A fire pump is generally operated at specific pressures to obtain the proper pressure and flow for safe and effective fire fighting. Controlling the discharge pressure of an engine-driven fire pump mounted on a fire truck is vital. The pump must supply water at a continuous rate and steady pressure so that firemen operating hand nozzles at a fire scene can control the reaction force generated by their nozzles. This is important since a detrimental discharge pressure surge can pull a nozzle out of a fireman's grip, or even throw him off a ladder or a ledge leading to delay, injury or even death.
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In one exemplary embodiment, a method of operating a pump driven by an engine is disclosed comprising steps of adjusting and measuring a speed of the engine with a controller electrically coupled to the engine, the controller having encoded therein a computer-readable method of controlling a pump; receiving a first discharge pressure at a first engine speed from a discharge pressure sensor coupled to a discharge line of the pump, and a second discharge pressure at a second engine speed from the discharge pressure sensor; determining a pump constant and an intake pressure in response to the first and second engine speeds; determining a maximum engine speed in response to the pump constant, the intake pressure, and a desired discharge pressure; and, limiting the engine speed to the maximum engine speed.
In another exemplary embodiment, a pumping system driven by an engine is disclosed comprising a pump coupled to the engine and operable to forcibly push a fluid received at an intake line to a discharge line; a discharge pressure sensor coupled to the discharge line of the pump and operable to measure a discharge pressure; a controller electrically coupled to the engine and operable to measure and adjust a speed of the engine, the controller having encoded therein a computer-readable method of controlling a pump; and, the controller operable to receive a first discharge pressure at a first engine speed, and a second discharge pressure at a second engine speed, determine a pump constant and an intake pressure in response to the first and second engine speeds, determine a maximum engine speed in response to the pump constant, the intake pressure, and a desired discharge pressure, and further limit the engine to the maximum engine speed.
In another exemplary embodiment a computer-readable medium having encoded thereon a method is disclosed comprising the steps of receiving a first discharge pressure from a discharge pressure transducer coupled to a discharge line of an engine-driven pump rotating at a first engine speed; receiving the first engine speed from an engine module; receiving a second discharge pressure from the discharge pressure transducer with the pump rotating at a second engine speed; receiving the second engine speed from the engine module; determining a pump constant and an intake pressure as functions of the first and second engine speeds and the first and second discharge pressures; determining a maximum engine speed as a function of the pump constant, the intake pressure, and a desired discharge pressure; and, sending the maximum engine speed to the engine module to limit the engine speed.
BRIEF DESCRIPTION OF THE DRAWINGS
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Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.
FIG. 1 is a simplified diagram of an embodiment of a fluid pumping system 10;
FIG. 2 is a flowchart of an embodiment of a system calibration method; and
FIG. 3 is a flowchart of an embodiment of a method of operating system 10.
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The system and method described herein are operable to limit the engine speed to a maximum speed based on several real-time inputs to an engine controller.
In pumping a liquid, the intake flow of the liquid to a pump may be interrupted by a restriction in supply, or by a volume of air being drawn into the intake of the pump. One example, without limitations, is found in fire trucks which pump a liquid from various sources such as, for example, a lake, tanker truck, hydrant or internal tank. An air volume or slug may be introduced into the pump when transferring the pump intake from one fluid source to another. An air slug may also result from a suction intake being near the surface of liquid where a vortex might form and draw air into the pump with the liquid. Another cause of limited supply can occur if a passing vehicle rolls a tire over an intake hose.
Restriction in flow, air entrainment or introduction of an air or gas slug can cause a pressure drop downstream of the pump. This occurs because the intake pressure drop leads to an increase in engine speed and pump speed. Once the slug of gas has passed the pump, the liquid will enter the pump which is then operating in an over-speed condition. This will cause an undesirable and sometimes dangerous pressure surge downstream of the pump and further shorten the life of the pump and engine.
A simple prior art device for controlling the pressure output of a fire pump is a mechanical relief valve which opens to discharge excess water when the incoming pressure is higher than the desired output pressure. However, a shortcoming of using such a valve is that it requires the use of a human operator to manually set the relief point of the valve. Furthermore, the relief valve only dissipates excess incoming pressure, and has no utility in situations where the incoming pressure is too low, such as when the water source is being depleted or another hose is connected to the system. In addition, if the pump engine continues to operate at full speed after the relief valve is opened, water will be continuously dumped from the system, resulting in needless waste, as well as flooding of the area where the fire truck is located.
More recently, electronically operated pressure control systems have been introduced. Three such systems are disclosed in U.S. Pat. Nos. 3,786,869, 4,189,005 and 7,040,868 (herein referred to as the “'869 patent,” “'005 patent,” and “'868 patent,”) to McLoughlin, the subject matter of which is incorporated herein by reference. In the '869 and '005 patents, the desired output pressure is dialed-in, or otherwise transmitted to a control box on board the fire truck, where it is compared to the actual output pressure as measured by a transducer. Any difference between the desired and actual output pressures is converted to an electrical signal which is fed to a DC motor which increases or decreases the RPM of the centrifugal pump as needed until the desired output pressure is reached.
These prior systems may require a long response time in a servo-mechanism controlled engine. Too much time can pass before the appropriate RPM and correct discharge pressure are reached. This is especially troublesome during transient events, such as overpressure spikes, where the system's response time is greater than the length of the event. Furthermore, no allowance is made for situations such as when the engine is already at idle and the incoming pressure suddenly increases, or is higher than desired, such as can happen when the pump is connected to a hydrant.
The '868 patent discloses a system that is able to detect a sudden pressure drop in less than one second and override the governor to maintain or decrease pump speed. Thus, some hydraulic shock may be prevented by the system's prevention of an over-speed condition of the engine. The engine speed may be adjusted by the governor using various means such as servo motor, electric signal or electronic data bus. However, a problem arises when the pump is asked to discharge more water than is available from the primary source such as when a fire truck switches its intake from an internal tank to a hydrant. When this happens, the discharge pressure requirement is not met and the governor instructs the engine to speed up. The engine will speed up but the required pressure will not be met, and the engine will continue to speed up until maximum engine speed is reached (engine runaway condition). Thus unnecessary wear and exertion is put on the system. And as previously mentioned, if an additional source of water is obtained and fed to the pump, with the engine at over-speed or runaway speed, the actual discharge pressure can exceed the desired discharge pressure by a factor of several times.
Referring to FIG. 1, a fluid pumping system 10 is shown arranged according to one exemplary embodiment. System 10 has an engine 12 coupled to a pump 14 via a pump transfer case 16. An engine controller 18 or governor is electrically linked to engine 12, an optional intake pressure transducer 20 and a discharge pressure transducer 22.
In operation, fluid enters pump 14 through an intake inlet 24 and exits pump 14 through a discharge outlet 26. Intake inlet 24 of pump 14 may be coupled, via an intake hose 27, to a fluid source 28, which may be a plurality of different fluid sources, such as, without limitation, an internal tank, a tanker truck, a lake and a fire hydrant. As pump 14 is preferably a centrifugal pump driven or rotated by engine 12, an increase in engine speed may cause an increase in the discharge pressure and discharge volume. A fire nozzle (not shown) is typically connected to a discharge hose 30 of pump 14, allowing a firefighter to control the fluid stream. The nozzle restricts fluid flow through discharge hose 30 causing a discharge pressure to build at pump discharge outlet 26. Discharge pressure transducer 22 measures and sends a discharge pressure signal 23 to controller 18 in real-time. Controller 18 compares the measured discharge pressure to a predetermined target discharge pressure. If the measured or actual discharge pressure is too low, then controller 18 may speed up engine 12 accordingly until a predetermined engine speed limit is reached. Thus, controller 18 has an internal memory (not shown) capable of storing measured parameters such as measured discharge pressure and current engine speed, as well as user-defined parameters such as target discharge pressure and maximum engine speed. User-defined parameters may be presented to controller 18 by an input means know in the art. Exemplary input means include, but are not limited to, a keyboard, serial interface, parallel interface, mouse, keypad, touchpad, dial, rocker switches, portable media interface, and the like.
Controller 18 has a processor (not shown) for performing calculations based on the aforementioned and other parameters. These calculations are described in greater detail below with reference to FIGS. 2 and 3. Controller 18 also comprises a memory for storing data such as the user-defined parameters, measured parameters, calculated values, computer readable code implementing the pump-operating method described herein, and other data.
When computing desired engine speeds, controller 18 may optionally receive an actual intake pressure signal 21 generated by intake pressure transducer 20.
It is also noted that engine controller 18 is operable to control the speed of engine 12 by sending a control signal 32 to engine 12 directly or to a module 34 coupled thereto. In turn, module 34 is operable to measure and send an actual engine speed signal 36 back to engine controller 18.
Referring now to FIGS. 2 and 3, the method of operating system 10 according to one embodiment is described. FIG. 2 is a flowchart showing what can be described as a setup or system calibration method 100. Method 100 proceeds to step 102 to ask the operator to verify that an adequate fluid supply exists for controller setup. It will be understood in the art that an adequate fluid supply comprises a sufficient volume of fluid at a consistent head, or intake pressure, such that no gas or air slugs are introduced into the pump during the setup method 100. If an adequate supply, or intake source is not available, then the operator is instructed at step 104 to attach an adequate source before continuing with the setup method.
After securing an adequate supply in step 102, engine 12 is allowed to rotate pump 14 at idle speed in step 106. Controller 18 then records the measured or actual engine speed (n1) and the measured or actual discharge pressure (p1) at step 108. Next the system checks for an intake pressure transducer at step 110, and if found, records the measured intake pressure (hm) at step 112. An alternate setup path is described below for systems without an intake pressure transducer.
A pump constant (k) is then computed in step 114 as a function of engine speed, discharge pressure and intake pressure (n1, p1 and hm). For example, the following exemplary equation may be solved for the pump constant, k:
k=the pump constant,
p1=the first discharge pressure,
hm=the measured intake pressure, and
n1=the first engine speed.
In step 116, controller 18 receives a desired discharge pressure (pin) either as an input from the operator or from a previously stored value in internal memory. An engine speed limit (nlimit) is then computed in step 118 as a function of the desired discharge pressure, the intake pressure and the pump constant in, h and k). For example, the following exemplary equation may be solved for engine speed limit (nlimit):