Constant Displacement Glycol Pumps

The intensity and delay time of the chemical reaction which occurs in an ignition sphere between potassium permanganate (KMnO4) and ethylene glycol (C2H6O2) is controlled by a number of variables. Two of the more important variables are the quantity of glycol, and the temperature. If either one of them varies too much from its optimum value, the reaction can occur poorly. If both of them vary too much from their optimum values, the reaction will not occur at all. Obviously, we cannot control variations in temperature, but we can control the amount of glycol injected into the sphere.

The first generation plastic sphere dispensers (and newer clones) use a pump and dispense valves to inject the glycol. The pump is always running to keep the glycol system under pressure. When a sphere is present in the injection cavity of the shuttle, the shuttle forces the sphere against an activation arm which opens a dispense valve allowing glycol to be pumped into the sphere. The amount of glycol that can flow is controlled by an adjustable needle valve.

The problem with this type of system is that it only works correctly at one speed. If the speed of the dispenser is decreased, the activation arm holds the dispense valve open for a longer amount of time, and more glycol is pumped into the sphere. With a 2:1 speed ratio of the dispenser drive motor, twice as much glycol is injected on slow speed as on fast.

In practice, the needle valves are adjusted to find a point where there is more than optimum glycol on slow speed, and less than optimum on fast speed, but the spheres will still just ignite. This is a workable solution as long as everything else stays the same. However, if the ambient temperature changes, the reaction may not occur without re-calibration of the needle valves to match the conditions of the current burn operation.

The Dragon and PyroShot ignition dispensers have overcome the problem of glycol quantity variation by introducing constant displacement pumps. This type of pump delivers the same quantity of glycol regardless of the dispense rate. The amount of glycol is always at the optimal value so that even with variations in air temperature, the reaction occurs normally.

The glycol pump assembly consists of a cylinder with a piston, an inlet check valve, an outlet check valve, and an injection needle. After the tip of the injection needle pierces the sphere, a mechanical linkage compresses the piston in the pump cylinder. The inlet check valve is closed, preventing glycol from returning to the reservoir. The fluid pressure in the pump cylinder rises, the outlet check valve opens, and the glycol in the pump cylinder flows through the injection needle and into the sphere.

When the needle is extracted, the mechanical linkage retracts, and an internal spring in the pump extends the piston. The outlet check valve is closed, preventing air from entering the cylinder through the needle. The extending piston creates a partial vacuum in the pump cylinder, and the atmospheric pressure in the glycol reservoir causes the inlet check valve to open, and the pump cylinder is refilled with glycol ready for the next cycle.

The amount of glycol that is pumped is fixed by the piston displacement and remains constant regardless of how fast the dispenser operates. The piston displacement was selected to provide the optimal amount of glycol for an ambient air temperature of 60°F (15°C). The air temperature can vary by as much as 30°F (15°C) higher or lower and the reaction will still occur.