Steam Main Jargon Buster

Absolute humidity

Is a measure of the amount of moisture in the air. This is expressed by mass, in kg of water, per kg of dry air.

Absolute pressure

Is a measure of pressure in a system. It is usually expressed as; bar a. It is defined as pressure measured from the datum of a perfect vacuum (which is 0 bar a). Therefore 0 bar a will be less than normal atmospheric pressure. Or expressed another way absolute pressure is the sum of atmospheric and gauge pressures.

Absolute zero

Is theoretically the coldest possible temperature. At this hypothetical temperature there is a total absence of heat. Since heat is a result of energy caused by molecular motion, there is no motion of the molecules with respect to each other at absolute zero. The temperature at absolute zero equals -273.15°C (-459.67°F).

Absorption system

This is a refrigeration system in which the refrigerant gas evolved in the evaporator is taken up in an absorber and released in a generator when heat is applied.

AC current

Alternating current is a type of electrical current that reverses polarity at a uniform frequency.


This a vessel used for storing thermal energy to suit production or operational needs. A steam accumulator will contain both water and steam at saturation temperature relative to the accumulator pressure.

Accumulator charging valve

This is an automatic valve used to control the steam flow rate into the accumulator itself. It would typically work so as to maintain the pressure in accumulator at the desired level.

Accumulator discharge valve

This is an automatic valve used to control the steam flow rate out of the accumulator itself. It would typically work so as to maintain the pressure out of the accumulator at the desired level.

Acid cleaning

The process of cleaning the interior surfaces of steam generating, and heat exchange units by filling the unit with dilute acid accompanied by an inhibitor to prevent corrosion. Once this has been done, the unit will be drained and washed. Then a further neutralizing wash of alkaline water will take place to complete the process.

Activated carbon filter

A filter which is used to remove minute particles from gases.


Is a mechanical device for controlling a mechanism or system. On a steam line actuators are used to operate a control valve or a ball valve. Actuators can be energised electrically or pneumatically using compressed air. The actuator moves the valve plug or valve ball to the position dictated by the control signal. The control signal could be on/off, or continuously variable depending on the demand of the system.

Adaptive control

This is where control algorithms in one or more of the parameters vary in real time, to allow the controller to remain effective in varying process conditions.


Without loss or gain of heat.

Air cooled condenser

A piece of equipment which allows heat to be transferred from the condensing coils in the condenser itself to surrounding air. This may be done either by natural convection or augmented by a fan or blower.

Air handling unit (AHU)

Is normally a large box which is used to condition and circulate air into a building or process. The box will normally contain a fan, heating/cooling element, filter, and humidifier.

Air heater battery

This consists of a bank of heated tubes. It is used to increase the temperature of the air flow, typically in an air duct which forms part of an air handling, or air conditioning system.

Air vent

Is a device that vents air and non-condensable gases. Air entrained in a steam system dramatically reduces efficiency as well as potentially leading to corrosion issues. When a steam system is shut down it will cool, and the steam within the system condenses. This condensing creates a 'vacuum' which will result in air being drawn into the system. When the steam system is re-started, the air inside the system must be expelled before steam can refill and reheat the process. Air vents automatically open to vent air but shut off once the system is full of steam and all the air is removed. Air vents work thermostatically, in essence they are normally open against the colder air, but once the much hotter steam comes into contact with them they expand and shut off.


Alkaline hardness is also known as 'temporary hardness.' It is known as this due to the fact that the hardness is removed by boiling. Calcium and magnesium bicarbonates are responsible for this alkaline hardness.


A chemical which is used in water treatment as a filming or neutralizing agent to protect wetted metal parts. It is sometimes referred to as a filming amine.


An acronym for the American National Standards Institute. This institute lists standards for industry to drive standardisation. In a steam system examples of this would be; pipe fittings (e.g. flanges) and pipework.


An acronym for the American Petroleum Institute. It is a trade association for the oil and natural gas industry. This institute stipulates technical standards and certification for the industry. An example of this might be API 611 which provides a standard for general purpose steam turbines in the oil and gas industry.


An acronym for an automatic pump trap. This device which uses steam as a motive force removes condensate under all operating conditions. It can work as a pump or a steam trap depending on operating conditions.


Model number for a type of automatic steam operated pump trap manufactured by Spirax Sarco. (Please also see APT).

Armstrong International Inc.

A company which manufactures a range of steam products (steam traps and valves etc.). It is well known for its inverted bucket type steam trap. The company was founded in Michigan USA.


The acronym for the American Society of Heating, Refrigeration, and Air Conditioning Engineers.

ASME standards

An acronym for American Society of Mechanical Engineers. This professional society has codes and standards which are used for boilers and pressure vessels.


The acronym for the American Society for Testing and Materials.


This is an EU Directive; 94/9/EC, which covers equipment and protective systems for use in potentially explosive atmospheres.

Atmospheric pressure

The force exerted per unit area by the weight of the atmosphere.


A piece of equipment used for reducing and controlling the temperature of a superheated vapour.


A term used when referring to a corrosion resistant grade of stainless steel in the 'AISI 300' series. For example; 304, 304L, 316, 316L, 321, 347 etc. stainless steel.


A pressure container. For clinical application it is typically used to sterilise equipment and porous material by using saturated steam to kill bacteria. For industrial applications it is used to 'cure' natural materials (for example rubber) with saturated steam.

Automatic pump trap

This is a device which uses steam as a motive force to remove condensate under all operating conditions. It can work as a pump or a steam trap depending on operating conditions.

Auxiliary feedback potentiometer

Used on a control valve to provide an analogue signal to report valve position.

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