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# Flange thickness calculation

## Flange thickness calculation

(OP)
Hi,
We have an exchanger of which we are trying to do a nitrogen test. The btm nozzle of exchgr cannot accomodate the thkness of the blind flange b'cos of limited space. So I have to calculate the new thkness of bld flg which can serve the test sufficiently. How can I perform this? Any leads?
Thanks.

### RE: Flange thickness calculation

harith07

The calc is t(thickness) = SQ RT of  3 * P / 16 * SE

where d = inside diameter of gasket, P = design pressure, SE = allowable stress.  The blindes should be designed to withstand any pressure-temperature combination listed in the ASA B16.5 flange design specifications.

Normally A 285 Gr C plate is used for CS up to and sometimes through 2" thickness; A 515 Gr 70 plate, A 181 Class I or II forging, and A 105 Class I or II forging are normally used for higher press or large line blinds.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website: www.oil-gas-consulting.com

### RE: Flange thickness calculation

(OP)
Hi Greg,
Thanks for the tips. Two q's:
1) Where is the 'd' in the formula?
2) Is the sq rt for the whole thing or just the numerator?
2) Where is this formula from?
harith07

### RE: Flange thickness calculation

harith07

I'm sorry, I'm a bit technically challenged and have not yet learned how to post forumula's on here.

It should have read d * SQ RT of  3 * P / 16 * SE
So it would be the sq rt of the whole thing * d.

It comes from B31.3, Sect 304.5.3.  You'll see it written there properly!!  I usually don't add corrosion allowances to it.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website: www.oil-gas-consulting.com

### RE: Flange thickness calculation

(OP)
Greg,
Thanks alot for your help. FYI, I did flip through B31.3 but just simply didn't see it!
:)
harith07

### RE: Flange thickness calculation

jte

I don't have a copy of the report on your link, but a question - we normally use the calcs above and then use the closest standard blind.  What's the basis (calcs) for using a "substantially thinner" blind for the test which is typically higher than your operating pressure (90%-110% SMSY) and what would be the justification (Code reference)?

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website: www.oil-gas-consulting.com

### RE: Flange thickness calculation

Greg-

In a nutshell, the B31.3 basis is work tracable to Poisson in the mid 1800's. This is a reasonable approach but has limitations such as the presumption that the plate does not deflect more than about half the plate thickness. All of the load is carried by shear and bending. If you take a nonlinear approach, first from a geometry perspective, then from a material yield perspective (it deflects before it yields) then you have essentially formed a dished head. This dished shape carries its load thru membrane action which is much stronger. The paper makes makes the presumption that some permanent deformation is acceptable and goes from there. Bottom line: it provides a rational means of determining slip blind thicknesses which is based on a different acceptance criteria than the Code rules which are based on no permanent deformation.

To answer your question on justification with a counter question: Which code specifies what flavor of slip blinds must be used for temporary hydrotest closures?

jt

### RE: Flange thickness calculation

jte

Appreciate the information you sent by the way.  I understand where you're coming from, but I guess I'm old fashioned and tend to look at the dim side of things.  Personally, and this is a personal opinion, I would use the codes to spec the blind flange for the air test, the reasons are:

1) the codes are more conservative, no doubt, but that would be the reason for using them in this case due to this being an air test and the inherent risks & dangers due to a rupture.
2) again, looking at the dim side, if there were a rupture involving the blind and someone were injured, I would find that a difficult one to live with and I would probably be open to the criticism of not relying on the code.

Some may say that's CYA, I look at it as mitigating risk.  Granted one can take it too far and get into the ridiculous of over designing everything, but on an air test, I just  prefer to reduce the chances of an error, especially in the case of air testing.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website: www.oil-gas-consulting.com

### RE: Flange thickness calculation

(OP)
Hi All,
Test is done, all is well.
Thanks for everything.
By the way, I had a discussion with some PE and they agreed with the caln, it's cool.
Thanks again

### RE: Flange thickness calculation

harith07

Appreciate you following up, many times the thread just stops and the results are never known.  Thank you for the update, glad all went well.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website: www.oil-gas-consulting.com

### RE: Flange thickness calculation

Greg-

I agree with your concern since this is a nitrogen test instead of water. However, from a risk perspective, I don't think its likely that someone could get hurt if the blind fails: The application is at a bottom nozzle of an exchanger with limited clearance. What that sounds like to me is that the nozzle connects to a stiff pipe spool with limited clearance. Thus, if the blind were to rupture (and with the thicknesses in the paper that is very unlikely) the blast would be contained in the pipe below the exchanger.

Harith-

Glad to hear the test was sucessful. But how did you solve the problem? Did you run numbers per B31.3 and use a blind (fabricated from plate?) thinner than a B16.5 blind or what?

jt

### RE: Flange thickness calculation

Gentleman,

Please advise whether in your engineering/standard practice. On shutdown turnaround, in case entry of static equipments, should we install temporary slip blind that the thickness comply to ASME B31.3 para 304.5.3 if there is the block valve already.
Is thin blind with block valve be acceptable in your standard practice?

Pongsagon.

### RE: Flange thickness calculation

(OP)
Pongsason,
In my opinion, the valve has to be tested first to see if it passes. If yes, then a blind is certainly required.
In my case, the minimum blind thickness is acceptable which has to be in accordance with ASME B31.3 para 304.5.3.
As to whether you will need a thin one will depend on the spacing available due to the configurational design of the pipe.
As to whether you will need one ultimately depends on your company's safety procedure. In any case, safety should prevail.

### RE: Flange thickness calculation

(OP)
jt
sorry i missed out on your q. yes, i used b31.3 for the calcs & the thkness was successful.
cheers

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