Openings on Blind Flanges (Bolted Covers) 1

[carlosfrm]

I'm working on a vessel that will have a 6" Class 600 (1000 psi @ 400F) Blind Flange on a nozzle that requires a Centered Opening (1 1/2" NOM) and (6) 1/2" NPT Tapped Holes surrounding this center opening (Image attached).

I've read ASME B16.5 Table 6 and the 1 1/2" Centered Opening complies with the reducing outlet required if I wanted to use a B16.5 Blind Flange for this, but there's nothing regarding additional openings. I'm open to do a custom Appendix 2 flange for this, but when trying to design this in Compress I'm unable to add a Tapped Hole to the bolted cover instead of welding a pipe/coupling and threading that... I'd be OK with using the couplings but the weld requirement would overlap the reinforcements of all the openings I'm trying to add...

I've seen this configuration before in Commercial Plasma Reformers so there should be a way of doing this per Code, I just don't know where to look for Tapped Openings on Bolted Covers... Any help is greatly appreciated.

Note: I've read in another thread that 1/8" - 1/4" Tapped NPT openings don't require reinforcement calculations, but there was no source and I'm unable to find that reference, anybody heard of this? I might be able to change my 1/2" to 1/4" NPT's if this avoids reinforcement/calculation issues.

 

[pennpiper]

What are the six holes for? Please explain.

Sometimes its possible to do all the right things and still get bad results

 

[prex]

You can't of course neglect such multiple openings, even if they were 1/4". If you look at a section through a diameter crossing three openings, then you see that you have at least an equivalent opening with a diameter equal to the sum of the three diameters.
You should go through UG-39(b) or UG-39(d) to determine the reinforcement requirements and consequently the cover thickness.

prex
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[carlosfrm]

We are to plug those with a custom part, not really relevant for the Bolted Cover design.

If I go the path of designing the Blind as a Bolted Flat Head instead of a B16.5 Blind Flange with openings and considering that the center nozzle would be welded top and bottom to be a reinforced opening; Per UG-36(c)(3)(d) would the 1/2" unreinforced openings be considered a cluster? Would I have to consider the distance between centers to be 2.5*(d1+d2) or can I assume them to be isolated as per UG-36(c)(3)(c) and just have a distance between centers of (d1+d2) and the distance between the tapped hole to the reinforcement material of the centered nozzle at least of one diameter of the unreinforced opening (Since the Centered Opening would be a reinforced opening, per UG-36(c)(3)(d) the distance between the tapped holes would just have to be one diameter of the tapped hole to the reinforcement material)... Am I right?

I've attached an quick sketch to try to make it clearer.

 

[racookpe1978]

Why will you use NPT (tapered threads) in the 6x holes? They will NOT give you any consistent force when tightened, because every thread will bottom out at a slightly different depth of engagement.

 

[carlosfrm]

It's because of what goes through them, we are to insert fire rods through those holes for a burner, needs to be removable.

 

[racookpe1978]

Seems like the fire rod goes through the hole in the center of the 6x NPT threaded holes. Not the 6x holes. Again, I recommend using straight threads on those tapped holes. Not NPT.

 

[prex]

carlosfrm, if you do not take credit for the insert in the center opening, then the distance of the center op. to a peripheral one is smaller than 2 times their average diameter, so none of your openings is isolated (per UG-39(b)(2) or UG-39(e)(1)).
Otherwise, by taking credit of the insert (suitably welded), the diameter of the center is the inner diameter of the insert (nozzle) and that condition is likely satisfied, so all your openings are isolated and none of them requires reinforcement (provided the vessel is not subject to rapid fluctuations in pressure, see UG-36(c)(3).

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[carlosfrm]

racookpe, regarding the threads, I'm not 100% sure at this point if the ones on the fire rod are npt or straight, I'll have to double check, but it's not a factor for determining the separation between the openings. The fire rods actually go on the 6 holes since we will have 6 fire rods (commonly seen on plasma reformers).

prex, and that is one reason why I need the centered opening to be reinforced instead of taking the exception UG-36(c)(3)(a), so I can apply UG-36(c)(3)(c) to the other 6 and consider them isolated. My concern now is the interpretation of the following sentence on UG-36(c)(3) "The centerline of an unreinforced opening as defined in (-a) and (-b) above shall not be closer than its finished diameter to any material used for reinforcement of an adjacent reinforced opening". Does that refer to the weld material added for reinforcement OR does it also include the Flat Head material that is within the limits of reinforcement?

 

[prex]

That sentence is clear: "to any material used for reinforcement" means what it tells and includes the material in the flat head.
However if the center opening doesn't need reinforcement, that statement doesn't apply. Can't understand why you don't want to apply UG-36(c)(3)(a), it is there to be used.

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[carlosfrm]

prex, the thing is, I don't understand at what point multiple unreinforced openings are considered a cluster by UG-36(c)(3)(d)... Where does it say in the Code how to determine if it's a Cluster or Isolated openings? The diference of misinterpreting this is (d1+d2) VS 2.5(d1+d2)...

I found that I was using the diameter of the opening in the flange to calculate the separation by UG-39(e)(1) instead of the finished diameter, so since I'm using a 1 1/2" Sch 160 pipe I just needed a minimum distance of 2*[(1.687+0.84)/2]=2.527". By this I just need to calculate the thickness of the cover by UG-39(d) and as long as the distance between centers is higher than 2.527" I should be fine having them unreinforced, correct? I'll end up with a 3/4" thicker cover but at least I'll be sure that I didn't screw up interpreting UG-36 separation requirements...

 

[prex]

Let me try to rephrase everything as we (me included) made a bit of confusion:
1) As the opening CL distance is greather than 2.5/(d₁+d₂), per UG-36(c)(3)(d) you can't classify them as not needing a check of the reinforcement
2) So you need to check the reinforcement of all the openings
3) As the opening CL distance is greather than d₁+d₂, you can check their reinforcement as if they were isolated ( UG-39(b)(2) or UG-39(e)(1) )
4) Now you need to choose between UG-39(b)(1) and UG-39(d)
5) If you choose the former you'll certainly get a lower increase of the thickness (possibly nothing), also because you will profit of the material in the nozzle; however you'll have to check also against the last paragraph of UG-36(c)(3)(d) (unlikely to occur, remember that you don't need to take the available limit of reinforcement, but only what you need to get sufficient reinforcement)
6) With the latter everything is simpler, as you noted, you just calculate an increased thickness with respevt to the minimum required without openings

prex
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[carlosfrm]

Thanks prex, that's really helpful