This paper was written by a team at Premier Farnell. Premier Farnell is a distributor of electronics technology.
The power generation industry has gone through many changes in the last 50 years or so and the controls and safety features that were based primarily on the pneumatic controls are now taken over by electronic controls (with its own set of integrated systems resistors, and capacitors) and/or the Digital Control Systems, Furnace safeguard Supervisory Systems (FSSS) and computer controlled systems. The automation systems have improved over the years and now a standard has emerged for the power generating industries. The general improvements in the systems can be enumerated as below. The systems in a boiler control are generally divided in five sections; wiz, drum level controls, steam temperature control, boiler pressure controls, and furnace safeguard Supervisory Systems (FSSS) as also auxiliary interlocks; apart from the boiler water chemistry control. The boiler water chemistry is a separate control not normally associated with other controls and will not be discussed here.
System for drum level controls
Earlier the controls were based on water level in the drum. Sometimes when the steam demand went down the decrease in water level (because of increase of pressure) would tend to supply more water to the drum. This anomaly was rectified by the introduction of a three level control for the boiler drum. In the new system, the difference in the steam flow and the water flow was the main element for control of drum level with correction from drum level adding to the better control. The drum level system can be independent control with its own electronic controls consisting of resisters transistors and capacitors or integrated circuits.
The system was further refined with pressure and temperature correction from steam parameters. This resulted in better management of drum level. The three element control with the compensation has not undergone any major change over the last 50 years and is now the industry standard.
Control for boiler pressure
The pressure control has changed from just pressure control to the addition of other parameters like air flow, fuel flow, and the fuel calorific value for pressure control. The online efficiency calculations are also now integrated along with fuel pressure controls. The primary air, secondary air measurements also assists the control of boiler pressure in the system.
Furnace safeguard Supervisory Systems (FSSS)
The FSSS system makes sure that there is no uncontrolled fuel that can cause explosion in the boiler at any stage. The flame is sensed by photo sensors and as long as the flame is available, the supply of fuel is continued to the burners. The absence of flame shuts off all the fuel in the furnace system. Of course the flame sensing is done by two out of three sensors so that at any stage at least two sensors are working and any dependence on a single flame sensor is avoided. Failure of two sensors to see a flame is a signal to close all the fuel supply to the furnace. The fuel can be in solid, liquid or gaseous form. All the fuel in any form will be cut off irrespective of the condition of boiler pressure. The fuel supply cannot be re-started unless the complete purging of fuel is ensured. For ensuring of absence of any fuel in furnace, the furnace is purged with 30% of air flow for 5 minutes and then only the new supply of fuel can be introduced. The purging is done irrespective of the reason of trip. Here also better flame sensors and better
The functions of FSSS are,
- Starting of furnace purge after conditions of auxiliary interlocks are satisfied
- Permit starting of fuel introduction
- Making sure that burners start working only when an auxiliary flame exists
- Stopping of all fuel to boiler when flame is extinguished or no longer detected.
FSSS has three units, wiz indicating and operator’s console, relay and logic cabinet with its own electronic circuits of transistors, resistors and capacitors, relays, timers, AC and DC supplies and the fuel trip system.
Steam temperature controls
The steam temperature controls ensure that the metals used in construction of the boiler are below the safe limits under which they can operate. The control of steam temperature can be achieved by many means, but the most effective control is achieved by attemperation of steam in the area between the primary and the secondary superheaters. Any increase of temperature can affect the life of parts and may even cause failure of metals of the final superheater.
Boiler alarm panels that were previously hard wired have started to become much more sophisticated panels. For example, previously changing alarm settings was only possible with the help of an instrument engineer, now the operator of the panel is able to fine tune the alarm setting.
All these systems were operated as independent systems with no outside communication. These were known as single loop controllers with their own logic. The trip systems operated independently and had no effect on the other entities in a power plant like the turbine, generator or the electrical systems. Additional reliability was introduced with the help of two out of three systems of primary sensing elements. With all of them agreeing on the value of the sensing element, the average value was used for control. When one of them gave a value beyond a permissible error, it was ignored and the other two were used for calculations and an alarm about the third one going out of service was given to the operator. This increased the reliability of the system, something that was not possible with the pneumatic systems.
Other systems like the water management and fire fighting systems could not be integrated with the overall systems, even though operators could get information through hard wired systems. In emergencies, some of these systems were often ignored, resulting in less than ideal ways of handling the emergency.
The next major change was the introduction of a Digital Control System (DCS) and the use of software in the boiler control system. All the above controls were previously independent of one another, but the introduction of DCS and computer software in alarm and emergency handling systems brought the safe shut down of the boiler, turbine and generator. The system of programmable logic controls and the mechanical relays for protection of generators slowly gave rise to the electronic relays in electrical systems and its integration with the DCS.
The architecture of the DCS is local control supervised by additional layers that overlook the entire system. The individual control level of the drum, pressure control, FSSS of boiler, as well as the control of auxiliary interlocks turbine and generator protection, are individual systems which have their own logics and system of alarm generation for decentralized controls, but the information is sent to a higher level where the safety system takes over in emergencies.
In case of emergency situations in individual areas, the system can have its own set of controls over the change in parameters to handle the situation. The safety of the entire system takes precedence over the individual systems and the safe shutdown of the entire system gets activated.
While the individual controls are meant for the control of a single parameter, the alarm system signals the operator about the abnormal condition. The operator can take over the situation and bring the system back to normal. In case the operator is unable to do so and the situation threatens to get out of hand, the shutdown system comes into play. At this stage the operator cannot have any involvement and can only oversee the system shutting down safely.
Overall, the change from individual controls, to dedicated single loop controllers with redundant sensing elements to the DCS formed the line of change to the present method of state of control and safety in manufacturing and power generating industries.