How Are Flange Bolts Tightened?

Bolted flange joints are used widely in water mains. It is the reason why a proper bolt tightening sequence in flanged joints is a must. This precaution helps in controlling the stress variation in flange joint components. If not done right, the chances of leakage are higher at flanged pipe joints during the operating conditions.

There must be a correct bolt placement by tightening them properly and that of gaskets and their placement in a flanged joint that will help ensure a leak-free joint. With the help of proper bolt tightening, it will also help you make sure that there is uniform load distribution in all the bolts without injuring the gaskets. In this blog, you’ll learn how you can properly tighten the bolts of the flange.

Before you begin with the actual flange bolting, the bolts/nuts, gasket, flange need some observation.

Checking points of flange before bolt up

If there is any leakage through the flanged joint, the layout of gasket material prevents the flange surface from any type of imperfection and seals them. You can see the limitations that a gasket can successfully seal. You must review the flange surface thoroughly and make sure there are no gouges or dents. The flange surface finish must exist as per the recommendation of the manufacturer for particular gasket materials.

The recommended values of the flange surface roughness areas follows:

Metal or Jacketed Clad Gaskets: 63-80 rms
Spiral Wound Gaskets: 125-250 rms
Solid Metal Gaskets: 63-80 rms

The meaning of rms stands for root mean square. The specification of roughness is usually millionths of an inch as the average of valleys and peaks measured from the midline of the flange surface. The recommended radius of the flange is usually 1/16″, and the round-nosed tool must have 30-55 serrations per inch in a spiral or concentric pattern.

How Are Flange Bolts Tightened

Now here are a few points to check before the flange bolts are installed.

Ensure proper cleaning of the gasket seating areas before insertion. In case of serration damage, re-serration is a must.
In the case of ANSI B31.3 not meeting the requirements, do not forget the rectification of the piping components.
According to clause no. 335-C in ANSI B31.3, permitted tolerances are as given below:

1. The alignment of the flange faces must be as per the design plane within 1 mm in 200 mm (1/16 in./ft) measurement across any diameter.
2. The alignment of the flange bolts must be within 3 mm (1/8 inch) maximum offset.

Check if the flanges are co-axial or parallel before you insert the gasket in between the flange pipe.

Checking points for the gaskets before tightening flange bolts

Examine the gasket making sure it is free from all defects.
The location of the gasket must help to ensure the full sealing. For instance, the gasket that is on the tube sheet of the exchanger requires a proper location so that the outer periphery of the gasket can match with the OD of the tubesheet at every point.
Make sure that the gasket is of the right dimension with the usage of specified materials.

Bolts for pipe flanges

Bolts must create compressive pressure on the gasket and flanges so that you can prevent leakage. So when you want to select, the temperature variations in service are considered.

Do not avoid the usage of short bolts on the flange joints.
The bolt’s lubrication with molybdenum disulfide is recommended, especially in all the bolts of size 7/8″. You can check the application as the bolt load is developed by torque and depends on the lubrication of threads.
The cleaning of nuts and bolts with suitable solvents is advisable, such as CTC/Diesel with the use of a wire brush, especially in the threaded portion.
Make sure that the material of all the nuts and bolts is specified.

Procedure for flange tightening

It is essential to tighten all the bolted joints in a uniform and diametrically staggered pattern.
In the case of pipe flanges from 12″NB to 8″NB and a pipe with spacer piece with at least two gaskets, the distance measured between the two flanges must be at four locations like 6’o clock, 9’o clock, 12’o clock, and 3’o clock positions and require equal distance for even tightening.
The stud bolts of 7/8″and above can be tightened with the use of a torque wrench. Make sure that the tightening load to every bolt is applied uniformly as possible. You can carry the tightening in three to four stages in the steps of 70, 50, and 30 percent of the final torque value. The fourth stage uses 70 percent again for the final torque value.
For the exchange of heat, final torque can value up to 100, 70, and 30 percent of the final torque value.
Follow the guidelines by the manufacturer wherever you can find the recommendation to use the hydraulic tensioner for bolt tightening.

Wrong practices for tightening flange bolts

There are many times when the plant operators end up following wrong practices for the flange bolt tightening. These are as follows:

Usage of rusted/dirty fasteners without lubricating
Usage of ordinary fasteners and not high tensile ones
Using a lot of gaskets for filling larger gaps between the flanges
Improper storage of gaskets
Reusing old gaskets
Improper sequence of bolt tightening


Flange bolt tightening is not an easy task. You must take care of all the elements required to understand the proper flange bolting, be it flange thickness, bolt sizes, or any other. Also, make sure not to go forward with wrong practices as it’ll put you at risk later.

If you need assistance to know more about the flange bolt tightening or choosing the right type of flange, contact Texas Flange today!

Guide to Properly Measure Flanges

Flanges are the best and easiest way to connect pipes, pumps, and valves. They come in different types and shapes. Flanges are in high demand and are the most convenient and cost-effective components in the piping system.

Flanges and their Advantages:

Flanges are ring-shaped devices designed to substitute for welding in various piping system components. A flange is integrated into the piping system either by welding or screwing. Flanges are highly flexible and can be easily fixed or removed if any modification is required. So the industries that require frequent maintenance rely on this piping system component. While purchasing flanges, make sure they possess high-pressure ratings and a good capacity to tolerate extreme temperatures if your application requires this.

Flange Faces and Its Types:

Before moving on to the different flange types, it will be productive to know about the different flange facings.

Flat face

Raised face

Ring-type joint face

Pipe flanges are made up of materials like stainless steel, cast iron, aluminum, bronze, brass, and even plastic. The most preferred material is the forged carbon steel. Each flange is different and has different characteristics, and so when choosing a flange, always double-check to make sure that it meets the specific requirements.

Flanges are available in different types, and so it is easy to choose according to your specific needs. Most commonly they are blind flanges, slip-on flanges, and welding neck flange, and less often they are socket welding flange, threaded flange, lap joint flange, and custom. Among these, welding neck flange is preferred in piping systems of a more complex nature.

How to Find Out the Right Flange Dimensions?

It is common knowledge that flanges can join the pipes to other parts of your assembly or vessel, but to get the best out of flanges, you must ensure that the flanges are correctly measured. Only then, the rest of the process can be done smoothly. Finding the apt flange dimensions is the primary and essential step.

A few efforts are required to get the right flange dims. The right measurement is completely dependent on understanding the accurate pressure ratings and the size of the corresponding pipe.

First, to measure the standard pipe flanges, measure the outer diameter of the flange (O.D.). It can be achieved by measuring from the outer edge of the flange to the opposite outer edge and then measuring the flange’s inner diameter from these two outer reference points.

Similarly, while taking flange dimensions, you should consider the flange base thickness too. For this task, you only need to analyze the thickness of the attaching outer rim (the base where the bolt pattern is located).

Calculating the Flanges Weight:

While calculating the weight, it is necessary to understand that the construction of pipe flanges is directly connected to the material grade and the pressure it can handle.

The weight calculation of pipe flanges is based on the model of the flange. Now calculating the flange weight is pretty simple as most of the manufacturers have provision for this. Just providing few details like outside diameter and inside diameter, the thickness of the base, number of drill holes, the diameter of the holes, and chamfer can easily help calculate the flange weight in seconds.

Measuring Flange Thickness:

Flange thickness calculation can be achieved through some techniques. In standard flanges, by understanding the design conditions, especially the pressure and temperature of the piping system and the kind of piping material, the thickness can be found. In this situation, no calculations are required to find the flange dims. All you need is to check the ASME pressure vessel design calculations and determine the ratings that match the design condition. Once you find out the flange rating (class), it is easy to find the standard flange dimensions from the ASME B16.5 table.

Flange Specifications:

Many standard flange specifications are available depending on the variety of industries. Flange specifications indeed vary according to the industries and their applications. Most of the standard manufacturers offer popular flange specifications like ASME/ANSI flanges, DIN flanges, API flanges, and AWWA flanges. ANSI standout best and most popularly used among the various flange specifications because of its ubiquitous mechanical designs amongst ASME engineers.

ANSI Flange and its Dimensions:

ANSI (or the American National Standards Institute) is the most standard flange specification and the most preferred flange in various industries. The standardized ANSI flange dimensions are 150, 300, 400, 600, 900, 1500, and 2500, in sizes NPS 1/2 through NPS 24. Furthermore, ANSI B16.47 covers pressure classes ranging from 75 to 900.