An air lock is a restriction of, or complete stoppage of liquid flow caused by

vapour In physics, a vapor (American English) or vapour (Commonwealth English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R. H. Petrucci, W. S. Harwood, and F. G. Herring, ''General ...

trapped in a high point of a

liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...

-filled

pipe Pipe(s), PIPE(S) or piping may refer to: Objects * Pipe (fluid conveyance), a hollow cylinder following certain dimension rules ** Piping, the use of pipes in industry * Smoking pipe ** Tobacco pipe * Half-pipe and quarter pipe, semi-circular ...

system. The

gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...

, being less dense than the liquid, rises to any high points. This phenomenon is known as vapor lock, or air lock. Flushing the system with high flow or pressures can help move the gas away from the highest point. Also, a tap (or automatic vent valve) can be installed to let the gas out. Air lock problems often occur when one is trying to recommission a system after it has been deliberately (for servicing) or accidentally emptied. Take, for example, a

central heating A central heating system provides warmth to a number of spaces within a building from one main source of heat. A central heating system has a Furnace (central heating), furnace that converts fuel or electricity to heat through processes. The he ...

system using a circulating pump to pump water through radiators. When filling such a system, air is trapped in the

radiator A radiator is a heat exchanger used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...

s. This air has to be vented using screw valves built into the radiators. Depending on the pipe layout – if there are any upside down 'U's in the circuit – it will be necessary to vent the highest point(s). Otherwise, air lock may cause waterfall flow where the loss of

hydraulic head Hydraulic head or piezometric head is a measurement related to liquid pressure (normalized by specific weight) and the liquid elevation above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22, eq.3.2a. It is usually meas ...

is equal to the height of airlock; If the hydraulic grade line drops below the output of the pipe, the flow through that part of the circuit would stop completely. Note that circulating pumps usually do not generate enough pressure to overcome air locks.

Undulating pipes

reservoir A reservoir (; ) is an enlarged lake behind a dam, usually built to water storage, store fresh water, often doubling for hydroelectric power generation. Reservoirs are created by controlling a watercourse that drains an existing body of wa ...

feeds a gravity distribution system, for drinking water or

irrigation Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...

, with an outlet pipe going into the ground from the bottom of the reservoir. This pipe undulates with a series of alternating low points and high points. When the water drains from the reservoir, it will pass the first low point and fill as far as the first high point. If the water flow velocity is below the rising velocity of air bubbles, then water trickles down to the next low point and traps the remaining air between the first high point and the second low point. As more water flows down, the upward leg after the second low point fills up. This exerts a pressure on the trapped air from the water on either side. If there is more water pressure from the section after the trapped air, then a full air lock is formed, and the water level stops at the first high point and no further water can flow. If there is more water pressure from the section before, then some water can flow, but the full pipe hydraulic head will not be reached and so flow is much less than expected. If there are further undulations, then the back pressure effects add together. Long pipelines built across fairly level, but undulating land are bound to have many such high and low points. To avoid air or gas lock, automatic vents are fitted which let air or gas out when above a certain pressure. They may also be designed to let air in under

vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

. There are many other design considerations for design of water pipeline systems, e.g."UNDERSTANDING GRAVITY-FLOW PIPELINES" Water Flow, Air Locks and Siphons
published by British Columbia, Ministry of Agriculture and Lands

Plumbing traps

The air lock phenomenon can be used in a number of useful ways. An 'S' trap (a pipe that descends from a reservoir, then curves back up, then down again) allows liquid to flow from top to bottom unhindered, and gas cannot flow through the trap unless it has enough extra pressure to overcome the liquid head of the trap. This is usually about 75 to 100 mm of water and prevents foul smelling air coming back from water drainage systems via connections to toilets, sinks and so on. 'S' traps work well unless the drainage water has sand in it – which then collects in the 'U' part of the 'S'.

References

{{DEFAULTSORT:Air Lock Plumbing Water physics

Undulating pipes

Plumbing traps

See also

References