Spectacle blinds used with brittle FF flanges

[jistre]

Okay, let's pick those brains out there.

I have an application where I need a spectacle blind in a plastic FRP piping line. The pipe spec calls for flanges made out of the FRP material that are flat faced and built to the dimensions of B16.5. I have a copy of B16.48 for spectacle blinds and see that this spec deals with blinds designed to fit concentrically with the pipe bore with their ODs just smaller than the ID of the bolt circle of a standard B16.5 flange so they can kind of "float" and rely on clamping forces to hold it in place once the joint is made up.

Here's my problem: The FRP flanges I have are extremely brittle and weak compared to CS and the insertion of the B16.48 blind effectively adds a raised face to the flanges. The brittle flanges would then lose the support benefits of the full flat face and would have a stress riser where they press against the blind. If the bolts are overtorqued, the flanges can easily be cracked, and then we've got a whole world of new problems.

I know that there are spacer rings that will allow a flat faced flange to be mated with a raised face flange to prevent cracking the FF flange, but I've never seen anything to handle the same problem when using a B16.48 blind with FF flanges. In any case, I would like to avoid the use of an additional spacer ring.

What I really need is a spectacle blind where both sides are dimensioned the same as the blinds of B16.5 to provide the full support through the bolt circle to keep the FRP from cracking. Then, the open side of the spectacle blind would have the bore cut to the inner diameter of B16.48. I really don't care how large the web would be or what standard it would be designed to. Then, I could get a flat faced spectacle blind that would be large enough to support the brittle flanges it goes between.

Has anyone come across a standard, code, interpretation, anything that provides for this, or am I looking at jumping off the cliff on my own here with a custom job?

 

[pennpiper]

Just my thinking.
The purpose of a "Spec Blind" is positive shut-off.
The motive is Safety
So.....
Are there any options for accomplishing the same thing.

Go to a Double Block and Bleed set-up with the ability to "Lock-out-Tag-Out" on the valves in the closed position when required.

Think about it.

 

[tickle]

I offer these thoughts as a possible solution.

Positive isolation is required. This could be achieved with a blind flange. A blind flange will be designed to cope with any pressure loads inside the pipe. Therefore you have got a thickness required. Just get a fabricator to make up the correct sized spectacle blind from either FRP or steel.

 

[gr2vessels]

jistre,
For the application you mentioned, is quite frequently used a pair of backing rings behind the flanges, to help the sandwiched spectacle blind seal, whilst preventing excessive bending moments on the FRP flanges. Typically, the sealing forces are low, but I have seen trough like supports to hold the two pipe ends and spectacle blind assembly in place...I'm sure you have seen those backing rings for the FRP tank's nozzles or the connecting piping...
cheers,
gr2vessels

 

[jistre]

Thanks for the responses. A couple of thoughts...

Pennpiper:
I considered that, but this isn't a safety matter, and the client wants a spectacle blind there. I have a situation where precursor product is made in separately owned plant A next door to our plant B. It is delivered to plant B via pipeline. This spectacle blind blocks a branch off of the main pipline that allows the precursor product to be trucked in for use in plant B if plant A ever goes down. The clients absolutely do not want anyone to be able to turn a valve and open this line, and they want the positive indicator the spectacle blinds provide.

Tickle:
The question isn't so much the pressure holding capabilities of the flange, as B16.48 flanges will handle the pressure. The question really is a geometric one due to the low ductility of the flange materials combined with the loss of full face sealing. We'd get high bending moments on the portions of the flange cantilevered out past the spectacle blind. I am looking at drawing up a custom spectacle blind with a B16.5 flange on each side, but with a B16.48 lumen machined into the other to be able to get the support we need on the flanges.

gr2vessels:
Do you have a source for these backing rings? Are they anything like the split backing rings you see used with elastomeric expansion joints? I want to look into this, as it sounds like it might be something that'll be helpful to know about for both this situation and future projects.

Thanks for the input, everyone! I'll probably be able to finish up with this now.