3.5 Stud bolts / Hex nuts and their relation to Flanges

Stud bolts and nuts

Stud bolts and nuts are the most commonly used fixtures for joining. 

They are most commonly used to join pipe flanges, heat exchanger shell girth flanges and many more. 

Reasons for replacement ?

This topic came to my mind, as I am required to purchase replacement Stud bolt and nuts for several heat exchangers girth flanges. The more common reasons for replacement in the process plant and specifying them correctly are due to the following:

1) Corroded stud bolts and hex nuts, 

a) Corrosion sometimes can be so severe that the diameter of the bolt is below minimum required diameter handle the bearing stresses. 

b) Corrosion is common occurance at the crevice between the flange bolting hole and bolt surface. 

c) Due to the recessed surface of the pitch on a bolt, they tend to catch dirt and corrosive particles more easily.

- The norm is to just replace the studbolt/nuts with same material/spec on wear and tear

- An upgrade can be performed to reduce maintenance cost, but remember take care of galvanic corrosion when using bolts of a seemingly better material on susceptible pipe lines, to avoid ill effects of "fixing problems to cause more problems".

2) "Frozen" bolts

a) This can be the resulting combinations of either corrosion, thermal expansion, over torque, loss of lubricant, crossed threaded bolt-nut, etc etc etc.

b) this is very common on very hot services approx above 80degC

 - These are usually broken free by cutting the bolt due to ease, which means replacement. 

- The better & toughy fitter may attempt to break free for you after applying anti-seizure lubricant but will take up much time.

3) Needs to be specifically defined to the correct vendor 

a) Contractors may not check or calculate for you, if your drawing does not show the exact length or details at all.

b) Flange/Pipe stockist may not carry stud bolts and nuts.

c) Different equipment uses different flange standards, which means different size and lengths of bolts and nuts. We shall discuss this later.

Standards for Bolts/Nuts

The most commonly used standards for defining Stud bolt and Hex nuts in the process industry are the following, due to the wide range design temperature it can handles and its superior market availability.

ASME B193 Grade B7 - Steel Stud bolts

ASME B194 Grade 2H - Steel Hex nuts 

For ASME B193 B7, it will handle from -29 to 427degC.

For higher temperatures, we will need to move up to grade B16, grade B8C1 and so on.

For cryogenic temperatures, use A320 L7, grade A320 B8C2 and so on.

For ASME B194 Grade 2H. it can handle -29 to 537degC.

The grades of the stud bolts and hex nuts will be marked on their body, provided they have not corroded.

Standards for its threads
Most commonly used standard for stud bolt threads is the Unified Thread Standard to ASME B1.1:
UNC - UN Course thread, common for up to 1", where bolt diameter size increases with decreasing thread pitch.
Others include:
8UN - 8 pitch thread, common for bolt/nut larger than 1")
UNF - UN Fine thread
UNEF - UN Extra Fine thread
UNS - UN Special thread

Thread pitch refers to the number of turns per inch length.

Standards for Flanges

Most commonly used standards for defining pipe flanges in the process industry are the following:

ASME B16.5 - Flanges from 24" and down

ASME B16.47 - Flanges from 26" - 60"

Read these standards and they will have tables to guide you on the number of bolts, length of bolts and size of bolts required for each type of flanges/pressure rating. You could also request a vendor version of these references from your Flange stockist.

Take note for ASME B16.47, there are Series A and Series B flange which do not share the same bolt hole sizes. Series A is adapted from MSS SP-44, and type B is adapted from API605 as part of ASME effort to standardize the dimension of flanges. 

Reference: StandardNE B16.5 Bolt Chart

Calculation for bolt length

L = 2(n+f+rf+s) + g + etc

Where

n = Nut thickness

f = Flange thickness

rf = Raised face thickness

s = Free length (1/3 diameter of bolt)

g = Gasket thickness

etc = additional fixtures such as tube sheets, orifices, additional gaskets, etc.

Additional Information

Collar /Jack bolts

These are additional bolts which sometimes or rather most of the times comes with heat exchangers.

The purpose of these bolts is to secure the tubesheet on the shell, should maintenance be only carried out on the channel heads.

But these bolts are usually more corroded or "frozen" than the regular stud bolts due to the irregular shapes it may have. Some of the irregular collar bolts may not be available off the shelf, as such do pre-fabricate some spares in times of need.  Check out the reference below for pictures which may better explain things.

Reference: Wermac Collar Bolt Information Page

Torquing the Flange bolts

Why torque the bolt to the recommended setting? You will tend to hear these from the contractors first:

- "I did not torque any bolts/nuts for the last blah blah years and they worked fine"

- "You don't need to torque, just go for the tightest"

- "It will not explode, dont worry"

My opinion goes to say that I agree to their advice for most services operating under ambient temperatures with exception to what to be mentioned next. One shall start obeying the rules of torquing the flange bolts when things starts going extreme ie.

- Very high temperature & pressure (HP boiler steam drums and reactors)

- Very large variation in operating temperature (Cryogenic loading/vaporizer lines)

- Very large variation in ambient temperature (Temperate location experiencing 4 seasons)

- Very large variation in operating pressure (Pulsating line, swing adsorption, etc)

Be very worried, unless you are prepared to seal the leaks on-line which can be very costly. Costlier than a 3ft torque wrench. 

Theory underlying the need for torquing:

- The forces from the bolt torque will affect the way gasket seats on the flange faces

- Uneven seating faces due to random tightening will cause leaks

- Over tightening will cause some solid material gaskets to yield or crack.

Reference: Bolt torquing tables & bolt tightening sequence