A keg is a large metal cylinder used to hold beer. A keg typically has two types of valve systems: the ball-lock and pin-lock valves. You can find these types of valve on some home appliances such as refrigerators and dishwashers.
The ball-lock type is opened with a hollow metal ball, which you insert into a small opening on the top of the keg and rotate to open it. The pin-lock type of valve has a metal pin, which you insert through a small opening on the top of the keg and then twist back and forth to open it.
The methods that apply for tapping a keg will depend upon whether you have a gas or electric tap system.
Gas-valve system: In a gas-valve system, you open the valve by turning on a built-in tank regulator to let CO2 flow into the keg and force out any old beer. The gas valve also allows for fast carbonation, because it only requires minimal contact with keg valves and no tubing or other fittings.
Electric-valve system: In an electric tap system, CO2 is forced directly through the top of the keg and thus must pass through a length of beer tubing and out the faucet before it bubbles up in your glass. Electric taps are best suited to low-gravity beers, which require very little contact with the tap valves and tubing to achieve ideal carbonation.
For both types of tap systems, you can pressurize a keg using either an external CO2 tank or an internal alternative such as a beer ball , which you fill with air from your compressor. Units that attach directly to your regulator will work with either ball lock or pin lock keg valves.
So this is how to tap a keg!