Headloss Old Pipelines

[Einzo]

Hello,
As part of a report i'm preparing for a training session, i must calculate the headloss in a pipeline.
The problème is, i only have access to the chacteristics of the pipeline in its newest state, but the pipe is 21 years old.
I tried using the data but the results were way too optimistic compared to reality (higher flow for same headloss).
Research taught me that headloss increases as the pipeline ages (mainly due to its roughness increasing over time and diameter reducing).
I tried to use the Hazen–Williams equation, only after I learned that it is only viable for water, the pipeline is for LPG transportation.
Is there a way to correct my calculations in accord to pipe age?

 

[SNORGY]

Diameter reduction will be difficult to quantify; to me, that would suggest wax or other deposition or sedimentation. If you can still pass a pig through the pipeline (if it's set up for pigging), then there shouldn't be any appreciable diameter reduction.

That leaves you with the increase in roughness over time. You state that when you calculated the head loss based on new pipe, "the results were way too optimistic compared to reality". So, if your fluid properties are not varying between Case A and Case B, you are left with the variation in the roughness, friction factor and Reynolds Number (which are correlated together in Colebrook-White).

There could also be a local restriction that has either been developed - or installed - somewhere, so inspection of any maintenance records or capital additions would be useful to you. Have there been changes over 21 years?

Is there more than one source of flow into / out of the line and, if so, are all of the flows accurately accounted for?

I think in Crane TP-410M, Page 1-7, Figure 1-7, treatment of this very problem is captured.

 

[LittleInch]

Can you give us some idea of numbers here.

If the current use is LPG then are you sure the whole line is staying as a liquid?

How much do you have to adjust the roughness figure to get reality matching calculation?

What's been flowing in the line all this time?

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[Einzo]

@Snorgy

The last pig that was sent down the pipeline dates from 2005. Our LPG contains a black powder that labs were not able to indentify, it also has megnetic properties. So there is surely some sedimentation build up but as you said, it's really hard to quantify.

The full pipeline has multiple flow injections, but the portion i'm calculating for is situated between two pumping stations with the stations being respectively the only flow input/output in both ends.

The flow is accurate for it is used for transaction purpose.

I live in an african coutry, we don't have access to online purchase yet, so i can't buy the Crane TP, but thanks for the info, i'll try to search for the figure or any subject treating the matter.

Thanks for the reply, Snorgy.

 

[Einzo]

@LittleInch

THanks for your reply.

For the 21 years the pipe transport LPG and only that and it is kept at a minimum 20-25 bars to keep it liquid (at 18 bars we get an alarm & at 17 bars emmergency shut down goes on, to avoid pumps cavitation).

For the numbers, flow rate goes from 1100 m3/h up to 1550 m3/h.

As for the roughness adjusting, I actually did't try to do that but it's a great idea. I should adjust the roughness & calculate for multiple flow rates/headlosses and see if i get a correction coefficient.

do you think adding a section talking about the effects of pipeline aging on flow/headloss would be enough to justify the use of a correction that I calculated myself?

 

[LittleInch]

I think so as long as the adjustment you get isn't silly. A standard roughness is 46 micron. I wouldn't go much more than 200 even for a highly corroded line. Those deposits might be a significant cause of extra friction though

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

 

[SNORGY]

Sorry Einzo.

I'll tell you some highlights from that section of Crane.

For 1000 litres per minute of water at 15 C through 102.26 mm I.D. pipe with roughness of 0.046 mm, if roughness increases to 0.115 mm, the head loss increases by 14%. If the pipe I.D. Is reduced to 97.2 mm, head loss increases by 29%. If the two changes occur simultaneously, head loss increases by 50%.

If you take that as true, you should have enough data to start with a reasonable baseline.

I wrote a MathStudio script on my iPad a while back and checked the numbers reported in Crane, and I get the same results.

 

[BigInch]

Use the Colebrook-White friction loss equation.
You can then use the density and viscosity for LPG, or water, gasoline, or crude oil, whatever.

New steel pipe can have an absolute roughess of 0.023mm.
Old steel pipe is generally assumed to have an absolute roughness of 0.046mm
A lot of times when you calculate hydraulic losses for old steel pipe, it seems to still have a roughness value close to that of new pipe.

Steel pipe in various locations along the pipeline does not rough up evenly all along the length. Some spots will be rougher than others. Regions with high fluid velocities, or cleaner locations along the pipeline (tops of mountains) may be significantly better than towards the ends of pipelines and at bottoms of valleys, or river crossings.

Do not change the ID, use the same ID as when it was new, unless you have specific measurements saying something different.

 

[shvet2008]

to Einzo

Try to find in Google "Handbook of Hudraulic Resistance" of Idelchik. In spite of it seems rather old this book is based on a huge research of real pipes behaviour. And you know pipes never change.
For roughness reduction in old pipes you can see para 6.1 in Norsok P-001 5th ed. There is not so much info, but something... As for me I agree with BigInch - of course there is some roughness reduction in old pipes, but reaaly it has some maximum. I would recommend to use 0.5mm from Norsok P-001.

 

[georgeverghese]

Also check your flow and have it confirmed for both ends of the line. If the meter you are reading now is at the downstream end, and if you have a sizable leak in between, you may be pushing more than 1100-1500m3/hr through the line.

 

[Einzo]

Well, I was already using a roughness of 46 microns, I didn't know it was suited for old pipes. In the mean time I spoke to a crew member of the LPG transportation team & they confirmed that the possiblity of a reduced diameter is pretty high, mainly due to a strange black powder present in the LPG, they even had to reduce their filters from 100 microns to 50 and also the pigging being delayed for quit a time (last is from 2005).

It seems that my probleme is coming from the diameter, which is pretty hard to quantify. I'll try to find a way to correct it by lowering it, recalculate for an interval of flow rates & see if there is consistancy in the results compared to the real mesurements.

I greatly appreciate your help guys, thanks a lot.

 

[SNORGY]

Let us know what that black powder is and where it's coming from...

From what I've read, black powder contamination in gas and NGL pipelines is quite common, and the lags between pigging, over time, have likely contributed to its severity now. Once it starts, it is very difficult to get rid of. That stated, georgeverghese makes an excellent point about possible leakage from the pipeline, which is worth ruling out.

 

[LittleInch]

It will be a compound of iron, usually a sulphide. Eliminating water in the pipeline or the product is key to reducing the inlet of material, but sweeping it out of the line is difficult if there is so much that it's impacting flow. See attached for some info and other references.

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

 

[BigInch]

That old black power is probably iron sulfide. It can be hard to get rid of, at least until it is exposed to air.
Then it catches fire!
It is "pyromorphic". Be careful!

https://www.google.de/url?sa=t&rct=...jz8bnh5u_QAhXEhiwKHWD8D6oQFghJMAY&url=http://

http://www.texasgas.com/wp-content/...tHYe9qibQ&sig2=zrr_7-i5IEcSi3qUeGEpOg&cad=rja

https://www.aucsc.com/_aucsc speake... Period 9 & 10 Difficult to Pig Pipelines.pdf

Google It,

 

[Einzo]

Well, they actually have pictures of it, exposed to air, yet it did not react.

 

[BigInch]

Sometimes yes, sometimes just heats up, sometimes no.
Just be prepared. It is possible. Probably when you least expect it.

 

[Einzo]

I shall keep that in mind

 

[LittleInch]

Found this while looking for something else. Although based around gas lines, some items would be suitable for LPG and also gives a few more references.

Use of gels between pigs to pick up more debris seems like a good place to start. I didn't realize the stuff was so heavy - SG 4.7 - and tends to coagulate and drop out very quickly unless you do something about it.

LI



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