Inspection of FRP tanks in service 2

[robsalv]

Hi folks.

We're looking at installing an FRP tank for non aerated, ambient temperature, 28% conc sulphuric acid service. At this point in time, it's my understanding that at this acid strength the tank won't require a corrosion liner.

One of the questions that I can't seem to get a straight answer on is in regards to the later inspection of the tank after it has been in service.

AE is one available technique, but it will only identify cracks. What are other inspection techniques that can be employed to find other types of defects that the service may be introducing/causing? We have little experience in visual inspection of FRP tanks, so early visual signs/indications might be missed.

We're also intending to paint the outside of the tank for UV protection.

Thanks in advance.

Rob

 

[robsalv]

... for clarity, I should add, that it's a fibre glass based tank that we're looking at...

Thanks.

Rob

 

[EdClymer]

Greetings

With GRP (or what you call FRP) vessels, the first step is to ensure item is correctly and accurately specified in terms of duty and then correctly designed and manufactured to a recognised code (BS4994 in the UK)

Have the proposals verified by a competent person who understands the code.

Pay a fair price!

During service, the item can be checked, visually and/or ultrasonically. If the vessel was lined ie PVC-U (what you call dual construction) then the welds can be checked by high frequency spark testing.

A big advantage of a lined vessel, is ease of construction, adding branches, checking integrity etc. (and it saves the cost of mould)

Have been designing/buliding/testing vessels for 30 years - provided they were correctly built usually not a problem.

We have a UK Health and Safety Executive (HSE) guidance note PM75 on 'advice to users'

Regards

Ed

Birmingham- England (where plastics was invented)




Ed Clymer
Resinfab & Associates
England

 

[EdClymer]

Greetings again

I have just re-read your post and noticed that you intend to paint exterior as anti UV protection. Not a good idea - low adhesion..

Using the correct resin - coloured - the flow coat (final coat after tissue) will give the protection you seek.

As regards frequency of testing: initialy a water test simulating the SG: after that monitor external surface: say 12 - 18 months have item fully inspected. In the UK such inspections are required by the plant insurers.

Regards

Ed

Ed Clymer
Resinfab & Associates
England

 

[ghopkins]

I recommend getting a copy of the latest ASME RTP-1 standard, which will answer all of your questions about fiberglass construction for vessels rated from full vacuum up to 15 psig.

http://members.asme.org/catalog/CategoryView.cfm?Category=CS&Designator=RTP

A lot of chemical companies (such as Dow, Bayer, Dupont) have been purchasing this type of equipment for over 30 years and have very detailed specifications and a very short approved supplier list. However, other companies who are not familiar with fiberglass construction and have no coporate standards (General Electric and Exxon) always buy their equipment with ASME RTP-1 stamps. This saves them big $$ in terms of not having to develop their own standars/specifications and assures them that they will be getting a high quality vessel.

If you don't want to purchase a copy of the RTP-1 standard, go to ASME's website to find RTP fabricators that are certificate holders and buy your vessel from one of them (with the RTP stamp).

http://www.asme.org/cns/departments/AccredCertif/RTP.htm

Getting the stamp assures your vessel will be built with certified labor, certified materials, using an ASME certified QA program (which inspects the tank during its various stages of construction).

I would highly discourage exclusion of a corrosion barrier in your tank. The corrosion barrier, usually 100 to 200 mils thick, protects the structure from the chemicals. For example, if the structure of your vessel is filament wound (very economical), the sulfuric acid might be able to wick along the fibers and shorten the service life.

 

[EdClymer]

Sulphuric acid requires a corrosion resistant barrier ie a non woven fabric embedded into a resin rich inner layer and backed up with lightweight mat. Then the structural layers applied.


Regards

Ed

Ed Clymer
Resinfab & Associates
England

 

[robsalv]

Much obliged to you Ed and GHopkins.

Since raising the question, I've had the opportunity to discuss the inspection issue with a local manufacturer [possible bidder]. He pretty much promoted visual inspection and possible hardness testing as an inspection regime.

The RTP-1 tip is not something prevalent in Australia, so I'll have a look. However Australia does have its own construction standard, AS2634.

I'm now a little wiser about this type of tank construction.

Thanks again for your valued contributions

Regards

Rob

 

[gvanbeek]

Rob:

Tank Inspection:
Nothing replaces a good internal inspection by an inspector experienced in FRP construction. An inspection should be performed at installation to form a base line.

The inspection should include (1) barcol hardness readings on various internal components (bottom, shell, welds, etc), (2) photos of the interior, and (3) description of inner surface (e.g. exposure of glass surfacing veil).

Chemical attack can be identified though (1) reduced barcol hardness readings, (2)roughening or loss of gloss of interior surface , or (3) exposure of glass reinforcement that was previously encapsulated.

Materials Selection:
One other point that should be made is resin selection. We had a facility that had a tank designed for <30% H2SO4. They then later decided to increase the concentration to 60% for logistics. The manufacturer provided the tank with an isophthalic polyester resin, which was suitable for 30%, but would require tank replacement in order to increase the acid concentration.

A vinyl ester resin would have handled the 60% H2SO4, and would have increased the tank cost by < 20%, and installed cost by <10%. If your temperatures will be low, and no exotherm from water addition will occur then a bis-A vinyl ester such as Derakane 411 / Hetron 922 would be sufficient. At higher temperatures(up to 170F) or concentrations (up to 70%), then move up to a novolac such as Derakane 470 / Hetron 970.

External Protection:
I agree that the tank should not be &quot;painted&quot;, but rather a pigmented wax coat with uv absorbers should be used. To increase the UV resistance add a nexus veil to the exterior. The addition of nexus works well for unpigmented tanks (10+ years without fiberbloom from my experience).

 

[robsalv]

Thanks GVANBEEK for that. Sounds like pretty useful advice.
Much appreciated.

Rob

 

[suzuki]

We have an agent in Australia who has a lot of experience in this type of application , e-mail me your address and I will forward company and name on to you.
klk@viatec.com

 

[uriel]

Who would rent/sell accoustic emissions instrumentation for inspecting FRP tanks?

 

[gvanbeek]

Physical Acoustics Corporation is the leading manufacturer of AE equipment. The software that is used for the evaluation is also important. PAC is also involved heavily in this (e.g. Mon-pac, tank-pac, etc.)

This is an art as much as a science, I would recommend using someone who has experience and is ASNDT certified.

http://www.pacndt.com