Flanges, gaskets and bolting

Why it matters

Most leaks in a chemical plant come from flanged joints, not from shells. A flange joint must be tight enough to seal when first bolted up, stay tight when pressure tries to pull it apart, and survive temperature cycles. Designing the gasket and bolting correctly, or choosing the right standard flange rating, is therefore one of the most practical skills in equipment design.

Key ideas

What a bolted joint does. Bolts squeeze a gasket between two flange faces. The gasket must first be crushed enough to flow into the surface irregularities (seating). In service, internal pressure produces an end force that tries to separate the flanges and unload the gasket, so the bolts must carry that end force and still leave enough residual compression on the gasket to keep it sealed.

Gasket factors. Codes characterise each gasket material by two numbers, both taken from the code's gasket table:

• y, the minimum seating stress (MPa) needed to make the initial seal;
• m, the gasket factor: the residual gasket stress needed in operation, expressed as a multiple of the internal pressure. Soft gaskets (rubber, PTFE, compressed non-asbestos fibre) have low y and m and suit low pressures; spiral-wound, metal-jacketed and solid metal ring gaskets have high y and m and suit high pressures and temperatures.

Effective gasket width and diameter. Flanges rotate slightly under load, so only part of a wide gasket is effectively compressed. From the basic width b0 (half the contact width N for a plain flat ring) the code defines an effective width b (b = b0 up to 6.3 mm; for wider gaskets b = 2.5·√b0, in mm) and a load-reaction diameter G (the mean diameter for narrow gaskets; the gasket outside diameter minus 2b for wide ones).

Bolt loads. Two conditions are checked and the larger bolt area governs:

• Bolt-up (seating) at ambient temperature: W_m2 = π·b·G·y, using the bolt allowable stress at ambient.
• Operating: W_m1 = H + H_p, the hydrostatic end force plus the gasket load needed to stay sealed, using the bolt allowable stress at design temperature. Bolts are used in multiples of four (for symmetric tightening and alignment with the centrelines), and bolt spacing must leave room for a spanner yet be close enough to give even gasket compression. A check is also made that the gasket is not crushed by the full bolt load.

Flange types.

• Weld neck (integral): long tapered hub butt-welded to the pipe or shell; stress flows smoothly, so it is used for high pressure, high temperature and cyclic service.
• Slip-on: slips over the pipe and is fillet-welded inside and outside; cheaper, for moderate duty.
• Lap joint: a loose flange over a stub end; easy bolt-hole alignment, and it saves alloy because only the stub end need be alloy.
• Socket weld and threaded: small-bore lines.
• Blind: a flat cover that closes a nozzle or line end, designed as a flat head.

Facings. Flat face (with full-face gaskets, for cast iron or plastic), raised face (the general choice), tongue-and-groove or male-female (confine the gasket), and ring-type joint (metal ring in grooves) for very high pressure.

Standard versus designed flanges. Pipe and nozzle flanges up to large sizes are selected from standards such as ASME B16.5 (pressure classes 150, 300, 600, 900, 1500, 2500) using their pressure–temperature rating tables; you do not calculate them. Large girth flanges on vessels and exchangers are designed by the code procedure (ASME VIII Div. 1 Appendix 2 or the corresponding IS 2825 appendix), which sizes the gasket, the bolting and then the flange thickness from bending moments.

Formulas

b0 = N / 2 (basic gasket width, plain flat ring) b = b0 for b0 ≤ 6.3 mm; b = 2.5·√b0 for b0 > 6.3 mm (b and b0 in mm) G = mean gasket diameter if b0 ≤ 6.3 mm; G = gasket OD − 2·b if b0 > 6.3 mm W_m2 = π·b·G·y (bolt-up seating load) H = (π/4)·G²·P (hydrostatic end force) H_p = 2·π·b·G·m·P (gasket load to stay sealed in operation) W_m1 = H + H_p (operating bolt load) A_m = max(W_m1 / S_b, W_m2 / S_a) (minimum total bolt root area) N_min = A_b·S_a / (2·π·y·G) (minimum gasket width to avoid crushing)

• N = gasket contact width (mm); b0, b = basic and effective widths (mm); G = gasket load-reaction diameter (mm); y = seating stress (MPa); m = gasket factor (–), both from the code's gasket table; P = design pressure (MPa); S_a, S_b = bolt allowable stress at ambient and at design temperature (MPa); A_b = actual total bolt root area (mm²); W in N.

Worked examples

Example 1 (standard): narrow gasket, operating load governs Given: compressed-fibre gasket, ID 500 mm, OD 520 mm (N = 10 mm), m = 2.0, y = 11 MPa (given; from the code table in practice), P = 2.0 MPa, bolt allowable S_a = S_b = 140 MPa, M20 bolts with root area 225 mm².

1. b0 = 10/2 = 5 mm ≤ 6.3, so b = 5 mm and G = (500 + 520)/2 = 510 mm.
2. H = (π/4) × 510² × 2.0 = 408 600 N.
3. H_p = 2π × 5 × 510 × 2.0 × 2.0 = 64 090 N; W_m1 = 472 700 N.
4. W_m2 = π × 5 × 510 × 11 = 88 120 N — smaller, so operation governs.
5. A_m = 472 700 / 140 = 3376 mm²; bolts = 3376/225 = 15.0 → 16 × M20 (a multiple of four).

Example 2 (GATE level): wide spiral-wound gasket, seating governs Given: gasket ID 1000 mm, OD 1040 mm (N = 20 mm), m = 2.5, y = 69 MPa (given), P = 1.0 MPa, S_a = S_b = 138 MPa, M24 bolts with root area 324 mm².

1. b0 = 10 mm > 6.3, so b = 2.5 × √10 = 7.91 mm; G = 1040 − 2 × 7.91 = 1024.2 mm.
2. W_m2 = π × 7.91 × 1024.2 × 69 = 1.755 × 10⁶ N.
3. H = (π/4) × 1024.2² × 1.0 = 8.24 × 10⁵ N; H_p = 2π × 7.91 × 1024.2 × 2.5 × 1.0 = 1.27 × 10⁵ N; W_m1 = 9.51 × 10⁵ N.
4. Seating governs: A_m = 1.755 × 10⁶ / 138 = 12 720 mm²; 12 720/324 = 39.3 → 40 × M24.
5. Crushing check: N_min = 40 × 324 × 138 / (2π × 69 × 1024.2) = 4.0 mm < 20 mm ✓. Note how a hard gasket with high y makes seating, not pressure, the governing case.

Common mistakes

• Using the full gasket width instead of the effective width b, or the gasket ID instead of G.
• Checking only the operating load and forgetting the seating load (or vice versa).
• Using bolt shank area instead of root (thread minor) area.
• Rounding the number of bolts to an odd number or a number not divisible by four.
• Calculating flanges for nozzles that should simply be selected from a standard rating table.
• Choosing a gasket for pressure alone, ignoring temperature and chemical compatibility.

For GATE CH

Expect conceptual questions on gasket factors m and y, flange types and facings, and why bolts come in multiples of four, plus numericals on hydrostatic end force, operating and seating bolt loads, number of bolts and minimum gasket width. Practise identifying which load governs.

Quick check

1. What do m and y describe?
2. Which flange type would you choose for high-temperature cyclic service?
3. For b0 = 4 mm, what is b?
4. A joint needs 3900 mm² of bolt root area and each bolt has 225 mm². How many bolts?

Answers: 1. m is the residual gasket stress in operation as a multiple of pressure; y is the minimum seating stress. 2. Weld neck. 3. 4 mm (b = b0 when b0 ≤ 6.3 mm). 4. 3900/225 = 17.3, so 20 bolts.