Vacating Ammonia Pipeline of Nitrogen Gas?

[MAPower]

I have a startup situation which is quite puzzling:

A ~10 mile, 4 inch anhydrous ammonia pipeline supplies ammonia to our plant process. The line is ~4 foot underground throughout except where it crosses under rail crossings and in two locations where it goes under river.

Initially, the line was pigged and had a nitrogen blanket. Supply pressure is about 180 psig via product pumps from supplier. Pressure at our facility is ~90-100 psig. This pressure drop is not expected and dows not meet our requirements. We are confident that because we are taking low flows, <1200 lb/hr, that there is a nitrogen bubble at the river crossings, causing excess pressure drop. At one river the line goes 60 foot under ground.

Does anyone have any ideas how to vacate the nitrogen? We have tried running continously, 24+ hours, at about 300-400 lb/hr. We can not get higher flow because our process downstream can not take lower pressures.

I have donw some research on the Froude number as someone indicated it may determine what velocity or flow I need to "push" the bubble through, but I can not find anything good on this number????

 

[BigInch]

Is there any high point in the line, or all more or less flat except for the dips?

What velocity do you have now, with the 400 lbs/hr ? Is this line supposedly sized for 1200 lbs/hr normal ops?

I don't know offhand, what's the density and the vapor pressure?

(Increase the velocity and/or run another couple of pigs.)

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[dcasto]

I can not imagine the line being vapor locked. At 1000 psig, the ammonia would have .9% nitrogen and eventually the N2 would be gone. Do your test indicate N2, ie is the vapor pressure over 209 psia?

I'd think you had some pig parts stuck in the line from the N2 fill.

 

[Latexman]

P, psig Sat. T, F
90 58
100 63
180 94

Hmmmm. What's the pressure class/rating of the line? Where's this at, climate-wise?

Good luck,
Latexman

 

[MAPower]

BigInch - there are no high points in the line, and yes it is pretty much same elevation except for the dips.

Velocity @ 400 lbs/hr = 0.03 ft/s

The pipeline is oversized, the supplier wanted to be able to supply "other customers" in the future. Maximum flow for our process is 5,000 lbs/hr.

Temp. Vapor Pres.
° F (psig)
40° 58.6
50° 74.5
60° 92.9
65° 103.1
70° 114.1
75° 125.8

dcasto - Our line is not at 1000 psig, it is at 100 psig. I do not understand your questions about vapor pressure??? We have done no specific test for N2, we have spoken to various ammonia folks and they all agree. I have asked multiple times if they left a pig in the line, they say no.

 

[Latexman]

dcasto,

Where'd this 1000 psig number come from? Reread the OP.

Good luck,
Latexman

 

[MAPower]

Latexman, the pipe class is all 300# in sunny Florida. The temperature of the ammonia, should roughly follow the ground temperature.

 

[Latexman]

I'm in Texas and our NH3 line runs at 300 psig and we have no problems. Depending on where you are at, you may have no problems (the Artic) to having nothing but problems (the Equator).

Good luck,
Latexman

 

[MAPower]

Originally the line was suppose to run at 250 psig, but the supplier is using different pumps currently. We definitely have problems downstream of where our line comes up with the vapor pressure. But now, I'm just trying to resolve this problem. Out of curiosity, what is your pipeline used for?

 

[Latexman]

Can the supplier raise the pressure? 180 psi is probably going to be problematic in the heat of the summer (my opinion).

I'd stop all flow and get the line up to 180 psi first so it's as full of liquid as possible. Then, start flow, stop flow, start flow, stop flow, etc.

If that fails, can they trap a high boiler between two pigs and send it through?



Good luck,
Latexman

 

[Latexman]

Out of curiosity, what is your pipeline used for?

Anhydrous ammonia.

Good luck,
Latexman

 

[MAPower]

The supplier can not raise pressure until they start using the original pumps, which will be some time from now. We'll look at trying some of the things you suggested. What is a high boiler?

I agree with you, summer time will be a drag, and it is around the corner.

What I meant, is what is the end user (process) for you ammonia?

 

[Latexman]

A "high boiler" is a chemical that has a very high boiling point relative to the application at hand. For example, in a flash drum (a 1 stage separation based on volatility) a "high boiler" in the feed will not appreciably partition into the vapor stream. In this case, it would be a chemical that is nowhere near a T and P in the pipeline that would cause it to form a two-phase mixture.

There are many end users on the ammonia pipeline we draw from. Knowing their or our particular end use is not going to help solve your immediate problem, but telling this forum your end use may lead to many other useful suggestions to solve your problem.

Good luck,
Latexman

 

[BigInch]

Just hazarding a guess, at that velocity and pressure i agree, you just might be trapping bubbles.

That's way too slow.

I say lower the pipeline outlet pressure and (chill it?) boost it, but you'll probably start gasifying.

Perhaps your best solution for now is to truck it in. Ugh!

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Pipelines4me]

The two suggestions that I have is to run a cupped pig or two down your line (Enduro can help size) to confirm there are no obstacles in the line. A transmitter in this tool could help locate the pig at preselected passage points to determine where it last passed in the event it gets stuck. This cupped pig passage could push out any nitrogen vapor pockets as well. The line pressure limit and flow rate are certainly limiting factors. A 4" line holds about 10 tons/mile of product, so at the 1,000lb/hr rate it would be a 200 hour pig run.

Other options are to weld TORs on the top of the pipe at the high points and flare off any nitrogen mix in the line, but use of a flare will burn any anhydrous coming out of the connection.

A big key is the delivery pressure. Have you considered adding a back-pressure control valve (some simple nitrogen bladder type exist) to keep the pipeline pressured to above the vapor pressure throughout the line?

 

[dcasto]

I just picked a number, shouldn'y assume anythying. A 10 mile 4" line is normally a high pressure system. Either get the line all vapor at 100 psig or all liquid at 300 psig. Thats it. You can't operate to close to the vapor pressure.