NATIONAL FIRE PROTECTION Association 1901, Standard for Automotive Fire Apparatus, requires each pumper have a “pressure control system,” an integral safety feature to the fire pump system, which for our purposes includes the pump and its discharge outlets. The discharge pressure control system prevents dangerous/excessive pressure rises to protect fire service personnel from injury.
The automatic pump pressure governor (APPG) changes the engine speed to control pump discharge pressure. With modern electronically controlled engines, the pressure governor throttles the engine to speed up or down to maintain the pressure set by the operator.
Most modern fire apparatus around the world use centrifugal fire pumps. Generally, at the same engine speed, as discharges open and flow increases, the pressure goes down. In pressure-control mode, the governor increases engine speed to maintain the pressure; when discharges close and flow decreases, it reduces engine speed to maintain discharge pressure.
Many operators use the pressure control mode to control the pump discharge pressure. Using the rpm mode, the operator controls the engine speed directly. The pump discharge pressure is not actually controlled in this mode by the governor. Even in rpm mode, some governors actually prevent pressure from rising above a certain limit. But this is a relatively new development, so many existing systems will simply hold the preset engine speed and the pressure can follow the pump curve wherever it leads.
In a training exercise, use the rpm mode to establish a common operating pressure of 150 psi operating from the onboard water tank. Next, hit a hydrant or supply hose with a residual of 75 psi. With most governors in service today, the resulting pressure is 225 psi. Does your equipment have a pressure control point to limit the pressure increase even in rpm mode?
Some of the latest governors will hold the pressure to less than 200 psi in this evolution; they monitor pressure even in rpm mode and keep pressures within “safe” limits. Are your operators aware of this feature on newer equipment, and do they even run the system in rpm mode in training to find out the mode’s characteristics? Even if your department guidelines call for operating primarily in pressure governor mode, pump operators should be familiar with the rpm mode; they shouldn’t have to learn it at an emergency response.
In the rpm mode, the pumper continues to operate even if a pressure sensor clogs or fails, and the mode allows constant engine speed operation for certain types of engine-driven equipment that need a relatively fixed speed for best performance.
A governor in pressure governor mode maintains the pump discharge pressure as long as the pump has an adequate water supply and is within the operational engine speed range. If the operator needs a 100-psi discharge and the normal “idle” pressure on the engine is 50 psi, this is not an issue if operating from the tank. However, when the pumper is on a 100-psi hydrant, the minimum discharge pressure will be 150 psi no matter what mode the governor is in. The residual inlet pressure is added to the net pump pressure to get the resulting discharge pressure.
If the water supply is limited and the pump cavitates, the governor will attempt to maintain pressure by increasing engine speed. The operator must recognize these situations and signs of cavitation to avoid potential problems. A pressure spike may result when a pump pressure governor-equipped pumper is placed in a marginal cavitation situation.
Train your operators to ensure all air is out of any supply lines when changing from tank to positive-pressure supply lines. Standard operating guidelines (SOGs) should call for making the tank-to-supply line change in rpm mode to avoid creating a cavitation situation. The operator slowly opens the intake valve while reducing the engine rpm manually to maintain pressure. Run this evolution at a pumping drill using lower “safe” pressures, and make sure your personnel understand these issues. Note that the same dangerous pressure spike can occur with some equipment combinations when flowing beyond the limits of a hydrant or relay water supply. Establish your operational guidelines to match your equipment characteristics.
Different engine/pump/governor combinations do not all react the same. Some equipment combinations produce almost no pressure spike while others can produce more severe spikes; train on these scenarios to avoid unpleasant surprises. In departments where an operator may run any one of a number of pumpers on any given call, do the available apparatus share any common operating characteristics? Do your operators know the warning signs of cavitation and how to avoid problems?
The modern APPG-equipped apparatus provide much better control if the operator is trained to avoid problem areas. Pump pressure governors operate differently depending on the manufacturer and the governor/engines/pump combinination. Train your operators to know how your equipment will perform in unusual or error situations before they happen; procedures and training should be in place to prevent problems before they start.
MICHAEL A. LASKARIS, PE, is the director of engineering for Hale Products Fire Suppression Division in Conshohocken, Pennsylvania.
