Pipe Coatings for Cavitation Erosion 3

[Bambie]

At our plant we have PVDF (KYNAR), Polypropylene, Polyvinylidene Chloride (SARAN) and Teflon lined carbon steel pipe and fittings available for use.

The carbon steel pipe and elbows downstream of our lake water strainer backwash valves are being aggressively pitted due to cavitation erosion.

The lake water temperature is 33 to 73 degF and the 3”nps sch 40 pipes/fittings are A106B/A105.

The backwash valves are 3”nps resilient seated wafer lug butterflies, (a good source of vapour bubbles) and should be replaced, however, in the interim, I am looking for a recommendation on which pip ecoating would last longer in this service.

 

[hacksaw]

Bambie,

run a material balance on your system, showing the normal flows and flush flows, calculate the various piping drops, valve drops along with the valve settings,

specify the details of the flushing cycle (timing and how many strainers flush at any time).

you also need to specify the percent open of all valves

 

[tbuelna]

It is incorrect to describe the damage done to a metal surface by cavitation as "erosion". Erosion would imply some form of abrasive wear produced by the fluid flow. The pitting and loss of material produced by cavitation on a metal flow surface is spalling due to compressive fatigue failure. All other things being equal, a metal flow surface with higher compressive strength (ie. hardness) will be more resistant to spalling failures, and thus cavitation damage, than one with lower compressive strength. The reason materials like stainless steel or bronze often exhibit better tolerance to cavitation is due to their greater corrosion resistance. The surface spalling process resulting from cavitation can be greatly exacerbated by surface damage caused by corrosive effects of the fluid on the metal.

 

[LittleInch]

That is a good start point and it will be interesting to see what the vendor says. To be fair to them, the filters are often run at relatively low pressures of a 2-3barg so they need to have the ability / size in the back wash to have a good flow rate under those conditions. It would be worth trying to find the original data sheet from both you and the vendor to see what they had assumed for the operating pressure.

What the filter vendor sometimes does is on commissioning sets up a throttle valve or some other type of flow restriction downstream to get an acceptable backwash flowrate (say 5-10% of normal throughput). What then happens is that either that valve or restriction quickly erodes or gets "readjusted" by someone who doesn't realise what it is doing or gets blocked and then the operator just removes it as it "keeps getting blocked". The reason for it then gets lost in the mists of time until the elbows start to erode...

Given the interest in this post, please let us know how you get on.

My motto: Learn something new every day

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

 

[Ripz]

Are you seeing just localised pitting on the extrados? Or does general wall loss also accompaniy this? Perform UT to confirm, not just visual.

Looks to me to be erosion. You can see the grooves and gullies being caused by it.

If it was MIC, the entire piping system would have been affected. Doesn't look to be cavitation either, but there's only so much one can tell from one pic.

...

But in order for you to fix the problem, you need to know the causes. Cut out an elbow and send it for failure (metallugical) analysis. This would conclusively tell you the cause.

If it's cavitation or erosion, upgrading to Stainless Steel elbows should fix your problem.

Or you can go further and look at eliminating the root causes of the erosion or cavitation.

 

[Bambie]

Ripz,

Elbow replacement is planned very soon (sample photo attached). I will obtain better photos then.

LittleInch and hacksaw,

I have little faith in being able to eliminate cavitation via throttling - it will remove material somewhere in the system. Moving it downstream to the isolation valve or common header would be more disruptive to replace.

 

[keyepitts]

In looking at the original photo, I also tend to think that you are seeing erosion damage rather than cavitation damage. Probably some corrosion issues also. It will be interesting to see more photos of the affected components.

 

[rconner]

First, I am not an expert in cavitation but have had some past interest in the subject. While I work in the piping field, I think I furthermore have only run across only a time or two of instances I can remember where I was told some lining/piping destruction problem was caused by "cavitation" in a great many years. While I have probably been some sheltered, this could also mean at least the real deal happens rather infrequently.
When I first opened up this photograph reported as cavitation damage, I likewise have to admit however what I thought I saw in the picture (some sort of what appeared to be axially oriented flow lines?/grooving? and some deep, but widely separated/discontinuous "pits"?, and perhaps even with much more normal metal thickness surrounding?) did not appear to fit the image I had in my "minds eye" of cavitation caused by very abrupt pressure drop etc.
While that certainly doesn't mean this is not cavitation, my previous minds image, right or wrong, was of a porous, pock-marked surface deeply scarfing an area of a valve or very closeby piping, I thought to be caused caused by sort of blasting or splattering due to implosion of gas bubbles.
Incidentally, in the one past instance I thought I remembered where I was advised someone had encountered severe cavitation, I thought I read or heard in my research way back then that the violence of the vapor implosion events were akin to tiny "nuclear explosions" that really no lining or pipe could withstand (sort of like what I have read from at least a few on this thread). While I had no idea then where or how they had come up with that analogy (and it seemed to be a sort of non-obvious over-statement to at least a few of us sort of semi-educated folk back then), I now read a more specific statement in another technical reference,

"When the cavitation bubble collapses it generates a temperature of 5,000 degrees C and a shock wave that travels over 500 miles per hour."

If this is in fact true (and for someone to even quantify such in this fashion I suspect they would have had to have some rather heavy duty brainpacks, $ and instrumentation behind them!) I'm not sure I would assume that the problem (cavitation or erosion) would be magically solved economically or functionally by just throwing in instead at least common austenitic steel pipes and fittings, I think that have about a maximum 88-95 HRB (maximum Brinell ~151-175 hardness). I guess this is what LittleInch talked about in treating the symptoms as opposed to the problem.
As butterfly valves are apparently involved with low discharge pressure, you may also be interested in the guidance at

http://uwrl.usu.edu/facilities/hydraulics/projects/ASCE.pdf

I also noticed in the latest pic that the 90 elbows are obviously VERY "close-coupled" off what appears to be a discharge valve, a practice I think had also seen mentioned in past in some literature.

On a definitely lighter note, there is homework for the weekend at

http://www.youtube.com/watch?v=gy7PTl6UJfI

. Everyone have a good one!

 

[3DDave]

In at least one location, I'd try putting in a Tee or Wye so that cavitation bubbles will either collapse against a more easily replaced capping plate or when they reach the increased stagnant pressure in the dead end, rather than carrying into the bend of the elbow. There is recognition of an increase of back-pressure, but this may be small enough.

Anecdotal: I saw a desk-toy in the form of a sealed glass tube partially filled with water, the remainder water vapor. If the tube was rocked back and forth the water would slosh, but if it was shaken it made a plink sound, very much like a ball bearing slamming into the ends of the tube. As the vapor and liquid are in equilibrium any mechanical change causes an instantaneous phase change at the interface.

 

[Bambie]

rconner,

The youtube link demonstrates the cavitation mitigating potential of aeration (beer carbination in that case).

Could this be achieved by locating a flanged orifice plate, with pressure taps connected to an air injector, downstream of the motorized butterfly valve? The orifice plate would need to be sized to provide 26 psid in order to generate the 5 usgpm required for air entrainment (by the model attached).

 

[hacksaw]

Bambie, glad to see you finally understood, the product you have selected is used to increase D.O. and thus to reduce MIC, not cavitation...

 

[Bambie]

hacksaw,

I always try to hear what the machine, rather than the tool, is trying to tell me... "Maybe Its Cavitation?"

 

[bimr]

The Lancaster product is used to add oxygen in order to oxidize soluble iron to insoluble iron so that the insoluble iron can be filtered out.

 

[3DDave]

http://www.youtube.com/watch?v=vRk1SWucy1s

"Cavitation in a globe valve"

Video of cavitation into a stub-Tee.

 

[hacksaw]

Bambie,

Oxide formation is essential in reducing Fe corrosion, and Dissolved Oxygen is also an essential step in removing the microbes that solubalize iron in the first place.

Some of the posters dealing with water treating can better explain what happens in increasing the D.O. and reducing the D.O. demand.

Good luck with your planned changes, they are right on target.

 

[Bambie]

hacksaw and bimr,

I love a good teasing...you're the best.

PS I attached an excellent paper on "Mitigating Irritating Cavitation" thru aeration.

 

[bimr]

Here is a tease for you from Bela:

"Because no known material can remain indefinitely undamaged by severe cavitation, the only sure solution is to eliminate it."

 

[Bambie]

bimr,

I didn't answer your question about "Mr. Bernoulli's velocity term" and I'm not certain he should get the credit, however, I was thinking of the following:

A long siphon connected to many filters will have unsteady flow.

From first principles: delta P = rho x c x delta V.

For water: delta P (psi) = rho (62.4 lbm/cuft)/32.2/144) x c (4720 ft/s) x delta V (ft/s)

delta P = 63.5 delta V

You only need a quick change in velocity of 1.6 ft/s to drop 100 psi in a water filled pipe.

The siphon drops static pressure close to vapour pressure and the shunting columns of water continuously open and close vapour pockets, which ravage my pipe.

Would you agree with this argument?

 

[LittleInch]

Don't know about bimr, but I don't agree with that. Your "syphon" won't do any of these things and, IMHO, the thing ravaging you pipe isn't cavitation, but a high velocity of liquid containing dirt and sand.

Did you get an answer back from your filter vendor on the flowrate anticipated through the back wash line at 120 psig to zero? You anticipated some very high veleocities.



My motto: Learn something new every day

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

 

[Bambie]

LittleInch,

I have learned the following:

1) These strainers and pumps are normally on "standby" and only see flow once per month during testing to confirm availability, which is done without starting the pumps.
2) There could, therefore be an accumulation of silt on the ouside of the 750 micron (.03" mesh) elements, but how would this get through?
3) The vendor suggested that velocities would be approximately 40 ft/s

Perhaps one month is time enough for MIC to build tubercles and create pits which are then removed via cavitation erosion?

 

[LittleInch]

Bambie,

Can you just provide us with a clear set of information on these elbows.

we need to know:
How often are the filters back flushed?
what is their purpose / normal operation?
Is there any biological inhibitor injection?
Does the discharge pipe remain full of water (stagnant) in what you now say is one month between flushing operations

your points above
1) I cannot understand how the system is tested without the pumps being operated
2) Any dirt etc would be on the inside of the filter (normal flow in to out) which then gets flushed out during back wash (Out to in)
3) That sounds about right, but is a high veleocity when it also has particles in it

MIC can occur quite quickly and if it builds any items would simply get flushed away by the flow and any dirt particles in the back wash - the cavitation issue is a red herring.

My motto: Learn something new every day

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