1.3 Industrial Boiler Control

We have seen in the basic diagram of a boiler that there are two main systems, the water-steam system and the fuel-air-gas system.

This second system, for the sake of clarity, will be further divided into the fuel system and the air-gas system. The systems detailed below are primarily developed from a control perspective. Therefore, while not all the instrumentation necessary for continuous operation or safety features is shown, the local instrumentation, manual valves, filters, drains, etc., included may not be exhaustive.

1.3.1 Water-steam system

Figure 1-9 shows the water-steam circuit of a water-tube boiler with a steam drum. The instruments belonging to the continuous control system are distinguished from the safety instruments by marking the former with a circle and the latter with a diamond, both inscribed within a box.

In this circuit, we can observe how the feedwater, at the appropriate pressure and temperature, from the pumping system, is supplied to the boiler drum through an economizer where its temperature is increased. The saturated steam generated is separated in the boiler drum and, upon exiting, is sent to a primary superheater where its temperature is partially raised. This superheated steam is then tempered before being sent to a final superheater, after which it is supplied to the existing consumers.

The steam circuit to the blowers is also shown. This steam will be drawn from different locations depending on the installation.

The variables that need to be monitored in this circuit from the point of view of boiler safety will be the following:

a) Since it is a boiler with a defined water level, there must be at least low-level protection to prevent damage to the boiler tubes due to insufficient cooling. In some cases, depending on the boiler or the steam usage, there is also high-level protection to prevent water from entering the superheater or contaminating the steam used in turbines. Differential pressure transmitters (LT) are used to measure the water level. Their readings must be corrected for the pressure in the boiler drum, as this affects the water density and, therefore, this type of measurement.

Figure 1-9. Water-steam circuit (Click to enlarge)

b) The boiler is a pressure vessel and is therefore subject to the Pressure Vessel Regulations (RAP) in force in Spain. The RAP requires that the heat supply be cut off when the pressure in the boiler tank exceeds a set value. This value must be below the threshold for the activation of the first safety valve in the boiler tank. These measurements are displayed using pressure transmitters (PT).

The variables to be controlled to maintain continuous steam production under the desired conditions will be the following:

a) As this is a boiler with a defined water level, the water level must be maintained at its normal operating level to ensure proper boiler operation. To achieve this, the following measurements will be taken: the water level (LT), the pressure (PT) for level correction if necessary, the feedwater flow rate (FT), and the steam flow rate (FT). These flow rate measurements must be corrected for temperature and for pressure and temperature, respectively, if their measurement is based on differential pressure and there are significant variations in these variables. The amount of water supplied at any given time will be controlled by a control valve (LV).

b) The steam produced must be maintained at optimal pressure and temperature conditions. Therefore, measurements will be taken of the final pressure of the superheated steam (PT) and its temperature (TT). The pressure will be maintained at the desired value by supplying the necessary fuel. To maintain the appropriate steam temperature, a control valve (TV) will be used to add the required tempering water. This valve will be installed between the two superheating stages. Finally, for control purposes discussed later, the temperature (TT) of the steam at the outlet of the first superheating stage will be measured.

c) The steam circuit for the blower system includes a shut-off valve that allows or prevents the passage of steam, a drain valve used to evacuate condensate as well as to heat the line, and a pressure (PT) and temperature (TT) gauge used as safety features of this system.

1.3.2 Air-gas system

A typical air-gas circuit of a forced draft boiler can be seen in Figure 1-10. The instruments of the continuous control system have been differentiated from the safety instruments in the same way as in the previous case.

In this circuit, we can observe how the air is propelled by the forced-draft fan. After passing through the air box and the burner registers, where the turbulence necessary for proper combustion is generated, the air enters the combustion chamber. Once inside, and after combustion has occurred, the gases leave the chamber and pass through the economizer, releasing some of their heat before reaching the chimney.

Figure 1-10. Air-gas circuit (Click to enlarge)

The variables that need to be monitored in this circuit from the point of view of boiler safety will be the following:

a) One of the most important safety variables is the combustion air flow rate supplied to the combustion chamber. To ensure that this supply is continuous and above a minimum, it will be monitored that the forced draft fan is operating (a signal obtained from the fan contactor) and that the combustion air flow rate is not low (FT). Additionally, this measurement will determine the minimum flow rate required to properly purge the unit before ignition. This measurement must be adjusted if there are large variations in air temperature and it is based on differential pressure.

b) Since the combustion chamber is designed to withstand a specific pressure, it must be protected against high pressure (PT) to prevent possible deformation of the casing. If the boiler is induced draft, the protection should be against negative pressure, and if it is balanced draft, against both.

c) If there is a damper or inlet vanes for modulation of the fan, the closed position must be provided for starting the fan, and the open position for monitoring its correct positioning after the maximum natural draft has been established in case of fan failure. If there are more dampers in the circuit, or in the case of air registers, at least the open indication for each of them will be necessary to determine the natural draft.

The variables to be controlled to maintain an adequate airflow that guarantees proper combustion under the best conditions of efficiency and safety will be the following:

a) Since the boiler is capable of operating at varying loads between a minimum and a maximum, the combustion air flow rate must be adjusted to the appropriate amount for the quantity of fuel being supplied. This requires measuring the air flow rate (FT) and using an actuator to control the fan's inlet vanes, thereby modifying their opening and, consequently, the airflow. If the air temperature is likely to vary significantly, it is advisable to measure the air temperature (TT) to correct the air flow rate measurement if it is obtained using a differential pressure reading.

b) In order to make combustion as efficient as possible, keeping the excess air as low as possible, the excess oxygen present in the gases will be measured.

c) Depending on the type of burners used, their air registers may or may not need to be modulated, according to the load, to generate the turbulence necessary for good combustion. In this case, an actuator will be needed to change its position.

1.3.3 Fuel System

Figure 1-11 shows the fuel circuit of a four-burner combination boiler for fuel oil and natural gas. The igniters are assumed to be for natural gas as well. The instruments for the continuous control system are distinguished from the safety instruments, as in the previous examples.

The circuits shown are based on the NFPA 8502 standard for multi-burner boilers.

In this circuit, we can observe the fuel distribution for the four existing burners. The gas line, used as fuel for the main burners and the igniter, branches into two lines, one for each purpose. Each of these lines then branches into four, with each branch going to one of the burners and ending in a vent that allows for manifold depressurization. The fuel oil and atomizing steam lines, necessary for the proper atomization of this fuel, also branch into four, one branch for each burner. There is also a recirculation line that allows the fuel oil to be conditioned to its operating specifications.

From a safety standpoint, the fuel circuit must not only ensure that critical fuel variables are maintained within the necessary limits, but also, given the need for reliable fuel shutoff, require the installation of a series of valves to guarantee this cessation when required, either individually for each burner or globally for each fuel type. Therefore, the following shutoff valves must be present:

a) Ignitors. A main shut-off valve, with its corresponding vent valve in the manifold, will be installed, and two shut-off valves in series, with their intermediate vent, for each of the igniters.

b) Gas burners. The same valves as those used for igniters will be installed. In addition, a small manifold charging valve will be installed in parallel with the main shut-off valve to allow for leak testing. All valves in this system must have at least a closed position indicator. In the case of vents, this indicator is usually replaced by an open position indicator.

c) Fuel oil burners. A main shut-off valve and a return or recirculation valve will be installed to allow the fuel to reach its appropriate operating conditions. A small circulation valve will be installed in parallel with the main shut-off valve to allow leak testing and to circulate the fuel for conditioning.

Figure 1-11. Fuel circuit  (Click to enlarge)

Additionally, there will be an individual shut-off valve for each burner. All valves in this circuit must have a closed indicator. Furthermore, since the lances of fuel oil burners are usually removable during operation for proper maintenance, limit switches are typically installed on them to indicate their engaged position.

Furthermore, the variables that need to be monitored in this circuit, from the point of view of boiler safety, will be the following:

a) Gas pressure to igniters (PT) not low and not high that ensures proper combustion.

b) Gas pressure to burners (PT) not high and not low, ensuring proper combustion and not exceeding the maximum contribution of this fuel.

c) Fuel oil pressure to burners (PT) not low enough to ensure proper combustion. It is also advisable to monitor the pressure to ensure that the maximum supply is not exceeded.

The fuel oil temperature (TT) should not be too low to ensure the fuel has the correct viscosity for proper combustion. It is also advisable to monitor the temperature to avoid high temperatures that could cause fuel vaporization.

e) Differential pressure of fuel oil-vapor (PT together with PT of fuel) not low enough to ensure proper atomization of the fuel.

f) Existence of a flame (BE) that ensures that the fuel being introduced into the home is being burned.

The variables to be controlled to maintain an adequate fuel flow rate for the existing boiler load at any given time will be the following:

a) Natural gasl. To properly modulate the natural gas supply according to the boiler load, a gas flow meter (FT) and a control valve (FV) are required to adjust the flow rate. If gas pressure and temperature conditions can vary significantly, it is advisable to measure these variables to correct the gas flow rate measurement if it is obtained from a differential pressure reading. A control valve (PV) is also typically installed in parallel with the main control valve to ensure minimum pressure to the burners (PT), especially during burner ignition and shutdown sequences.

b) Fuel oil. To properly modulate the fuel oil supply according to the boiler load, a fuel oil flow meter (FT) and a control valve (FV) are required to adjust the flow rate. If the fuel oil temperature is likely to vary significantly, it is advisable to measure these variables to correct the flow rate measurement for temperature if it is obtained from a volumetric reading. As before, a control valve (PV) is usually installed in parallel with the main control valve to ensure minimum pressure at the burners (PT), especially during burner start-up and shutdown sequences.

c) Atomizing vapor. To properly atomize different fuel oil loads, a constant vapor-fuel oil differential pressure must be ensured. This requires measuring this differential pressure (PDT) and using a vapor control valve (PDV) to modify the atomizing vapor flow rate to maintain this differential pressure.

1.3.4 Auxiliary systems

Figure 1-12 shows a typical feedwater circuit. In this circuit, water/condensate is mixed with steam in the deaerator to remove dissolved gases. This deaerated water is then pumped to the boiler.

This figure also shows the boiler blowdown circuit. Water from the boiler drum is sent to the pressurized continuous blowdown tank, where it acts as a seal before being sent to the intermittent blowdown tank, which is atmospheric.

Although none of the variables in this circuit directly influence the safety of the boiler, the deaerator level measurement (LT) should be considered for the safety of overflow due to high level and for the safety of starting the pumps due to low level.

The variables to be controlled for the correct functioning of the degasser and the continuous purge tank will be:

a) Deaerator. To maintain an adequate water level that guarantees a continuous water supply to the boiler, a water level gauge (LT) and a control valve (LV) are required to adjust the water supply according to needs. Furthermore, to maintain the pressure at the operating value that ensures deaeration, a pressure gauge (PT) and a control valve (PV) are needed to adjust the steam supply.

b) Purge tank. To maintain a stable water level that maintains a water seal, we will need a tank level (LT) measurement and a control valve that modulates the drain flow rate (LV).

Figure 1-13 shows a fuel oil preparation circuit. In it, fuel oil from the tank is pumped through heaters, where its temperature is increased to the operating temperature, before being sent to the burner fuel circuit.

Figure 1-12. Feed and blowdown water circuit (Click to enlarge)

Figure 1-13. Fuel oil preparation circuit (Click to enlarge)

The variables to be controlled for the correct functioning of this circuit will be:

a) In order to maintain the fuel oil supply pressure at its operating point, it will be necessary to measure this at the circuit outlet (PT) and a control valve (PV) in a return to the tank that allows this flow rate to be modified.

b) In order to maintain the temperature, a temperature measurement (TT) and a control valve (TV) will be needed to modify the flow of steam through the heaters and therefore the temperature of the fuel oil.