This invention relates to a method for controlling the pressure output of an engine-driven pump system. Specifically, this invention relates to a method of controlling the discharge pressure of an engine-driven pump for use in a fire truck.
It is vital to control the discharge pressure of an engine-driven fire pump mounted on or in a fire truck. The pump must supply water at various rates and steady pressure so that firemen operating the hoses at a fire scene can control the reaction force generated by their hose nozzles. Fire pumps as used here are centrifugal pumps. These pumps add pressure to the incoming source of water. Therefore pressure changes in the supply are pressure changes in the discharge. This is problematic because even slight variations in pressure in the supply line leading to the intake of the pump are amplified by the pump on the discharge side, causing surges or oscillations in the water flow discharge at the nozzle and corresponding changes in the reaction forces. Such changes are extremely dangerous, as they can pull a nozzle out of the fireman's grip, or even throw him or her off a ladder or ledge.
The simplest prior art device for controlling the pressure output of the fire pump is a mechanical relief valve which opens to discharge excess water when the pressure is higher than the desired output pressure. A shortcoming of such a valve, however, is that, the relief valve only functions to dissipate excess pressure, and has no utility in situations where the 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 recirculated in the system, resulting in needless waste and wear and tear on the pump and engine. Overheating of the pump and engine is also more likely.
Electronically operated pressure controlled systems have been developed. Two such systems are disclosed in U.S. Pat. Nos. 3,786,869 and 4,189,005 to McLoughlin, the subject matter of which is herein incorporated by reference. In these systems, the desired output pressure is dialed in or otherwise transmitted to a control box on the board of 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 pressure 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. A shortcoming of this type of system is that, because the response time of the servo-mechanism controlling the engine is slow, 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 what can happen when the pump is connected to a hydrant. Recent engine technology has replaced the servo with direct commands to the engine computer or an electrical throttle control which can improve response times.
Another control system of interest is disclosed in German Patent No. 1,274,402 to Mueller and Company, which discloses an engine-driven pump which responds to an over pressure in the supply line by simultaneously opening a pressure relief valve and mechanically reducing the engine speed. The shortcoming of this purely mechanical system is that by its nature, in cases of over pressure, the relief valve will always be open to some extent, allowing some fluid to always bypass the relief valve, and the engine rpm will always be above its idle setting to a certain extent.
Another pressure control system of interest is disclosed in U.S. Pat. No. 5,888,051, which discloses an engine-driven pump which responds to an over pressure in the supply line and lowers the engine rpm and simultaneously controls a pressure relief valve which may be commanded to open and dump water for short durations to relieve over pressure spikes, or for longer duration to relieve excess water coming into the pump. The shortcomings of this system are that the change in engine speed and the relief valve may be operated at the same time resulting in a waste of water. In addition, operating the engine and relief valve simultaneously results in a needlessly complicated response system.
Accordingly, a need exists for a new and improved electronically operated fire pump discharge pressure control system for quickly and safely responding to drops or increases in the incoming pressure of a fire pump, which change the discharge pressure required, as well as changes in discharge pressure due to the opening or shutting off of various valves downstream of the pump.