Steam Main Jargon Buster


A heat exchanger which heats the boiler feedwater before it is fed into the boiler (on a shell boiler the economiser is located after the boiler feed pump so temperatures in excess of 100 °C are possible). It does this by using the heat in the flue gas as it is discharges up the chimney.


A type of pipe fitting. It can come in long or short elbow types.

Electric actuator

A device fitted to a valve that allows the valve to be operated remotely by an electrical signal.

Electro pneumatic positioner

This is a device attached to a pneumatic actuator converting the electrical signal from the controller to air pressure that moves the actuator, whilst guaranteeing a constant valve position relative to that signal.

Electrode boiler

A type of boiler which generates steam or hot water by using immersed electrodes.

Electronic positioner

This is a device which is fitted to a control valve that moves an electrical actuator relative to an electrical control signal, whilst guaranteeing a constant valve position relative to that signal.

Enthalpy of evaporation

The heat energy required to change water at its saturation temperature into saturated steam. It involves a change of state rather than a change in temperature. It can also be referred to as 'latent heat'.

Enthalpy of steam

The total energy of saturated steam. It is calculated by adding the enthalpy of water, and the enthalpy of evaporation (to change water into steam).

Enthalpy of water

This is the heat energy required to heat water from a datum of 0 °C to the temperature it is at. It can also be referred to as 'sensible heat'.

Entrained condensate

This is condensate which is carried along with steam flow.


This is a thermodynamic property that is used to determine the energy not available for work in a thermodynamic process.


A type of synthetic rubber. The E refers to ethylene, P to propylene, D to diene and M refers to its classification in ASTM standard D-1418.

Equal percentage valve

A valve with a flow characteristic produced by a certain shape of valve trim such that the percentage change of flow with respect to spindle lift at any point is proportional to the existing flow at that point. In practice this means that the change in flowrate with respect to travel will be relatively small with the plug near the valve seat, and relatively high when the plug is approaching wide open.

Expansion vessel

An expansion vessel or tank is used to protect closed heated water systems.

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