PVC Lined GRP tanks

[rsauk]

BS4994 requires pvc lining of GRP tanks to be radiused at intersection between tank wall and floor.
With a sloping base(floor) it is very difficult to fabricate such a radius on pvc liner.
One fabricator overcomes this difficulty by providing square edge (shell to base) configuration.
Does anyone have experience of such a solution being acceptable to Client.
Contents of tank is Sodium Hypo.

Exactly why is this square edge configuration allowed (in code) when using fabric backed thermoplastic material.

 

[suzuki]

We are a company that designs, engineers,and fabricates chemical processing equipment worldwide such as storage tanks,process vessels, process columns,processing piping,heat exchangers,stacks,scrubbers,fume duct systems and related equipment, we fabricate the process equipment with a wide range of materials of construction , such as: GRP Composites,PVC,PolyPro,PE,CPVC,PVDF,ETCFE,MFA,PFA,State of the art Dual-Laminates and related fluoropolymers.

You construction is what we refer to as a DUAL-LAMINATE, being a PVC inner liner and a GRP exterior overwrap, the PVC being the corrosion barrier and the GRP being the structual outter shell and bonded to the PVC.

The area of your concern is what we call the "knuckle radius" , where the shell wall meets the bottom, we form a radius in the pvc sheet material prior to the GRP overlay, I believe we use a 1 1/2 inch radius(Min) in our designs and fabrications, being a slopped bottom is not a problem, any fabricator that uses a "square" corner for such an application is just asking for futute problems and probably doesn't know anything about goog design and fabrication practices.

http://www.viatec.com

 

[ghopkins]

The applicable code in this case is ASME RTP-1, which makes special provisions for thermoplastic linings of GRP - called dual-laminates as suzuki mentioned already. The ASME RTP-1 code does NOT allow square edge joints between the shell and end closure.

It specifically states that a 6% knuckle radius be used - as a percentage of the diameter. This type of construction is not easy and usually requires thermoforming of the plastic to achieve the desired shape. The thermoforming operation must not reduce the lining thickness by more than 10% - also called out in the code, see section M12D-310 of the ASME RTP-1 code.

A square edged joint WILL FAIL - welds should not be placed in the high stress areas. DO NOT do this type of construction and let me reiterate that this is not permitted according to ASME RTP-1.

Lastly, you mention fabric backed sheets. PVC is actually not fabric backed. This type of thermoplastic liner is a chemical bond to the fiberglass corrosion barrier and structure (vs. fabric backed sheets which are mechanical bond). There are two types of bond tests for these materials - a climbing drum peel test and a lap shear test. The climbing drum test is for fabric backed sheets and the lap shear is for chemical bond such as with PVC. These tests are required for each lot of lining material received and the procedures spelled out in RTP-1, Mandatory Appendix 12.

Check with ASME to determine the approved/certified fabricators of RTP-1 stamped dual-laminate equipment. Or you can contact me at 513-874-2800x119 for more information.

Best regards,
Greg Hopkins

 

[ghopkins]

rsauk,

I apologize, but I need to make a correction. The 6% knuckle radius requirement is only for joining dished heads to a shell. For flat bottoms, part 3A-120(d) of the ASME RTP-1 code calls out a minimum radius of 1&quot; for vessels <=4' in diameter and 1.5&quot; for vessels >4' in diameter. Using a vacuum forming process to achieve this radius provides the most durable option and can easily be accomplished with sloped bottoms. Again, please do not use a square joint.

Thanks,
Greg

 

[EdClymer]

BS4994:87 permits a 90° corner, ie base disc/shell junction.

The whole is reinforced with an external knuckle of nominal double thickness.

In the UK we have been making like this for only the last 30 years: do it right - no problem!

Regards

Ed

 

[EdClymer]

I forgot to add, that the reason for sharp corners in fabric backed PP/GRP construction, is that the sheet will not take compound corners.

Regards

Ed