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.

 

[Bambie]

rconner,

You are quite right, I used 4”nps sch40; thanks for this verifiable correction.

The literature states that high hardness, yield, tensile and work hardening properties reduce the rate of material loss from cavitation. One paper determined the highest cavitation resistance to be steel with 0.1% carbon and 6% chromium, with a microstructure of 32% martensite and 68% austenite (attached).

 

[tbuelna]

Cavitation produces compressive fatigue stress failures in the pipe flow surface. Cavitation damage is surface spalling rather than "erosion". Cavitation occurs when the local flow conditions temporarily fall below the vapor pressure of the fluid, creating air bubbles that expand and rapidly implode, which produce extremely high compressive stress on the pipe flow surface.

If there is a cavitation condition in the pipe system, then no type of coating will resolve the problem. Switching to a different type of metal for the pipe will not help much either. The only thing that will effectively resolve the cavitation problem is correcting the flow deficiency.

 

[LittleInch]

II just don't understand why you're obsessed with cavitation. You might well be producing some bubbles in the water and this continues in the water, but cavitation damage occurs at the point where the bubbles are formed and then collapse within a very small distance and time. The high velocity of liquid with particles in it would appear to be a much more likely cause of your erosion. you might be getting some collapse of some larger bubbles, but I find the whole system difficult to follow. Can you sketch it up as a corss section so we can see what is happening?

Can you advise where you get the flow figures from? My experience of back wash filters is that the flow is directly related to the operating pressure of the filter and given that the pressure loss is essentially taken across the filter elements can lead to much higher flows than you think. Also remember that the initial flow from each filter element will contain the largest particles. Hence if you operate your filters at pressure much above 1 or 2 barg, the flow can increase from that which the vendor has assumed.

My motto: Learn something new every day

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

 

[hacksaw]

this discussion is a perfect example for the need for getting a piping/mechanical/civil engineer involved in the system design, rather than patching a bad design after the fact.

Materials selection, piping layout, coordination with the pumping/backwash system and where you route the backwash, is only the beginning of the design issues that come up

good luck

 

[Bambie]

LittleInch,

Cavitation erosion has a history of producing through wall pits (not large areas of wall thinning) in the extrados of the first elbow downstream of the strainer backwash discharge MVs (sketch attached).

My task is to reduce the elbow replacement frequency, not re-design the configuration.

 

[Bambie]

Hutton's paper on cavitation erosion mitigation through aeration is also attached.
I am impressed with the tiny amount of free air that is required to significantly reduce erosion rate.

Now if I could only find someone who has employed this technique in short backwash discharge lines.

 

[hacksaw]

Sounds like you'll be the first, give it a go! sketch up your design, test it and keep us updated

 

[bimr]

Thanks for posting the sketch as it clarified the issue. There were several posts that were misleading prior to the introduction of the sketch.


[li]The initial post said there was pitting in the steel pipe and elbows.[/li]
[li]The velocity through the 3-Inch elbow is 12.5 ft/sec at 275 gpm. The velocity is not excessive in a waste pipeline where you are discharging to atmosphere.[/li]
[li]The strainer is operating at 120 psig according to your sketch, which means that it is possible that you have cavitation at the butterfly valves. Cavitation in valves is caused by a reduction in flow area that increases the velocity head through the valve while at the same time decreasing the fluid pressure.[/li]
[li]The strainer backwash lines are connected to a common header which discharges to atmosphere and provides a good siphon, dropping the individual backwash lines below atmospheric pressure.” This statement is also incorrect. The siphon is not an issue at all. You only have a drop of 10.5-feet which calculates to a static head of 4.5 psi. That will not produce a significant siphon, especially at the fluid velocity that you have.[/li]

The problem is the valves.
[li]You need to replace the backwash discharge valves with valves that are a full port type.[/li]
[li]You need to increase the downstream backpressure by installing an orifice at the end of pipe. Size the orifice to pass the required flow while at the same time reducing the discharge pressure. [/li]
[li]Install sch. 80 steel pipe, it will last longer. I don't believe the pipe material is an issue at all.[/li]

http://www.valmatic.com/pdfs/Cavitation_in_Valves_7-22-08.pdf

 

[Bambie]

bimr,

You might have forgotten Mr. Bernoulli's velocity term...

 

[hacksaw]

Bambie,

Are you throttling the motor operated valves or opening them completely?

 

[bimr]

Please explain "Mr. Bernoulli's velocity term" and how it was overlooked.

Bernoulli states "The kinetic energy increases at the expense of the fluid pressure". Isn't that was posted above "Cavitation in valves is caused by a reduction in flow area that increases the velocity head through the valve while at the same time decreasing the fluid pressure."

In your application, the static pressure is inconsequential and does not need to be addressed.

Here is another reference for you:

http://www.engineeringtoolbox.com/cavitation-d_407.html

Cavitation can in general be avoided by:
"reengineering components initiating high speed velocities" ie components meaning valves
"increasing the total or local static pressure in the system" ie add backpressure valve or orifice plate

http://www.engineeringtoolbox.com/control-valves-cavitation-d_490.html

 

[rconner]

While I am trying real hard not to claim specific expertise with what you are running into here (and probably convincingly!), I noticed now your "turbine blade" paper advocating some specific chemistry/microstructure of metal for alleged best "cavitation" resistance came up with a conclusion that the best resistance was what sounded like really a sort of specialty steel formulation (w/ it appears "6% chrome", and 489 HB hardness) . I think the article you provided is even some confusing to me with its reference to "stainless steel", as I thought that term is generally applied to alloys with far more chrome than that.

While I guess I wouldn't argue that e.g. very high hardness steel (e.g. near 500 Brinell, if available reasonably in the piping configurations you need?) could conceivably provide at least a little better general wear resistance than softer, I don't think that's what you would really be getting with at least the most common grade stainless steel (flanged?) piping mentioned so far on this thread. E.g. I think you most common ferritic SS pipes have in range of instead 16-20% chrome, and are generally much softer than this.

As I'm not sure the point was understood, I will ask a couple more questions - while I understand the desire to backwash quickly (and therefore e.g. one might not want to just throttle flow quantity way back as perhaps another has suggested) and your statement your job is not to mess with "design", is this backwash discharge piping correctly sized given the resulting quite high flow velocity with it appears for at least some time with some sort of solids you've had? Regardless what the experts determine is going on here, I wonder since the piping and valves are to be replaced anyway would really hurt to also put an increaser/reducer and a little larger e.g. lined piping off this backwash discharge (that would appear to result in a much more normal water or slurry flow or whatever velocity beating around the elbows etc.)? I noticed the first thing the folks who have appeared to look at arguably at least some similar-sounding problems at length but in the energy field at

http://www.hse.gov.uk/research/rrpdf/rr115.pdf

mentioned under the heading "Design of Pipework" was:

"Pipework should be designed to minimise flow velocities..." [immediately followed by "...and avoid sudden changes in flow direction (e.g. at elbows, constrictions and valves).]

 

[Bambie]

bimr and hacksaw,

Okay, we all agree that cavitation is very likely the cause of pitting in the steel pipe and elbows.

The solution cannot be to increase backpressure - that will directly impact on strainer efficiency.

No matter how 'slippery' the valves are, vapour bubbles will still form on the filter elements, travel downstream and cavitate.

The solution is to live with cavitation by increasing resistance to cavitation erosion.

The possibilities are: stainless steel, aeration or polymeric linings.

Any ideas?

 

[Bambie]

rconner,

I agree, the materials in that paper were not readily available, the attachment illustrates that stainless steels have higher resistance to cavitation erosion than plain carbon steels.

 

[rconner]

OK, just trying to learn here - what was this last figure an "attachment" to (I guess I missed any reference to it in any of the papers I quickly glanced through on this thread)? Thanks.

 

[3DDave]

The damage done by cavitation is similar in nature to that done by electrical discharge machining. Hard or soft doesn't matter much. If it did the material would be common knowledge, and cavitation would not be a problem.

If you can't up the backpressure enough to stop cavitation, then put in a sacrificial piece or change the routing so the bubbles can't collapse against what you don't want eroded. Since the pressure is low, you do have the chance to put in a viewing window to watch them to determine a method that works for your circumstance.

If you are looking to replace the elbows and materials hard or soft still erode, either pick something cheap or something thick.

The cavitation problem I was involved with was in a hydraulic manifold where one of the valves would whistle a bit as it opened. The bubble stream went across a passage and drilled an extra path through the manifold block to produce control actions that weren't appreciated. The ultimate fix was for the valve maker to redesign the valve, but a short term bandaid was a steel sleeve, which would wear through a little slower than the aluminum.

 

[bimr]

Where did you get this idea? "The solution cannot be to increase backpressure - that will directly impact on strainer efficiency."

Have you discussed this issue with the manufacturer of the filter?

Are the valves used for control or are the valves on/off?

If the valves are used for control, replace the valves with a control valve that has a cage allowing the bubbles to collapse within the cage.

http://www.cla-val.com/documents/pdf/B-KO_Anti-Cavitation.pdf

If the valves are used for on/off, replace the valves with types such as full-port ball valves, plug valves, or pinch valves.

 

[LittleInch]

Bambie,

You clearly have an issue with your elbows on the backwash side and appear to be trying to solve the symptom, not the cause. We disagree about what is causing your erosion, but no matter - use a Teflon coated elbow or a HDPE elbow and you will replace them much less often.

If you want to fix the problem rather than just replace elbows on a less frequent basis, you need to start from the basics. First - What exactly is the flow rate coming out of these things. You are running a fairly high filter pressure (120 psig / 8 barg). Given that your backwash is effectively at 0psig, you need to determine from the filter vendor what the flowrate is for that DP. When I worked on a similar system, we worked out that during the backwash cycle, we lost about 10% of the flow... Now if your butterfly valves were manually set to some partial opening to reduce the flowrate, then your flow may be less than that, but given that the valve is being subject to impact with dirt and hard bits, there may not be much of the valve left by now. Any cavitation effect will be on the valve itself, not further downstream. Your assertion that providing some sort of flow restriction further downstream would impact on the filter performance is not correct. You have 8 barg driving the flow - 1 barg back pressure won't make any difference

It is a well known issue on "dirty" flow that any changes of direction need to be as gentle as possible and as slow as possible.

So to fix the problem you really need to stick in a reducer and go to 6" in a straight line before bending down into your header. By the way your discharge piping looks Ok, but it is not a syphon. ?I can't see if your discharge pipe goes below the lake level, but if it does then as bimr has noted, your static pressure in the header is still about 10psia - nowhere near low enough to cause an issue when flowing.

I've done my best to try and look beyond your current issue, but there are so many holes in the data that I can do no more so good luck and keep the spanners handy for changing your elbows on a regular basis.

My motto: Learn something new every day

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

 

[hacksaw]

1. strainer blow-down in not continuous but intermittent, the op's presumption is that "bubbles" are being formed in a liquid-full pipe, with insufficient pressure drops to cause cavitation...yet claims the raw water is not capable of support MIC...

2. the mov's, if full opened have neligible pressure drop and can only support incipient cavitation at best if partially opened.

The discharge piping sizes are responsible for neglible pressure drop<15 psi at best.

The op has not addressed the valve operation; though butterfly style valves rarely used or needed for high pressure drop control

3. Strainers or strainer flushing do not support cavitation processes.

4. The low cost alternative is to hire a qualified engineer!

 

[Bambie]

The vendor literature states that 7000 usgpm will pass through 15, 750 micron filters or 467 usgpm per filter with 7.2 psid when dirty, which gives each one a Cv of 174 and flow of 1905 usgpm at 120 psid.

These values are based on inside-out filter flow, so I am endevouring to find out if this is also the outside-in flow characteristic. That would be a discharge velocity of 87 ft/sec...yikes.