API 620 and fiberglass

[jtk921]

I have a large silo constructed to the API 620 standard which is rated for 3.6 psig MAWP at 176 F. The top of the silo is 100 ft. up and we have a very heavy 20" 150# manway cover on top that we would like to replace with a fiberglass manway cover to make access easier and also because we would like to mount a new conservation vent on it. Is this allowed under API 620 as long as the thickness is adequate?

 

[TD2K]

I don't know the answer to your fiberglass question but couldn't you design a thinner flange that will meet your design conditions? A 20" class 150 blind flange is nearly 1 3/4" thick, you don't need that much metal for 3.6 psig.

 

[JStephen]

That's a good point. However, I think API-620 does specify a minimum design pressure of 50 psi for cover plates, so it won't be as thin as you'd expect. One of the appendices addresses flange design.

 

[TD2K]

Can the customer decide not to have the silo designed to API 620 anymore and then they can design a flange cover how they want? Obviously you have to ensure it's properly designed for the pressure.