Using Jam Nut as Load Bearing Component, Calculation...

[McDermott1711]

Hi everybody,
One of valve suppliers in a project, mistakenly used jam nut (instead of regular heavy hex) in a series of DBB valves (due to limited space). Now Clinet asked him to present calculation which shows it is still satisfactory. The valve is designed under API 6D and the jam nuts are made of ASTM A194 Gr. 2H. Is it ok to make the calculation on the following basis?
1- Find worst (largest) gasket area, multiply design pressure by this area to get total force.
2- Divide total force by total nuts to get per/nut force.
3- Add this force to the tightening pre-load.
4- Compare the result with proof load of corresponding one from ASTM A194 for a Gr. 2H, jam nut.

If so,
1- What design pressure shall be considered for a class 1500 valve?
2- What tightening pre-load shall be considered for, say, a 9/16 nut?
Any help will be highly appreciated.


Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has. Rene Descartes

 

[SPDL310]

What were the jam nuts used for? Your procedure looks sound to me, but I can't give you a definite without knowing how they are used. You also need to also use the appropriate API safety factors for bolting with the proof strength of the jam nut. There will be multiple design conditions to consider including hydro test, normal operation, and maximum temperature operation. Check the standard for bolting rules they may only accept heavy hex nuts for certain applications. The pre load will depend on the vendors tightening procedure. API publishes a maximum pretension load for bolting but that assumes a nut that is capable of developing the full proof strength of the bolt, a jam nut will not do that.

At the end of the day I suspect that the vendor will have to replace with a design that uses a proper nut; because, the vendor probably based their requirement on the amount of bolting area need when using the full strength of the bolt.

 

[McDermott1711]

Thanks SPDL310 for your reply.

Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has. Rene Descartes

 

[desertfox]

Hi McDermott
Your procedure is not correct, you cannot simply add the force at point 2 to the preload because the bolt won't see all of the external load. The external load is shared between the components clamped and the based on their relative stiffness's.
see this site:-

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[McDermott1711]

Thanks desertfo for your reply. Actually, I got more involved in this calculation and reach to the question of "what is yield strength of the ASTM A194, Gr. 2H nut?". Most of the materials in ASTM have their mechanical properties defined but I couldn't find them for A194, Gr. 2H. Why it is so?

Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has. Rene Descartes

 

[desertfox]

Hi McDermott

I found this site gives values well proof stress anyway:-

http://www.portlandbolt.com/technical/specifications/astm-a194/

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[McDermott1711]

There are lots of things in this link, but nothing about yield and tensile strength.

Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has. Rene Descartes

 

[desertfox]

Hi
When I click the link it lists hexagon nuts and shows the proof load stress as 175ksi for the nut you specified in your last post, proof stress is used when yield stress is not a clear cut value.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[McDermott1711]

Sorry to bother you, but would you please define proof stress, or proof load for me?

Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has. Rene Descartes

 

[desertfox]

http://www.portlandbolt.com/technical/faqs/proof-vs-yield-strength/

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[McDermott1711]

And what's the relation between design load and this proof load? I mean, what's the safety factor for a valve's bolt designed under API 6D?

Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has. Rene Descartes

 

[desertfox]

Hi McDermott1711

I am not familiar with valve design so its hard for me to comment, however we would normally use 70% to 90% of the proof stress for any particular fastener either nut or bolt.
We use 70% if we want to reuse the fastener after parts have been taken apart for maintenance and 90% if the components don't come apart for maintenance and if they do the fasteners are replaced with new ones.

Do you work for the said valve supplier? If not you should be directing questions at them.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[McDermott1711]

Thanks desertfox for your collaboration!

Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has. Rene Descartes

 

[desertfox]

Hi

You're welcome.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[SPDL310]

1.) I read your post too quickly and missed the error in your procedure that was caught. I'm sorry for that.
2.) Thank you to desertfox for quickly correcting the error that I overlooked.
3.) The allowable bolt stress for an API 6D valve will vary depending on the design code used. 6D mentions ASME section VIII division 1 or 2 and ASME B16.34 as well as EN codes. You can find allowable bolting stress in ASME section II part D for customary units. API 6A limits bolt stress to .83 Sy. Where Sy is the materials yield strength. There are no published Ys for nuts. In API 6A table 62 states "ASTM spec. and grades, heavy" for A194 which looks to me that API will only accept Heavy hex nuts for closure bolting.

 

[McDermott1711]

Thanks SPDL310 for your reply and sorry for not a quick response because I was away for a while.
You said in item 3 that

"which looks to me that API will only accept Heavy hex nuts for closure bolting."

But it seems the best place for strict forbidding of using jam nut (or "only using heavy hex nut") is API 6D or ASME B16.34, isn't it?
If you look at ASME B16.34, there is a formula which says "...Threaded body joints shall, as a minimum, satisfy the following thread shear area requirement:
Pc*Ag/As<=3300
Where:
Pc= pressure rating class designation
Ag= area bounded by the effective outside periphery of a gasket (for ring joint, it is defined by pitch diameter of the ring
As= total effective thread shear area "

1- What is "effective" thread shear area of the jam nut (please see attached configuration for the valve and jam nut)?
2- What's the purpose of the ASME B16.34 by saying "as a minimum" in para. 6.4.2.2 (restated above)?
2- Why there isn't any effect of material of the male and female threades in this formula?


Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has. Rene Descartes

 

[McDermott1711]

Sorry, I couldn't upload my image by my phone, but suppose the jam nut is attaching valve flange to the pipe flange.

Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has. Rene Descartes