API 6A " STUD AND NUT SELECTION BASED ON API 6A"

 STUD AND NUT SELECTION BASED ON API 6A

1. SCOPE

This Engineering Standard provides guidelines for selection of stud and nut based on API 6A latest Edition.

2. STUD LENGTH CALCULATION

This section defines how studs lengths are calculated. The calculation differs slightly from Appendix C of API 6A latest edition. Both the API method and the method stated in this Standard are acceptable.

The formula:

Flanged Connection, Lcsb = 2 * (T + t + d) + S + n

Studded Outlet connection, Lcsb = T + t + 2 * d + S + 0.06 + n

- If Lcsb is .010 or greater than 1/4 in increment, round off upward to the next

1/4 in increment.

Where:

Lcsb = Required stud length

T = Flange thickness

t = Plus tolerance on the flange thickness

D = Bolt diameter

S = Flange standoff

n = Negative tolerance of length of bolt

1/16 in for lengths up to 12 in

1/8 in for lengths over 12 in to 18 in inclusive

1/4 in for lengths over 18 in

Example:

Flange size 2 1/16-2000 psi WP:

T = 1.31 in

t = .12 in

D = 5/8 (.625 in)

S = .47 in

n = 1/16 (.0625 in)

Calculation:

- Through bolt connection, Lcsb = 2 * (T + t + d) + S + n

= 2 * (1.31 + 0.12 + 0.625) + 0.47 + 0.0625

= 4.643 in

- Studded connection, Lcsb = T + t + 2 * d + S + 0.06 + n

= 1.31 + 0.12+ (2 * 0.625) + 0.47 + 0.06 + 0.625

= 3.273 in

As far as possible, the length of thread engagement should be at least 1x diameter of the stud size.

*Note: Standard length to be used is RX connection except specified by customer requirement

2.3. MATERIAL SELECTION

2.3.1. API 6A latest edition, Table 49 lists the material requirement for Exposed and Non-Exposed bolting. Bolts are considered exposed only if there is possibility of retained fluid permanently trapped

around studs and nuts.

2.3.2. Table 3 reduces the number of choices provided by API 6A latest edition. This is to allow standardization of bolts to reduce inventory and should be adhered to unless otherwise requested by

customers or if design requirements does not allow pressure derating.

2.3.3. Provision should be made to prevent galling if 660 or CRA nuts are used.

2.4. 2HM nuts when used in conjunction with studs of yield strength at or above 105ksi need not be pressure derated. This is because stress on female threads is much lower than that of male threads of similar size.

2.1. COATING SELECTION

2.1. Standard coating shall be zinc plating except A660 and 718 which shall not be coated.

2.2. If requested by customer, Xylan coating can be used for extra lubrication.

2.3. If requested by customer, Sermagard may be used for extra anticorrosion resistance. As Sermagard coatings are quite thick, both nut and stud must be of the same material and purchased from the same vendor in order to ensure that the bolts can be made up.

Coating studded holes with Sermagard is not recommended.

Consult Engineering Manager if there is such a need.

2.4. For fire resistant equipment, studs and nuts shall be phosphate and oil only. Zinc plating or any other coating that may produce toxic gas when exposed to flames shall not be used.

3. REQUIREMENTS

3.1. Material for bolting shall meet API 6A latest edition and ASTM requirements.

3.2. Dimensions and thread pitch shall meet ASTM A193 for stud and ASTM A194 for nuts.

3.3. The allowable design stress is listed below. Take note that when nuts and studs of different materials are analysed, the lower yield strength material should be used in the calculation.

3.3.1. Hydrostatic body test: Tensile Stress ≤ 0.83 x Yield Stress

3.3.2. Working Pressure: Tensile Stress ≤ 0.67 x Yield Stress Tensile stress is based on the tensile area of the minor diameter. Refer to

Table 4: Tensile stress area of UNC bolts

3.4. The following API flanges must be derated when used with B7M and L7M.

This is because from API 6AF2 the combination load graphs for these flange sizes at zero bending moment and tension barely meets the pressure rating of the flange at 40,000psi bolt makeup stress. API 6A latest edition, Table 49 also lists the flange sizes that will require pressure derating due to low strength bolting. Refer to API 6AF2 for the acceptable derated pressure.

3.4.1. 5,000psi: 13-5/8” and larger

3.4.2. 10,000psi: 4-1/16”

3.4.3. 15,000psi: 2-1/16”, 2-9/16”, 3-1/16”, 4-1/16”, 7-1/16”, 9”, 11”, 13-

5/8”, 18-3/4”