Hydro Pneumatic means air over water. These type tanks usually have two pipes, a pipe on one side comes from the well, and a pipe on the other side of the tank goes to the house. These are plain tanks with nothing separating the air from the water. Since air is touching water inside the tank, the air will mix with the water and be taken out with the flow of water. These type tanks require some kind of air make up system to continually replace the air. The most common air make up system is the bleeder orifice style. A bleeder orifice is a fitting that goes down in the well about 5' below the surface. This fitting has a small hole that is covered with a flap or a ball from the inside of the pipe when pressure is applied. This keeps the bleeder orifice closed when the pump is running and there is pressure in the pipe.
There must always be a one way check valve on a submersible pump. The bleeder orifice works in conjunction with an additional check valve that is installed above ground before the pressure tank. On the inlet side of this check valve there should be a "schrader valve". A "schrader valve" looks like the valve stem on a car tire. When the pump shuts off, the above ground check valve closes and keeps the pressure in the pressure tank from going back down the well. As soon as this above ground check valve closes, the pressure in the pipe down hole will be at low or no pressure, and the ball or flap in the bleeder orifice will open. As water drains out of the bleeder orifice into the well, the schrader valve will allow air to be sucked into the pipe. The water in the pipe will then drain down to the level of the bleeder orifice in the well, so all the pipe from the bleeder orifice to the schrader valve will be filled with air. .
When someone uses water from the other side of the tank, the pressure will drop from 60 to 40 PSI as the amount of water available from the draw down in the tank is being used. When the pump starts at 40 PSI, the pressure in the pipe closes the bleeder orifice and the air that was in this top section of pipe is forced through the above ground check valve and into the pressure tank. Each time the pump cycles on, more air is injected into the pressure tank. If too much air is injected into the tank, soon air will start to come out the faucets and will blow a glass out of you hand or blast air at you in the shower. Therefore another device called an "Air Volume Control" or AVC is used to bleed any excess air out of the tank. The AVC is usually installed about half way up the side of the tank through a 1 ½ inch fitting. Sometimes the pressure gauge or even the pressure switch will be attached to the AVC as well. Although sometimes the pressure switch and gauge are attached to the pipe going into the tank, and the AVC will only have a small brass fitting sticking out of it.
Inside the tank on the AVC there is a float on a 12" rod that drops when the water level is low. When this float is in the low position, the AVC allows excess air out of the tank through the little brass fitting outside the tank. When the float on the AVC is up, this little brass fitting is closed so as not to allow water out of the AVC. All four of these items, the bleeder orifice, the above ground check valve, the schrader valve, and the AVC must be working properly for the air make up system to function. If the AVC is not working, you will get air in the faucets in the house. If either the bleeder orifice, above ground check valve, or the schrader valve are not working, the tank will become "waterlogged" and the pump will cycle on and off very rapidly.
There are also other types of air make up systems that use a air injector or "micronizer", or a small air compressor to add air instead of a bleeder orifice system. These are not as common and so we won't discuss them here.
To manually add air to a "hydro pneumatic" type system you should first shut off power to the pump. Opening a faucet will not completely drain the tank as it is like holding your finger over a straw full of water. You must remove a fitting half way up the tank or higher to allow air in. The air will gurgle as the tank slowly drains of water. You can also use a compressor to add air at the schrader valve while a faucet is open. This will force air in, and water out of the tank without needing to remove any fittings. Once all the water has been blown out of the tank, or has been drained out of the tank, replace any fittings that were removed, close the faucets and restart the pump. Having to do this means your air make up system is not working and should be repaired, or this maintenance procedure that should be done very regularly.
Hydro pneumatic tanks are one of the oldest style pressure tanks. These type tanks are now only used in certain areas where water quality requires them. Mixing air with water in these type tanks is used to reduce "rotten egg" smell from sulphur or other things in the water. If your system does not require mixing air with water for water quality purposes, then a bladder type pressure tank can eliminate several moving and wearable parts, which can make the system more reliable and require less maintenance.