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Glossary of pump terms: D

Damping: Where vibration is occurring, physically touching a vibrating component is the process of damping.

Darcy-Weisbach equation: An equation used for calculating the friction head loss for fluids in pipes, the friction factor f must be known and can be calculated by the Colebrook, the Swamee-Jain equations or the Moody diagram.

DCI: DCI is an acronym that stands for Disassembly, Cleaning and Inspection. This is the first step in any maintenance, repair or upgrade.

Dead ending: A procedure that isolates the stuffing box. There are no flushing lines or recirculation lines coming in or going out.

Dead head: A dead head is a dangerous situation occurring when the pump is running but there is no discharge, either due to blockage in the line, or an inadvertently closed valve. The pump will continue to operate until a safety shutoff pressure is reached. Prior to this, recirculation within the pump may cause overheating and damage to the pump.

Decanting: Gradually pumping from one container to another.

Deflection: Movement of an axially situated shaft in a radial direction.

Density: The mass per unit volume of a material. Often measured in g / mL, or g per cubic centimeter.

Design duty point: This is the point on a pump’s efficiency graph for which the design has been optimized. It will be expressed as a particular capacity at a head or pressure of the process fluid. Ideally, this point should coincide with the pump’s best efficiency point.

De-staging: In multi-stage pumps with multiple impellers, de-staging refers to the removal of one or more impellers to reduce the head.

D-frame adapter: A device used to align the shaft on the pump to the shaft on the motor. D-frames are used when the system in question is manufactured in metric dimensions, while a C-frame is used with standard dimensions.

D-Gun process: An alternative to the plasma process for metal deposition. Normally used to put hard metal over softer metal.

Dial indicator: An indicator used to test how well a shaft is aligned axially. Any displacement will cause the indicator to deflect.

Diaphragm pump: Also known as a positive displacement pump, or an air-operated diaphragm pump. Diaphragm pumps with double diaphragms have smooth operation. These pumps are exceptional at pumping liquids that are viscous, toxic, chemically aggressive, or abrasive.

Differential Pressure: The differential pressure is the difference between the pressure of the system at the inlet and outlet of the pump.

Diffuser: Set within the discharge region, the diffuser reduces turbulence of the discharged liquid by promoting a gradual decrease in velocity. It’s usually a set of vanes that are an integral part of the pump.

Dilatant: A liquid that behaves in a non-Newtonian fashion in that it increases in viscosity as the shear strain applied increases. As an example, a mixture of cornstarch and water behaves as a dilatant.

Dilatant liquid: A liquid whose viscosity increases with increased shear rate e.g. agitation. A liquid that behaves in a non-Newtonian fashion in that it increases in viscosity as the shear strain applied increases. As an example, a mixture of cornstarch and water behaves as a dilatant.

DIN standard: A standard published by the “Deutsches Institut für Normung” or German Institute for Standardization.

Disaster bushing: Used in API glands to support the shaft in the event of a bearing failure, or to prevent product from rushing to atmosphere after a seal failure. The close clearance (0.025 inch or 0.5 mm.) directs most of the leakage through a drain connection in the seal gland to an appropriate container.

Discharge head: The discharge head is the pressure at the discharge or outlet.

Discharge recirculation: Discharge recirculation is when the recirculation, or flow reversal happens at the discharge tips of the impeller vanes. Connecting a line from the discharge side of the pump to the stuffing box. Should be used with a close fitting bushing in the end of the stuffing box to increase the stuffing box pressure. A common application when pumping a fluid close to its vapor point.

Displacement: The change in position of an object over time usually with reference to its starting position. Can also refer to dynamic movement when it usually refers to peak-to-peak displacement.

DN factor: Do not use precision bearings if the bearing bore (millimeters) x rpm. is 300,000 or greater.

Dosing: A controlled method of pumping in order to discharge exact amounts of fluid.

Double balanced seal: A seal that is hydraulically balanced in both directions. This is a desirable characteristic, but one that is rarely provided by manufacturers.

Double row, deep groove, ball bearing: A ball bearing configuration in which the rows of balls are positioned to allow the load to be transmitted in an outward converging angle, to increase the thrust load capacity.

Double seal: A pump with two mechanical seals on the drive shaft. Designed to prevent the process fluid from leaking. Used when pumping dangerous liquids.

Double suction pump: In this type of pump the process fluid is channeled inside the casing to both sides of the impeller. This is a very stable configuration as the hydraulic forces are balanced.

Double suction impeller: An impeller that has two inlets, where the liquid enters the impeller from both sides. Stable configuration, as the hydraulic forces are balanced, but expensive to implement.

Double volute: A centrifugal pump in which there are two cutwaters to prevent shaft deflection when the pump is not operating at its best efficiency point.

Drawdown: The decrease in volume of a liquid in a container due to a pump’s removal of the liquid. Usually refers to the decrease in liquid height along the container’s walls.

Drive collar: In a cartridge seal, this is the part that connects the seal sleeve to the shaft. It transmits rotation and prevents axial movement of the sleeve.

Drive lugs: A more robust method of transmitting the torque to the seal face, rather than setscrews.

Drooping curve: A pump efficiency curve in which at low flow, the head rises and then drops as it gets to the shut-off head point.

Dry critical speed: This is the definition from API610: "a rotor's natural frequency calculated assuming that the rotor is supported only at its bearings and that the bearings are of infinite stiffness." In layman’s terms this just means that this calculation does not take into account any of the damping affects of the process fluid. See wet critical speed for a comparison.

Dry running: Running without any process fluid when referring to a pump, or without any lubricant for a seal.

Dual seal: Two seals in one of several possible configurations. They can be back-to-back, face-to-face, tandem or concentric.

Dual mechanical seal: In a dual mechanical seal configuration, there are two seals in the same chamber arranged in one of several possible positions, tandem, concentric, face-to-face, or back-to-back.

Ductility: The ability of a metal to deform without breaking. Often characterized by how easy it is to stretch the material into a wire.

Dynamic elastomer: The elastic components of the seal that must compensate for seal face wear or shaft movement. Usually made of rubber or other elastic polymer.

Dynamic head (system head): The dynamic head is the component of the total dynamic head caused by friction in the system, due to interaction of the process fluid with the pipe walls.

Dynamic unbalance: A situation where the axis of inertia of a rotor (impeller) is not coincident with its geometric axis. Also known as two-plane unbalance because correction is required in two planes. This is the most common type of mass imbalance in centrifugal pumps.

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