Slip on flanges for normal fluid services

[Xk333]

Is it a good practice to use slip on flanges for normal fluid services for flange rating 150# instead of weld neck flanges to save cost?

Would you recommend slip on flanges for steam service of 10 bar and 30 bar pressure?

 

[GD2]

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.

GDD
Canada

 

[Xk333]

GD2,

Thanks for your reply.

I would like to clarify my query was regarding flanges in Normal fluid service piping system in accordance with ASME B31.3.

Can you please suggest when is it absolutely necessary to use weld neck flanges and avoid slip on flange?

 

[XL83NL]

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.

 

[akpipelineengr]

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.

 

[Xk333]

XL83NL,

Thanks for the reply.

I belive the temperature and pressure range from GD2 is from 16.5 pressure temperature ratings from 300# ratings.

 

[Xk333]

akpipelineengr,

Thanks for your reply.

I think I better discuss this with client itself mentioning possibilities of errors in Slip on flanges as compared to weld neck flanges.

 

[LittleInch]

The key question is whether SO flanges actually save you any time or money.

You should be making two fillet welds, one on the outside and one on the inside vs one butt weld.

Personally I like the look and guaranteed integrity of a butt weld and they can be xrayed if needed to check.

It's too easy for a SO flange to be made up without the inner weld and when bolted, no one wants to unbolt it to check.

I've never liked them, but accept they exist for what should be low pressure, low hazard materials. But that's personal preference.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TugboatEng]

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.

 

[r6155]

Never use socket welding for high or low temperatures.

Regards

 

[Xk333]

LittleInch, TugboatEng,

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.

 

[Xk333]

r6155,

Thanks for you response.

What temperature range do you suggest for socket welded fittings?

 

[GD2]

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.

GDD
Canada

 

[r6155]

(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.

Regards

 

[Xk333]

GD2, r6155

As mentioned by you I'll refer B31.3, hopefully it will help me make a decision.

 

[1503-44]

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."

 

[david339933]

Using good engineering sense...yes. Done all the time.

 

[1503-44]

But often prohibited by short-sighted bean counter budgeting.
This is mostly 1" stuff here. How much money you gonna save???
False economy.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Xk333]

1503-44,

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.

 

[Abloom]

SS lap joint stub ends and flanges. 150# NB2in and up. This is your process, utility, & waste service.
Steam & condensate cs socketweld sch80