The principle of flange bolt hole positioning

ASME B16.5 has clear regulations on the number of bolt holes and bolt hole diameter of the flange. The number and diameter of flange bolt holes are different due to different flange pressure levels, but all types of flanges of the same pressure level are the same. The flange bolt holes are evenly distributed on the bolt hole diameter, and the number of holes is always an even number (4, 8, 12, 16, etc.).

In the manufacture of prefabricated parts with flanges, the positioning of flange bolt holes is particularly important. Perhaps you will see the following sentence in a drawing:

  • All Flange Bolt Holes Straddle the Centerlines.

This means:

For the vertical flange sealing surface (the flange surface is vertical, the pipeline runs horizontally), the bolt holes should span both sides of the vertical line and the horizontal center line.

straddleverok - The principle of flange bolt hole positioning

The above is the correct vertical flange positioning.

straddleverno - The principle of flange bolt hole positioning

The above is wrong vertical flange positioning, this positioning is prohibited.

For a horizontal flange face (the flange face is horizontal and the line is vertical above or vertical down) the bolt holes want to be orientated to straddle the Plant North centerlines. See below on this page, a image of a plant north situation.

straddlehorok - The principle of flange bolt hole positioning

The above is the correct horizontal flange positioning.

straddlehorno - The principle of flange bolt hole positioning

The above is the wrong horizontal flange positioning, this kind of positioning is prohibited.

It is very important, that is not deviated from the standard bolt hole orientation. Only on explicit request, e.g. of the customer, may be a different orientation be applied. In 99 percent of all cases, where you will see a different orientation, you can assume that it is a mistake. This centerline rule for flanges, understood and followed by all responsible equipment manufacturers and piping fabricators.

What are flange bolts?

Flange bolts are mainly a type of fastener consisting of a hexagon head and a flange (the gasket under the hexagon is fixed together with the hexagon) and the bolt (cylinder with external thread), which needs to be matched with the nut , used to fasten the connection of two parts with through holes.
This type of connection is called bolt connection. If the nut is unbolted from the bolt, the two parts can be separated, so the bolt connection is a detachable connection.
Flange bolts are a common type of fasteners. Flange bolts are widely used on highways and railway bridges, including heavy machinery such as industrial and civil construction, cranes, and excavators, with a wide range of applications.

Hole size of flange bolt

The first bolt specification selected is the standard specification: M4, M5, M6, M8, M10, M12, (M14), M16, etc.
Secondly, the other side leaves holes. The bolts you choose must be smaller than the holes. The other side leaves 14 holes. You can only choose the bolts smaller than 14. According to the specifications, the closest one is M12. So this is your only choice.
Of course, you can also choose M10 or M8 bolts to satisfy your mechanics, but you can check whether the M10 or M8 bolt spring washers can not be completely larger than the flange hole and cannot be fully functional. This is called unreasonable structure. In layman’s terms, the hole does not match the bolt.
Generally, the structural relationship of mechanical design is that the hole is one level larger than the bolt, and the diameter of the hole is generally 1 to 2 mm larger than the bolt, which is convenient for assembly.
But there are exceptions. When the bolt is too small, it is generally smaller than M6, and there is a choice of 0.5 mm larger. This is also determined according to the free tolerance of the assembly;
There is also a complete matching relationship between the hole and the bolt, and the difference is only one tenth, one hundredth of a millimeter. This is called a reamed hole bolt connection;
When the bolt is larger than M20, the hole can be appropriately widened to be 3 mm larger, and even larger to 4 mm. This is generally the case. The main consideration is to facilitate installation and reduce installation difficulty. When bolt holes are particularly difficult to install, designers also consider relaxing the requirements. Consider relaxing requirements when there are too many bolts.

Classification of flange bolt

  • 1. Types of hexagonal bolts: There are two types of hexagonal heads, one is flat head and the other is concave head.
  • 2. Surface color category: According to different needs, the surface has white, army green, color yellow, and Dacromet that will never rust.
  • 3. Flange category: According to the place where the flange bolts are used, the size requirements of the disk are different. There are also flat bottom and toothed ones, and the toothed ones play a non-slip effect.
  • 4. According to the force mode of the connection, there are ordinary and reamed holes. The flange bolts used for reaming holes should match the size of the holes and are used when subjected to lateral forces. In addition, in order to meet the need of locking after installation, there are rods with holes, and these holes can prevent the bolts from loosening when they are subjected to vibration. Some flange bolts without threaded polished rods should be thin, called thin rod flange bolts. This flange bolt is conducive to the connection of variable forces. There are special high-strength bolts on the steel structure. The head will be larger. The size will also change.

Common materials for flange bolts

  • 1. High carbon steel C%>0.45%. Hexagon flange bolt product materials are basically not used in the market at present;
  • 2. Medium carbon steel 0.25% hexagon head bolts Medium carbon steel materials are usually called No. 35 and No. 45 steel in China, and are basically called 1035, CH38F, 1039, 40ACR, etc. abroad. Mainly used for grade 8 nuts, grade 8.8 bolts and grade 8.8 inner hexagon products;
  • 3. Low carbon steel C%≤0.25% is usually called A3 steel in China. It is basically called 1008, 1015, 1018, 1022, etc. abroad. Mainly used for grade 4.8 bolts, grade 4 nuts, small bolts and other products without hardness requirements. (Note: Drilling nails mainly use 1022 material.)
  • 4. Alloy steel: Add alloy elements to ordinary carbon steel to increase some special properties of steel: such as 35, 40 chromium molybdenum, SCM435, 10B38. Hexagon flange bolts mainly use SCM435 chromium-molybdenum alloy steel, the main components are C, Si, Mn, P, S, Cr, Mo.

Fastening method and requirements of flange bolt

1. No-torque Torx wrench or hammer wrench

It is suitable for the fastening of general equipment and pipeline flanges. It is selected according to the bolt size and flange pressure rating. The tightening requirements are as follows:
1) The maintenance unit formulates a tightening plan, symmetrically tighten the flanges, and number the tightening sequence.
2) Use 4 bolts as washers at the positions 1, 2, 3, and 4 to make sure that the center of the wound washer is within the flange edge.
3) Tighten the positioning bolts by hand, then insert other stud bolts and tighten them to balance the load, ensuring that at least two threads are exposed at each end of the nut.
4) According to the on-site equipment and flanges, a tightening circle is calculated once, and the number of tightening (at least 3 times) and the hammering load (strength) of each tightening are reasonably set. Tighten in order from small to large (such as 50%, 80%, 100% increasing), and the load cannot be loaded too fast or too large to prevent gasket seal failure.
5) The sequence of each tightening of a torqueless torx wrench or hammer wrench:

  • a. Fasten the two diametrically opposed bolts to the predetermined hammering load (strength) of the bolt;
  • b. Fasten another pair of bolts about 90 degrees away from the previous two bolts along the circumference;
  • c. Continue to tighten until all other bolts are tightened to the predetermined hammer load.

6) Finally, tighten all the bolts in a clockwise or counterclockwise direction with 100% hammering load (strength).

2. torque wrench

It is suitable for important equipment and pipeline flanges such as high temperature and high pressure, inflammable and explosive. The tightening requirements are as follows:
1) The maintenance unit needs to formulate a fastening plan, formulate an appropriate torque, and conduct a design review according to the bolt strength, the initial seal specific pressure of the gasket, the working seal specific pressure, and the medium pressure to prevent the bolt from breaking and the gasket from being compressed. Excessive force and loss of elasticity cause seal failure.
2) Fasten the flange symmetrically, and number the fastening sequence.
3) Use 4 bolts as washers at the positions 1, 2, 3, and 4 to make sure that the center of the wound gasket is within the flange edge.
4) Tighten the positioning bolts by hand, then insert other stud bolts and tighten them to balance the load, ensuring that at least two threads are exposed at each end of the nut.
5) According to the field equipment and flanges, a tightening circle is calculated once, and the number of tightening (at least 3 times) and each tightening torque are reasonably set. The tightening torque is tightened in order from small to large (such as 50%, 80%, 100% increment), the load cannot be loaded too fast or too large to prevent gasket seal failure.
6) The sequence of each tightening of the torque wrench:

  • a. Tighten the two diametrically opposed bolts to the predetermined torque of the bolts;
  • b. Fasten another pair of bolts about 90 degrees away from the previous two bolts along the circumference;
  • c. Continue to tighten until all other bolts are tightened to the predetermined torque.

7) Finally, tighten all the bolts in a clockwise or counterclockwise direction at 100% torque.
8) Record the torque value for reference for future maintenance.

3. bolt tensioner

It is suitable for important equipment and pipeline flanges such as high temperature and high pressure, inflammable and explosive. The tightening requirements are as follows:
1) The maintenance unit formulates a fastening plan, formulates an appropriate tensile force, and conducts a design review based on the bolt strength, the initial seal specific pressure of the gasket, the working seal specific pressure, and the medium pressure to prevent bolt fracture and gasket pressure. Excessive tightening force and loss of elasticity lead to seal failure.
2) When the bolt tensioner tensions and tightens the bolts individually (step by step), according to the principle of uniformity of bolt tightening, refer to the tightening sequence of the torque wrench.
3) The bolt tensioner tension tightening process, reasonably set the number of tension tightening, pressurize from small to large (such as 50%, 80%, 100%), uniformly pressurize, and every time a certain pressure is increased, It should be increased after the voltage is stabilized to avoid excessive impact tension and affect the pre-tightening effect of the bolt.
4) Record the pressure value for reference for future maintenance.

Source: Network Arrangement – China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@metallicsteel.com

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straddlehorok - The principle of flange bolt hole positioning
Article Name
The principle of flange bolt hole positioning
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ASME B16.5 has clear regulations on the number of bolt holes and bolt hole diameter of the flange.
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www.metallicsteel.com
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