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]

cavitation, as it is usually defined, is not common in that service, suspect that you are dealing with line velocity being too high, particulate erosion, or corrosion common to raw water services

 

[MJCronin]

For lake water erosion/corrosion --I vote for 316 stainless -- schedule 40S --- or schedule 80S if you got it

This may be the cheapest solution also....

MJCronin
Sr. Process Engineer
Venture Engineering & Construction

http://www.VentureEngr.com

 

[Bambie]

hacksaw,

I have attached a photo taken of the inside elbow extrados and, although fuzzy, you can see pitting - some through wall.

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.

Vapour bubbles form at the valves and collapse at the first elbow where the static pressure recovers, generating a continous growling noise.

 

[Bambie]

The 7000 usgpm continuous flow strainer manufacturer specified 416SST body and Teflon seat material for the 3"nps 150# backwash discharge butterfly valve. Perhaps teflon coating is the way to go?

 

[hacksaw]

looks like bacterial pitting,

if you specify the pressure temp and flow rates, you should get responses that will help you resolve the problem

 

[bimr]

Will agree that looks like MIC probably due to lack of disinfection.

It is not likely that this is cavitation as your system is under pressure. The valves are typically used to throttle down the pressure before discharge.

Consider adding some chlorine to stop the MIC.

 

[Bambie]

hacksaw and bimr,

I am not familiar with growling bacteria or mircrobes...

MJCronin,

I agree with stainless, a lining could disbond.

I just finished reviewing "Erosion of Concrete in Hydraulic Structures" ACI 210R-93 and am intrigued with the idea of supressing cavitation through aeration using a radial array of vacuum breakers.

Any thoughts?

 

[bimr]

Stainless steel is NOT the answer. MIC will chew up stainless as well:

http://www.nickelinstitute.org/~/Me...sedforCoolingandHydrostaticTesting_10085_.pdf

Here is what MIC looks like"

https://www.google.com/search?q=mic...%3B343%3BRhFCxnSznS7sSM%3Bhttp%253A%252F%252F

http://www.bti-labs.com%252F&source...2Fwww.stroudsystems.com%2Fpm_fss.html;851;570

The corrosion look familiar doesn't it?

You do not have cavitation, you have MIC. If you want to end it, you will have to add chlorine to your water to kill the microorganisms that are responsible for the MIC.

Google "MIC":

http://www.corrosionclinic.com/type...influenced_biological_microbial_corrosion.htm

The "growling" is simply the sound caused by the pressure reduction being taken across the valve. It is not the cause of the problem.

 

[bimr]

I reviewed the paper that you mentioned. Here is a link:

http://civilwares.free.fr/ACI/MCP04/210r_93.pdf

There are a number of reason that your proposed solution would not work:

The system is under pressure and you will not have cavitation. They are talking about gravity spillways and flumes, not pressure pipes.

"A good way to avoid cavitation erosion is to make u large by keeping the pressure high, and the velocity low".

A vacuum breaker will also not produce fine bubbles. You would need some type of bubble diffuser.

You should reread the paper again, specifically section 4.3.2 (Bacterial action) where MIC is discussed.

 

[Bambie]

bimr,

Thank you for all the links, however:

1) I see absolutely no common features (except a hole) in the photographs
2) there is no sign of biological matter in the elbow, just a scoured and pitted moonscape
3) MIC could not survive in a turbulent strainer backwash discharge pipe - it needs normally stagnant or very low flow conditions to thrive
4) When the backwash discharge valve opens, the inner volume of a filter element is exposed to the sub-atmospheric pressure in the siphoning discharge header, causing vapour pockets to form and highly turbulent flow, conditions anathema to MIC
5) gravelly, growling sounds from the elbow are audible symptoms of cavitation
6) dousing the backwash flow with chlorine as it is pumped back into the lake would, I think, be unpopular...

 

[bimr]

Common features of MIC are "scoured and pitted moonscape", not biological matter.

It is not possible to see the biological agents that cause MIC, just the results, pits, which are the result of biological attack. M stands for microorganism which would indicate that the biological matter is quite small. Studies have shown that a millilitre of lake water commonly contains nearly 10 million microorganisms.

You say there is no stagnant water, but then "When the backwash discharge valve opens."

What is cavitation? Cavitation manifests itself audibly as a grinding noise, a noise that closely resemble gravel being moved around in a cement mixer. It can be heard at pump inlets and control valves. It is due to the fluid being vaporized because of low pressure and then suddenly collapsing due to high pressure produced by a pump impeller for example or the increase in pressure that occurs at the outlet of a control valve. Where in your system is the higher pressure that should be causing the bubbles to collapse?

Here is a recording of cavitation:

https://www.youtube.com/watch?v=1Lbxtjfdat4

Why don't you put a pressure gauge on the filter to determine the pressure. One would be highly skeptical that a "vacuum condition" and cavitation could exist. A pump rotating at thousands of rpms will not cavitate unless you are pulling more than 15 feet of water.

One would think that dumping backwash water into the lake would also be unpopular.

 

[Bambie]

bimr,

Why would putting the silt and organics back in the lake be unpopular?
The product information sheet is attached for your edification.

 

[bimr]

It is not legal where I am located to discharge filter backwash to the environment. It depends on the regulations where the facility is located. Here is an example:

"SEPA considers that routine discharges to the water environment from the water treatment works are ‘activities liable to cause pollution’ under Section 20(3) of the Water Environment and Water Services (Scotland) Act 2003"

http://www.google.com/url?sa=t&rct=...cd=7&cad=rja&uact=8&ved=0CF4QFjAG&url=http://

http://www.sepa.org.uk/water/regula...PzKpygRJRScz-S_LQ&sig2=S4xxoGxPgZsGjBx4wkgi9A

 

[Bambie]

bimr,
To summarize then:
- cavitation cannot occur in 'pressure pipes' attached to strainers
- fresh water MIC builds transparent tubercles in turbulent intermittent flow
- chlorine laced backwash will solve the problem but is probably illegal
?

 

[LittleInch]

I've been watching this thread and I go back to hacksaws comment.

The thing missing here is any idea of what your flowrate is as you discharge this product and hence your velocity. Also how many minutes in an hour does this discharge - many filter systems work on both a differential pressure to initiate the cleaning and a regular flush every hour or four hours or once a day? what is the "average" time and duration for your system.

I haven't seen any of these details except a discussion, very useful though it is, on MIC

The "growling" sounds to me like all the dirt, small creatures, gravel etc from the filter hitting your elbow at high velocity. The differential pressure in these systems can be very high as the filter sits at a few bar pressure, but the 3" outlet basically at about atmospheric.

If this line is low pressure why don't you use PE? It's used quite a bit in slurry lines which essentially is what this is.

My motto: Learn something new every day

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

 

[Bambie]

LittleInch,

I appreciate your interest in the strainer operation, however, I feel no need to discuss further the improbability of bacterial infestation.

I modelled the backwash lines from three strainers and demonstrated that the elevation drop in the backwash header alone (no dynamic pressure gradient) can remove flows of 275 usgpm per strainer when all three MVs are open up and 400 usgpm when only one strainer MV is open. This equates to velocities of 7 to 10 ft/sec, respectively.

The backwash flow through each strainer element and MV must keep up with this siphoned flow rate or vapour pockets will form at surface irregularities, move downstream and spontaneously collapse (cavitate) at locations where the static pressure increases sufficiently (elbows).

Aerating the flow downstream of the MV using vacuum breakers could introduce too much air and reduce strainer efficiency. Buoyancy forces and secondary vortice flows would make it impossible to ensure the air arrives at the extrados where most of the cavitation is taking place.

I think sch80 stainless is the most cost effective way to prolong the replacement period.

 

[bimr]

Bambie,

If what you are theorizing is correct, the simple and inexpensive solution is to throttle the backwash discharge with a valve or fixed orifice at the end of pipe discharge point. With this fix, you would maintain pressure in the backwash discharge pipe and eliminate the possibility of cavitation.

That would be the fix to remedy the system problem if your theory is correct. However, the theory is improbable at those fluid velocities.

 

[Bambie]

bimr,

Unfortunately throttling like aeration increases backpressure which reduces strainer efficiency.

Would you care to elaborate on the improbability of cavitation 'at those fluid velocities'?

 

[rconner]

While I guess I could have done something wrong, I just did a very quick calculation that came up with a velocity of more than 17 fps at 400 gpm in 3" sch 40 Stl pipe. If the velocity has been in that range, and also with non-clear backwash flow conveyed, is it really surprising that a steel elbow has exhibited some metal loss? Also, am curious what is there to lead one to believe that "stainless steel" pipe will survive such conditions really any differently than carbon steel?