A gasket seals a joint between two parts that do not move, such as a pair of flanges, by being clamped between them; a mechanical seal seals the point where a rotating shaft passes out of the pump, with two flat faces running against each other. A gasket is a seal, then, but a static one, and the two cannot stand in for each other: each works only on its own kind of joint.

You will also see gaskets called mechanical seals, in the broad sense of any seal held tight by mechanical force. In the pump trade a mechanical seal is the rotating-shaft face seal.

What Is a Gasket

A gasket is a seal that fills the space between two fixed mating surfaces, most often a pair of bolted flanges, so that fluid or gas cannot escape between them. Gaskets fall into three categories by material: non-metallic or soft gaskets, semi-metallic gaskets such as spiral wound, and metallic gaskets of solid metal.

What Is a Mechanical Seal

A mechanical seal is the device that stops the pumped fluid from leaking out along the rotating shaft. It seals with two rings called seal faces: one is mounted on the shaft and turns with it, the other is fixed to the housing, and the two run pressed together. By construction, mechanical seals are usually grouped into O-ring pusher seals, elastomer bellows seals and metal bellows seals.

A mechanical seal also contains static seals of its own, such as the gasket or O-ring between the gland plate and the seal chamber, so the two words often turn up on the same parts list.

The Difference Between a Gasket and a Mechanical Seal

Every difference between a gasket and a mechanical seal follows from the kind of joint each one seals. Fitting and replacement differ too: a gasket is normally renewed whenever its joint is opened; a mechanical seal has to be set to its working length on the shaft, or comes preset as a cartridge seal, and is replaced when its faces wear.

Static vs Dynamic Sealing

A gasket seals a static joint, where the two sealed parts do not move relative to each other; a mechanical seal seals a dynamic joint, where the shaft turns inside the fixed housing. On a pump, the suction and discharge flanges and the casing joints and covers are sealed with gaskets or O-rings, and the mechanical seal sits on the shaft where it passes out of the casing.

How Each One Seals

A gasket is held tight by bolt load that crushes it into the joint; a mechanical seal is held tight mainly by the pressure of the pumped fluid, with springs keeping its two faces closed when that pressure is low, at start-up and shutdown.

A mechanical seal’s faces are lapped flat, often to fewer than three helium light bands (about 0.8 µm), and run on a film of fluid as thin as about 1 micron and usually a few microns thick.

Leakage

A gasket in good condition is meant not to leak at all, while a mechanical seal always leaks a small amount across its faces, normally as vapor too faint to see. On volatile fluids that leakage is measured as vapor concentration at the seal, and the European Sealing Association’s guidance on seal emissions puts a reliable target for a general-purpose single seal with a plain gland at 1,000 ppm.

Leakage at a mechanical seal is a question of how much is acceptable, while a gasketed joint is held to a stricter limit.

Materials

A gasket is made of a single material that deforms under bolt load, or a soft material combined with metal; a mechanical seal is an assembly of several materials, with the sealing done mainly by a pair of lapped face rings. Gasket materials include rubber, compressed fiber sheet, PTFE, flexible graphite and metal. In a mechanical seal the springs, drive parts and gland plate are metal, and the sealing parts are:

  • Seal faces: commonly a carbon-graphite ring running against a hard face of silicon carbide, tungsten carbide or alumina; both rings can also be hard faces.
  • Secondary seals: elastomer O-rings, or flexible graphite rings in metal bellows seals built for very high or very low temperatures.