Whalen method question

[Gorpomon]

Hi,

I was doing some reading on a method of developing bolt load for flanged joints, and I came upon a simplified procedure created by a fellow named Whalen. The book is "Handbook of Fluid Sealing" by Robert V. Brink, Daniel E. Czernik and Leslie A. Horve. The section is on page 3.9.

My question is this, in the description of this method, they offer two methods of getting bolt load, one by gasket seating stress, the other by hydrostatic end force, just a simplified version of the ASME BPVC guidelines.

However, this method goes on to say that the bolt load calculated by seating stress must be higher than the bolt load calculated by hydrostatic end force. If it is not, then, "gasket design must be changed, the gasket's area must be reduced, or the total bolt load must be increased."

It was my understanding that in BPVC, you just use the higher of the two, that makes sense to me. But it seems here its saying seating stress bolt load must always be higher than the hydrostatic calculation.

I have been trying to figure out a reason why it would say this, but I can't think of a good one. Can anyone explain why this might be the case?

Thanks everyone!

 

[SnTMan]

Gorpomon, it's so you still have a seating load applied after applying the hydrostatic load, which unloads the gasket. Appendix 2 calculations account for this.

Regards,

Mike

 

[Gorpomon]

If the hydrostatic force unloads the gasket, then wouldn't you want the required hydrostatic bolt load to be higher than the seating bolt load? This way you go above and beyond what's required to seat the gasket, so when it is subjected to forces, it still maintains its seating.

To me it seems Whalen got it backwards. I would think you would always want a lower seating stress, so that way when you torque above and beyond it to resist hydrostatic loads, you don't have to worry about the gasket unseating.

 

[SnTMan]

Maybe a litle fuller reply is in order. I am not familiar with Whalen, but as described in your first post, it "sounds" right, that is, you want the bolt load that produces the seating stress higher than the bolt load that just retains the end force, so that the gasket stays seated.

I am curious how Whalen assigns a stress or load to the gasket.

The Code calculation uses a slighty different approach, the operating bolt load consisting of both the hydrostatic end force and a load on the gasket proportional to a multiple of the design pressure, the "m" value.

Code seating bolt load is found from the (somewhat fictional) "y" value, at zero pressure. The higher value of the two is used in further calculations.

Regards,

Mike

 

[Gorpomon]

Whalen simplifies the design from the BPVC, his first equation is for gasket seating stress, and he simplifies it to:

Fb1 = Ag * y ; where Fb1 is required bolt load, Ag is gasket contact area, and y is required seating stress

Fb2 = K * P * Am ; where K is a safety factor from 1.2-4.0 depending on situation, P is operating or test pressure, whichever is higher and Am is gasket area which hydrostatic pressure acts on, which he says is generally mid-diameter

Whalen says that Fb1 should be greater than Fb2, if not redesign the gasket, reduce area or increase your bolt loads.

To me it sounds right that you just go with the higher force, and if its the bolt load to resist the end force, then that's fine, because its already higher than the seating force, so the gasket should still stay seated.

It doesn't seem necessary to do a redesign, or reduce area as Whalen says. This is where my confusion lies.

 

[SnTMan]

Whalen's expression for Fb2 does not contain an explicit term to account for retained stress on the gasket, in excess of the end force. This is where Code differs.

If bolting were selected per Fb2 then, ignoring the safety factor, at design pressure all the bolt load is taken by the end force, none is available to retain any gasket seating stress.

If Fb1 is the higher and bolting selected per it, then the difference between Fb1 and Fb2 is available to apply gasket seating stress at design pressure.

Regards,

Mike

 

[Gorpomon]

I understand now, thank you!

Could you give me some insight as to how I could redesign if Fb1 is not greater than Fb2, Whalen says "gasket design must be changed, the gasket's area must be reduced, or the total bolt load must be increased" What exactly does he mean by that?

At large NPS's like 84 I couldn't expect a ring style to be used to reduce area. It seems my only options are to swtich to a higher seating stress gasket or to just set a S.F. on my torque so I've applied enough, or set torque to 90% yield on my bolts. Selecting a higher seating stress gasket seems a bit counterintuitive.

 

[SnTMan]

Not being familiar with the method, I don't know what may be used for gasket and bolt stresses, but, bolt load can be increased by adding bolt area or by increasing the bolt stress. Reducing gasket area, by say, making the gasket narrower will increase gasket stress for a given bolt load. "Gasket design must be changed" means who knows what.

As discussed previously Code supplies values for gasket seating stress and bolt stress, so that you know what values to use. Does Whalen offer values, or is that up to the designer?

Regards,

Mike

 

[Gorpomon]

As far as I can tell, Whalen's method is not very commonly cited or popular, its just a simplified method I found in the book I mentioend. It was originally published in the magazine (or journal) "Product Engineering" in the October 1960 edition, the article is titled "How to Select the Right Gasket Material".

In my book, it does not mention Whalen giving any values for gasket seating stress or bolt stress, it just gives those two equations and a brief explanation of appropriate safety factors.

Since these are standardized flanges, I can not make the bolts bigger, or add more bolts, it seems my only option would be to put more stress on them, probably 75-90% yield.

Can you clarify making the gasket "narrower" do you mean reduce its width? With standard off the shelf full-faced I don't think that's possible.

 

[SnTMan]

Well if you are using standard stuff, why not forget Whalen?

Actual, applied bolt stresses are well known to be above, and often far above, Code allowables used in flange design rules.

You might contact the gasket manufacturer for a recommended stress and get a bolt load from that, or you might get hold of ASME PCC-1.

Regards,

Mike

 

[Gorpomon]

My interest in Whalen lies in how much easier it is to do and explain to people than the ASME spec. Sometimes when I'm asked about bolt loads, it would be much easier to show someone Whalen than BPVC.

The math itself using Whalen is just quite simple and intuitive, its only when he mentions re-designing a gasket does his method get abstruse.

I have read PCC-1 and use it frequently, its an invaluable resource, however it doesn't really explain its values too well, nor have I found it gets read as much as it needs to.