Nozzle opening calculations

[maxh]

Hi

I am wondering how to consider an opening in a pipe manifold.

I have a 12"NB pipe manifold that at the end terminates with a standard pipe cap. However, what I wish to do is cut a 6" i/dia hole centrally in the 12"NB pipe cap, then weld a 8"NB pipe onto the pipecap in such that the 6" hole in the pipe cap acts as a restrictor orifice, and the 8" pipe acts as a continuation of the manifold to elsewhere.

Flow would then pass along the 12"NB pipe through the 6" hole and exit into the 8" discharge pipe.

I am aware that I would need to do some reinforcement calculations if this were a classic nozzle opening, however, in this case I am unsure of how to apply code rules, or even if they apply in such a case.

I do know that normally you wouldn't put an 8"NB nozzle into an 12"NB cap, however, this seems an elegant cheat, but perhaps not a legitimate one.

Any ideas / advice would help.

 

[panduru]

The basic principle was supposed to be that you only have to replace the quantity of metal lost in the opening by providing a reinforcing pad of the same mass. If this is the only criterion, then one ought to have no doubts on the reinforcement requirement calculations.

Further comments/ corrections most welcome.

 

[JStephen]

The reinforcement rules vary somewhat by the design code used. One thing that is usually the case, though, is that the reinforcing that is required is equal to the required metal that is removed, not the "as built" metal that is removed. In something like a pipe cap, if the operating pressure is near the maximum for the fitting, you probably need to look at it in more detail. If the operatating pressure is far less than maximum, you may be able to show adequate reinforcing in the nozzle neck and the excess plate thickness around the opening.

B&PV code may prohibit an opening in a head this large, or have special design requirements for it, but there again, it would vary by code and application.

 

[panduru]

@JStephen: Thanks for reminding me, you are correct: we can utilize excess thickness available in the shell at least to partly compensate for the loss in the opening, such that only the balance of mass needs to be made-up by way of the pad. [speaking in terms of the BPV Code]

 

[SnTMan]

maxh, how about a 12 x 8 reducer with an orifice welded inside? Avoid the whole reinforcement issue.

Regards,

Mike

 

[maxh]

Would love to use a reducer SnTMan but we have some internals welded inside and we are scratching for every last mm of straight length. We are even having debates about few mm for this or that part, and this is essentially a fabricated product !!

I will check the excess in the shell and nozzle, hopefully they will give me the required numbers.

As a thought, in this situation if my orifice hole were very small, say 1/4", rather than 6" would that affect the way people thought of this problem ?

 

[SnTMan]

maxh, philosophically, you might think of this as similar to a stainless nozzle (zero corrsion allowance) "set on" to a carbon component (positive corrosion allowance). This produces a case where the "finished opening diameter" is different from (larger) than the actual nozzle ID.

I don't think there is any problem with doing such a thing, its just that most canned programs will not handle the various dimensions properly in the calculations.

However if the "finished opening diamater" is very small, the actual attachement may be outside the limits of reinforcement. Again if the calculations are done properly and the design meets Code in all other respects, I don't see any real problem.

Regards,

Mike

 

[gr2vessels]

Maxh,

The Code does not prohibit your design / arrangement. It is only to solve the process probems (hydraulic calcs, cavitation, erosion, pressure drop, design pressure and design temperature, etc) and the mechanical problems (estimate the loads on the 8" nozzle / pipe extension, calculate the stresses at the junction with the cap and compare with the limits set in WRC 107 or BS 5500). Estimate the required / available reinforcement and weld-in the pad, if required. If needed, you can include vibration analysis or any additional external loads you can imagine (and quantify), all this is acceptable per any PV Code, provided that the allowable stresses set in the Code are not exceeded.

 

[GenB]

with 8in you are at near the nukle and best would be the 6 in on the cap and then increase it to 8 in,
then you can cut and inset the 6in with full penetration and if still need reinforcing, you reinforce it, so what?
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genb