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How much forcing is allowed when connecting flanges

How much forcing is allowed when connecting flanges

How much forcing is allowed when connecting flanges

(OP)
We have burner fuel gas piping to be connected to the burner inlets and it was found that the flanges aren't perfectly level. Production people of course pull and push and manage to get the flanges closed but my question is in this case, the line and the bolting connection itself is under tension, so how much of this tension can we tolerate and how does it change depending on pipe size or pressure rating?
More info: the line size is 3/4" (haven't got all the papers on me right now and do not remember pipe class and w.thck.)

RE: How much forcing is allowed when connecting flanges

(OP)
Forgot to add length of the line, which is around 400mm

RE: How much forcing is allowed when connecting flanges

Cold sprung? Depends on how much thermal expansion it will have at operating condition.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: How much forcing is allowed when connecting flanges

(OP)
BigInch, we do not have any cold springing procedure or any plans like that, the misalignment of flanges is simply due to poor pre fab and not a planned thing, so they just pull it in place and tighten the bolts. This is a very short and smail line with no high pressure or high temps - I do not think there is any thermal expansion at this item. I lean towards this not being a big concern -do you agree?
Thanks

RE: How much forcing is allowed when connecting flanges

No. A relatively small deflection on a relatively short pipeline can still translate into a very high stress.

2 mm stretch / 400 mm = .005
Stress = 0.005 * E = 150,000 psi
... rip.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: How much forcing is allowed when connecting flanges

(OP)
BigInch, sorry for my complete ignorance - can you give me a reference to a code where I could find this formula?

RE: How much forcing is allowed when connecting flanges

Any strength of materials text.
elongation, Δ
Δ = P * L / A / E
P = axial load
A = cross-sectional Area
L = length
E = Young's modulus = 30,000,000 psi for Grade B steel pipe.

P/A = stress = {Δ]/L * E

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: How much forcing is allowed when connecting flanges

The question of permissable coldspring in piping systems has been around for a long time. Coldspring was permitted by the older ASA B31.1 Power Piping code has been occasionally used in construction of high temperature piping systems. The idea was to coldspring the system in the "cold condition" and reduce the piping reactions on nozzles and equipment in the "hot condition.

The problem, of course, was that the very systems where coldspring would do the most good (large diameter steam systems) were the most difficult to physically "spring". Come-alongs and hydraulic jacks were used to perform the "spring" along with welded lugs attached to the pipe

Coldspring was discussed in the "Piping Handbook" by Nayaar

The US nuclear power program has also struggled with this issue during the plant construction salad days of the 1980s.

As I recall, the de-facto ruling from the utilities/USNRC was that the spring could be ignored if it could be performed by one man without any assistance from jacks, come-alongs, levers or similar devices....

Just my recollection....

RE: How much forcing is allowed when connecting flanges

(OP)
Gents thank you for your valuable comments but you two bring up sort of a contradictive statements. From BigInch I understand this is a concern 150,000 psi seems awful lot of stress but from MJCronin's experience it seems that USNRC wasn't too much concerned about it :))
I think I'll just go with the flo...

RE: How much forcing is allowed when connecting flanges

What MJ Cronin said was the NRC wasn't concerned if one person could do it without "any assistance from jacks, come-alongs, levers or similar devices." (I also don't know if that's entirely true -- I've seen cold sprung pipes at nuclear plants where it took multiple people and come alongs and ... ummm, I won't mention where or the problems they had and ...)

BigInch just gave you the equation (awfully nice of him wasn't it, there's a way you can thank him); you need to make a decision as to whether that's ok or not -- your pipe, your decision. You have a small diameter pipe (3/4") and a short length (400 mm.) Is it within your piping allowable? You also don't mention how far out of alignment it is, which is a pretty critical dimension.

Which leads me to another question -- whenever I see mixed units and a new user, I start wondering if it's a student problem. Please come back and reassure us that this isn't a homework assignment.

Reason I'm asking is that I know

RE: How much forcing is allowed when connecting flanges

It's not that common to see mixed units in homework, so I doubt it is homework.

There was no rotational misalignment in my example. You'd have to add bending stress to correct a rotational misalignment. Moment required to do so /section modulus.

Even if cold spring is allowed, you cannot overstress the pipe (even though that would be in the opposite sense from the loaded condition) during installation. So, no. This pipe is far too stiff to consider rectifying any misalignment by coldspring.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: How much forcing is allowed when connecting flanges

(OP)
EnergyMix, thank you too for your input and I thanked BigInch as well for his formula - yes it is very kind of you guys to share your knowledge with me. And yes, I'm a student and will always be one till the grave perhaps but no, I'm out of study rooms already and on site where not many are concerned really about quality and therefore I have all these questions, because everyone says Ok to almost anything contractor does, as long as it finishes the job on time.
The misalignment isn't that great 2mm max I would say. Almost always one guy can manage to get the job done without any help of come alongs or chain blocks.
Perhaps I should communicate this data to someone in our head office...(which always aggravates the CM on site of course).

RE: How much forcing is allowed when connecting flanges

Plug in 2 mm and solve for P to find the load required to stretch it 2 mm (given that the opposite end is held rigidly, fixed in space). If you drag the opposite, "fixed" end along, then P is much reduced.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: How much forcing is allowed when connecting flanges

OCJT, thanks for clearing that point up.

With only a 2mm displacemnt and a 3/4" line, I wouldn't be that worried. That seems to be within the boundaries of what could be nudged in place without much problem. But as a long term fix, can the piping be moved over by the 2 mm?

Interesting cases are when you see people trying to align two 12" diameter pipes off by 2" -- and one of them is rigidly restrained at one end. Crew of six big guys sweating with come alongs. It totally amazed me.

RE: How much forcing is allowed when connecting flanges

(OP)
Thank you BigInch. EnergyMix, it is indeed a bit too much - and did your QC accept that?

RE: How much forcing is allowed when connecting flanges

I think the "thank you" hint was to star his post ;)

RE: How much forcing is allowed when connecting flanges

It depends on if you overstress or not during the installation procedure whether QA/QC accepts. Installation combined stress should be held to less than 0.9 * material yield stress. With a very large diameter pipeline it can take a D8 to move it over just a hair, if there is just a short length sticking out of the dirt. We limit most deflections to what you can get only by allowing some sag from the weight of the pipe alone. You don't have much of an option when the length is only 400 mm,even if the diameter is only 3/4". A "trick" we use a lot is to wait until the afternoon after the warmth of the sun finally closes the gap for us. If your pipe fitter is good, he'll cut it at the right length in the morning so that it fits nicely when you're ready to close the weld later on in the day.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: How much forcing is allowed when connecting flanges

Everyone here has addressed the issue about the coldspring and its effect on the pipe. There is, whoever, the additional issue of the effect on the flange.

RE: How much forcing is allowed when connecting flanges

No one thought about ASME B31.3 para 335 and ASME PCC-1 appendix E?
Seems to answer the OP's original question...

RE: How much forcing is allowed when connecting flanges

I haven't found many cases where, if the pipe works, the flange fails, so I don't often go there. In the case I've assumed, the pipe failed, so I didn't really have to check the flange, but that is the next step.
Most common for me are high compression loads anyway.

Go for it.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: How much forcing is allowed when connecting flanges

BigInch - I would generally tend to agree with you, provided that by "flange fails" you mean that the flange breaks. My experience is that the flange problems are generally that they leak, especially under external bending moments. This can be overcome by appropriate assembly bolt loads, but...

RE: How much forcing is allowed when connecting flanges

Forcing flanges to fit can generate leaks. A little is OK (inevitable actually) unless the pipe is totally inflexible, i.e. a short, straight spool- then fit-up has to be as good as you can get it. How much you can force the pipe to fit without overstressing the flanges is difficult to calculate, because you're talking about two different states as mentioned above: the cold condition under which you're doing the "forcing", and the hot, pressurized condition. Chances are you are not going to do any kind of accurate measurement of what total forces and moments were necessary to get things to fit. If both fit-up and thermal expansion exert forces and/or moments on the flange pair in the same directions such that the effects are cumulative, watch out. If one cancels the other out, you may be in luck. Personally I don't like to count on luck, but it's tough to make this anything other than an experiential judgment call.

RE: How much forcing is allowed when connecting flanges

you must have both axial and rotational flexibility to be successful. Short pieces of pipe,even of small diameter, arn't very flexible. Rotational flexibility means in this case that if the pipe bends, very little rotation is actually seen at the flange.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

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