As flanges carry or transfer both flammable and non-flammable liquids and gases for major industries like oil and gas, petrochemicals, subsea, ports, etc. they always need to be checked for leakages and must maintain joint-integrity.To ensure safety, after every bolting and machining job during maintenance, the flange is pressure tested at high pressures to ensure the accuracy of the job. In this article, we will provide a step by step guide on what factors need to be considered to ensure accurate bolting with torque tools on flanges.
Before we talk about the different factors involved in ensuring joint-integrity, based on the flange specifications, choosing the right torque wrenches for the job is very important as both over-torqueing and under-torqueing can cause leakage. In order, to choose the right torque wrench for controlled bolting, the engineer needs to be experienced to suggest the right torque required. Also, the operator needs to be experienced in operating a robust tool to ensure his safety and complete the job without damaging the flange. Understanding the importance of experience in controlled bolting, following are some of the factors that need to be taken into consideration.
Torque Loss caused by torque wrenches
Torque loss is very common and found in every bolting job done with torque wrenches . The loss is nothing but the loss of torque during tightening. It is a combination of bolt relaxation, vibration of the system, gasket creep, thermal expansion, and elastic interaction.When torque losses exceed, the internal pressure exceeds the force holding the gasket in place causing a leakage or the flange to blow out. Another factor contributing towards leakages is the gasket thickness which can be avoided by installing a 1.6mm thick gasket on a standard ASME flange.
Such instances can be avoided by installing the gasket properly i.e. by bringing the flanges closer slowly and parallel during installation. Also, following the right bolt tightening sequence by taking at least four bolt tightening passes can reduce future maintenance costs and increase safety.
What are the four bolt tightening passes and the sequence?
For tightening a four-bolt flange, we use a crisscross pattern as determined in the image below.
In the first pass, lightly tighten the first bolt and then move 180 degree towards the second bolt, followed by 90 degrees on the third and finally directly across the fourth. Continue this sequence till all the bolts are tightened by the torque tools.
Lubricating the bolt is also very important to reduce friction during tightening. It not only increases the life of the bolt but also decreases bolt failure rate during installation. High friction leads to low torque which eventually leads to preload. Therefore lubricant is applied on both male threads and nut bearing surface.
Preparing a flange for bolting
With the most common cause of leakage being improper installation, following are the inspection checklist to ensure accurate bolting.
- The flange faces must be clean and scar or defect free.
- Inspect for any damaged fasteners or corroded threads. Replace them if necessary.
- Lubricate the threads of the bolt and the surface of the nut adjacent to the washer or flange.
- Ensure gaskets are properly centered.
- Check the flange alignment according to ASME B31.3 process piping.
- Adjust the nut’s position to ensure that at least 2 threads are visible above the top of the nut.
The checklist proves that experience is required to inspect, understand, and then use the appropriate torque tools to achieve joint-integrity among flanges.If you are looking for experienced controlled bolting service, ABS has more than a decade of experience in resolving complex flange maintenance jobs.Visit us next time for more knowledge sharing articles.