Frequent Bleach Pump Premature Failures with FRP Materials 1

[Yousuf Alshuhail]

The service is Sodium Hypochlorite 0.2 % and seawater. We experienced impeller vane fracture, and other vane cracking at leading edge and casing cracking. There are quality issues such as porosity, inclusions, deep cracks and coating pealing off at different location. Also, the vane inlet and outlet have sharp fillets 5 mm. The pump is quite large with flow is 3055 GPM, head is 98.4 ft, power is 90 kw and speed of 1800 RPM. We are currently conducting investigation to identify root causes of this failure but we welcome your input and feedback on possible causes in such application.

 

[LittleInch]

What is the pump made of exactly?
"FRP" is rather vague?

Who is vendor?

Any photos?

Was the pump operated in accordance with the data sheet?

Not much to go on here.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JJPellin]

Not enough information. Old pump? New pump? Materials of construction? Pictures? Other vanes?

Johnny Pellin

 

[EdStainless]

Is it continuous duty or intermittent?
Where on the pump curve is it operating?
Do you have records for flow/pressure/power?
Do you monitor this pump (vibration, noise, other)?


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P.E. Metallurgy

 

[Yousuf Alshuhail]

New pumps. We have 4 pumps 2 continuous operation and 2 on intermitted operation. The vibration is 4 mm/s which is within vendor manual 6 mm/s. There is no up normal noise. The pumps are operated very close to rated point and within motor rated current.

 

[Yousuf Alshuhail]

Our IT system prevent me to upload photos. The vendor name is Affetti and the model is CFG and the size is 10x8x15. There are a lot of technical details about this model in the internet. The material is FRP-VES for seawater with fiber glass reinforcing and Vinyl Ester resin, the manufacturing process is injection molding. We are in the process of purchasing spare parts to run the plant and we are looking for acceptance criteria's of how much we can accept of molding defects such as holes diameter and depth , crack length and depth and inclusions sizes and depth. The vendor does not have any criteria's for such manufacturer standard pump. The only acceptance criteria is visual inspection, hydrotest and testing at reduced speed 1500 RPM.

 

[LittleInch]

Well straight away looking at the website for the pumps, you see that FRP-VES is "standard~" and FRP-VEC is for "Bleaching"

https://www.affetti.com/en/horizontal-centrifugal-pumps-series-cfg-ansi-asme-b73-1/

So what did the data sheet say about the fluid?

Why do you seem to have a material which is not compatible with the fluid?

Have you followed ALL the recomendations in the manual?

http://www.europumps.sk/materials/files/affetti/User manual CFG.pdf

Note page 22 where is states No solids of any type.

There are lots of warnings about not running dry so it is clearly quite sensitive to that. Did they ever run dry?

Also FRP materials are not great at resisting damage / over tightening etc.

If your technicians are used to metal units, there is a potential for damage. These things need to be handled and treated very carefully. They are not metal pumps and the material behaves differently.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JJPellin]

LittleInch hit on all the points I would have made. We tried fiberglass pumps in sour water service many years ago. They did not work out. Our mechanics and pipe fitters over-tightened the bolts causing cracks. The resin in our fiberglass compound was incompatible with something in our sour water resulting in impeller hub failures. The pipe strain was too much for the fiberglass case to take. I gave up on fiberglass entirely. I would select a suitable metallic pump. Your service does not seem very difficult.

Johnny Pellin

 

[EdStainless]

We ran a lot of pumps with FRP impellers and cast metal housings, just for the reasons mentioned already.
We always insisted on testing new pumps at full speed across the entire operating range. If the performance was off or there was vibration we rejected the impeller. Static balance does not tell you much, but dynamics can.

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P.E. Metallurgy

 

[Strong]

Cavitation can be a killer, so be sure there is adequate NPSH. High frequency acceleration vibrations and contact ultrasound can detect the presence of cavitation. Audible sound and measurements with a microphone can also be used for detection; check throughout operating range.

Walt

 

[itsmoked]

Don't Sodium Hypochlorite solutions have a very high out-gassing bubble creation rate? Seems there are lots of problems with feed lines often being blocked by "mystery bubbles", likely chlorine. Could this lead to exceptionally easy cavitation as Strong suggests?

Perhaps the NPSH needs to be higher than water alone would need it to be?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Yousuf Alshuhail]

Dear All. Thanks a lot for your valuable feedback. I will Try to answer some of your questions. The data sheet say about the fluid is Sodium hypochlorite 0.2 % and seawater. The vendor in a conference call has no explanation to the same question also raised by our non-metallic material engineer on why FRP-VES was supplied while the data sheet specify bleach and the manual specify FRP-VEC for bleach. The pumps have strainers and our field people found minor dirt only . The existing strainer has cylindrical basket constructed from sheet double metal that has 1/8" perforations. So small particles will pass our strainer since we are in commissioning stage. Also, salts from seawater and sodium hypochlorite in solid state found inside one of the pumps. There are lots of warnings about ot running dry so it is clearly quite sensitive to that. Did they ever run dry? Our field people did not report any dry running condition. Also FRP materials are not great at resisting damage / over tightening etc. I will inform our field people to handle the pump with care since FRP is easy to crack. Vendor claim the failures are due to cavitation issue. The suction pressure is 0.5 - .08 bar(g). The data sheet liquid vapor pressure is 1.7 Kpa(abs) and NPSHr is 4 m at rated flow. Our field people did not report any cavitation issues during vibration collection. . We are verifying the cavitation issue on the running pump.

 

[LittleInch]

Thanks for the feedback.

Could those inlet strainers have become bloodied and caused low NPSH? Where was that 0.5 barg measured? Upstream or downstream the filters?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LittleInch]

Also NPSH limit is NOT the same as cavitation limit.

Usually the cavitation point is anywhere from 1 to 5m ABOVE the NPSH curve.

You need to ask the vendor to give you the cavitation curve.

Typically like this

At cavitation initiation it is very difficult to hear it without very good microphones and you need to start from a high inlet pressure and then slowly reduce in order to identify the high frequency sounds of collapsing bubbles.

but having no answer about the materials is pretty damming so you might have a get out of jail card right there.


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Also: If you get a response it's polite to respond to it.

 

[Yousuf Alshuhail]

Thanks for your valuable feedback. The suction pressure is taken down stream of the strainer. I will consider your comments and investigate the cavitation issue in more details.

 

[LittleInch]

Also check out itsmokeds comments about the true vapour pressure of the fluid you actually have with the bleach in it. The figure you quote looks to me like "normal" seawater. If you're getting bubble formation (fizzy liquid) at atmospheric conditions, your pumps will be cavitating even though they shouldn't be if you use the standard Vapour pressure curves for seawater



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Also: If you get a response it's polite to respond to it.

 

[itsmoked]

The following was gleaned from the excellent handbook:
OxyChem Sodium Hypochlorite Handbook
by Oxychem Corporation

Warning - sodium hypochlorite
solutions must be stored in vented
containers, or in containers equipped
with adequate relief devices due to O2
gas generated from decomposition. If
venting rate is exceeded by the
decomposition rate, swelling or
damage to the container may occur.

Materials Selection
The following materials are compatible
with sodium hypochlorite solutions or
as linings for non-compatible
materials. Some may not be suitable
for use in processes that manufacture
sodium hypochlorite.
 PVDF (polyvinylidene fluoride)
 PTFE (polytetrafluoroethylene)
 Titanium (Warning: titanium must
not be used in contact with dry
chlorine)
 Ethylene propylene rubber
 EPDM rubber (ethylene propylene
diene monomer rubber)
 Chlorobutylene Rubber
 Polypropylene
 PVC (polyvinyl chloride)
 CPVC (chlorinated polyvinyl
chloride)
 Tantalum
 Viton® A with a minimum durometer
(hardness) of 70
 FRP

FRP is allowed so it's probably not a material issue but a cavitation issue.

I think this is a lot lower than H₂O but I could be wrong. It's kind-of over my head.. the water.. never mind.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[Yousuf Alshuhail]

Thanks a lot for of your feedback. We verified with the sodium hypochlorite system licenser and he confirmed that the concentration is of sodium hypochlorite 0.2 % and the vapor pressure is 1.7 kpa.a which is very low. The suction tank is continuously vented by blower.

 

[LittleInch]

Youssef,

It's not the vapour pressure per se that we're talking about. It's whether when mixed with your seawater does it create micro bubble of gas due to the chemical reaction. This has caused bubbles and vapour locks in other similar systems.

It will be a transistory thing as the chemical reaction takes place so taking samples and testing them won't be simple.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.