Flange Rating 9

[darrenyee88]

1. How do we decide the flange rating for pressure vessel? Based on the design pressure and MAWP?

2. If the flange rating is 150 psig and the hydrotest is UG-99(b) excluding the footnote 34, the hydrotest values came out higher than the flange rating of 150 psig. Will this detroy the flange?

Please answer my queries. Thanks.

Regards,
Darren

 

[GenB]

class 150 not 150 psi. the psi is actually for a higher design. check Codes.

 

[XL83NL]

1. Based on design pressure. MAWP is calcualted once the max allowables of all pressure retaining components in a vessel, incl flanges, are known.
2. Flanges can be hydro-ed up to 1.5 the ambient rating; see B16.5 para 2.6.

 

[SnTMan]

What KIND of flange?

Regards,

Mike

 

[LittleInch]

XL83NL has it right.

just to expand a bit as it seems you're not very familiar with this, which worries me more than a little when you're messing about with pressure vessels, but flange rating is defined in ASME B 16.5 based on material type, flange class rating (150,300,600 etc) and Design temperature. There are then tables which give you MAWP for your flange. You can interpolate between the temperatures which go up in steps to get your particular temperature.

However hydrotest can be up to 150% of the pressure seen by the flange at test conditions (usually ambient pressure), hence usually higher than the MAWP at higher temperatures

I suspect you won't have a problem as your vessel MAWP and hence test pressure will be limited by the weakest component, which is not normally the flanges.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[darrenyee88]

Then if the thickness of the vessel increase , does schedule of the nozzles has to increase and revise also ? The vendor told me that it has to be change because the hydrotest preasure increase which nozzle thickness.cant handle . I find it weird.
Can someone explain so that i can understand .

 

[TGS4]

You really shouldn't be changing components because of a hydrostatic test condition. If the wall thickness of the shell is based on design pressure, then so too the smaller components (e.g. nozzles). Without further information we can't tell why your shell thickness increased, but unless it was driven by a design pressure change, there should be no change to the other components.

 

[Shmulik]

UG-99: Hydrostatic test pressure in the vessel is at least equal to 1.3 times the maximum allowable working pressure multiplied by the lowest stress ratio (LSR) for the materials of which the vessel is constructed.

You haven't indicated your vessel's design conditions and materials of construction.

Maybe your vessel's design temp. is high and the LSR for construction materials derive a high hydrostatic test pressure. In such case your vendor's explanation can make sense.

 

[Duwe6]

Vessel 'Philosophy' - do not buy a vessel where the MAWP is limited by the flanges of the nozzles. Reasoning - at some time in the future, the plant may want to 'debottleneck' and it will require an increase in the pressure of the vessel. Try to ensure that your limiting factor is something major like the shell, or expensive like the body flange on a Hx Bonnet. Never allow something small and cheap to be your limiting component.

RE: flange hydro pressures; the max hydro pressure of a Class 300 SA-105 flange is 1080 psi, for 304 s/s it is 900 psi. see the part of B16.5 that XL83NL referenced.

 

[XL83NL]

RE: flange hydro pressures; the max hydro pressure of a Class 300 SA-105 flange is 1080 psi, for 304 s/s it is 900 psi

As per B16.5:2013 that'll be (acc. my calcs) 1110 psi (1125 pis rounded) and 1080 psi (1100 psi rounded), respectively.

 

[sanshu1111]

Do not agree with the philosophy to increase the equipment nozzle flange rating just to meet MAWP requirement. All the counter flanges on the piping shall also require higher rating compare to all other flanges and valves on the same piping.

All the piping flanges and valves also require higher rating if any future rerating/ debottlenecking by increasing pressure necessiate higher flange rating.

 

[moltenmetal]

Duwe6- I actually prefer vessels to have their MAWP limited by the B16.5 flange class selected for nozzles when that is possible without a lot of extra cost. This is the case just about any time a small vessel is made from pipe rather than from plate. I'd rather have a nozzle flange limit than a nozzle neck or nozzle to shell/head attachment, for sure. When exposed to excessive pressure, the likely failure mode of such a vessel is flange leakage rather than shell or head failure, and that's worth something in my opinion.

I will let a shell or head thickness limit the MAWP if there's a big jump between flange classes and hence big savings to be had in vessel cost by keeping the shell and head materials thinner. ASME VIII will let any component in the pressure envelope limit the MAWP- it doesn't tell you which one must limit.

If the selected plate thickness for the required MAWP is already sufficient to go up to the limit of the next higher flange class, then sure- why not go up a flange class on the nozzles? They don't cost much, usually. But it's rarely so in my experience.

 

[sanshu1111]

Moltenmetal,

Going for next higher flange rating is not a small cost since up-gradation of only equipment flange and counter piping flanges will not help to achieve a higher rating for the complete system.

Complete piping flanges, valves and instrument shall also require upgradation for next higher rating which is going to cost a lot.

 

[racookpe1978]

But, you have to keep "uprating" the flanges and pipes downstream, or make a "otherwise unneeded" pipe schedule+designation" change downstream of every PV nozzle. Yes, such a scheme (to uprate PV nozzles so the nozzles are higher rated) does do what you state, but each mating pipe flange and pipe schedule have to match the PV nozzles. And those pipe changes create extra expense, and a little bit of extar weight as well.,

 

[moltenmetal]

sanshu111: as you see from reading the whole of my post, we agree for the most part- and both of us disagree with Duwe6's point of view. But as I said, there is a balance between the cost of the vessel and associated flanges and instruments now, versus the potential re-use of the vessel later- not in debottlenecking so much as in the boneyard when the vessel's original function is over. Some customers do make good use of their boneyard The steps between flange classes are pretty large, so as I mentioned it is rare that a shell required for one class is good for the next highest one. The exception is for vessels made from pipe rather than plate, especially when sch40 is the thinnest schedule commonly used (i.e. in carbon steel), where the minimum shell is often good for a very considerable pressure (depending on corrosion allowance).

The difference between 150# and 300# in the cost of valves, instruments and (small) pipe flanges plus the nozzle flanges themselves is typically pretty small relative to the cost of the vessel in total. It might be worth the jump in nozzle flange class between 150# and 300# if the shell, heads etc. are all good for it already. But if you're jumping between 300# and 600# or between 600# and 900# (or 1500#- we rarely use 900#), or if you have to make a body flange thicker too, THAT would be a BIG jump in cost and availability and would be pretty much a pure waste of money in my opinion.

 

[jte]

moltenmetal and sanshu-

An interesting perspective. I've been brought up to agree with Duwe's philosophy but I could see that in some cases, particularly with relatively small vessels, your perspective would make sense. For the most part my company's spec's have been changed such that this won't be an issue; otherwise I might consider changing the "minor components shall not govern MAWP" requirement for small diameter vessels.

One concern I have with the "piping" approach is that it is not unusual to have high forces and moments at a vessel flange - often other flanges in piping can be designed in locations with less force and moment. One way some folks account for the forces and moments is to take an "equivalent pressure" approach and add that to the design pressure / MAWP to provide some margin against flange leaks. With the flanges max'ed out on pressure, there is no margin left for piping loads.

Another reason I like Eng-Tips: Philosophy can be much harder - and more interesting - to discuss than relatively simple code compliance. Just so long as everybody recognizes that with philosophy there will most likely not be one "right" answer!

 

[XL83NL]

With the flanges max'ed out on pressure, there is no margin left for piping loads.

Not sure if thats really the case. Ill see if I can dick up some articles/graphs that counterprove that. Just not sure where I left those ...

Also you may wish to purchase this useful paper which an updated formula for the overconservative equivalent force/moment method.

 

[sanshu1111]

We never follow the philosophy of "MAWP not to be limited by flange". In case some one wants to follow this philosophy, needs to review the entire system/ process before implimeting this philosophy. Some of the main reasons for not following this philosophy are as below;

1. Assuming that MAWP is limited by the flange rating which we do not want, we have following options;

a) Upgrade all the nozzle flanges to next higher rating. All the piping counter flanges, valves and instrument directly mounted on the equipment also require to have higher rating to connect with the nozzle flanges (rating of complete piping, flanges valves, instruments not upgraded). This will however not help in case of future de-bottlenecking as the complete system is not suitable for the higher rating.

b)Upgrade all the nozzle flanges along with the complete piping system (pipes, flanges, valves, instruments etc) to make it suitable for higher rating of the vessel. This may have a very big cost impact unless the piping circuit is a small circuit with low pressure rating and very few instruments, controls and valves. A big system even in low pressure rating (eg Nitrogen or Instrument air which is generally in 150# class and running all through the plant), if required to be upgraded to next higher rating just to meet vessel MAWP criteria, will have a big cost impact. Main process, high pressure systems with a lot of controls, instrumentation, valves will definetely have an extraordinary cost impact if we need to meet the criteria of "MAWP not to limit by flanges"
We also need to realize that most of the time piping class (pipe thickness)are based on flange rating pressure and not the process design pressure and change in flange rating will most likely require heavier pipe.

2. Above uprating and extra cost can not be justified for an unforseen future de-bottlenecking which may never happen also and if we are confident about future de-bottlenecking with increased pressure, why not select a higher design pressure from the day one?

3. Many times, I have personally seen that process design pressure is selected by based on the flange rating and not based on the actual operating pressure which may be much less compare to the design pressure. This is to allow any future de-bottlenecking upto the flange rating pressure without any change in the complete system (equipment, piping, valve, instruments etc). In such case, MAWP is mostly limited by flange and if I apply the criteria "MAWP not to be limited by flange", Its certainly overkill.

 

[moltenmetal]

The only flanges that would need to be bumped up a class in my example are the ones connecting to that particular vessel alone. Bumping a whole piping system's flange class up one unit in the hope of future debottlenecking would be a complete waste of money in my opinion.

As to nozzle loads, the problems we encounter are usually with the nozzle necks rather than the flanges. And we find that the equivalent pressure method and all other methods we've examined for determining the effect of pipe stresses on flange leakage are excessively conservative below about 6" NPS.

 

[Duwe6]

What moltenmetal said.

The actual cost increase by using flanges of a high enough class to not be the limiting factor in the MAWP of a new vessel in the 'Real World' is so small that it disappears into the cost. The $$ may be less than the value of the scrap engendered in fabricating the vessel from stock sizes of pipe and plate. Don't believe me, call up the estimater at a vessel fab shop.