Flange Bolt-Up - Torque Tightening, Preload, Torque Procedures, Torque Sequence, Torque Wrenches,...

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Definition and Details of Flanges - Torque Tightening -

Torque tightening

To obtain a leak-free flange connection, a proper gasket installation is needed, the

bolts must be assign on the correct bolt tension, and the total bolt strength must

be evenly divided over the whole flange face.

With Torque Tightening (the application of preload to a fastener by the turning of

the fastener's nut) the correct bolt tension can be realized.

Correct tightening of a bolt means making the best use of the bolt's elastic

properties. To work well, a bolt must behave just like a spring. In operation, the

tightening process exerts an axial pre-load tension on the bolt. This tension load

is of course equal and opposite to the compression force applied on the

assembled components. It can be referred to as the "tightening load" or "tension

load"

Torque wrench

Torque Wrench is the general name for a hand-guided screwing tool, and used to precisely set the force of a fastening

such as a nut or bolt. It allows the operator to measure the rotational force (torque) applied to the bolt so it can be

matched to the specifications.

Image of manual and hydraulic wrench.

The selection of the proper flange bolt tigtening technique requires experience. The successful application of any

technique also requires qualification of both the tools that will be used and the crew who will do the work. The

following summarizes the most commonly used flange bolt tightening techniques.

Manual Wrench

Impact Wrench

Hammer Wrench

Hydraulic Torque Wrench

Manual Beam and Gear-Assisted Torque Wrench

Hydraulic Bolt Tensioner

Torque loss

Torque loss is inherent in any bolted joint. The combined effects of bolt relaxation, (approximately 10% during the

first 24 hours after installation), gasket creep, vibration in the system, thermal expansion and elastic interaction

during bolt tightening contribute to torque loss. When torque loss reaches an extreme, the internal pressure exceeds

the compressive force holding the gasket in place and a leak or blow-out occurs.

A key to reducing these effects is proper gasket installation. By bringing the flanges together slowly and parallel when



installing a gasket and taking a minimum of four bolt tightening passes, following the correct bolt tightening sequence,

there is a payoff in reduced maintenance costs and increased safety.

Proper gasket thickness is also important. The thicker the gasket, the higher the gasket creep which in turn can result

in torque loss. On standard ASME raised face flanges a 1.6 mm thick gasket is normally recommended. Thinner gasket

materials can take a higher gasket load and therefore higher internal pressures.

Lubrication reduces friction

Lubrication reduces the friction during tightening, decreases bolt failure during installation and increases bolt life.

Variation in friction coefficients affect the amount of preload achieved at a specific torque. Higher friction results in less

conversion of torque to preload. The value for the friction coefficient provided by the lubricant manufacturer must be

known to accurately establish the required torque value.

Lubricant or anti-seizure compounds should be applied to both the nut bearing surface and the male threads.

Tightening sequence

The first pass, lightly tighten the first bolt then move directly across or 180 degrees for the second bolt, then move

1/4 turn around the circle or 90 degrees for the third bolt and directly across for the fourth. Continue this sequence

until all bolts are tightened.

When tightening a four-bolt flange, use a criss-cross pattern.

Tightening Sequence

Preparation flange bolt-up

In a flanged connection, all components must be correct to achieve a seal. The most common cause of leaky gasketed

joints is improper installation procedures.

Before beginning a bolting process, the following preliminary steps will avoid future problems:



Clean the flange faces and check for scars; the faces must be clean and free of defects (burrs, pits, dents, etc.).

Visually inspect all bolts and nuts for damaged or corroded threads. Replace or repair bolts or nuts as necessary.

Remove burrs from all threads.

Lubricate the threads of the bolt or stud, and the surface of the nut face adjacent to the flange or washer.

Hardened washers are recommended in most applications.

Install the new gasket and be sure gasket is properly centered. DO NOT REUSE old gasket, or use MULTIPLE

gaskets.

Check flange alignment ASME B31.3 Process Piping:

...flange faces must be parallel within 1/16" per foot of diameter, and flange bolt holes must be aligned to within

1/8" maximum offset.

Adjust the position of the nuts to insure that 2-3 threads is visible above the top of the nut.

Irrespective which method of tightening is used, above determined checks and preparations always must be done.

Remark(s) of the Author...

My own experiences about...Torque WrenchesIn the past I have assembled hundreds leak-free flange connections, from NPS 1/2 to NPS 24 and bigger.

Rarely I have used therefore a Torque Wrench.

In practice, "normal" pipe flange connections are almost never assembled with a Torque Wrench. The

difficultest connections for me always were the "little ones" and then especially the Raised Face type above

Class 300 (RF Height = approximately 6.4 mm).

Relationable the flange faces from a NPS 1/2 flange are smaller then for example a NPS 6 flange, and the

chance on misalignment, in my view is much bigger.

In practice I encounter regularly flange connections, where the alignment do not in the tolerance limit. If

simply the order of Tightening Procedure is followed the Mechanic is not well busy. Possibly must be begun

on bolt six instead of on bolt one. Use your eyes during flange assembly, it is very important and

contributes very probably to a leak-free connection

Improper flange connections - the bolts are too short !

What can you do?

The picture show a improperly bolted flange,

because two bolts are too short, and the nuts are

not completely on the bolts. This means that the

joint may not be as strong as it should be. Flanges

are designed so that the entire nut-bolt combination

holds the forces on the flange. If the nut is only

partially screwed onto the bolt, the connection may

not be strong enough.

If your job includes putting equipment together,

assembling flanged pipe, bolting manhole covers or

other bolted connections on equipment, or other

equipment assembly, remember that the job is not

complete until all of the bolts are properly installed

and tightened.

Some equipment requires special bolt tightening

procedures. For example, you may have to use a

torque wrench to correctly tighten the bolts to the

specification, or tighten the bolts in a special order.



Make sure that you follow the correct procedure, use the correct tools, and that you are properly trained in

the equipment assembly procedure.

Check pipes and equipment for properly bolted flanges as part of your plant safety inspections. As simple

guidance, bolts that do not extend beyond the nuts should be reviewed by a plant piping craftsman or

engineer.

If you observe improperly bolted flanges in your plant, report them so they can be repaired, and make

sure the required repairs are completed.

Inspect new equipment, or equipment which has been re-assembled after maintenance, to make sure it is

correctly assembled and properly bolted before starting up.

What is the proper length of a Stud Bolt?

As a rule, you can use: The free threads of the bolt above the top of the nut is equals to 1/3 times the bolt

diameter.

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Flange Bolt-Up - Torque Tightening, Preload, Torque Procedures, Torque Sequence, Torque Wrenches,...

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