Keg

A keg is a small cask used for storing liquids. Wooden kegs made by a cooper were used to transport nails, gunpowder, and a variety of liquids. Nowadays a keg is normally constructed of stainless steel, although aluminium can be used if it is coated with plastic on the inside. It is commonly used to store, transport, and serve beer. Other alcoholic or non-alcoholic drinks, carbonated or non-carbonated, may be housed in a keg as well. Carbonated drinks are generally kept under pressure in order to maintain carbon dioxide in solution, preventing the beverage from becoming flat.

Beer keg

Beer kegs are made of stainless steel, or less commonly, of aluminium. A keg has a single opening on one end, called a "bung". A tube called a "spear" extends from the opening to the other end. There is a self-closing valve that is opened by the coupling fitting which is attached when the keg is tapped. There is also an opening at the top of the spear that allows gas to drive the beer out of the keg. The coupling fitting has one or two valves that control the flow of beer out of and gas into the keg. The keg must be in the upright position, that is, with the opening on top, for the beer to be dispensed. Kegs can be contrasted to casks, which have two or more openings and no spear. Most major breweries now use internally speared kegs. Beer that is sold in kegs is fully conditioned. Beer intended for local use may be kegged without sterilization, however, the beer must stay cold.

Size

Historically a beer barrel was a standard size of, as opposed to a wine barrel of, or an oil barrel of. Over the years barrel sizes have evolved, and breweries throughout the world use different sized containers. Even when the content capacity of two kegs are equal, e.g. metricized to 50 liters, the keg shape and tap system may differ greatly.

U.S. keg sizes

Most U.S. brewers sell beer in barrels of 15.5 gallons, barrels of 7.75 gallons, and barrels of 5.17 gallons.
Since keg sizes are not standardized, the keg cannot be used as a standard unit of measure for liquid volumes: despite this, a number of people still refer to kegs as if they were a unit of measure. This size standard varies from country to country and brewery to brewery, with most countries using the metric system rather than U.S. gallons.
Image:Keg1.jpg|right|thumb|A typical keg with a single opening in the center of the top end
In common parlance, the term keg refers to a half-barrel vessel as this is the most common size used in restaurants, bars and limited home use. A quarter-barrel has a volume of 7.75 U.S. gallons. Generally, a keg is a vessel smaller than a barrel; thus, it is 30 gallons or smaller.
In the U.S. the terms half-barrel and quarter-barrel are derived from the U.S. beer barrel, legally defined as being equal to 31 U.S. gallons.
A 15.5 U.S. gallon keg is also equal to:

Exactly 58.673882652 liters
124 U.S. pints
165 twelve U.S. fluid ounce drinks
6.875 24-unit cases of 12 fl oz cans
1,984 fluid ounces
≈12.90645 Imperial gallons
≈103.2516 Imperial pints
177 330ml drinks
7.375 24-unit cases of 330ml cans
≈2065.032 fluid ounces

However, beer kegs can come in many sizes:

Size	Size	No. of 12 fl oz drinks	No. of 16 fl oz drinks	No. of 20 fl oz drinks	Weight of full keg	Also known as
1.32	5	14	10	8	—	Mini Keg / Bubba
3.875	14.67	41	31	24	—	Eighth Barrel
5	18.92	53	40	32	55–60	Soda syrup / Corny Keg / Home Brew
5.17	19.56	55	41	33	58–60	Sixth Barrel / Torpedo Keg / Sixtel / Log
6.6	25	70	52	42	—	"Half Barrel"
7.75	29.33	82	62	49	90	Quarter Barrel / Pony Keg
13.2	50	140	105	84	—	Import Keg
15.5	58.67	165	124	99	140–170	Half Barrel / Full Keg

Specifications for a U.S. barrel keg

Accepted specifications for a standard keg are:

Height of keg	23.3 inches
Diameter of keg	16.1–17.15627 inches
Contents	1984 U.S. fluid ounces
Contents	15.5 U.S. gallons
Contents	12.90645 imp. gallons
Contents	58.673882652 liters
Full keg weight
Empty keg weight
Beer weight
24 × 12 fl oz case equivalent	6.875 cases
12 fl oz servings	165
16 fl oz servings	124

DIN-keg and Euro-keg

In Europe, the most common keg size is 50 liters. This includes the UK, which uses a non-metric standard keg of 11 imperial gallons: by coincidence, this is exactly.
The German DIN 6647-1 and DIN 6647-2 have also defined kegs in sizes of 20 and 30 liters.

Content	DIN 6647-1/-2	DIN-Keg diameter	Euro-Keg	Euro-Keg diameter
	310 mm	363 mm	216 mm	395 mm
	-	-	327 mm	395 mm
	400 mm	381 mm	365 mm	408 mm
	600 mm	381 mm	532 mm	408 mm

Taps

Kegs are sealed to contain the liquid and pressurized gas for storage and transportation. Most beer is served carbonated, and this is most easily accomplished by storing it carbonated as well. Beer kegs are designed to maintain the carbonation in a beverage by storing it with pressurized carbon dioxide in the headspace above the liquid. The liquid is also dispensed using pressurized gas; the pressure of the gas provides mechanical force to overcome friction and gravity to push the beer to the dispensing location.
To serve the beverage, a keg must be tapped to breach the container so that pressurized gas can be added and the liquid can be dispensed. Originally, this was done by hammering a tapping rod through a cork bung, similar to how a keystone is still used today to tap unpressurized cask ales. Tapping a keg this way would often waste a bit of beer, which would be forced out under pressure until the tap was secured. By the 1950s and 60s when metal kegs had replaced wooden ones, common tap systems included Golden Gate, Hoff–Stevens, and Peerless taps, which all had one or two couplers for pressurizing and dispensing the beer but retained a separate bunghole for cleaning and filling the keg which was sealed with a wooden bung. These made it easier to tap the keg, but still had sanitation problems and tended to leave some beer inaccessible at the bottom of the keg.
In the 1960s and 70s, several similar styles of tap were created in Europe and America which used a single hole for cleaning, filling, pressurizing, and dispensing. A single bunghole at the top of the keg is used to clean and fill the keg, after which it is sealed with a metal assembly containing a ball bearing which acts as a stopper, held in place by the gas pressure inside the keg. The tap is twisted or slid into place atop the keg, and a lever provides the mechanical force needed to push the ball bearing down, providing access to the keg's contents. These taps, or "couplers", are more sanitary and easy to operate, and were adopted by major U.S. breweries like Anheuser-Busch in the 1970s and quickly displaced other taps to become the industry standard. One such system was referred to as Sankey after its designer. The term Sankey, often misspelled "Sanke", has become a generic name for all of the similar industry standard couplers.
Today there are six industry standard couplers:

The D System is used by most breweries in the Americas.
The S System is used by many breweries in Europe. It is similar to the D System but has a longer probe.
The G System is less common and is used by some British and Irish breweries and beers including Tennent's, Boddingtons, and Fuller's ESB.
The U System is uncommon and is used for a few Irish beers and Magners cider.
The A System is used by many large German breweries. It slides into place rather than rotating.
The M System is very uncommon and is used by only a few breweries in and around Germany. It also slides into place.

There are two different types of tapping equipment that are available for kegs. A "party tap" or "picnic tap" is a hand-operated pump that utilizes outside air, thus introducing oxygen and bacteria into the keg. This causes the beer to oxidize, affecting the taste; the partial pressure of CO₂ will also decrease, causing the beer to go flat. Kegs dispensed with a party pump should be used within 18–24 hours so that the beer does not become unpalatable. Commercial installations, as well as some home users, use pure pressurized gas; these can preserve a keg up to 120 days with proper refrigeration. In simpler installations only CO₂ is used to pressurize and dispense the beer, but in installations with very long lines between the keg and dispensing location, the pressure needed to pump the beer for dispensing would over-carbonate the beer. In these situations, "beer gas" or "mixed gas" is used which combines CO₂ with another gas, usually nitrogen. Nitrogen is 80 times less soluble in water than CO₂, so it can provide additional pressure without noticeably affecting flavor. Typical beer gas is 70-75% nitrogen and 25-30% CO₂, but the ideal ratio depends on the beer being served and the installation; more advanced installations blend the gas on site so it can be adjusted for each beer. A few beers like Guinness are required to be pressurized and dispensed with mixed gas; they usually also require the use of a special faucet that deliberately creates additional friction to force the nitrogen out of solution, creating a thick frothy head.
As with any pressurized container, a keg can cause injury, even at normal operating pressure, whether with compressed air or carbon dioxide:
Commercially, kegs are usually kept in a refrigerated room for storage and dispensing, and liquid lines run from this room to all of the faucets in the restaurant. Kegs are too large to fit in a typical home refrigerator. A kegerator can be used, but as these are somewhat specialized they are cost-prohibitive for the average consumer who has only occasional use for one, and are obviously impractical to bring to a beach or campsite. Instead, in the US and Australia, kegs are usually kept in a bucket of ice and/or water to keep the beer cool. Alternately, the keg can be kept at ambient temperature and served using a "jockey box", consisting of a cooler with beer coils and filled with ice, which acts as a heat exchanger to cool the beer to serving temperature by the time it reaches the faucet. European consumers, when not using a party pump, mainly use inline beer chillers, essentially the same concept as a jockey box but using a refrigeration unit instead of ice. Those chillers usually also have their own air compressor for keg pressurization.