OP,
Mechanical strength of slip-on flanges are typically 30% lower than WN flanges.
You can use these flanges for 150# and 300# class flanges.
For Group 1.1 material, you can use class 150 upto 300 C for 10 bar, class 300 upto 400C for 30 bar.
One reason for weld neck is the difference in weld detail (assuming a full pen weld). I wouldn’t go beyond approx 350 deg C using fillet or partial pen welds, so for me that is one of the reason to switch from SO to WN.
I have no idea where the boundary of 30 bar and 400 deg C as mentioned by GD2 comes from.
Application of high pressure, to me, in itself really isn’t a constraint for fillet welds.
Huub
- You never get what you expect, you only get what you inspect.
I assume you would double weld your SO flanges? If not, there are a number of potential issues (see 308.2.1).
In general, SO flanges are more susceptible than WN flanges to fatigue failure due to the fillet weld configuration, as well as installation errors like failing to weld the inside fillet. At least in oil & gas, many operators' piping specs lean heavily towards WN out of good practice as the cost differential is low unless you are installing a large number.
Another drawback of slip on flanges is the inside bore is larger diameter than the pipe bore. Expect additional erosion and corrosion at the flange faces. A lot more.
Thanks for your reply, I'm leaning towards using weld neck flanges for 50NB and higher sizes for process lines and slip on flanges for utilities like cooling water. For 15Nb to 25NB, socket welded flanges seems my only option.
OP,
You said the fluid category is "Normal Fluid Service". That means you have less limitations in terms of material use and fabrication requirement. Read Part 3 of ASME B31.3 for limitations.
The steam at 10 Bar even qualifies for Category D Fluid Service (limited to 150 psig at temperature limits of -20F to 366F). 366F is essentially the steam saturation temp at 150 psig.
You are right. My temperature and pressure limits drawn in my my earlier comment is drawn from ASME B16.5 for Group 1.1 material which I hope you will use for the Normal Fluid Service.
(OP), please read "low temperature" in B31.3
The gap in the socket is like a crack, so you need to consider how this crack represents a risk in your design.
A similar concept is that of the Slip-on flange.
It's not often you save money. Usually you just delay maintenance costs until it breaks, then and parts, collateral damage and shut down time costs more than a new unit.
--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."
I'm using weld neck for normal fluid services now. Just for cooling water, Potable water, Nitrogen, Fire water I'll be using Slip on flanges. Slip on flanges are generally from 50NB to 600NB in sizes.