Article Archive
Electronic Pressure Governor
Electronics solve historic pump control issues
Volume 24, No. 4

Fire fighting operations call for a stable water supply to the firefighter and any deviations from the expected pressure and flow can have disastrous consequences. Managing pressure typically involves starting off with too much and then limiting it by routing the excess pressure back through the pump, gating individual discharges, modulating pump speed manually, and resetting the discharge relief valve and discharges to fine tune the operation. This works until pressure drops below the desired set point since these are "pressure limiting" actions.

Controlling pressure requires a different method. This is where a pump pressure governor comes into the picture. Historically there have been many different attempts to control pump speed automatically with mixed results. With the arrival of electronically controlled diesel engines, control of pump speed became more accurate and reliable.

The "National Fire Protection Association (NFPA) 1901 Standards for Automotive Fire Apparatus 2009 Edition" calls for a pressure control system test consisting of specific pressures and conditions.?Under all conditions specified, the pressure shall not rise more than 30 psi from the established pressure.?This standard is typically met by using a mechanical discharge relief valve or using a pump pressure governor.

The evolution of diesel engines has progressed to the point that modern engines can be controlled precisely using external devices.?Pump pressure governor methodology varies from a pressure governor routine built into the engine control module (ECM), which is no longer available to the more prevalent system that uses the same secondary throttle control circuitry as a vernier style throttle. Basically these act as a remote throttle position sensor (TPS) almost identical to the "foot feed."? A third method is just now coming into the picture that uses J-1939 controller area network (CAN) messages to control engine/pump speed. This J-1939 control method provides control approximating that of the built in routines.

Electronic pressure governors use a pressure transducer fitted to the discharge side of the pump and regulate engine speed to maintain a "set pressure" transmitted by the transducer. This equates to the pressure governor acting as "cruise control" for the pump.?If the pressure governor is looked at in this light, many of their operating characteristics become quite clear.

To set pressure, increase engine speed to the desired pressure and release the increase switch (cruise control SET).?The governor records the transducer output at that time and increases or decreases its output to the engine control module (ECM) to maintain that set point.?The idle switch is identical in operation to pressing the brake pedal or cancel switch for cruise control and the preset switch functions as the resume switch.?Increase and decrease switch raise or lower the pressure set point.?While operating in pressure mode at a set pressure, the governor reacts to increased load caused by opening another discharge or nozzle or driving up a hill, increasing its output (RPM). It reacts to a decreased load -- again closing another discharge or nozzle or driving downhill by decreasing output (RPM). This scenario is identical to cruise control when it encounters a hill.?Increasing output is like driving uphill while decreasing output is like driving down hill. While using cruise control on wet roads, there is a potential for hydroplaning. The result is an uncontrolled increase in wheel speed that results in the cruise control shutting itself off. The governor performs the same action if ran away from water or if the pump cavitates. Transitioning to a pressurized water source without bleeding the air off can produce this problem.

Newer electronic pressure governors like Class 1's Total Pressure Governor Plus (TPG+) add features to the package, making it desirable from a cost-to-benefit standpoint.?The TPG+ incorporates the "Master Gauges" into the unit thereby reducing required panel space and giving an immediate governor diagnostic feedback.? If the discharge gauge is working, then the governor can control the engine since it is the primary input for pressure control.?The TPG+ also incorporates engine information negating the need for separate tachometers, oil pressure and temperature gauges.?Also included is a pump hour meter, transmission temperature gauge, fuel flow meter check engine and stop engine indications.

Class 1 develops a control algorithm specifically to handle conditions where the water supply cannot meet the discharge requirements.?The TPG+ recognizes when an rpm increase does not result in a pressure increase and performs a handshake to maximize water output while minimizing engine speed.?

Couple all of these features with advanced engine control using SAE J-1939 rather than analog methods and Class 1's new insufficient water supply control, this becomes a "must have" unit for fire suppression.

Dick Smith is Class 1's Fire Electronics Product Manager. His background in electronics, automotive technology and customer service provides him with the experience and technical expertise to shape product development to customer's needs. He has been with Class 1 for over 15 years and has assumed roles ranging from engineering to customer service manager. Holding degrees in Computer Science and Business Management along with a Six Sigma Black Belt and Master Automobile Technician Certification from the National Institute for Automotive Service Excellence (ASE) provides him with a unique perspective on problem solving in the fire industry.


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