Steam Main Jargon Buster


Another word used to describe insulation. Lagging insulation is usually a fiberglass or other type of fabric. It is used as a thermal protection on steam pipes, condensate pipes, valves, heat exchangers, boiler feedtanks, and in other high heat environments. Correctly lagged equipment can lead to less heat loss, better process operation, and energy savings. Lagging insulation is normally protected from physical damage by external cladding.

Laminar flow

This is defined as when the streamlines remain distinct from one another over their entire length. A streamline being a line which lies in the direction of flow at every point at a given instant. For laminar flow the streamlines need not be straight or the flow steady, as long as the above criterion is fulfilled.


A computing acronym which stands for; Local Area Network.

Lapped in

This is a term which is used to describe a procedure for reducing the leakage rate of, metal to metal seated valves. The plug and seat are lapped together with the aid of an abrasive compound in an effort to obtain a better seating surface than would normally be achieved by means of machining.

Latent heat

When a body changes from the solid to liquid state or from the liquid to the gaseous state a certain amount of heat is used to accomplish this change. This heat does not raise the temperature of the body and is called latent heat. When the body changes again from the gaseous to the liquid of from the liquid to the solid state, this quantity of heat is given out by it.

Latent heat of evaporation

This is the heat that must be supplied to a liquid at the boiling point to transform the liquid into a vapour.

Level control system

A system which comprises of; sensor, controller, and valve or pump. Its action can be on-off or modulating.

Level probe

A sensor that detects level in the system being controlled.

Lift check valve

A type of check valve which stops reverse flow. When flow in the desired direction goes through the valve, the valve disc floats vertically off a seat, and the fluid flow is 'up and over'. Reverse flow causes the valve disc to fall back onto the seat and block the flow path.

Liquefied natural gas

LNG is a natural gas that has been condensed to a liquid through a cooling process. The composition of natural gas, and hence LNG that is formed from it varies according to its source and processing history, but consists almost entirely of methane (CH4), the simplest hydrocarbon compound. Typically the composition of LNG is 85 to 95+ percentage methane along with a few percentage ethane, even less propane and butane and possibly traces of nitrogen. Like methane, LNG is odourless, colourless, non-corrosive and non-toxic. It is a fuel and therefore highly flammable. At atmospheric pressure methane will condense to a liquid when it is cooled to -16 °C.


The definition of the state of a substance. When a substance is in liquid state it exhibits a characteristic where it will flow with little or no tendency to disperse, and relatively high incompressibility.

Liquid petroleum gas

This is colourless, odourless gas. LPG is used as a fuel source and is therefore highly flammable. Its main hazards are fire and explosion.

Low level alarms

On a typical shell steam boiler there are two low alarms, the first shuts the burner off but allows it to re-fire if the water level is re-established. The second also shuts the burner off but does not reinstate the burner if/when level is re-established, a boiler person must manually check the boiler and associated plant, and then they can re-set the burner and boiler.


An acronym for Low Temperature Hot Water. Water that is typically below 100 °C. Common heating flow temperatures being 82 °C and return temperatures being 71 °C.

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