Draught systems: Key elements

Here we’ll learn about four key elements in most draught dispense systems – the keg, coupler, Foam on Beer or FOB detector, and beer tap/beer faucet.

How does a basic draught dispense system work?

When a keg is tapped by a coupler, dispense gas (typically CO2) enters the keg and applies pressure to the beer. This then forces the beer out of the keg and through the beer lines to the faucet.

(FOB detectors are only used in specific systems, which you’ll read more about below.)


Kegs are containers that enable beer to be transported in bulk and dispensed by the glass while maintaining its quality and integrity. Keg design protects beer from the damaging effects of both air and light, while enabling it to be easily and rapidly dispensed.

Most brewers use kegs made from stainless steel, but single-use plastic kegs are now becoming popular. Regardless of the material used, all kegs are pressurised vessels, so should be handled with care.

Keg sizes vary, as seen below. The standard keg size in the U.S. is the 1/2 barrel (at 15.5 U.S. gallons or approximately 60 litres).


A keg is tapped using a coupler.

A coupler attaches to the keg valve, allowing gas to flow in and beer to flow out. (As a note, gas always flows in through the side, while beer flows straight up through the centre.)

When tapped, the keg’s valve admits dispense gas into the head space, applying pressure to the beer. This then forces the beer to travel up the keg’s spear, or downtube, and through the beer lines to the faucet.

Keg valves vary and each has its own coupler type, as seen in the image below. Most couplers are fitted by being seated into the keg valve and twisted a quarter turn clockwise, but there are some exceptions.

  • Type D, also called Sankey, is the most common in the U.S.
  • Type S, or Euro-Sankey, is most common across Europe
  • Types A and M are sliders, meaning they slide into place instead of being seated and turned; these are typically used in Germany
  • Type G is specific to Grundy-type kegs
  • Type U is specific to Guinness and its related brands

It’s important for bars and pubs to have the correct coupler for each type of keg.

FOB Detector

FOBs, or Foam on Beer detectors, are a common feature in long-draw draught systems, in which beer has to travel through beer lines that are at least 25 feet or more to reach the faucet.

A FOB stops the flow of beer through the beer line once the keg empties, preventing the beer line from filling with foam. This keeps the beer line full of pressurised beer while the keg is changed, reducing beer loss.

Beer tap/ beer faucet

Taps or faucets dispense beer to the glass. And, they often hold the tap marker to identify the type of beer being dispensed.

The two most common faucet types are vented rear-sealing faucets (the Standard or European faucets in the image above) and ventless forward-sealing faucets.

Vented faucets are most common in the US. The vents allow for smooth beer flow and permits the faucet to drain between pours. However, the interior of the faucet is susceptible to microbial growth, so the faucet and its vent holes need to be carefully cleaned and regularly inspected.

The ventless, or forward-sealing faucets, are easier to clean and have a lower susceptibility to microbial contamination, but may have a slightly more turbulent flow of beer.

As seen above, other faucet types exist, but the only additional one we’ll mention here is the nitro faucet. Nitro faucets are specifically used for nitro beers. These faucets contain a restrictor plate that forces the beer through tiny holes to facilitate the breakout of nitrogen gas from the beer, giving it that cascading effect and smooth, creamy mouthfeel.

Discovering Beer is not affiliated with or endorsed by the Cicerone® Certification Program.

Brought to you by Beer with Nat

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