Craftsman 4HP air compressor with new improved drain valve. Shop Stuff:

Replacing A Compressor Drain With A Ball Valve

In This Article:

The original air compressor drain valve is removed and replaced with ¼" NPT pipe fittings and a ball valve.

Related Articles:
Skill Level: 2-3 (Moderate) Time Taken: About 30 Minutes

By , Editor



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 It?

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 worse.

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.


Standard drain valve on air compressor. 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 used: 
  • Street elbow, 
  • 4" nipple, 
  • Ball valve,
  • 2" nipple.


The drain valve was threaded into an adapter, which is probably a ¼" x ½" bushing.


Removing original drain valve on Craftsman air compressor. Using an adjustable wrench, I unscrewed the drain valve.


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.


Installing new iron pipe in drain hole on air compressor. Then I connected a 4" nipple to the female end of the street elbow.


I screwed the ball valve onto the end of the nipple. Installing ball valve for new drain on air compressor.


And I screwed the 2" nipple into the ball valve.


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 floor...


... 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.


Health Hazards:

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 compressors.



Tools Used:

  • Small Pipe Wrenches (2)
  • Adjustable Wrench
  • Pliers
  • Screwdriver


Materials Used:

  • Ball Valve, ¼" NPT
  • ¼" Nipples, 2", 4"
  • ¼" Street Elbow
  • ¼" 45º Elbow
  • Pipe Thread Compound



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Written September 3, 2003