Surge Capacity of an ASME B16.47 Flange 2

[Everynameistaken]

Hello All,

I am looking for some info on what the surge capacity is for a ASME B16.47 flange.

We are designing a high pressure piping system for a hydro plant, typically when the pressure is this high (6000 kPa) we only use CJP welded steel pipe. I have a situation where the client is asking about the use of flanges for the last connections.

We defined three performance levels for the system:

Normal operation
Emergency operation and
collapse prevention.

For these performance levels we have simple stress criteria of the pipe and run a full transient analysis for the system and can compare the von mesus stress to the limits.

My question comes when dealing with the flange! I understand we can compare the working limit of the flange for a given pressure class, material class and temperature against the normal operating condition. I know the test pressure for the flange is 1.5 times the working pressure.

What is the limit for the flange pressure when dealing with different performance surges? typically for emergency we use 0.75 fy for the steel pipe limit and collapse prevention increase further to 0.7 or 0.8 Fu, not sure how to work with surge and a flange, any help would be appreciated!

 

[LittleInch]

I don't believe there is such a thing.

Normally you design a system to Design Pressure to a particular design code or standard. That design pressure should not be exceeed other than hydrotest or an allowance for short term upsets. In most of those there is an allowance for surge ( between 10 to 30% above the Design Pressure depending on the code)

At 60 bar you're in class 600 territory which is good for a max allowable working pressure of around 100 bar depending on temperature, but for hydro you're in low temperature sections.

By the way what's CJP welded pipe? We don't always know everyones abbreviations
Assume Fy is yield and Fu ulitimate stress?

Flanges are simple things which are rarely examined in terms of stress and when people do, they use ASME VIII and find out the B 16 family of flanges are over stressed...

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[1503-44]

"we use 0.75 fy for the steel pipe limit and collapse prevention increase further to 0.7 or 0.8 Fu," Who's we?
See your design code for allowable stresses. ASME/ANSI B31. 3 for the chemical and petrochemical plant piping systems specifies a maximum allowable surge pressure of 1.33 times the design pressure of the system. For long distance pipelines, ASME/ANSI B31.4 specifies 1.1 times the design pressure. B31.8 says 1.0

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[bcd]

Limit for flange pressure is usually stretching of the bolts and blow-out of the gasket. Not the structure of the flange.

 

[TGS4]

I agree with bcd. Leakage is your primary consideration for flanges, not mechanical strength. And if you have a fire event, not adding to the fuel load is a top consideration.

Use the rules in ASME Section VIII, Division 1, UG-44(b) to assess your risk of leakage - and do not take any additional exceedance.

 

[Everynameistaken]

Thanks for the info so far.

A couple of clarifications. We design the transmission piping using AWWA M11 and ASCE No. 79 which are guideline used for penstock design. Not strictly a code like some of the pressure and process piping codes noted above but covers lots of detailed information. When it comes to flanges it is not quite clear, why I am clarifying.

The normal operation performance pressure is the pressure we expect to see everyday or when the main flow is shut down in a planned controlled manner, there maybe an increase of 10% in pressure from static. The emergency operation occurs when the system needs to be shutdown quickly, but in a controlled manner, it may only happen a few times in the life of the project, the pressure could increase by 1.25 to 1.5 times the static. The collapse prevention is an extreme condition, a mechanical failure and is likely to never happen but if it does the surge could increase the pressure in the order of 2 times the static.

As far as I can tell ASME notes (this is a quote from ASCE No. 79 on ASME flanges) "the allowable pressure for each rating class, defined as the maximum allowable non-shock working gauge pressure"

Based on this I think there is an argument for that only Normal operating conditions are the only non-shock pressure. If the emergency condition (which can have quite a high surge or shock) is also must be under the allowable pressure rating then that would certainly bump up a flange class that may only be seen a few times every. The collapse prevention is definitely a shock condition, may never happen but if it did almost certainly only once, how far above the allowable pressure or working pressure rating could be considered acceptable?

ASCE No.79 has limits for other elements of up to 0.9 of the ultimate tensile capacity, this could be well above the yield of the material?

PS CJP stands for complete penetration weld

Thanks again

 

[LittleInch]

I'm not familiar with the particular codes you mention but I am troubled a bit by your theory that just because things may only happen "a few times in the life of the project" that this somehow would allow you to greatly exceed the maximum working pressure of the flanges.

This just doesn't work for me.

True - it may seem that the pressure rating of your flange is considerably higher than your "normal" operating pressure, but, IMHO, you need to design your system to be able to handle anything you can think of and calculate in the lifetime of the project. Now if that extreme event is only anticipated a few times and for short durations, design codes allow for a margin above maximum operation pressure for components like flanges by a range up to 30%. If your code doesn't explicitly state a figure then I would use something in that range, but not more. Whatever it is it should be less than your test pressure.

If ASCE79 has some allowable stress values up to 0.9 of UTS then this is really high and sounds like a plastic design strain based issue where the item is being allowed to yield but not break. At that point though it should then be replaced afterwards. Sounds rather hairy to me.

In short, flanges to ASME B16.47 are based on pressure not yield or UTS. That's all you have to work with unless you want to take a 16.47 flange and then do FEA on it to establish some other pressure rating based on yield or UTS max values in your penstock codes and COP. Then you're on your own and no longer have the fall back of compliance with the a recognised national standard when or if something goes wrong. Your choice, but a good attorney for the other side would have a field day with that approach unless it is really well documented and signed off by all parties including the owner.

Let us know what you plan to do.



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

 

[1503-44]

Agree with LittleInch. Surge pressures, no matter how infrequent, are normally limited to far less of a design factors than what you seem to propose, with pressure relief systems being required as well to prevent higher levels from being reached. And as others have said, the flanges themselves are probably not the most critical item, unless you are talking about flange separation.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[Everynameistaken]

Hi All,

Thank you again for your help and opinions.

I am looking into it a bit further but at this stage, I will be sizing the flanges (for initial budget pricing) based on the Emergency pressure as part of the maximum working pressure. This will put me in the 600# range, pretty high, and once we have our final transient analysis completed I can compare the finals values from the Collapse prevention with the Emergency to see what the increase is and make a judgement on then.

Either way we will be documenting things clearly and having owner sign off before installation.

Thanks again,

 

[LittleInch]

No problem,

For me class 600 isn't particularly high, but each to his own.

When you make a decision try and let us know what you do and why. It all adds to the knowledge. Each post stay open for 6 months for updates.



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

 

[1503-44]

For initial project budget estimates, erring on the higher side of cost of materials is far more favorable to your long term career plans than getting it too cheap now and giving everybody a big unpleasant surprise later on.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[saplanti]

Everynameistaken, I would suggest you to check the flange class of turbine emergency shut-off valve. If I was your customer, I directly compare those two. If yours is lower class I would question it, perhaps eliminate your quote eventually.

In hydroturbine structure every client would expect the shut-off valve and intake pipeline structure shall take all the loads without any problem in any condition. Otherwise they will face a disaster.

In my experience I used to use ASME VIII flange calculations with the company standardised dimensions using o-ring seals (metal-to-metal contact flanges). In this scenario bolt circle diameter was as close as possible to the outside diameter of the pipe which allows adequate manufacture, sealing and bolting. This application had given us lighter flanges for the same pressure classes than ASME B 16.47. We always limited the allowables within the ASME VIII rules under all type load conditions.

Hope it is satisfactory.

 

[Everynameistaken]

Saplanti,

That is a good idea, I can take a look and see what they have provided.

Typically we find that the turbine suppliers do not check the transient pressures associated with a catastrophic failure of their equipment (they always claim that never happens, however there is recorded evidence of it happening in the industry). In the case of a small turbine, this could amount to an almost instant stop of 50-100% of the flow and this can cause a huge pressure spike.

I do generally agree that the Normal and Emergency conditions (which will probably occur at some point in the project life) should be under the pressure rating. It is the catastrophic case where all we care about is the penstock does not collapse, it can yield but we do not want it to burst and empty the pipe. This case is the tricky one with respect to the flange pressure class, as per the text I have quoted above about non-shock loading.

Thanks, a good idea to look at that for sure!

 

[saplanti]

Please do not forget that emergency shut-off valve can be used in a routine check (I do not want to get into detail since I do not know the turbine type). Do not take it as a catastropic case for the pressure or stress calculation. I believe the water hammer pressure increase due to shut-off is included in the design pressure for shut-off valve, its flanges and all the intake equipment which includes the spiral case (if used). The effect of the total pressure is to be compared with the code allowables stresses ( Some specififations reduce the allowable stresses for the hydraulic structures).

I trust this should be found in all project specification with different or same wording. Please be carefull.