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Glossary of pump terms: L

Labyrinth seal: A mechanical seal that forces the sealed fluid through a labyrinth like path to slow its leakage.

Laminar: A situation when fluid particles flow in parallel layers and these layers don’t mix.

Lantern ring: A perforated ring in the middle of a ring set that is used to provide lubricant to packing. Commonly found in the middle of a packing ring set.

Lapping: A method of finishing machined parts that results in extremely flat, smooth surfaces.

Lateral critical speed: The speed at which the lateral deflection of the rotor is in resonance. The critical speed refers to the same property, but it is the speed at which the angular torsional velocity excites resonance in the rotating object.

L10 bearing life: A bearing specification that gives the lifetime of a population of bearings such that after this time, 10% of bearings will have failed due to fatigue failure.

L³ / D⁴: A metric used to predict how well a pump will perform. L is the length of the shaft measured from the center of the impeller to the center of the radial or inboard bearing and D is the the outside diameter of the solid shaft measured beneath the sleeve. For best performance, this number should be below 60 in imperial units and 2.0 in metric units.

Leakage concentration: How much of the process fluid leaks into the environment. Measured in parts per million.

Leakage rate: A rate (volume / time, e.g. litres / minute) of a fluid that passes through a particular part of the pump, for example a sleeve, seal, etc.

Life cycle cost (LCC): Life cycle cost is the total cost of the machinery including purchase, installation, operation, maintenance, repair and dispose of the machinery. Calculated when trying to decide whether to upgrade.

Linear pressure drop: In reference to a mechanical seal, refers to a pressure drop across all lapped faces. Unusual.

Line bearings: Bearings in a pump that position the rotor radially.

Line bored: A method of machining that causes the drilled (bored) holes to all line up.

Liner: A replaceable “liner” that fits into a pump casing to protect it. Often used when the pump processes abrasive material.

Lip seal: Also known as a radial shaft seal. They are used to seal rotary elements such as a shaft or rotary bore.

Load E: The total load on a bearing, including both radial, as well as axial loads.

Lobe pump: Similar to gear pumps in operation, except that the lobes do not make contact. Positive displacement pumps that are popular where the sanitary quality is important.

Low flow: Must be taken into account when selecting a pump. Low flow can cause excessive heat build up in the volute.

Low NPSH pump: A pump which has been designed to deal with a low Net Positive Suction Head. Prevents cavitation in the pump.

Lubricant: An introduced substance that reduces friction between moving parts. Usually a fluid.

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