B7 versus B16 bolts 3

[owassokings]

We are using B16 bolts for expansion joints in large exhaust ducts that have hot flanges. The temperature ranges from 800 F to 1000 F with an internal duct failure pressure of 20 inches of water.

What is the advantage in using B16 bolts in place of B7 bolts? The B16 bolts are 10X the price of the B7 bolts and the question was asked if we could use the B7 bolts instead. The only difference I can see in the 2 bolts is the B16 bolt includes Vanadium in the composition.

 

[STYMIEDPIPER]

Look at the allowable stress value for the temperatures that you specified.
At 800 deg F the allowable stress value between the B7 and B16 is small.
However, at the 1000 deg F the allowable stress values are nearly 2.5 times higher for the B16 as compared to the B7.
Therefore, at the elevated temperatures above 800 deg F the B16 is a more durable component

 

[owassokings]

Stymiedpiper,

Thank you for the information. Could you tell me what technical reference you are getting these allowables? Is it from the ASME Boiler Pressure Vessel code?

 

[owassokings]

Stymiedpiper,

Again, I want to thank you for the information. I found the allowables you spoke of in B31.1.

 

[unclesyd]

You can use B7 up 1000F using the aforementioned values as the tempering temperature used in making B7 is 1000F. Anything above 1000F and the strength could fall off. Being your application is duct work at a low pressure I can see no problem using B7 even if the duct insulated.

If by design you have to use B16 I would look at stud bolt/2nuts for about 25% of a B16 hex bolt cost. A B16 stud is 2X the cost of a B7, today's prices.

 

[owassokings]

Unclesyd,

That is great information. There are cases where we are required to use the B16 bolt and that would be a great alternative.

 

[jte]

owassokings-

Welcome to the forum! Don't forget to put in a little description of your background in the "personal profile" page (button in the top left box with light blue background). You can see my profile by clicking on the "jte" at the top of this post.

Not sure which design code your ducting falls under - low pressure ductwork may not fall directly within the scope of any code. If this is the case, then I have to ask how you are deriving your 800 to 1000°F design temperature for your bolting? Are those temperatures from a line list for the ductwork? Is it the fluid temperature? Or... since you seem to be in an operating plant, have you directly measured the bolt temperatures?

If I did not have a code mandating my design, I would choose B31.3 or VIII-1 simply because I'm familiar with them, and use them for guidance. Take a close look at B31.3 301.3.2(b)(4). Here you are allowed to set the design temperature of the bolts at no less than 80% of the fluid temperature. In most cases, it sounds as though this 20% drop in temp may be very beneficial for you. Now take a close look at 301.3.2(b): ...unless a lower average... tempeature is determined by test or heat transfer calculation... so if you have similar ductwork in service, and you go out on a hot summer day and measure the temperature of the bolting and add some margin to it (unless you pick a record hot day to do your measurements) then you can set the design temperature of the bolts based on the measured values of your "test" ductwork flange bolts.

B31.1 has a similar, though more restrictive, approach in 101.3.2(A).

jt

 

[owassokings]

jt,

Thanks for the information. I will review that specification you mentioned and see how it benefits us. Our ducting serves as an exhaust system for large ground power turbines. The temperature of the bolts was estimated from FEA models that have been performed on our expansion joints. The exhaust temperature is normally 800 F to 1050 F and the flange joint is not internally insulated, therefore, as you can imagine it gets very hot.

We prefer not to use the boiler pressure vessel codes or the power piping codes because our equipment is normally a low pressure environment. Sometimes our hands are tied because the customer has made it a specification requirement. However, for the cases where we can do things that make more since, we would prefer to use suggestions similar to yours. We want to use good engineering judgement but be cost effective as well. Thanks again for the help.