Fuel gas regulator

Let s now assume that I am driving the same compressor with a gas-fired turbine. The fuel-gas regulator to the turbine is 100 percent open. The turbine is spinning at 10,000 rpm. As the compressor s rotor fouls with salt, what happens to the speed of the turbine ... 

The horsepower output from a gas turbine is seldom limited by the position of the fuel-gas regulator, as I just described in the previous example. The limit is usually the exhaust temperature of the combustion or flue gases. The turbine s blades have a metallurgical temperature limit of 1100 to 1200°F (as designated by the manufacturer). The temperature of the exhaust combustion gases monitors the temperature of the turbine blades. 

The fuel-gas regulator can now be opened, because we are no longer constrained by the exhaust-gas temperature. 

Some fired heaters, especially boilers, have a device called a purple peeper, which is simply an optical device that looks at a flame. If it does not detect light with a wavelength in the high-frequency (i.e., purple) end of the optical scale, it interprets this as a flame-out. The fuel-gas regulator is automatically shut. 

What if the fuel gas regulator fails open or closed ... 

A fuel gas regulator for a control valve to a furnace should fail in the closed position on loss of instrument air pressure. 

A less common type of fuel-gas trip to a heater is a low-pressure trip. A pressure transducer generates a milliamp output from a boiler feedwater pump. Should this milliamp output fall below a certain level, the instrument air signal to the fuel-gas regulator actuator will be shut off. These fuel-gas valves are air-to-open, meaning that loss of instrument air flow causes the valve to close. 

Fuel systems can cause many problems, and fuel nozzles are especially susceptible to trouble. A gaseous fuel system consists of fuel filters, regulators, and gauges. Fuel is injected at a pressure of about 60 psi (4 Bar) above the compressor discharge pressure for which a gas compression system is needed. Knockout drums or centrifuges are recommended, and should be implemented to ensure no liquid carry-overs in the gaseous system. 

High or low fuel gas pressure ean have a dramatic effect on the operation of a firetube heater. Burners are typically rated as heat output at a specified fuel pressure. A significantly lower pressure means inadequate heat release. Significantly higher pressure causes overfiring and over heating. The most common causes of a fuel gas pressure problem are the failure of a pressure regulator or an unacceptably low supply pressure. 

Application The zirconia oxygen sensor is widely used for combustion control processes and for air/fuel ratio regulation in internal combustion engines. The closed-end portion of the electrode tube is inserted into the exhaust gas stream. In the control of industrial combustion processes, no out stack sampling system is required. 

After testing for soundness it will be necessary to safely introduce gas into the pipework displacing the air or inert gas that is in it. Similarly, if pipework is decommissioned for any reason fuel gas must be displaced by air or inert gas. This is a requirement of the Gas Safety (Installation and Use) Regulations, Regulation 21. Guidance on recommended procedures is given in the British Gas publication Purging Procedures for Non-Domestic Gas Installations (IM/2). 

In a simple single-loop system, we measure the outlet temperature, and the temperature controller (TC) sends its signal to the regulating valve. If there is fluctuation in the fuel gas flow rate, this simple system will not counter the disturbance until the controller senses that the temperature of the furnace has deviated from the set point (Ts). 

A cascade control system can be designed to handle fuel gas disturbance more effectively (Fig. 10.1). In this case, a secondary loop (also called the slave loop) is used to adjust the regulating valve and thus manipulate the fuel gas flow rate. The temperature controller (the master or primary controller) sends its signal, in terms of the desired flow rate, to the secondary flow control loop—in essence, the signal is the set point of the secondary flow controller (FC). 

In the secondary loop, the flow controller compares the desired fuel gas flow rate with the measured flow rate from the flow transducer (FT), and adjusts the regulating valve accordingly. 

Figure 10.1. Cascade control of the temperature of a furnace, which is taken to be the same as that of the outlet process stream. The temperature controller does not actuate the regulating valve directly it sends its signal to a secondary flow rate control loop which in turn ensures that the desired fuel gas...

To regulate the air flow rate with respect to the fuel gas flow rate, we can use ratio control. Fig. 10.5 illustrates one of the simplest implementations of this strategy. Let s say the air to fuel gas flow rates must be kept at some constant ratio... 

The oxygen from the dual gas regulator module flows directly through two ports into a closed-end manifold in the fuel cell stack to provide optimum oxygen distribution in the cells. All the oxygen that flows into the stack is consumed, except during purge operations. 

As shown in Fig. 14.24, a self-regulating oxidizer feeding mechanism is used to eliminate the liquid oxidizer pumping system. A flow of the pressurized fuel-rich gas generated in tlie primary combustor forces the oxidizer tank to supply the liquid oxidizer to the secondary combustor. Simultaneously, the fuel-rich gas is injected into the secondary combustor and reacts with the atomized oxidizer. The fuel-rich gas is injected from the primary combustor into the secondary combustor through the fuel gas injector under condihons of a choked gas flow. The pressure in the primary combustor is approximately double that in the secondary combustor. This system is termed a gas-pressurized system. 

Since both H2S and CO2 are acidic, they tend to come out together as mixed "acid gas" when the gas is washed to extract H2S. Various other compounds, including COS, may be extracted at the same time. Extraction of acid gas may not be needed if the treated gas is to be used only as fuel it may be sufficient to treat the raw gas by the Stretford process, converting H2S directly to sulfur. Such treatment removes virtually all H2S but leaves COS in the fuel gas, which may be unacceptable if air pollution regulations require more than about 95 percent removal of total sulfur. 

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