A–B–C–D–E–F–G–H–I–J–K–L–M–N–O–P–Q–R–S–T–U–V–W–X–Y
Data Highway
A single-cable link which provides communication between control stations which are physically separated.
DC (Direct Current)
Electricity which flows in one direction as produced by a battery.
DCS (Distributed Control System)
Central process control system distributed among two or more process control units coupled by a high speed communications network. Remote field devices such as valve actuators are connected to one of the process control units.
Deadband
The amount of power supply of an actuator can be varied without initiating valve obturator motion. The area of a signal range or band where no action occurs. The technical term for deadband is hysteresis.
Deflash
Process of removing excess material (flash) from the parting line of a molded rubber product.
Delta-P (ΔP)
See Differential Pressure and Pressure Drop.
Design Appraisal
A procedure by which a certifying authority appointed by the purchaser, appraises the design parameters of the equipment and/or materials he is buying. The supplier shall submit drawings, calculations and documents as required to the certifying authority in conjunction with those normally required for review and acceptance by the purchaser.
Design Temperature
The piping system in which the valve is operating in has a design temperature. The valve must be designed to this temperature.
Detent
A spring-return device which maintains the position of the spool of a directional control valve in position.
Diaphragm
A round, thin, flexible sealing device secured and sealed around its outer edge and sometimes around a central hole in the diaphragm with its unsupported area free to move by flexing.
Diaphragm Valve
Consists of a valve body with two or more ports, a diaphragm and a seat upon which the diaphragm closes the valve.
Differential Pressure
The difference in pressure across a valve in a pressurized line. The difference in pressure between any two points in a pressurized system under flowing conditions.
Digital
Data represented by a series of digits. Many functions/values can be transmitted over a single pair of cables.
Dip Tube
Extending the blow-down valve on large gate valves requires a tube which is located inside of the valve. This tube is called the Dip Tube and extends through the bonnet to the bottom of the body cavity.
Disc
The closure element of a globe angle or small regulator valve. The disc also termed plug, moves to and from the seat in a direction perpendicular to the seat face. Depends on stem force for tight shutoff.
Double Block & Bleed Valve
A valve with two seating surfaces that in the closed position provides a seal against pressure from both ends of the valve with a mean of venting/bleeding the cavity between the seating surfaces.
Double Piston Effect (DPE)
The sealing principle of ball valves whereby line pressure is used on both the upstream and downstream floating seats to effect a dead-tight seal simultaneously on both sides of the ball. With the DPE seat configuration when the upstream seat leaks, the pressure entering into the body cavity acts on the downstream seat, which being of the PPE design is then pushed against the ball and the valve seals in both directions.
Double-Acting
An actuator which requires power in both operating directions i.e. no spring.
Double-Isolation & Bleed Valve (DIB)
A single valve with two seating surfaces, each of which in the closed position provides a seal against pressure from a single source, with a means of venting/bleeding the cavity between the seating surfaces. This feature can be provided in one direction or in both directions.
Drain Plug
A fitting at the bottom of a valve, the removal of which permits draining and flushing the body cavity. The vent plug assembly on some ball and gate valves also serves as a drain valve.
Drive Pins
The pins which fit into the bottom of a ball valve stem and engage corresponding holes in the ball. As the operator turns the stem, the drive pins turn the ball. The drive pins transmit the drive torque by shear. The drive pins can be replaced by a tang-drive connection integrally machined in the valve stem.
Drive Train
All parts of a valve drive between the operator and the obturator, including the obturator, but excluding the operator.
Drop
A drop in set (outlet) pressure of a regulator or control valve due to the travel of it’s valve or poppet, as the required flow increases from low to maximum. A slight change in the control spring length due to the valve travel will result in spring force variations, translating into a change of set (outlet) pressure.
Dry Bearing
A sliding bearing consisting of a PTFE (Teflon) and lead powder lining bonded to a metal backing. Also termed DU® bearing, which is a trademark of GGB.
Dry Lube
Dry-film molydisulphide applied as a coating to sliding or rotating parts to reduce frictional drag. After application, the film is baked in an oven at high temperature.
Dual Seat
A combination of one unidirectional seat and one bi-directional seat installed in the same valve body. The directions of both seats shall be specified on a separate identification plate. This design provides two sealing barriers in one direction and one sealing barrier in the opposite direction.
Dual Seat (Both Seats Bi-Directional Valve)
Valve with two seats, each sealing in both directions.
Dual Seat (One Seat Uni-Directional & One Seat Bi-Directional Valve)
Valve with two seats, one sealing in one direction and the other in either direction.
Durometer
The Durometer (also called Shore Durometer) measures hardness in terms of material elasticity. A diamond-tipped hammer is used to strike the testing surface from a known height. The rebound of the hammer is measured. The harder the material, the higher the rebound.