Bolting for Flanges

Flanges are an important component to connect pipes, valves, pumps and other equipment to form a piping system. Flanges allow for easy operations incase of cleaning, inspection or modification. Flanges are usually welded or screwed.  With the proper combination of bolting tools and correct nut-bolts or stud-bolts as defined in the specifications; Flanged joints make a sealed connection between two flanges with a gasket which ensures leakage free movement.

To obtain a leak-free flange connection, a proper gasket installation is needed, the bolts must be assigned on the correct bolt tension, and the total bolt strength must be evenly divided over the whole flange face.

With bolt torquing - the appropriate bolt tensioning is realized by applying the preload through turning the fastener's nut at a determined torque value.

To get the best out of the bolt’s elastic properties a bolt needs to be accurately tightened. The characteristic of the bolt enables it to work like a spring to work well. While on-field the operator exerts axial preload tension on the bolt. The compression force applied is opposite on the assembled components whereas the tension load is equal. This is described as the "tightening load" or "tension load"

The tensile strength of a bolt is the primary factor to consider. The association between stress (force/area) as well as strain (change in length/initial length) is depicted on the plot, which is generated at the temperature that is considered "ambient." The plot displays this relationship when the bolt load is increased. At first, the relationship can be described as linear. Throughout this entire range, complete flexibility is preserved. The bolt will revert to their initial length after the load has been removed from it.

In appearance, flanged joints are deceptively simple in appearance; however when in real-time experience the mechanical science behind flange and joint sealing in onsite bolting is quite complex and continuously developing. Sound joints are critical to piping- joint integrity and plant safety.

Collective decisions about materials, procedural application, defined use of bolting tools and the skills and application of the operator has more effect on the reliability of a pipe joint for maximum integrity.

Flange Types -

The flange type can be determined based on the method by which it is fastened to the pipe, as shown below. 

A) Weld-Neck Flange (WN) 

Butt welding is used to attach the WN flange to the pipe. Flanges with the WN designation are typically used for demanding applications, such as those involving high pressures and/or toxic materials. During the fabrication process, the butt weld could be tested using radiography or ultrasonics, in addition to either MPI or DPI. As a consequence of this, there is a significant amount of faith in the reliability of the weld. In addition to having good fatigue resistance, a butt weld does not cause excessive local stresses to be induced in the piping due to its presence. 

B) Socket Weld Flange (SW) 

The maximum nominal pipe size (NPS) accommodated by socket-weld flanges is 1 and 1/2 inches. However, these flanges are frequently utilized for high-pressure and hazardous-duty applications. 

Filet welding is used to attach the pipe to the hub of the SW flange. Because radiography cannot be used on the filet weld, it is essential to ensure that the joint is properly fitted and welded. In most cases, the filet welds will be inspected by either the MPI or the DPI. 

C) Weld Slip-On Flange, Model(SO) 

Used most commonly on low-pressure, low-hazard services like fire water, chilled water, and other similar services. Because the pipe is " double-welded " both to the hub and the bore of the flanges, radiography cannot be performed because it is impossible. An MPI or a DPI will be utilized to check the weld's integrity. 

The SO flange will be utilized on pipe sizes more than 11/2 inches wherever it is stated, although the SW flange will be preferred for sizes up to and beyond 11/2 inches. 

D) Composite Lap Joint Flange 

Comprises a hub or "stub-end" that is welded to the pipe as well as a backing flange as well a capped flange that is used to bolt its joint together. The hub and the capped flange are referred to collectively as the "stub end." This particular style of the flanged junction is typical of the Conifer and other high-alloy pipework that can be encountered. It is more cost-effective to purchase an alloy hub with a galvanized steel backing flange than an entire alloy flange. A flat gasket, such as a CAF sheet gasket, is used to seal the raised face of the flange, which has a raised face itself. 

E) Swivel Ring Flange, Model 

A hub will be butt welded onto the pipe, just like it was done with the Composite Lap Joint Flange. A swivel ring is placed above the hub, which makes it possible to bolt the joint together. Swivel Ring Flanges are typically utilized for subsea services because of the ease with which the swivel ring facilitates flange alignment. An RTJ metal gasket is utilized to seal the flange.

The Effects of Temperature 

The stiffness of the bolting material, and thus its strength, is diminished when it is exposed to elevated temperatures. The result may be extremely harmful. The bolt will retain some of the stretches achieved during the initial load it was subjected to if the temperature increases. The bolt's tension is effectively reduced as a result of this loss of flexibility. This relaxation causes a reduction in the load that is transmitted to the gasket. 

Stud Bolts Components

The ASME B16.5 standard specifies the dimensions used when working with stud bolts. The many ASTM standards describe the material properties of studs, and each standard assigns a letter grade to signify those attributes. The grade A193 is typically used for thread rods, and the grade A194 is typically used for nuts. 

The scope of ASTM A193 includes alloy & stainless steel bolting materials for hydraulic systems, Valves, flanges, and fittings designed for high temp or high-pressure duty, as well as other applications that serve unique purposes. 

Nuts made of carbon, alloy, martensitic, & austenitic stainless steel are all included in the scope of the ASTM A194 standard. These nuts are designed for applications requiring either high pressure or extreme heat, or both.

Bolting - Do's and Don'ts

Do's

a)  Ensure Correct Size and Bolting Material is accurate to specifications. 

b) For good practice, the two threads need to be exposed outside the nut once it is tightened. The only exception is when hydraulic bolt tensioning is required to be used on the flange and the additional length  needs to be exposed outside the nut.

c) Rust-free nuts and stud-bolts are a primary factor which shouldnt be compromised..

d) Lubricate the stud, nut thread, the nut and flange surface for optimum usage.

Don'ts

a) Do not use worn-out or damaged stud-bolts, bolts or nuts.

b) Do not use nuts or bolts that are an incorrect fit together.

c) Do not use a tightening strategy that cannot be accurately identified.

d) Do not mix different coatings of stud-bolts on a particular flange. As some coatings are more lubricating than others; same coatings are essential to achieve uniform bolt tension.

e) Do not assemble nuts with a hard stamp or pad against the flange.

f) Do not use bolting  where the correct material grades are not mentioned - a wrong assumption could have significant consequences.

This is because the stress is typically supplied to them by the manufacturer. The knowledgeable end-user knows that there are many potential compensations of this kind. In any scenario, it is essential to have a solid understanding of the stresses placed on the bolts through  bolt tensioners or torque wrench & flanges. 

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