Patent Application
20210140635
This invention relates generally to control of fuel oil to fired equipment such as a boiler. More particularly, it is directed to the control of both the flow rate and the viscosity of a fuel oil using a flow control valve and a trim heater.
Residual fuel oils such as #6 oil vary in viscosity by batch and location of origin. The variation in viscosity can cause a change of flow rate of the fuel to fired equipment, resulting in either too much or too little combustion air. Furthermore, change in viscosity can also influence the atomization quality and the flame stability. In most residual fuel burning applications, a trim heater is used to heat the fuel to a desired temperature directly before ignition in the fired equipment. The trim heater is typically set to a pre-determined temperature setting (for example 180° F.) and will maintain that temperature regardless of flow rate or fuel properties. Using a flow meter to provide a feedback signal to a flow control valve can result in a more precise control of the fuel flow rate, but it does not address the problem of varying viscosity and its effects on atomization and flame quality. Using a viscometer to directly control viscosity of a fuel oil requires additional expenses for a viscometer and its maintenance. There exists a need for a control system for directly controlling the flow rate and indirectly controlling the viscosity of residual fuel oils.
It is a general object of the present invention to provide a method for controlling the flow rate and the viscosity of a fuel oil to fired equipment.
A more specific object of the present invention is to provide a method for directly controlling the flow rate and indirectly controlling the viscosity of a fuel oil to a boiler.
These objects are achieved by an apparatus for controlling a fuel oil using a combustion controller 70 and a trim heater controller 90, wherein the trim heater controller 90 uses a flow meter as a feedback mechanism to control a trim heater's temperature (instead of controlling the flow control valve position) to compensate for the deviation of the measured flow rate from the target flow rate set by the combustion controller.
Additional objects and features of the invention will appear from the following description from which the preferred embodiments are set forth in detail in conjunction with the accompanying drawings.
Identical reference numerals throughout the figures identify common elements.
Residual fuel oil 5 is drawn by an oil pump 10 to be pressurized, and goes through a pressure regulator 15 to reach a constant pressure downstream of the regulator 15, before going through a trim heater 20. A temperature sensor 30 measures the temperature of the fuel oil 5 and provides a feedback signal to a controller 90. Fuel oil 5 then goes through a flow control valve (FCV) 40 and a flow element 50 before it passes through the oil gun 61 to be atomized and burned in a burner 60 in a boiler 80. Boiler 80 could be a firetube boiler or a water tube boiler. The burner 60 produces a hot flue gas in boiler 80. The hot flue gas goes through the boiler and exits the boiler as flue gas 81. The flow element 50 measures the flow rate of fuel oil 5 and provides a feedback signal to the controller 90.
A combustion controller 70 takes a call-for-heat signal 71, calculates the firing rate needed for the heat demand, and sends out a signal to the flow control valve 40 and sends out a signal to a combustion blower 62 to draw combustion air 65 according to combustion curves established during a commissioning period by a commissioning engineer, see Table 1.
These combustion curves may include index (for example, points 1 to 6), flow control valve positions (0-100% open), target flow rates (in gallon per hour), combustion blower VFD settings (0-60 Hz), combustion blower louver settings (0-100% open) and FGR (Flue Gas Recirculation) valve setting (0-100% open). During commissioning, a heavy fuel oil (for example #6 oil) typically available for the site is used and the trim heater 20 is set at a temperature that is expected to be the average operating temperature of the trim heater (for example 180 degrees Fahrenheit). Combustion controller 70 also sends out a signal to inform the controller 90 regarding the target value (setpoint) for the fuel oil flow rate. The controller 90 is a PLC or loop controller. Note that controller 70 is a combustion control system (CCS) commonly used for dual-fuel boiler applications, while controller 90 is an add-on device to controller 70. Being an add-on device, controller 90 is not meant to function without controller 70. Since controller 90 does not have to perform all the functions of controller 70, its function can be simplified, and its costs can be kept low. While controller 70 is considered a prior art widely in use today, controller 90 and how it controls the trim heater 20 to compensate for the viscosity of the heavy oil are unique, and form the foundation of the current invention.
Controller 90 functions to maintain the actual fuel oil flow rate at the target flow rate specified by controller 70 by modulating the temperature setpoint of the trim heater 20. It is well understood that the viscosity of a heavy fuel oil decreases as the temperature of the heavy fuel oil increases, and a lower viscosity tends to increase flow rate. In other words, a high viscosity is an impedance to flow. The controller 90 gets a feedback signal from the temperature sensor 30 located immediately downstream of the trim heater 20. The controller 90 uses a PID loop to change the fuel temperature setpoint to maintain the “curve” established by the technician. The “curve” here refers to the relationship between the Flow Control Valve (FCV) 40 settings and the flow rates.
For instance, if the viscosity of the heavy fuel oil is higher than the viscosity when the control system was initially commissioned, the effect of viscosity will manifest itself as a measured flow rate lower than the target flow rate at a given FCV position, and the controller 90 will automatically change the heater temperature setpoint to a higher value to compensate for the difference in flow rates, and vice versa.
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The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well known devices are shown in block diagram form in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, the thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.
