Do your boiler plant controls need an IQ injection? New electronic control systems can help you stay on the information highway.
Modern electronic controls offer the central plant more power than ever before to control the big job steam boilers found in a campus central plant. These boilers, typically in the 40,000 lb/hr to 100,000 lb/hr range, are critical to the mission of the enterprise. Unfortunately, many of these big boilers are suffering from a lack of reliability due to the obsolescence of old pneumatic or electric controls. The time has come to learn how to bring modern electronics to bear on the reliability issue, and reduce energy bills at the same time.
Today's boiler plant is equipped with electronic (digital or analog) controls for each boiler to control flame monitoring, fuel and air valves, and startup and shutdown sequences. Further, the plant is equipped with a separate microprocessor, the plant master, which coordinates all boilers and equipment while recording data and providing a communications link. The modern plant master controller may be a proprietary combination of hardware and software specially designed for boilers or it may be a general-purpose industrial controller programmed for the needs of the plant.
Individual boilers have always had safety circuits to monitor the presence of the combustion flame and ensure that the fuel supply is completely shut off in the event of a flame failure. Redundant automatic valves in the gas supply piping or fuel train (Figure 1) shut off the fuel supply in the event of a boiler trip. Without such a system, the boiler could fill with combustible fuel and present the danger of explosion when the burner is restarted. Most boiler safety controls use a photoelectric sensor to confirm the presence of a stable, efficiently burning flame. If this flame is lost or becomes unstable for any reason, the fuel is shut off and an alarm is sounded.
In addition to flame loss or instability, the following can also cause a boiler shutdown (Figure 2):
* High steam pressure;
* Low or unstable fuel pressure;
* Low or high boiler water level;
* Low or high feed water temperature;
* Unstable boiler water level (foaming);
* Low or unstable combustion airflow;
* Incorrect combustion air damper position;
* Incorrect combustion air fan status;
* High, low, or unstable firebox pressure;
* High or low stack gas temperature; and
* High or low stack gas oxygen content.
In the case of a boiler safety trip in the fully automated boiler plant, the plant master controller takes over to increase the burn rate of other boilers or bring new boilers on-line, as required, to meet the plant load. The use of the plant master controller is not necessarily intended to replace the boiler operator, but it gives the operator time to safely determine the cause of the boiler trip and automatically transmits an alarm to others if required.
CLEARING THE AIR ABOUT REIGN REIGNITION
Central plant steam boilers typically produce steam at pressures upwards of 15 psig and are considered high-pressure steam boilers. Boilers rated at over 15 psig are labeled by the American Society of Mechanical Engineers (ASME) and must conform to required standards of material and construction. Pressures may run to 100 psig in a typical community steam distribution system and up to several hundred psig if the steam is used in a steam turbine cogeneration system. The additional pressure is required to overcome the pressure drop of the distribution system. Steam at these pressures is up to 200[degrees]F hotter than the steam used in single buildings heated with dedicated boilers. This means that the interior of these large central boilers is at least that much hotter than their smaller counterparts.
The real danger that lies in the loss of burner flame is the potential of an explosion, and the higher internal temperatures in large central plant boilers aggravate this danger. An explosion can be caused by either the burner attempting to relight the flame in a furnace full of combustibles or by the fuel-air mixture coming into contact with a hot surface or hot particle in the boiler.
To guard against either event, the fuel source must be shut off and kept off until reset by the operator at the boiler. The other controls on the boiler see that the water level is maintained as the unit cools and also that combustion air is circulated through the boiler to dilute the remaining fuel-air mixture below the threshold of combustion. This dilution or purging of the boiler combustion space must continue until the boiler is cooled safely below the fuel combustion temperature.
When purging is complete, controls confirm that fuel pressure is sufficient for burning. The controls then turn on the combustion air fans, monitor the air volume, and open the fuel valves in conjunction with energizing the ignition device.
All of the above procedures are controlled by the local boiler controls. Whereas older control systems were based on a combination of pneumatics and electric relays, their modern counterparts are either analog or digital electronic circuits. The newer, microprocessor-based controls are faster and monitor more elements of boiler operation than their electromechanical predecessors and improve the safety of the plant.
