Slip On Flanges in Steam Service 3

[engineeringguy]

I have a client requesting slip on flanges for a steam application. I have another client who is adamant that this poses a safety concern and will only accept butt or socket type connections in steam service (ANSI 1500).

I've tried to get a full answer to why this is but I cannot get a rational response. The best I can get is that slip on flange connections are not as strong as weld neck or socket type connections. Why?



"If experience was always the only factor, how would we get to the moon?"

http://www.piping-designer.com

 

[mtngreen]

I do not work with steam pressures that high but in my high pressure water applications I use SO flanges for two reasons:

1. The connection to the pipe is via an external weld and an internal weld...this is twice the connection of a BW or SW.

2. Also, when the welder installs the flanges/gaskets/valve they typically hand-tighten the bolts and tack the flanges in place before completing the full welds, ensuring a perfect mating of flange/gasket/valve.

I'd be interested in others opinions as well.

 

[pennpiper]

There are a lot of unknowns here.
We do not know the pressure or temperature of the steam system in question.
We do not know what type of plant this is.
We do not know where this plant will be built.
We do not know who the client is.
and
We do not know if this project was bid (by your company) as a Lump-Sum or Cost-Plus contract.

All of these things can and often do have a bearing on material choices.

Since I do not know the answers to all these questions I will offer the following.

Piping R. O. T.
(Rule Of Thumb)
1 - There is a rule for everything
2. There is an exception to every Rule
3. The Boss is always right
4. Remember Rule #1

In the case of your Steam project your Client is the boss. If the Client wants weld neck flanges for the steam system then put in weld neck flanges.

If the project was bid as a Lump-Sum project and your bid included Slip-On flanges then you have a valid reason for a Change Order.

 

[engineeringguy]

I like the rules of thumb. The client is always right. In this case, the client is a major oil company who is injecting steam at ANSI 2000 psi & 500°F.

I did find a bit of answer to my question by searching and re-searching and searching the re-searching. (From this site:

http://www.eng-tips.com/viewthread.cfm?qid=154470&page=1)

Don't do it. Main reason is that 31.3 only gives you 30% of the fatigue strength/life allowance for a slip-on vs a buttweld. Plus you can't RT the slip-on weld.

Note that the piping codes do not consider a fillet weld to be a strength weld in the same vein as a full-pen buttweld, ergo the reduction in fatigue life allowance.

So my understanding is that the fillet weld on a slip on does not have the same strength as a full penetration butt weld on a weld neck.

I would like to try to round up some numbers though. . .



"If experience was always the only factor, how would we get to the moon?"

http://www.piping-designer.com

 

[PAN]

engineeringguy,

You have both high pressure and temperature steam. Welding neck flange is a good choice in my opinion. This requirement is not only from some client. Some process licensors also give the same comment.

 

[mizzoueng]

The common thought is that SO flanges are easier as they "slip on", but they are actually harder as you now have double the welding to do and this is twice the risk of having a bad weld which could lead to failure.

You found some good info, a full penetration weld is much stronger than a fillet. Also, WN flanges have higher pressure ratings than SO's.

Depending on your size, you could use socket weld fittings, but again you are just using a fillet weld to hold that on. But on SW fittings, the fillet is equal (should be) to the thickness of the mating flange that the pipe is going in, which is usually 1/4 to 1/2 inch. For small diameter pipe this makes a very large fillet and will hold nearly forever if done correctly.

As you are handling steam at this high pressure and temp, are you following B31.1 or B31.3 code? I would assume you are getting steam from a nearby generator, if this line is coming off a steam generator then you need to follow B31.1. If it is a simple steam dump line coming from a steam turbine, I think you can get away with B31.3.

Either way, use WN flanges.

As for the SO client, call your local flange rep (NIBCO or someone) and get the flange catalog. Tell them what you want to use the flange for and they will let you know right away if it is acceptable for the code you are using.

 

[11echo]

I work in the oil fields in central Calif. We do alot of steam generation and steam well injection in this area, AND I see S.O. flg.s used all the time. They can you a bit of flexibility when tie-in to existing systems. AND if you'll notice, most vessel fabricators use S.O.flg.s for nozzle connections.

 

[engineeringguy]

11echo

I'm in the central valley as well. I'd be willing to be that we've crossed paths at one time or another.

One of my previous clients had this hard written into their Safety In Design Manuals. Another allows only Slip On at low pressures (300 and below) of non sour processes. I agree that it gives you flexibility when you tie-in to existing systems.



"If experience was always the only factor, how would we get to the moon?"

http://www.piping-designer.com

 

[bosse1950]

Just a general comment - SO Flanges, although common in the past - are a maintenance departments nightmare - during the lifespan of a piping system (20 years or so) wall thickness checks & weld joint integrity checks are routinely performed on all systems - the S.O. Flange does not allow testing on the internal weld. I would ask the question - Why is a slip on flange required? When in doubt - leave it out.

 

[DSB123]

missoueng,
You state "Also, WN flanges have higher pressure ratings than SO's."

Can you point me to where in ASME B16.5 the pressure/Temp tables show higher values for WN flanges? A 300# WN Flange has the same press/temp rating as a 300# slip-on flange as far as I can see. The press/temp tables in ASME B16.5 do not differentiate.

 

[doct9960]

engineeringguy,

Also take note that from Class 600 through Class 1500, you can only use B16.5 slip-on flanges up to NPS 2 1/2". At Class 1500, you can only use socket welding flanges up to NPS 2 1/2". Welding necks you can use up to NPS 24" at those rating classes. As probably mentioned by other forum members, SO are not as strong as WN for cyclic conditions.

 

[vesselfab]

echo,

""most pressure vessel manufacturers use so flanges"" is a pretty broad statement.

I can't remember the last time I used a slip on flange for anything other than a non-pressure application.

We use WN as our standard flange, and LWN and their HB, V1-3, F variants as required.

Kind of like a studding outlet. Some use them, but we choose not to

 

[moltenmetal]

There are definitely code limitations on the use of single-welded slip on flanges, but I'm not a code expert and am not aware of any limitations on SO flanges where both welds are completed. As a standard practice, we just plain don't permit slip on flanges to be used unless you do both welds.

I'd differ with what was stated earlier: two fillets ARE easier and quicker to do than one butt weld completed 100% from the outside and which must pass RT. Both welds are available for inspection visually and by MP/LP etc. when the piping is disassembled- just not RT. And contrary to another poster's assertion, you have 1/2 the risk of a single weld leading to a failure since you have two welds EACH of which should be strong enough for the job.

Some folks drill a small hole through the flange to vent the cavity between the two welds: it helps vent trapped air during completion of the 2nd weld and helps detect cracks in the face weld earlier.

At 2" and smaller we use socket weld flanges: one fillet weld is quicker to do than two.

There's a lot less material in a SO than in a WN- a benefit if you're using something other than carbon steel and LJs are off the table.

All of that said, I just plain STOP using either SW or SO beyond 600# class on piping. As body flanges on small vessels made of piping, where the flange never sees moments or forces other than hydrostatic and bolt loading, I'll permit their use beyond 600 class.

In piping, the stress concentrations on the fillet welds and the thermal stress/cycling fatigue risk, and the residual stress on these welds due to the heat input during completion of the 2nd weld and subsequent shrinkage make a SO flange riskier than a single butt welded WN, and make a SW flange very risky. Hence the reduced fatigue life stated previously.

Don't forget that they also need gaskets with a bore equal to the OD of the pipe, NOT its ID. Use the wrong spiral-wound gasket and you could have hte gasket unravelling inside the piping...

 

[mizzoueng]

DSB123,

I will see where I saw that. I primarily work with B31.1 and rarely use SOs due to this.

 

[ApC2Kp]

To: engineeringguy, and mizzoueng

Where the ASME B31.1 Power piping code would apply (inside a electric generating plant, or on Boiler External Piping),

Para 104.5.1

104.5.1 Flanges — General
(A) Flanges of sizes NPS 24 and smaller, that are manufactured in accordance with ASME B16.1 and B16.5,
shall be considered suitable for use at the primary service
ratings (allowable pressure at service temperature)
except the slip-on flanges to ASME B16.5 shall be limited
in application to no higher than Class 300 primary pressure
service rating. Refer to para. 127.4.4.

Para 122.1.1 (Boiler External piping)

(F) American National Standard slip-on flanges shall
not exceed NPS 4. Attachment of slip-on flanges shall
be by double fillet welds. The throats of the fillet welds
shall not be less than 0.7 times the thickness of the part
to which the flange is attached.

(H) American National Standard socket welded
flanges may be used in piping or boiler nozzles provided
the dimensions do not exceed NPS 3 for Class 600 and
lower and NPS 2-1?2 in Class 1500.

 

[Stainlessteve]

The main reason for never using SO on HPS is turbulance,the least resistance cause's the the least wear.Just as over penetration and misalignment of fanges.

 

[11echo]

vesselfab ...Well I guess I must amend my statement to ..."Most vessel I SEE HERE (central Calif.) use S.O.Flg.s for nozzels". World wide I don't know if this is the norm, but as I say what I see around here it's pretty Std.!

 

[vesselfab]

ohhh...that explains it.

oil field production stuff
little separators and that kind of equipment

 

[jte]

An interesting thread, to be sure. I'll throw in my perspective: WN's beat SO's except for ease of field fitup and sometimes the thought that SO's can be "hydrotested" without putting water in the pipe.

I've worked with some folks who prefer SO's for field welds since the cavity in between welds can be drilled and tapped and pressurized to "hydrotest" the welds. The state of stress may be completely different, but yes, the "hydro" may detect a gross flaw in the welding. On the other hand, since the pressure is on "the other side" the pressure may put a crack into compression and not show the flaw. A SO is definitely easier to fit up to existing piping, no argument there. Perhaps "most" vessel fabricators will use SO flanges as their standard approach when they are not prohibited since they are cheaper and the fabricator will not have to deal with the problems later anyway. I don't recall ever accepting a vessel with SO flanges, much less specifying one. Much of the time in California, by the way.

Last time I checked, we don't use SW couplings often in refineries and upstream piping. Most of the time when I have seen pipes welded to pipes it is with a full pen butt weld. Same with vessel shell courses and head to shell joints. Makes one think there may be a reason. Makes one think maybe a butt welded pipe to flange joint might be more reliable. From a fatigue perspective, a butt weld vs two fillet welds is no contest. The fillet welds have a fatigue factor of as much as four applied to them when assessing them for fatigue life. Also consider thermal shock issues. Butt welds will simply handle the shock better than one fillet weld "fighting" the other in a SO.

So, in general, and in particular for high pressure steam, there is no way I'd support SO flanges when WN are a reasonable option. How many responders here have actually walked past a high pressure steam leak? Low pressure is one thing, pressures requiring ASME B16.5 CL-1500 are a totally different, and much scarier leak. Much harder to put a temporary patch on also. When the consequence of failure go from "annoyance" to "likely injury or fatality", the "cheaper and easier to fit up" arguments tend to wither.

Some will agree with me; some will disagree. That's ok. An open dialogue / vigorous discussion can be very enlightening.

jt

 

[DSB123]

missoueng,
Still waiting for the reference which shows a 300LB WN can sustain a higher pressure than a 300LB slip-on!! Have you managed to find it yet??