

If I had a gauge pig come in looking like that, I wouldn't have loaded the smart pig.

If you are stuck in geometry (e.g., too tight a turn, to big a dent, etc.) then it doesn't matter how hard you push, you are not going to make the pig turn the corner. Do an experiment: get a 2 m length of 3-inch (75 DN) PVC, a 90° elbow, and another 2 m piece of PVC. Glue the three pieces together. Get a 2 m long closet rod (about 1.5 inch diameter). Shove the rod into the pipe. When it hangs up get another closet rod and try to push it forward. Push harder. Push even harder. Now go to the other side and try to push it back out. Unyielding material simply does not yield.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[LittleInch]

I agree with zdas04. That guage plate has taken a big whack from something and needed to be investigated before you stick what looks more like a 6m long pig, unless the picture shows two in line...

Your issue is that once a pig gets stuck and then starts passing fluid - what's the flow you're able to get past it by the way? - the pig cups have "flipped" or have had a big piece ripped out of them by whatever dented your guage plate and are now pointing the wrong way. Increasing delta p will do nothing for you other than increase the velocity past the pig creating even more damage. Reducing pressure downstream the pig will only flash butane across it. You can clearly do the simulation and tell me what the gas temperature is going to be.

The real risk is that by some miracle it might actually work. What then?. Your pig then has a volatile liquid at high pressure behind it, gas in front of it at low pressure and hence a constant pressure across it as the butane behind the pig flashes off. Result - a pig travelling at very high velocity constantly accelerating.

What do you call a cylinder travelling at high velocity inside another cylinder. An artillery shell.

That's your real risk. Think about it.

You can always load a tracker pig which has a spring loaded relief valve it to not totally block the line when it hits the back of this mess, but at least you will have found it.

I agree that this pig train won't go backwards, but articulated units like this can easily get very kinked and jammed tight.

Let us know what your next move is.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[LittleInch]

PS. It might seem like we're criticising you but clearly you're being put in difficult position not of your making and all we're trying to do is help you make the right decision.

Once a pig train like this gets stick and passes fluid, you really have run out of options and what you really need to start thinking about is how to vent it down and cut the thing out once you know for sure where it is.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[if7005]

Pipeline usually design for 20 or 25 years. 27 years in service without regularly ILI and cleaning ??? What kind of material used ? Have company planned replacement pipe after 25 years or integrity ?
Anyway this stuck area shall be cut and replaced. Then you dare to proceed pigging with stuck again at next location or not, it depend on historical and maintenance record possessed during 27 years..
My opinion..

 

[austsa]

If the pig suddenly becomes unstuck at high dP, it reach very high speeds and cause damage to the smart pig and/or pipeline.
I agree with comments from other members. I'm a little surprised that your gauging pig didn't get stuck. Suggest that for any future gauging plates, you use a 'petal' plate design.

 

[11echo]

I was working on the Alaska Pipeline back in 79 when the "super" pig was stuck in mainline at check valve #29. We ended up doing a stopple by-pass to maintain through-put and isolated the check valve so we could open the valve up to remove what was left of the super pig. From what I read here and you need to stay in operation, try looking at using this stopple by-pass system.

 

[EnergyMix]

It's been over a week. What happened with this?

Want to know the do's and don'ts of Eng-Tips? Read FAQ731-376.
English not your native language? Looking for some help in getting your question across to others or understanding their answers? Go to forum1529.

 

[LittleInch]

See

http://www.eng-tips.com/viewthread.cfm?qid=355087

A partial update at least.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way