drip valve located under the steam chest of the engine

The cylinder drain valve, or the drip valve as it is more commonly called, is used in two ways. The first is when steaming up the car. When steaming up the car the burner is firing to heat water in the boiler. However the flames of the burner are also heating the superheater. Without steam going though the superheater on its way to the engine the superheater can get excessively hot. This can deform the superheater as well as weaken it. When the throttle is opened to move the car the much cooler steam runs through the superheater and quenches it. This rapid temperature change is also detrimental to the superheater.

The drip valve is located on the domed access cover to the engine's valve chest.  This is the lowest part of the engine's cylinder block and were any water reaching the engine from the steam supply piping or any water from steam condensing within the engine would accumulate.  A long handle attached to the valve stem extends through the left splash guard so that the valve wheel can be reached by the driver (the valve extension and wheel was located on the right side of the car for pre-1914 cars with right-hand steering).



A good practice during firing up is to open the throttle and the drip valve. Once the burner is burning kerosene, the burner valve should be opened a small amount (perhaps one-quarter to one-third of a turn of the valve wheel) so that the burner doesn’t fire at maximum heat. While this means the firing up process takes longer, it is better for the car since the boiler and the superheater are not subjected to extreme heat but rather are heated gradually. From when the car was shut down there will no doubt be water condensed in the superheater which will turn to steam and vent out the drip valve.

Once the water of the boiler starts boiling the generated steam will be released through the throttle. The steam will blow any water in the engine steam lines back to the engine and out the drip valve at the bottom of the valve chest. At this point the burner valve can be opened fully and the burner fired at its maximum rate. The steam running through the superheater acts to cool the superheater and thus protects the superheater from damage.

The steam flow has the added benefit that it serves to heat the engine valve chest and cylinders as it exits the drip valve under the engine. To protect the superheater keep a small amount of steam passing through the line, which is enough moisture to protect the superheater. As the burner heats the boiler and steam generation increases, the throttle will need to be closed to only allow a low rate of steam flow from the drip valve. Steam pressure will increase and as it does the throttle will need to be continually adjusted to close down the steam flow. Proper steam flow through the drip should be such that the exiting steam just hits the pavement under the car rather than floods the pavement under the car with an inverted mushroom cloud of steam.  Shown at the left is a 1908 Model K Stanley being fired up.  The steam flow cascading from the drip is nominal for keeping the superheater properly cooled and for preheating the engine casting.

The second and perhaps most important use of the cylinder drain valve is to provide a means for water to escape from the steam lines and engine valve chest when starting the car out from rest. When the car is stopped there will be some residual steam in the piping past the throttle. Anytime the car sits idle for a short period of time this steam will condense back to water. Additionally if the throttle leaks, the steam will flow into the superheater and piping to the engine. With the burner off these pipes eventually cool and the condensing water may eventually fill the superheater and engine steam piping with water. This water will collect in the superheater and piping to the engine.

When it is time to move the car again, opening the throttle will cause all this water will be blown back to the engine. If the drip valve is closed the water will be blown into the engine cylinders where clearances at either end of the piston stroke to the cylinder heads are tight. Too much water, which is non-compressible, and cylinder heads will be blown off and the piston shafts and crank arms can be bent. Opening the drip, along with always starting out slowly allows any accumulated water to exit the drip valve and not enter the cylinders. In fact a good practice especially when steaming up the car is to "rock" it back and forth. This relieves any water trapped in the engine cylinders. Rocking the car can be accomplished by moving 10 feet forward or so, stopping, then reversing back to the starting position. This should be done slowly where there is between 400 PSIG and 500 PSIG steam pressure and with the drip valve one-half turn open.

Generally for a car in good condition, where a strong pilot is maintained, there is little necessity for operating the burner for any length of time to build steam pressure before starting out after the car has been parked for awhile. Thus there’s little chance of damage to the superheater. Still, a good practice if steam pressure has to be raised after parking for car for several hours is to open the drip and allow steam-flow to cool the superheater while preheating the engine. When starting out, or any time there is reason to believe condensation might have accumulated in the cylinders and piping, the drip should be left open when starting out. The drip should be left open one quarter to one half a turn to allow water and steam to blow free. When the water no longer exits the drip it may be closed. Opening the drip too much allows too much steam to escape to the point that there’s not enough steam pressure for the engine to move. When used too long or too often, having the drip valve open too much can be detrimental as the high steam flow cleans out the saturation of cylinder oil from the steam line.

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