Glossary M

Mach number

  • The relationship between a moving body and the speed of sound in that locality which can vary with temperature, altitude and there fore pressure.

Magnetic drive

  • A type of seal less pump that is pretty much limited to pumping clean lubricating liquids. Similar in concept to a canned pump.

Magnetic seal

  • Uses magnetic materials rather than springs to keep the lapped seal faces together. Limited to non corrosive fluids because of the magnets.


  • Iron oxide ( Fe3O4). A protective coating the forms on iron pipe to prevent further rapid corrosion. Usually black or maroon in color. Can be very destructive to mechanical seal components until the oxide stabalizes. We often find these iron pipes in new hot water systems

Mating ring

  • Another name for the hard face in a mechanical seal. It can be either rotating or stationary.

Mean Diameter

  • The middle diameter. Usually refers to the center of the seal face. The term is commonly used with metal belows seals to describe the middle diameter of the bellows plate.

Mechanical seal

  • A positive sealing device used to seal all fluids (liquids and gases). The primary seal is a set of lapped seal faces that are installed perpendicular to the shaft.

Metal bellows

  • Used in mechanical seal designs to eliminate the need for a dynamic elastomer.

Metal fatigue

  • A breakage of the metal caused by the bending and flexing of a metal part beyond its endurance limit.

Micro Organisms

  • Used in a variety of chemical processes. Can cause corrosion of stainless steel if they penetrate the passivated layer and attack the carbon content of the stainless steel.

Minimum flow

  • A condition that can cause excessive heat inside the pump volute. A temperature rise of 10°C (18°F) across the operating pump is considered excessive. Normally caused by throttling a pump discharge valve.


  • When one liquid mixes or blend with another liquid. Same as soluble.

Modulus of elasticity

  • Refers to the stiffness of the material. The higher the modulus the stiffer the shaft. Most pump shafts have similar modules numbers. Also called Young’s modulus. The relationship of stress to strain. If seal faces have a high modulus they are less likely to distort under pressure.

Moment of inertia

  • Referring to rotation about an axis. In the pump business it refers to a formula that describes the shape of the shaft. A solid shaft would have a different moment of inertia than a hollow shaft.

Monochromatic light

  • A single color light used with an optical flat to read seal face flatness.