The "water" governors on Pumper trucks are a safety feature designed to protect the firemen from overly high hose pressures. Say a fire engine is pumping through two hose lines at 100 psi each. Then one of the men on one of the nozzles shuts his line down. Suddenly all the pressure developed by the fire pump is exerted on the one remaining hose, with a correspondingly higher nozzle reaction force - quite dangerous.
The governor used reacts according to the pump pressure exerted on the water from the pump. In other words, there's a water line from the fire pump to the governor, and, (if the governor is "enabled") when the pressure spikes in the pump, the governor closes the throttle correspondingly, to lower the fire pump pressure. It sounds like the same principal as a vacuum operated governor, only with water pressure. Used broadly on fire engines from the 30s on.
Not that anyone asked . . . ;-)
-Michael
