4.1 Gaskets and Packing

Why This Topic Matters on the Exam

Gaskets and packing form the "Packing, Seals and Gaskets" content domain of the AEN15MLWR05 assessment specification alongside O-rings/non-mechanical seals and mechanical seals — together worth 8% of the 125 scored items. This section covers curriculum module 15107, a 15-hour Millwright module that teaches gasket types, layout, cutting, and installation, plus compression packing for stuffing boxes. Every millwright breaks flanged joints and repacks valves and pump stuffing boxes routinely, so the exam leans on practical judgment: which gasket material belongs in which service, and how to torque a flange so it doesn't leak or blow out six months later.

Core Terms and Concepts

A gasket is a static seal placed between two mating flat (or near-flat) surfaces — most commonly a pipe flange — to fill microscopic surface irregularities and prevent leakage of a fluid or gas under pressure. Packing is a different animal: a compressible, braided or die-formed material installed in a stuffing box (the cavity surrounding a rotating or reciprocating shaft, such as a pump shaft or valve stem) to control leakage around moving parts rather than between two fixed faces.

Gaskets are classified by construction:

Spiral wound gaskets are the exam's favorite "step up" material: the metal winding provides mechanical resilience and recovery, while the soft filler (flexible graphite for the highest temperatures, PTFE where graphite would be chemically attacked) does the actual sealing against the flange face. A plain cork-rubber or solid rubber gasket used in that same steam or hydrocarbon service would extrude, harden, or char and leak.

Gaskets are also cut as full-face (covers the entire flange face and every bolt hole — used on flat-face, low-pressure flanges) or ring-type/inside-bolt-circle (cut to the raised-face diameter only, inside the bolt circle — used on raised-face ANSI flanges so bolts never contact the gasket material). Installing a full-face gasket on a raised-face flange, or vice versa, is a classic exam distractor.

Layout, Cutting, and Torquing

When a gasket isn't available pre-cut, module 15107 teaches laying it out directly from the flange: place gasket sheet stock against the flange face, mark the bolt-hole centers and inside/outside diameters, then cut with a gasket knife, hand punch, or hollow punch set — never an open flame or grinder near the raised face, which would score the sealing surface.

Installation always follows a star (crisscross) bolt-up pattern, never a clockwise/sequential pattern, and always in stages — typically 30%, 60%, then 100% of the specified torque value across multiple passes. This distributes compressive load evenly around the flange. Torquing straight around in one pass cocks the flange, over-compresses one side while under-compressing the other, and is a leading cause of new-gasket leaks. Manufacturer torque specifications must be followed; over-torquing crushes semi-metallic and metallic gaskets past their recovery point just as reliably as under-torquing leaves gaps.

Packing Materials and Installation

Common packing materials, in rough order of increasing temperature/chemical capability:

• Flax/cotton — economical, general low-temperature, low-pressure water service.
• Graphite/carbon-impregnated — self-lubricating, handles high temperatures and is the default for steam and many process fluids.
• PTFE (polytetrafluoroethylene) — broad chemical resistance, low friction.
• Aramid (synthetic fiber) — high abrasion resistance, often blended with graphite for abrasive slurries.

Packing rings are installed one ring at a time, each seated fully with a packing tool before the next ring is inserted, and the joints of successive rings are staggered 90–120° apart around the shaft circumference. Stacking every ring's cut joint at the same clock position creates a straight leak path through the stuffing box. Unlike a gasket, packing is not tightened to zero leakage — the gland follower is snugged only enough to allow a slight, steady drip. That controlled leakage lubricates and cools the packing against the shaft; over-tightening the gland to stop the drip burns up the packing and scores the shaft or sleeve within hours.

Exam Scenarios

A technician torques a new spiral-wound gasket flange in one clockwise pass straight to final torque, and it leaks at start-up. The most likely cause tested is uneven, non-staggered bolt-up sequence — not gasket material choice. Another common scenario: a valve stem packing is tightened until the drip completely stops to "keep the floor dry," and the packing burns up within a shift — testing that packing is designed to leak in a controlled way, unlike a gasket or mechanical seal.

Key Takeaways

• Gaskets seal static joints (flanges); packing seals dynamic joints (rotating/reciprocating shafts) inside a stuffing box.
• Spiral wound gaskets (metal winding + graphite/PTFE filler) handle high-temperature, high-pressure steam and hydrocarbon service that soft rubber or cork gaskets cannot.
• Always torque flange bolts in a star pattern across multiple passes (e.g., 30/60/100%), never straight around in one pass.
• Stagger packing ring joints 90–120° apart; never tighten a gland until the drip fully stops.
• Match gasket type (full-face vs. ring-type) to the flange face style (flat-face vs. raised-face).