Truck Tech: September 2013
My May column was about internal failures of fire pumps due to overheating, and how to prevent that from happening.
September 9, 2013
By Chris Dennis
My May column was about internal failures of fire pumps due to overheating, and how to prevent that from happening. I hope you have had a chance to try a couple of things and prove that the movement of water through a pump will help to dissipate heat. I thought since I was focused on fire pumps, I would move into the millennium and talk about electronic pressure governors (EPGs) and how much of a relief – or not – they have been. Essentially, I’m going to explain what an EPG does and why it is not a relief valve.
The NFPA 1901 Standards for Automotive Fire Apparatus 2009 Edition calls for a pressure-control system test that consists of specific pressures and conditions. Under all conditions specified, the pressure shall not rise more than 30 pounds per square inch (psi) or 200 to 215 kilopascals (kPa) from the established pressure. This standard is typically met by using a mechanical discharge relief valve or a pump pressure governor.
|Photo 1: The electronic pressure governor controls engine speed, and raises or lowers it to control water pressure. Use the mode button to switch between contolling the engine’s r.p.m. and controlling its pressure.
|Photo 2: An indicator light will illuminate when the EPG is in r.p.m. mode. When in r.p.m. mode, the operator can no longer control engine speed.
|Photo 3: Similarly, a different indicator light will illuminate when the EPG is in pressure mode. In this instance, engine speed can increase or decrease depending on the flow of water.
|Photo 4: Features on an EPG include a large control knob in addition to push-button controls and programmable pre-set settings, among others. All photos by Chris Dennis
Firefighters require a stable water supply. Any unexpected change in pressure, flow or supply creates a serious and dangerous situation. I often say and hear that firefighters are there to make a bad situation good; we did not create the situation and we do not want to be a situation. Health and safety always come first. Generally, water pressure starts off too high and is controlled by limiting it, gating back the discharges in use, sending excess water back into the fire pump, controlling fire-pump speed by manually turning down engine speed with a hand-controlled device on the pump panel, and resetting the discharge relief valve and those discharges. All these methods work well, until the pressure drops below the desired rate and setting. Why does that happen? Because these techniques are just pressure-limiting actions
So how do we control pressure? With the electronic pressure governor, or EPG. The EPG never would have been developed had diesel engines not become fly-by-wire operated. The EPG speaks to the diesel engine though the engine control module, or ECM, via an analog system, or through controller area network (CAN) bus lines and/or a serial line, such as J1939. In turn, the diesel engine, via sensors in the pump, will decrease or increase engine speeds to control pressure, rather than to relieve it.
EPGs are electronic devices mounted on the pump panel that control engine speed. The word electronic is the key; the EPG is really a microprocessor. Some models not only monitor discharge, but also monitor intake to help prevent cavitation. When the fire apparatus industry moved away from mechanical relief valves to EPGs, some of us felt that EPGs were really electronic relief valves, so no more yellow light, green light, pilot valve, on/off switch, or wheels to turn – just buttons to push. Well, many pump operators, when asked what an EPG does, will tell you that it electronically monitors water pressure internally, and if you got water hammer (a pressure surge), the EPG would recognize that electronically and would relieve the pressure in the pump, so that neither the truck nor any firefighters were hurt, or equipment damaged. No! Well, yes, an EPG is electronic and it does monitor something, but it is not a relief valve.
How does this EPG works in conjunction with a diesel engine? Our department is not afraid of innovation, so when the EPG was introduced, we knew a clear understanding of the internal function of the fire pump and the EPG was critical to be able to deliver the message. Just as I discussed in the May column, sometimes, being able to see and touch a piece of equipment makes it easier to understand its function.
As with all technology, the diesel engine has become so far advanced that it can be precisely controlled using externally mounted devices. The EPG control acts as a throttle position sensor, or TPS, only it is not on the cab floor, but on the pump panel somewhere.
The pressure governor has changed from when it was built into the engine’s electronic control module (ECM). The EPG today is a remote-mounted unit and can talk to the engine ECM through the J1939 CAN bus line or controller network line. It monitors engine functions and sounds an alarm if engine trouble occurs. This means simply that there is no longer a long, vernier hand throttle control cable attached to a mechanical injection pump lever to crank the engine’s idle up and down. The engine’s r.p.m. function is now controlled with the EPG. The EPG gets its signal to increase or decrease r.p.m. through a device known as a pressure transducer, which is mounted in a discharge port in the fire pump and, in turn, regulates the engine speed to maintain a previously set pressure on the EPG at the pump panel. The EPG signal is transmitted by the transducer to the engine, regulating and managing engine speeds – much like cruise control on your car.
When you are traveling down the highway at 100 kilometres per hour (kph) in your personal vehicle, you set the cruise control with the push of a button; it can be cancelled at any time by turning it off at the switch, touching the brake pedal or even pushing in a clutch. The road changes, traffic goes slower or faster, and there are curves and bumps and hills to be climbed up and driven down. Once you set the cruise control and the speed is recognized, it locks in that speed and will maintain it until you decide it needs to be changed. Around tight curves, up hills, down hills, the cruise setting sensor built into the car or truck increases and decreases engine speed to stay at its pre-set point of 100 kph.
The pressure transducer does the exact same thing. The pump operator (driver) turns the EPG to pressure mode from r.p.m. mode with the push of a button. The pressure transducer is the engine cruise control sensor, except that it sees discharge pressure, not road speed. The pump operator can stop or adjust that pressure at any time, just as the vehicle driver can control the road speed. This equates to the pressure governor acting as a kind of cruise control for the pump; if the pressure governor is perceived as a cruise-control device, it will be easier to understand.
Say you have a long lay of 100-millimetre (four-inch) high-volume hose from a pressurized source to your pumper; it is stretched out across the road from one side to the other. The hydrant is open and you are pumping to multiple lines. Two of those lines have firefighters on the inside fighting a stubborn fire. The EPG is set in pressure mode. The discharge pressures are all set and the rig is working flawlessly. Then, all of a sudden, the engine r.p.m. increases by 500 r.p.m. to 600 r.p.m., then ramps right back down to the set pressures and stays there. The firefighters on the inside may have seen a change in the streams they are managing and in nozzle reaction, due to the rise and fall of engine speed and pump pressure, while the pump operator has had to do nothing more than watch the gauges, listen to the engine, and be prepared to react if something should go wrong at the pump panel.
So what happened to cause the r.p.m. change? Well the street had not been closed down and a car drove over the 100-millimetre hose coming from the hydrant to the truck. Because the flow of water from the hydrant changed dramatically to the truck, the transducer detected an outgoing pressure change and increased the engine r.p.m. to compensate; then, just as quickly as the pressure was adjusted down, it had to go back up, until the transducer signal was at the original pump setting.
The EPG controls engine speed and raises or lowers it to control water pressure; it is not a pressure relief valve.
In photo 1, you can see several buttons. The mode button controls r.p.m. and pressure. Whether the EPG is made by Class 1 Fire Research or by another manufacturer, you must be able to switch from r.p.m. mode (see photo 2) to pressure mode (see photo 3). The mode will be illuminated as active. When in r.p.m. mode, the pressure transducer in the pump has been bypassed. This means there is no longer any engine speed control. The engine speed can rise and fall with the change of water coming in, or because of a lack of water being pumped out. Remember, the pressure transducer controls the engine speed, automatically controlling engine speed through the water being discharged. So, if not enough water, or too much water, is being discharged, the transducer will adjust the engine speed. In r.p.m. mode, this feature is not available, in which case there is a safety issue for all fire hoses flowing water, and for the fire pump itself.
Water hammer (a pressure surge) occurs when a water source has been cut off quickly. Slamming the bail of a nozzle closed causes the water that was speeding out of the nozzle to stop. However, all of that speed and inertia causes a reaction. The water bounces back, creating a hammer effect, which, in turn, harms the equipment. The old mechanical relief valve would have recognized that and redirected the water back into the fire pump on the intake side. The EPG pressure transducer, when in pressure mode, recognizes this action on the discharge side and instantly controls engine speed to soften the blow, not to redirect water or relieve the built-up pressure. Now do you get it?
The last thing I want to mention is doing a pump test on the apparatus apron. Be sure all safety-related actions are in place with correct PPE. The EPG check is simple. Turn the knob or push the button to verify that the engine r.p.m. changes; if it increases and decreases, it is working. Switch it to pressure mode, observing gauges and increase and decrease the pump pressure. Verify this against the master gauge if equipped. With the tank-to-pump valve partially closed, it will simulate no water to the pump. If the pump-to-tank valve is open (a discharge), the engine speed will change via the transducer as if you had run out of water. Open the tank-to-pump valve back up, allow a full rush of water into the pump volute, and the engine speed will readjust again and keep the pressure that was set in check. The apparatus pump test is done.
The EPG has removed some of the pump-operator processes but has added another process. Understand your equipment. Keep training. And remember, rubber side down.
EPG OPERATING FEATURES: CLASS 1 MODEL
- Audible alarm output
- Easy setup and configuration
- Large, easy-to-read alphanumeric display
- 12-volt and 24-volt compatible
- Analog engine control of J1939 CAN control for improved resolution and response
- Improved ergonomic tactile feedback button
- Totally integrated instruments including battery voltage, temperature, oil pressure and r.p.m.
- Integrated engine information reduces required pump panel space
- Programmable presets
- Displays intake and discharge pressure
- Dedicated check engine light and stop engine light
EPG OPERATING FEATURES: FIRE RESEARCH MODEL
- Large control knob – easy to operate with a gloved hand
- Push-button controls
- Automatic regulation of pump discharge pressure
- Single 300 psi discharge pressure sensor
- Programmable pre-set settings
- Always starts in pressure mode at idle r.p.m.
- Displays and LEDs automatically adjusted for day or night operation
- Limits increase of pressure when in r.p.m. mode
- Recognition of no water condition with automatic response
- No pressure or r.p.m. variation when changing modes
- Red idle button – brings the engine to idle r.p.m. quickly
- Manual control of pressure or engine r.p.m. settings
- Diagnostic capabilities
- Interlock signal recognition with throttle ready LED
- Retains warning alerts history
- J1939 CAN bus line for engine information and control
- High idle input for external switch
- Audible alarm buzzer (optional)
- KPa / bar (optional)
- Engine monitoring functions
Chris Dennis is the chief mechanical officer for Vaughan Fire & Rescue Services in Ontario. He can be reached at Chris.Dennis@vaughan.ca
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