MINIMUM NOZZLE SIZE

[PEHasan]

What is the minimum size of nozzle that can be
welded to any pressure vessel? As part of good
engineering practice, we usually take 2 inch as the
minimum size for any pressure vessel. Is there any
clause in ASME Code that refers to this matter?

 

[unclesyd]

I don't know what the latest edition says but we have thousands of 3/4" and quite a few 1/2" pipe nozzles in pressure vessels. Any thing smaller we use a tapped or flange boss in the shell. All the small nozzles under 2 1/2" are at least Sch 80.
Some problems with small nozzles are the attachment welds and the actual physical strength of the nozzle under external loads. We try to use a Flanged Pipet or Weld-O-Let if possible.
I would suspect that the limit to 2" nozzle is based on a strength requirement to prevent damage.

 

[deanc]

There is no requirement. Just what is practical.

 

[jte]

I agree with unclesyd and deanc. We have thousands of 3/4" connections, usually a 1/2 coupling. I don't have TEMA handy, but didn't TEMA require auxilliary connections in heat exchanger nozzles? We often have a 1" and a couple of 3/4" connections in our HX process nozzles. These are usually plugged and provide another leak point and deadleg for corrosion. I'm not a fan. So while there is no code limit on minimum nozzle size, you have to consider the service the vessel will be in. The way I was brought up in the oil refining vessel design business I was tought to use a 2" min nozzle size. Where I'm currently working our standards call for a 2" min (1 1/2" for thermowells) and I fight to upgrade "replace in kind" vessels to that standard. Sometimes we don't due to issues mating up to existing piping. If I'm connecting to existing 1 1/2" piping I make sure we're using LWN nozzles which actually are stronger (in bending) than a typical built up 2" nozzle. I'd attribute the minimum nozzle size to both mechanical strength (some clown will use the nozzle as a ladder etc) and plugging issues. Though I've seen 2" nozzles plugged, its a lot easier to plug up a 3/4" nozzle.

unclesyd- I can't think of any nozzles (of any size) which I've seen which are made of CS or low alloy pipe and Sch 40: With any CA it would be hard to meet UG-45(b)(4), though you could argue that the 12.5% tolerance provides some CA...

jt

 

[unclesyd]

The largest majority of CS piping we have is for service that is non corrasion and in not danger of plugging up. The biggest user of lines less that say 2" are on Therminol Vapor Systems as vapor and vent jumpers and the drains. As you state where possible we stay with the larger sizes if at all possible.
Where smaller nozzles get a little tricky is like in one of our processes where the process material will freeze or plate out on the walls if the velocity isn't kept up. So sometimes small is good.

 

[PEHasan]

For any nozzle below 3" size, we choose the flange type as Long Weld Neck Raised Face.