Nozzle reinforcement at head 1

[jtseng123]

Has anyone ever used head nominal thickness instead of "minimum thickness after forming" for nozzle reinforcement calculation? Assuming the nozzle size is less than 30% of the head diameter so it won't fall in the knuckle area.

This "minimum" thickness is not " code minimum thickness". It is thin out after forming, normally happens in the knuckle area, which is approximate 1/8" less than the nominal thickness.

 

[innovation2]

I wouldn't recommend this practice, however you could take an actual measurement of thickness and use that.

It is always best to error on the side of conservatism.

 

[CaedmonH]

Moreover, you should be careful with assuming that the knuckle region is the one with the highest thinning, depending on the forming process of the head it is also possible that the crown area is the thinnest. Only dished and flanged heads have the thinnest wall in the knuckle region, and this is only a rule with still a few exceptions. I'd propose to follow innovation2's advice and be a bit conservative here.
A thickness measurement might help, but you need to test several areas to have a trustworthy knowledge of the actual thickness in the nozzle area.

 

[gr2vessels]

It is a bit tricky question, because the code is not specific on this question. It is implied that you'd use the minimum calculated thickness for reinforcement calculation and the excess is between the minimum calculated thickness and the estimated minimum thickness after forming. The ASME BPVC does not have provision for recalculating the reinforcement after the reception of the heads. By then, your calculations have been reviewed and approved, finalised.
However, I have typically used the plate nominal thickness for the nozzles located in the centre of head, when I have had the confidence the thickness will not be altered. For other locations, there might be a chance of unexpected thinning.
I would say, you judge the location and the risk of thinner wall than expected.
Cheers,
gr2vessels

 

[EngAddict]

You can't use nominal but you can specify two different forming allowances if required. Specify something like not to exceed 5% over the 60% diameter limit and 15% for the remainder. The head manufacturer always does a dimensional check to verify compliance with your order. If they are already made then you probably will need to get the heads re-checked. I have seen the thinnest point vary a lot in dished and flanged ends, it can often be within the 80% diameter limit, therefore this would be a last resort if you can't change the design.

 

[jamesl]

It is interesting to see comments from experienced people on using nominal head thickness in nozzle reinforcement calculation. The Code rule is to use minimum specified head thickness as the "t" in UG-37 calc. This is the thickness that is to be shown on Form U-1 as the minimum head thickness, not the nominal plate thickness that is to be used to form the head, even if the head thickness at the nozzle location is close to this nominal thickness.

 

[gr2vessels]

You are correct jamesl unconditional.
However, the use of nominal thickness in some cases is a common practice outside USA when bending a bit the rule could save significant money, without compromising the safety intent of the code.
Cheers,
gr2vessels