Final answer:
While operating foam dispensing equipment, one should cautiously adjust pump pressure when changing from foam to water, due to potential increases in nozzle pressure and reaction. The exact percentage to decrease pump pressure is not provided, however, understanding the Bernoulli effect can help appreciate the need for this adjustment to maintain safety.
Step-by-step explanation:
When operating foam dispensing equipment, the transition from using foam to water in firefighting applications must be handled with care due to the risk of a sudden increase in nozzle pressure. The question implies a need to adjust the pump pressure in order to manage the resulting change in nozzle reaction safely. However, without a specific guideline provided in the question, it's challenging to recommend an exact percentage by which to reduce pump pressure before changing from foam to water. It is a common practice to reduce pump pressure to prevent a potentially dangerous spike in nozzle reaction, but the precise percentage can vary based on the equipment and the situation.
According to the principles such as the Bernoulli effect, pressure within a moving fluid such as water decreases as the speed of the fluid increases. When changing from foam to water, the altered flow characteristics can result in higher velocities and therefore a potentially hazardous increase in the kinetic energy at the nozzle. This kinetic energy can manifest as increased nozzle reaction, which is the force exerted on firefighters holding the hose. To guard against injury and maintain control, it's important to follow proper operating procedures, including the adjustment of pump pressure when switching flow types.