5.3 Gasket Layout, Cutting & Fitting for Field-Made Gaskets

Gasket Layout, Cutting & Fitting

Core Trade Concept: Section 5.2 covered engineered gaskets — spiral wound, ring joint, ASME B16.20 hardware bought to a specification. This section covers the other half of module 34106: the gaskets a boilermaker lays out and cuts by hand. Boiler manway and handhole covers, breeching and duct joints, non-standard equipment flanges, and obsolete castings frequently have no off-the-shelf gasket. Getting the layout right is a measuring and geometry problem before it is a materials problem.

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1. Sheet Gasket Materials

Asbestos. Any sheet gasket or gasket residue on legacy equipment must be assumed to contain asbestos until it is tested. Scraping old gasket dry is precisely the activity that generates airborne fibers. Wet methods, HEPA-vacuum housekeeping and a qualified abatement program are the controls; this is not a judgement call made at the flange.

Thickness selection. Choose the thinnest sheet that will seal, normally 1/16 in. for machined flanges. Thick gaskets creep and relax more, lose bolt load, and blow out more readily. Thicker stock (1/8 in. and up) is reserved for rough, pitted or warped faces such as an old cast breeching flange.

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2. Capturing the Dimensions

Five numbers define a flat gasket:

              +------- OUTSIDE DIAMETER (OD) --------+
              |                                      |
              |   o           BOLT CIRCLE (BCD)   o  |
              |        +---- INSIDE DIA (ID) ---+    |
              |   o    |                        |  o |
              |        +------------------------+    |
              |   o                               o  |
              +--------------------------------------+
              Also required: NUMBER of bolt holes (n)
                             DIAMETER of the bolt holes

• ID should match the flange bore or the raised face inner limit. A gasket ID smaller than the bore protrudes into the flow, erodes, and can break loose into a valve or a pump.
• OD for a ring gasket stops inside the bolt holes; for a full-face gasket it runs out to the flange OD and is drilled for every bolt.
• Ring gaskets are used on raised-face flanges. Full-face gaskets are required on flat-face flanges — most importantly on cast iron, because a ring gasket on a cast-iron flat-face flange puts a bending moment across a brittle casting and cracks it (Section 5.2).

Deriving the bolt circle from a measurement you can actually take

You can rarely measure a bolt circle directly, but you can measure center-to-center between two adjacent holes. With $S$ = that chord and $n$ = number of holes:

$\text{BCD} = \frac{S}{\sin\left(\frac{180^\circ}{n}\right)}$

Worked example. An eight-bolt flange, adjacent hole centers measured at 4.596 in.:

$\text{BCD} = \frac{4.596}{\sin(22.5^\circ)} = \frac{4.596}{0.38268} = 12.01\text{ in.}$

The flange is on a 12 in. bolt circle. Always verify by measuring across two opposite holes as well — on an even-numbered pattern that distance is the bolt circle directly.

The straddle rule

Standard flanges have their bolt holes straddling the natural centerlines — two holes placed symmetrically either side of the vertical and horizontal centerlines, never one hole sitting on the centerline. Lay the gasket out to straddle, or the holes will not line up when the flange is oriented normally.

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3. Layout and Cutting Methods

Hole cutting. Use a hollow punch (wad punch) of the correct diameter, struck with a mallet on a lead sheet, end-grain wood block or hard rubber cutting board — never on concrete or steel, which flattens the punch edge on the first blow. Punch bolt holes slightly larger than the bolt diameter so the gasket drops on without being stretched or torn during assembly.

Knife and blade safety. Cut away from the body with the workpiece secured. The predictable injury here is a hand laceration from a utility knife skipping across gasket stock that was not clamped.

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4. Fitting Rules That Get Broken

1. Never install two gaskets to make up a gap. Two gaskets double the creep and relaxation and roughly halve the blowout resistance. A gap means the wrong gasket thickness, misaligned pipe, or a flange that needs facing.
2. Never reuse a gasket. Compressed sheet, spiral wound and graphite all take a permanent set. The second install cannot generate seating stress on an already-crushed gasket.
3. Do not use sealant as a rescue. Pipe dope or silicone on a sheet gasket lowers the friction that keeps the gasket in place and can chemically attack the binder. If the joint needs sealant, the joint is wrong.
4. Centre the gasket. An off-centre ring gasket loads unevenly and can protrude into the bore on one side. On raised-face flanges use the bolts themselves to centre the ring during assembly.
5. Clean the faces properly. Old gasket must come off without gouging the serrated finish — brass or plastic scrapers, never a grinder on a sealing face. A radial scratch across the serrations is a leak path that no amount of torque will close.
6. Follow the controlled bolting sequence from Section 5.2 once the gasket is in. Perfect gasket geometry with an impact-gun tightening sequence still leaks.

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5. Boiler Manway and Handhole Gaskets

Boiler manway and handhole covers are an internal-pressure ("pressure-actuated") design: the cover sits inside the opening and pressure pushes it outward against the seat, so the yoke and bolt only have to hold the cover in place until pressure builds.

• Use the gasket type and dimension specified by the boiler manufacturer — commonly a pre-formed metallic or graphite ring for handholes, not a hand-cut sheet.
• Clean and inspect the seat. Steam cutting on the seat face is common on units where a leaking cover was tightened repeatedly rather than re-gasketed; a wire-drawn groove must be repaired, not gasketed over.
• Do not over-tighten the yoke bolt. The joint is not a compression joint; excessive yoke torque bows the cover and worsens the leak.

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6. Realistic Trade Scenario: A Cut Gasket for an Obsolete Breeching Flange

A crew must reseal a 1950s cast-iron breeching flange with a bolt pattern that matches no current standard: 14 bolts, flat face, 3/4 in. holes.

1. Material. Low pressure, hot dry flue gas, flat face on brittle cast iron: a 1/8 in. high-temperature fiber sheet, cut full face so the bolt load is carried outboard of the gasket and does not bend the casting.
2. Bolt circle. Adjacent hole centers measure 5.360 in. With $n=14$, $\sin(180/14) = \sin(12.857^\circ) = 0.22252$:

$\text{BCD} = \frac{5.360}{0.22252} = 24.09\text{ in.}$

Confirmed by measuring 24-1/16 in. across two opposite holes. 3. Layout. OD and ID scribed with a gasket cutter; bolt circle scribed; the 14 holes stepped around with dividers and checked so the pattern straddles the vertical centerline. 4. Cutting. Holes punched with a 7/8 in. hollow punch (1/8 in. clearance on 3/4 in. bolts) over an end-grain block. 5. Fit. One gasket only, dry, centered on the raised bolt pattern, then tightened in a crisscross star pattern in the staged passes from Section 5.2 — with reduced final torque appropriate to cast iron, which will crack long before a steel flange would.