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.
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!