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.

 

[bimr]

It not completely clear, for example what is the 4720 ft/s term?

It may be possible that this siphon occurs during transitions when the systems is starting up or shutting down. But you have witnessed the noise during continuous operation and the operation has been described as generally continuous rather than being a batch.

I am not confident that the siphon is the problem.

You said the problem is at the elbows. At the elbow, you have 12 ft/sec velocity which would generally be defined as full flow which would prevent the loss in pressure from traveling upstream. Since that location is also the location where the problem is the worst, one would suspect the problem originates with the valve not the siphon.

Your sheet also shows the siphon being approximately 10 feet or so, which has a significantly lower pressure than the inlet pressure.

If you were having the cavitation problem in the larger 6-Inch pipe, you may have an argument.

To resolve the siphon question, consider adding a siphon break on the 6-Inch pipe. The cost of the siphon break will be very inexpensive compared to the other solutions proposed.

 

[bimr]

Maybe you can explain the flow period of once per month? How long does the flow occur?

The 40 ft/sec velocity (suggested) is too high to be believable. If you know the flow and pipe diameter, it is simple to determine the velocity.

Since the strainers are backwashing to remove the crud and detritus that are present in the process water, this material will eventually be backwashed out into the backwash line.

 

[LittleInch]

I can believe 40 ft sec in the 3" backwash nozzle. The filter has 120 psig inside it remember and there's not a whole lot of resistance to the flow back through the filter. Use a PE or a Teflon lined pipe and it should make all the issues go away - or put in a bigger pipe / elbow to reduce the velocity.

My motto: Learn something new every day

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

 

[1gibson]

Looks like you might have room to weld two 45* reducing elbows together, with the large ends in the middle. Worth a shot to reduce velocity and see if life improves. That change might help supplement a better solution. Of course then you have a weld right in the middle and that feature could make things worse.

 

[hacksaw]

Let's see:

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

Curious to see how with the pumps and strainers normally on standby, and the pumps are not turned on for your tests, that you are having problems in all of the discharge piping.

It is also curious that you are only associating filter blowdown with cavitation, but not the pumps.

You've clearly stated that the blow down flow of 10 ft/s or so in the 3" pipe, though you later increased this to 4" piping, now it is 40 ft/s

Sound like you owe the forum a bit of an explanation

 

[rconner]

Until better pics and less contradictory information is available it may be hard for anyone to tell exactly what is going on here. While perhaps flow erosion or other may not end up being the dominant issue (since we have learned of some periods that could involve perhaps significant stagnation etc. as well?), you may be interested in the lined pipe flow velocity guidance e.g. on pages 9-1&2 (I saw PTFE listed) at

http://www.publications.usace.army.mil/Portals/76/Publications/EngineerManuals/EM_1110-1-4008.pdf

which states,

"Liners should not be affected by erosion with liquid velocities of less than or equal to 3.66 m/s (12 ft/s) when abrasives are not present. If slurries are to be handled lined piping is best used with a 50% or greater solids content and liquid velocities in the range of 0.61 to 1.22 m/s (2 to 4 ft/s)..." I think DIPRA suggests a similar magnitude/maximum of about 14 fps for lined ductile iron water pipes in normal service. For design numbers beyond such guidance, ​I guess ​it might be best to talk with the manufacturers involved.

Everyone have a good weekend. ​

 

[Bambie]

LittleInch,

How often are the filters back flushed? Once per month.
what is their purpose / normal operation? Standbye (not operating).
Is there any biological inhibitor injection? None.
Does the discharge pipe remain full of water (stagnant) in what you now say is one month between flushing operations Yes.

your points above
1) I cannot understand how the system is tested without the pumps being operated (Three normally operating pumps pressurize the common discharge header).
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). Since the pump is not run, there is no accumulation of organics or silt on the inside of the filter elements.

 

[Bambie]

hacksaw,
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. (Pump discharge header is pressurized by three normally operating 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?
(Backwash removes silt and organics from the inside surface accumulated during normal operation, not standbye mode. Silt and organics accumulated on the outside element surface would not enter the backwash discharge - to sandblast the elbows).
3) The vendor suggested that velocities would be approximately 40 ft/s
(This is due to 120 psig).

Curious to see how with the pumps and strainers normally on standby, and the pumps are not turned on for your tests, that you are having problems in all of the discharge piping.
(The two strainers at the end of the line, closest to the header discharge are on standbye, the three upstream pumps and filters are normally operating).

It is also curious that you are only associating filter blowdown with cavitation, but not the pumps. (no capeesh).

You've clearly stated that the blow down flow of 10 ft/s or so in the 3" pipe, though you later increased this to 4" piping, now it is 40 ft/s
(I clearly stated that the 10 ft/s - corrected to 17 ft/s was due to the syphon alone, not due to 120 psig)

 

[Bambie]

bimr,

4720 ft/s is the speed of sound in water (c).

Once per month each of the 15 filters elements is cycled thru the backwash in the two standby filters.

We are seeing cavitation pitting in the 154' long 6"nps header, just not as aggressive as in the 3"nps standby filter elbows.

I like your idea of vacuum breakers in the 6"nps header - that would prevent cavitation due to unsteady flow pressure transients in the header and, perhaps, the 3"nps elbows too.

 

[3DDave]

I'm puzzled as to what the speed of sound in a liquid has to do with delta P.

q=1/2*density*V^2. dq = density*V*dV. This neglects delta P due to change in height.

 

[Bambie]

3DDave,

This equation predicts decelerative head, which (I think) should be happening in a very long siphon of incompressible fluid with many sources of constantly changing flows (and no vacuum breaker).

It is attributed to Newton's law of motion and the continuity equation.

 

[3DDave]

The equation seems to apply with to instantaneous change in flow, not a continuous flow. It is related to sharply closing valves, which is not the condition previously mentioned.

 

[bimr]

Thank you 3DDave for answering the Bernoulli question.

I would add that the speed of the pressure wave in such situations (a) varies with the fluid, pipe size, and pipe material. For a medium sized steel line, it has a value of about 3500 ft/sec, slightly less than the 4720 ft/s that you used.

 

[Bambie]

3DDave,

The filter backwash on each of the three continuously operating pumps will cycle on whenever the filter pressure drop reaches 7.2 psid.

The motorized butterfly valves close in less than one second.

Inertance or rigid column theory predicts delta P = rho x v x L / delta t where:

rho = .0135 (lbfx(s/ft/in)^2, v = 40 x 9/36 (ft/s) and L = 154 ft

delta P = 21 psid per second at 3"nps backwash velocity of 40 ft/s, which is 10 ft/s in the 6"nps header.

 

[LittleInch]

What is going here - first you say the filters get flushed once a month and now three pumps are running continuously with regular flushing once they reach their diff pressure limit, flushing at 40ft/sec (12m/sec). Please stick to one story or the other. Is the damage occurring in all the elbows? DO you change around which filters / pumps are working?

Your system is not suffering from cavitation, but pure erosion in the working units and probably MIC induced corrosion in the virtually static units.

My motto: Learn something new every day

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

 

[Bambie]

LittleInch,

If you refer to my sketch, strainers STR1, STR2 & STR3 are backwashing whenever required by their continuously operating pumps. These elbows have never been replaced, which I assume means low wall thinning rate.

Strainers SRT4 & STR5 are backwashed once per month, however since their pumps are on standby, there may be organics but no silt in the backwash flow. The first elbows are being aggressively pitted.

 

[bimr]

I agree with LittleInc. If you are experiencing pitting in the strainer piping where the strainers are mostly in standby mode, the pitting is most likely due to MIC. The periodic backwashing will tend to wash off the exposed corrosion detritus exposing the cleaned areas to additional corrosion.