Replacing A Compressor
Drain With A Ball Valve
The original air compressor
drain valve is removed and replaced with ¼" NPT pipe
fittings and a ball valve.
About 30 Minutes
Bruce W. Maki,
When I worked in hotel maintenance many years ago, I noticed
several air compressors around the complex that had been modified to
make it easier to drain the water that accumulates in the bottom of
the tank. Somebody had simply removed the cheap drain valve and
replaced it with some ¼" NPT pipe fittings and a ball valve.
This made draining the water from the tank a simple matter of
turning the ball valve a quarter turn and waiting a few seconds for
the water to blow out.
How Does An Air Compressor Get Water In
First, air is a sponge. Air is capable of holding a
considerable amount of water vapor, even air that is colder
than the freezing point of liquid water can hold water vapor.
When air is squeezed it has less ability to hold water vapor,
so some water molecules clump together and form droplets,
which is known as condensation. You could also call it rain.
Yes, it's raining inside your air compressor.
Consider this: When repairing an automobile air
conditioning system, all traces of water must be removed.
But... there is no easy way to get inside the plumbing to mop
up the water. The solution is to connect a vacuum pump to the
air conditioning system and suck all the air out. When the
pressure inside the system is close to a perfect vacuum, water
will boil at a much lower temperature than the customary 212
degrees Fahrenheit. In fact, under a "hard" vacuum,
water will boil at room temperature.
Conversely, when air pressure is increased, the boiling
point of water is increased (the automobile is another example
of this... the engine cooling system pressurizes itself to
about 14 PSI, thus raising the boiling point of the cooling
water to around 250 °F.)
Although my college thermodynamics textbook did not have an
explanation for this phenomenon, it just makes sense to me
that if reducing the pressure of the air (by sucking out some
of the air molecules) causes liquid water to evaporate, then
raising the pressure of the air (by cramming more air
molecules into a tank) causes water vapor to condense.
|I've had this Sears Craftsman 4HP air compressor
for almost 10 years. After the first year it developed a very
small leak in the drain valve. And the drain valve didn't work
quite right... if I tightened it with pliers, the air leak got
Eventually this leak would stop... but that meant not draining
the water from the tank as recommended... every day or so. If the
water stays in the tank too long, the steel can rust and cause a
leak or rupture, which would ruin the air compressor.
So I decided to remove the old cheap drain valve and replace it
with a ball valve.
||First I unplugged the compressor and drained the
air. Note the water that came out the bottom drain valve.
|To make the task easier, I tipped the compressor
on its side.
||These are the ¼" NPT pipe components I
- Street elbow,
- 4" nipple,
- Ball valve,
- 2" nipple.
|The drain valve was threaded into an adapter,
which is probably a ¼" x ½" bushing.
||Using an adjustable wrench, I unscrewed the
|Just to be sure I bought the right size of pipe
fittings, I tested the fit of the street elbow.
||I applied some pipe thread compound to the male
threads. This is done for all pieces of pipe and fittings.
|I inserted the street elbow and tightened it
with a small pipe wrench.
||Then I connected a 4" nipple to the female
end of the street elbow.
|I screwed the ball valve onto the end of the
||And I screwed the 2" nipple into the ball
|This is the end result.
||When I set the compressor back on its feet, I
realized that the new drain valve was kinda close to the
|... like this.
||So I attached a block of 1x4 to the bottom of
the compressor foot. I think there might have once been rubber
feet on this metal bracket, but they are long gone.
|With the 1x4 spacer, the drain valve is not in
danger of being damaged when I move the compressor.
||The completed project.
|The new drain valve works great.
But... I didn't like the way it would blow air and dirty
water a couple of feet across the shop floor.
||So I added this 45 degree elbow fitting.
This stopped the blast of air from blowing across the
floor, but now it tends to kick up dirt into my face.
Caution must be used when opening a drain valve like this. The
pressure inside an air compressor tank can be as high as 125 PSI,
possibly as high as 175 PSI. At pressures like these, the air will
move very fast when a valve is opened, and the blast of air can blow
debris into your face.
The normal drain valves are so cumbersome to open that they can't
be opened quickly, which I suppose increases their safety.
Use some common sense and open the drain valve slowly. Keep your
face away from the outlet. And keep children away from air
- Small Pipe Wrenches (2)
- Adjustable Wrench
- Ball Valve, ¼" NPT
- ¼" Nipples, 2",
- ¼" Street Elbow
- ¼" 45º Elbow
- Pipe Thread Compound
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