Many water heaters have a drain valve that has a cone shaped handle. They spell trouble. Other makers of water heaters have a plastic faucet type drain valve. This type of drain valve is prone to fractures and leaks. The better types are the brass hose bibb, not unlike the normal garden hose bibb.
On most residential water heaters a 3/4 inch brass gate valve can be installed. Ball valves allow the largest opening for drain valves that provides you the best flushing of the inside of the drain valve. Shutting off a ball valve is as simple as moving the lever 90 degrees.
A hose bibb has to be turned around and around. Also, know that an adaptor bit has to be set up that extends from 3/4 pipe thread to 3/4 hose thread is going to be needed. Now you have the brass ball valve which is the least likely to clog of all of the drain valves. If the brass ball valve cannot be installed, the next best is a brass hose bibb. You can get all of the information you require about valve if you do a bit of research through http://www.nssteel.co.th/.
Plastic cone valves and plastic faucet valves drip, freeze inside, and clog with sediment the easiest. Plastic threads are very easy to cross when you wish to attach a garden hose to the drain valve. Even after opening a plastic valve one time, using it and closing it back up, the valve may drip.
To remove the plastic cone valve, unscrew it from the water heater by turning it counterclockwise when pulling it out. Six turns will most likely be sufficient. Then turn clockwise and keep pulling. Wrap Teflon tape around the back nipple.
A pint of water or less may come out of the water heater after all of the preparation has happened. Avoid being scalded by conducting a faucet in the house for five minutes prior to beginning this whole procedure. The cold water going into the water heater will lower the temperature inside. Be prepared to commence the procedure quickly shortly after though.