Local loss in welded plastic pipeline 9

[mod23]

Hi,
Do you know something about losses in connection of butt welded plastic pipe?

For example in 3000 km long pipe there will be about 600 connections. Even if it is a small loss, for 600 joints losses will no longer be negligible.

How to determine the loss of joint plastic pipe?

Thanks in advance

 

[brimmer]

What do you mean by losses? The weld does not fuse properly, or some other issue with the weld??? Can you use a different word?

 

[ione]

Probably he's asking for pressure drop....

http://www.performancepropumps.com/PDF_Files/FrictionLossChart.pdf

 

[DLite30]

One joint every 5km is insigificant. Modeling error associated with pipe roughness and elevation changes is what you should be focused on.

 

[Duwe6]

Heat-welded plastic pipe has a large internal and external weld projection -- root & cap. Ultrasonically welded has a lot less.

Just 'guesstimating' on thermal buttwelds, I would use something between the "Equivalent Length" for a gate valve and the 'run' side of a Tee. ione provided a small-bore pipe table. On a 4"NPS pipe this gives an Equivalent Length between an extra 3 to 8 feet. Meaning that the fluid friction of a thermal buttweld on 4"NPS pipe is the same as 3 to 8-ft of pipe.

This is *only* a SWAG, no experimental data to back it up.

SWAG = Scientific Wild-A$$ Guess

 

[mod23]

Yes I`m asking for a pressure drop. As Duwe6 said heat-welded plastic pipe has a large internal and external weld projection.

There is a mistake in example, I meant 3000 metres long pipeline, and when the pipe is welded every 5 metres, there will bee 600 joints. On each joint must be some pressure drop and I am asking how much it is.

I quess, then in large pipe (diameter) the pressure drop will be insignificant, but in smaller pipe could be indispensable.

Has anybody measured this pressure drop. Some experiments or verified data?

I apologize for my bad English

 

[blacksmith37]

Not what you asked ,but: Amoco spent a $ million (20yr ago) to find out there was no substitute for sanding each surface just before cementing .Firewater system for offshore platforms where reliability was important.

 

[Latexman]

Google is your friend! Look what I found from a HDPE pipe vendor we use:

Good luck,
Latexman

 

[rconner]

In reading all responses, it appears it is possible not all readers understand exactly what is involved here. Attached therefore is a picture of the external part of such a “bead” I took (of a 24” pipe size job fusion) some time ago, to hopefully more graphically explain what I think the OP is talking about. I also have another picture somewhere of a partial cross-section of a wall coupon, cut-out across an hdpe weld joint (I think cut from another, also 24” size job), that may have exhibited even a little more pronounced protruberances on the outside, as well as similar protruberance/constrictions to the flow path/area on the inside of that pipe. Unfortunately, I have not been able to readily lay my hands on that.
As to the hdpe pipe vendors claims (I think basically going back many years) that the bead be “ignored”, this appears to be disputed by some other folks (I believe also claimed to be far from ingénues in the polyethylene pipe field) at

http://www.ahmcelroy.com/products?page=shop.browse&category_id=3

. From that website, accessed this day,

“Internal bead removal is fast becoming an important step in the pipe fusion process.
Why Remove the Internal Bead?
One of many great attributes of HDPE pipe is its low friction factor. HDPE pipe is less impaired by flow reduction or pressure loss from the fusion beads than the turbulence in a comparable system made of PVC push joints. Using a material that has less flow reduction and pressure loss doesn't mean that there isn't impairment. It is this impairment that we are completely removing. Removing the internal bead can remedy the following problems:
• The internal bead can cause problems when pigging for blockage and biofilm or when HDPE is used as a casing for telecommunication conduits.
• Sediment can build against a bead during a slow flow rate, this smaller blockage is a place where as large debris become embedded and cause more severe blockage.
• In the potable water industry biofilm can stage a footing from under the fusion bead where bacteria or other particle entrapment is of concern.
• With increased flow the bead creates a turbulence that generates wear when slurries of abrasive materials flow through the system. Even though the bead will finally wear away and turbulence decreases, the final effect is a thinner wall cross-section in that location.”

I believe you will find, e.g. with a good web search of these and other forums that at least the above risks have not just been dreamed up, and the hdpe specialty bead removal company has not invested the design effort, money and tooling for nothing (I believe I remember a case e.g. reported not long ago on these forums where a small diameter hdpe pipeline had in fact unexpectedly gotten blocked with some sort of debris or detritus).
All that being said, I suspect a general answer to this question, like so many other questions on such forums, is “it depends”. Not the least of the factors might be whether or not the designer has assumed a reasonably conservative, long-term design flow coefficient for actual field performance and the size pipeline involved (i.e. e.g. backed by a database involving a significant number of pipelines actually working in the service involved, not just laboratory tests or vendor promotions trying to compete with larger flow area pipes).

While I'm not exactly sure how it is effectively accomplished, e.g. with the remote access that might be involved and also with pipe ovalled by earth/ring loading or path bending etc. and without damaging the pipe wall?, the aforementioned folks claim to be able to do it, I'm sure FOR A PRICE.

 

[rconner]

Oops!

 

[mod23]

Thanks for your answers.
Latexman: I used google a lot and found this article

http://stc.fs.cvut.cz/pdf/VeselskyJaroslav-306611.pdf

I don't agree with you, that fusion beads can be ignored.

Yes, best solution is remove internal beads, but sometimes you are not able to do this and then you have to calculate with some pressure drop (or it can have influence on total flow through pipe)

 

[katmar]

The Info Brief No 3 from PolyPipe that claims that the beads can be ignored is based on grossly wrong engineering. They try to calculate the pressure drop per bead by using the Hazen-Williams equation. This is nonsense. The pressure drop caused by the bead is not due to wall friction of the type that Hazen-Williams assumes in calculating the pressure drop along a straight piece of pipe. The pressure drop introduced by the bead is due to the changes in flow direction as the fluid accelerates through the reduced cross sectional area and then slows down again after the bead.

Using the bead dimensions given by PolyPipe in an orifice calculation gives much higher pressure drops than they claim. Where PolyPipe get a pressure drop of 0.005 psi for 25 beads in a 10" pipe, I calculate it to be 0.009 psi per bead or 0.225 psi for all 25.

In their example, the flow rate would not decrease from 1600 gpm to 1599 gpm but in fact would reduce to 1560 gpm - which is a 2.5% decrease. In a 4" line with welds every 20 ft (instead of the 40 ft assumed by PolyPipe) the reduction would be almost 12%.

The bead sizes measured by Veselsky are a bit smaller than those quoted by PolyPipe. This makes the effect a bit less than what my calculations above show, but the losses are still significant. Veselsky has published data which is very useful for standard pipeflow calculations. The values for [ζ] in Table 2 correspond with what Crane or most Western fluids authors would call "k" - the resistance coefficient - so thay can be plugged straight into a Darcy-Weisbach type calculation.

Unfortunately Veselsky's data is limited to pipes below 2" diameter. Using my orifice assumption as above I get k values of around 0.1 for a bead in a 4" line and 0.025 for a 10" line. These are small when taken alone, but mod23 has 600 of them which could become very significant, depending on the pipe ID.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[Latexman]

I think rconner nailed it with "it depends". My experience with electric fusion butt welds of HDPE is large diameter underground fire water lines of relatively short length (no where near 3000 km) in petrochemical plants. These systems are designed with considerable conservatism since they are for safety and loss prevention and, in the grand scheme of things in my little world, are cheap. I have several systems in operation that routinely pass performance tests, and I never included the pressure loss of the butt welds in those. However, I can see that for other applications (small diameter, more precise flows, long runs) these losses would be significant, as is the case for mod23. Each engineer must make the decision of when to include this loss, or not. Still, I have no intention of going back and adding in the butt weld pressure losses to a complete firewater system design I have in the project pipeline now, which starts-up 1Q13.

Good luck,
Latexman

 

[stanier]

The head loss through PE pipe is more dependent upon inside diamter than roughness. When you look at the standard for PE pipe manufacture you will see the tolerance on wall thickness is about 10%. The catalogues quote the mean wall thickness. The manufacture is flogging you PE resin and energy. The energy converts pellets to pipe.

How can the manufacturer maximise profit? Simply by using good measuring tools on line and a 3000m run can be made with the absolute minimum wall thickness and still be within standard tolerances. Hence your inside diameter will be larger than you think. Do the hydraulic calculations with the smallest wall thickness and see how drammatically the head reduces.

In fact this is one of the headaches with PE pipe. If friction is a major component of system losses you can end up running down the pump curve and operating at >115% of BEP. This will result in your pump vibrating and seals/bearings failing prematurely. Many a time I have had to machine the pump impeller to bring the performance back to the BEP.

Another thing to watch is when a manufacturer supplies a higher PN than ordered because it is in stock and he wants to shift it or meet a deadline that may incur penalties. The manufacturer has little understanding of power losses and your contractual obligations.

Lesson for all inpsect your PE pipe on site.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[chicopee]

Well, you could assume the whole 3000km to be corrugated piping, then friction loss factor would be at least double that of straight plastic piping.

 

[saplanti]

It may be extra job, and somehow timeconsuming during the construction, but cutting the welded joint fusion internal bead mihgt be a good option to eliminate or reduce the local pressure losses.
5-6 metres working distance with a automatised apparatus can reduce the cutting time. But I am not sure if there is a supplier for that. Therefore the HDPE pipe manufacturers might need to think about or do more research to eliminate the internal bead.

I think there are other HDPE pipe manufacturers that use welded fusion at the joints, but their welding is not butt welding, and the joint area wall thicknesses are larger than the pipe wall thickness. I was not directly involved in the discussion with manufacturer and do not know if this type joint gives the similar internal bead at the weld joint. It may be valuable to talk to them.

Kind regards,

Ibrahim Demir

 

[BigInch]

The theoritical velocity along the pipe wall is zero. This type of "restriction" is only producing a very localized velocity increase in the vicinity of the bead against the pipe wall, where flow velocities are very low in relation to the velocity farther away from the wall in the turbulent zone, hence the friction losses are very small and probably nearly fully recoverable as those velocities once again slow down after passing the beads.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso