AFFINITY LAWS: Predict how capacity, head, and horsepower are affected by changes in the centrifugal pump impeller diameter or shaft speed.

BALANCE RATIO: A 70-to-30 balance ratio means that 70 percent of the seal face closing area is seeing the stuffing box pressure and 30 pressure is not seeing the pressure.

BARRIER FLUID: The high-pressure fluid that is circulated between two mechanical seals. The fluid should enter the bottom and leave the top to prevent air pockets.

CARBONIZING: A reduction of hydrocarbons resulting in the formation of carbonaceous residue that will interfere with the movement of a mechanical seal. Often called “coking.”

ELECTROLYSIS: A process involving chemical change caused by the passage of an electric current through a liquid.

FLUSH: Putting an outside liquid into the stuffing box of the pump at a pressure higher than the stuffing-box pressure. All of this liquid mixes with and dilutes the pumped fluid.

GLAND: The part that holds one half of the mechanical seal and attaches to the pump stuffing box.

HEAD: The equivalent height of the liquid. 20 C. water is used as the standard where 10 meters (33.9 ft.) of water equals one atmosphere (14.7 PSI or one bar).

IMPELLER: Attaches to the end of the shaft to impart energy to the fluid being pumped. Available in open, semi-open, and closed designs.

JOULE: A metric unit for the measurement of heat. Defined as the energy required to move one Newton over one meter.

LOW 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. Usually caused by throttling a pump discharge valve.

NEWTONIAN FLUID: A fluid that does not change viscosity as it is agitated.

PIPE STRAIN: The strain on the pump volute caused by the piping. It will cause excessive mechanical seal movement and can cause contact between rotating and stationary pump and seal components.

ROTATING SEAL: When the spring-loaded or moveable portion of the seal rotates with the shaft.

SUCTION HEAD: The head on the suction side of the pump. You subtract it from the discharge head to determine the head being produced by the pump. It is a sum of the static, pressure, and friction heads.

THRUST: In a centrifugal pump it refers to the axial movement of the shaft. The thrust can be towards the wet or power end of the pump, and at start-up it thrusts in both directions.

VACUUM: Any pressure less than atmospheric. Can present a problem for the elastomer in many seal applications.

WEAR RING: Used with closed impeller pumps to restrict leakage from the high-pressure side of the pump to the low-pressure side. Should be replaced when the recommended clearance is doubled.

The terms and definitions for this issue’s Word Search come from a glossary of terms compiled by the McNally Institute (, a provider of technical information on pumps and seals.