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How Do I Choose a Bolt?

In the piping sector, bolted joints are commonly used. Flanged joints can only be considered with adequate bolting. As a result, the selection of the right bolts is essential for good joining and leak prevention. In this article, a few requirements that regulate the bolts used to construct flanged joints are explained.

The bolt selection criterion can be divided into two categories based on their limitations:

  1. Physical Limitations.
  2. Material Limitations

Physical Limitations in Bolt Selection

  • Threading Requirements: All threads must comply with ASME B1.1.
  • Jack Screws: Jack screws must be used to separate flanges for maintenance. Orifice plates, spectacle plates, spacers, screens, and drop-out spools are examples of joint assemblies that frequently require separation. The piping system must be constructed such that flanges may be detached without using too much force. Jackscrews must be placed so that they are accessible from both sides of the pipe. Jackscrews must be placed at 3 and 9 o’clock positions on orifice flanges. Jackscrews are not required when using flange separators. The jack screws must be made of the same material as the flange bolts.
  • Bolt Selection: Bolting for flanged joints must be selected for service based on service temperature and environmental corrosivity.
  • Bolt lengths and sizes: The flange standard determines bolt length and diameter. Each of the flange standards listed below has a technique for determining bolt length.
  1. ASME B16.1, 
  2. ASME B16.5, 
  3. ASME B16.47, 
  4. API 6A Type 6B, 
  5. MSS SP-44, 
  6. AWWA
  • Bolt Length: Bolts must be specified in inch diameters and millimeter lengths, rounded up to the nearest 5 mm.
  • Flanges Not Covered: Flanges that are not covered under the above standards are indeed non-standard, and bolting is handled on an individual basis.
  • Washers: Washers are classified into two types, which are listed below.
  1. Flat Washers: Flat washers under the nuts are only used in specific cases, such as insulating flanges and nuts bearing against plastic flanges.
  2. Belleville Washers: Belleville washers may be needed for severe cyclic operation and bolt service temperatures exceeding 4500 degrees Celsius.

Material Limitations for Bolt Selection

  • General Process: Bolting materials for process and general services shall be ASTM A193 Grade B7 stud bolts with ASTM A194 Grade 2H nuts for service temperatures ranging from -20 to +4500 C.
  • Low Temperature: The following bolting materials shall be used for low-temperature services:
  1. Stud bolts according to ASTM A320 Grade L7 with nuts in compliance with ASTM A194 Grade 4 or 7 shall be used for bolt service temperatures ranging from -18 to -101 degrees Celsius.
  2. ASTM A320 Grade L7M studs and A320 Grade 7M nuts can withstand temperatures ranging from -18 to -73 °C.
  3. Stud bolts according to ASTM A320 Grade B8 with nuts according to ASTM A194 Grade 8 shall be used for bolt service temperatures ranging from -101 to -195 degrees Celsius.

Bolting Materials for Upper Intermediate Temperature Services:

  • For temperatures up to 450 degrees Celsius, use ASTM A193 Grade B7 or B7M studs with A194 Grade 7 or 7M nuts.
  • For bolt service temperatures ranging from 450 to 510 degrees Celsius, use ASTM A193 Grade B16 stud bolts with A194 Grade 7 nuts.

When spiral wound gaskets are used for high-temperature applications, the bolts must generate sufficient pressure to seat the gasket.

  • When using B8 class 2 bolts at temperatures above 650 °C, strain hardened bolts are required to seat the gasket. The most common materials for high-temperature gasket windings are stainless steel 347 or Inconel.
  • Strain hardened steel will anneal at temperatures above 650 °C. The annealed bolts have enough strength to hold the joint together as long as it is not damaged.

Sour Service Bolting Materials:

Standard quenched and tempered ASTM A193 Grade B7 stud bolts with 2H nuts shall be used for sour wet services when the bolting is as follows:

  1. There is no direct contact with hydrogen sulfide.
  2. It is not buried or insulated.
  3. Lack of flange protectors, or lack of protection from direct atmospheric exposure.
  4. Under the same conditions, ASTM A320 Grade L7 stud bolts with Grade 4 or 7 nuts can be utilized.

Stud bolts according to ASTM A193 Grade B7M with nuts complying with A194 Grade 2HM must be used in the following circumstances:

  1. Direct exposure to sour environments
  2. The bolts will either be buried or insulated.
  3. The flange has flange protectors or is otherwise deprived of direct air exposure.
  4. Under the same conditions, ASTM A320 Grade L7M bolts and Grade 7M nuts can be utilized.

Machine Bolts: In a non-sour environment, steel machine bolts complying with ASTM A307 Grade B may be used on flat-faced cast-iron or non-metallic flanges. ASTM A563 Grade D nuts must be used. When the Grade D nuts are not desulfurized, this bolt and nut combination can be used in sour services. This technique is also appropriate when “weak” bolting is required to avoid overloading flanges. This bolt might be zinc plated.

Expansion and contraction: When fluid temperatures are less than minus 45 degrees Celsius, the bolting material or design must consider differential contraction between flanges and bolts so that changes in gasket seating pressure do not result in leakage. Similarly, differential expansion must be considered at operating temperatures above 300 degrees Celsius.

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