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Heating Boiler Operating Controls
Learning Outcome
When you complete this chapter you will be able to… Name and describe the various operating controls found on low-pressure heating boilers.
Learning Objectives
Here is what you will be able to do when you complete each objective.
- Describe the operation of the on-off control, the high-low fire control, the modulating control and the hight limit control found on low-pressure steam boilers and hot water heating boilers.
- Explain the operation of the common control switches found on a low-pressure heating boiler.
- Describe the operation of the safety switches found on the fuel supplies of low- pressure heating boilers.
- Explaing the required testing and maintenance of heating boiler controls.
Introduction
The safe operation of heating boilers is ensured by many controls on the boiler. There are operating controls which control the firing of the boiler, control switches, which can be operated manually or used by safety devices to prevent an unsafe condition, and high and low limit cut-off switches which control the combustion of the fuel heating the boiler. This chapter will describe the operation of the most common of these controls used on heating boilers, as well as the testing and maintenance required to keep them in safe working order.
Objective One
When you complete this objective you will be able to… Describe the operation of the on-off control, the high-low fire control, the modulating control and the hight limit control found on low-pressure steam boilers and hot water heating boilers.
Learning Material
Operating Controls for Steam Boilers
The operation of a heating boiler, and thus the operation of the firing equipment is controlled by an operating control that senses changes in boiler pressure or temperature and adjusts the burner operation accordingly. There are four distinct methods of burner operation control, they are:
- On-Off Control - The burner fires at a constant rate which is sufficient to maintain boiler pressure or temperature at full load. When operating at less than full load, the burner is shut off when the upper pressure or temperature limit is reached and it is lit up again when the pressure or temperature drops to the lower limit. Thus the burner operates on an on-off cycle. This type of control is the simplest and is commonly used on low capacity boilers. However, it has the disadvantages that the pressure or temperature swings between the upper and lower set limits and that the boiler will not operate at maximum efficiency unless fired continuously.
- High-Low Control - Two firing rates are available with the rate used depending on the boiler load. Operation with this type of control is more efficient than with the on-off control and pressure or temperature fluctuations are smaller.
- Modulating Control - The firing rate is continuously adjusted to match a varying boiler load. The boiler pressure or temperature is maintained at a constant value and operation is most efficient.
- High Limit Control (High Steam Pressure Fuel Cut-Off) - This control is used to shut down the fuel supply in the event of too high a steam pressure. This will help prevent a possible explosion.
On-Off Control
The pressure control is the device used on steam boilers to sense the boiler pressure and to start and stop the operation of the burner to maintain the pressure within set limits. A cut-away view of this control showing the internal parts is illustrated in Fig. 1. A simplified diagram of the same control is shown in Fig. 2. Figure 1. Pressure Control
assumes a more horizontal position. When the pressure increases still more, the lever will move so far over that the glass tube starts tilting to the left causing the mercury to flow to the left end of the tube. This breaks the electrical circuit to the fuel valve, the valve closes, and the burner stops firing. The boiler pressure will now begin to drop, the bellows starts to contract, the spring pulls the lever up and the tube starts tilting to the right again. When the pressure has dropped to the cut-in point, the tube will be tilted to the right sufficiently to allow the mercury to flow back and close the circuit, thus starting the light-up sequence for the boiler. The operating pressure of the boiler is controlled by the tension of the large spring in the pressure control. Increasing the spring tension will raise the operating pressure, decreasing the tension will lower it. However, when a control is equipped with only a single spring, the pressures at which the control starts the boiler (the cut-in point) and stops the boiler (the cut-out point) will be fairly close together. The difference may be less than 5 kPa, (0.75 psi). Under certain load conditions, the small pressure difference may result in frequent starting and stopping of the burner, called cycling, which is undesirable and should be prevented. To reduce cycling by obtaining a larger spread between the cut-in and cut-out points, the control is equipped with a second spring, called the differential spring. This spring has limited action, obtained by using a stop on the spring movement (see Fig. 2). When the control cuts in, energizing the burner circuit, the boiler pressure builds up. The left end of the lever starts to move downwards by the action of the bellows against the restraining force of the pressure spring only. However, when the lever has travelled about half way between its stops, the differential lever and spring are engaged as well. As a result, the opposing force to the lever movement is increased and more pressure is required before the switch can reach the cut-out point. The larger the tension of the differential spring, the greater the spread or differential between the cut-in and cut-out points. Thus the cut-in point of this control can be varied by the pressure adjustment while the cut-out point is controlled by the differential adjustment. The control has two scales with pointers on the outside of its housing. One indicates the setting of the pressure adjustment which is the cut-in pressure of the boiler. The other scale indicates the differential, the difference in pressure between cut-in and cut-out points. For example, the scale of the pressure adjustment in Fig. 2 shows a cut-in pressure of 10 psi (69 kPa) and the differential scale shows 3 psi (20.7 kPa). The burner will then be lit when the boiler pressure has dropped to 10 psi and it will stop firing when the pressure reaches 13 psi (89.7 kPa). NOTE: On some pressure controls the pull of the differential spring is applied opposite the way described above by a change in the lever
system. As a result, the cut-out pressure is then controlled by the large spring while the differential spring now controls the cut-in pressure. To avoid confusion, most controls give directions for adjustment on the outside of the cover. The operator is well advised to check these instructions before making any adjustments. In order to have the pressure control operate accurately at the settings shown on the control indicator scales, it should be mounted in a perfectly vertical position. To aid the mounting of the control in this position, a levelling indicator is fitted inside the housing. The pointer should be exactly in line with the mark in the back of the housing as shown in Figs. 1 and 2. The control should always be connected to the steam space of the boiler with a syphon between the control and boiler to protect the bellows or diaphragm against the high steam temperature or corrosive vapors in the steam.
High-Low Fire Control
Boilers equipped with multiple nozzle burners for high-low firing require two pressure controls similar to the on-off control described above. One control, called the "operating limit pressure control", starts and stops the burner but only controls the low fire nozzle. The second control, the "high-low fire control", is used to change the firing rate of the burner from low to high by engaging the high fire nozzle(s) and vice-versa depending on steam demand but it does not control the operation of the low fire nozzle. The diagram in Fig. 3 gives an example of how the cut-in and cut-out points of these controls may be set to maintain a fairly constant boiler pressure during operation and to ensure burner start-up on low fire only. Keep in mind that this is only an example, actual control settings on heating boilers may differ depending on operating pressure and pressure fluctuations allowed. Figure 3. Cut-In and Cut-Out Points of Operating Limit and High-Low Fire Controls
modulating control is illustrated in Fig. 4. This control operates in conjunction with a modulating damper motor which simultaneously adjusts the fuel supply to the burner and the air supply to the furnace. Figure 4. Modulating Pressure Control The operation of the control is quite similar to that of an on-off pressure control, however, it does not operate a switch. Variations in boiler pressure will cause the bellows to contract or expand. The movement of the bellows is transmitted to an operating lever which moves sliding contacts over electric resistance coils, varying the resistance of these coils. This change in resistance will cause an electric current to flow to a modulating motor which then, in turn, adjusts the fuel and air supply to the burner, thus changing the firing rate. An increase in boiler load will drop the pressure, the modulating control will sense this drop and increase the firing rate. A decrease in steam demand will increase the boiler pressure and the firing rate is reduced accordingly. The operating pressure of the modulating control as well as the modulating range can be adjusted in a similar manner as used on the on-off pressure control as indicated in Fig. 4. As with the high-low fire control, the modulating control should be used in conjunction with an operating limit control to start and stop the operation of the burner. The control panel is also equipped with an auto-manual switch and a flame control switch which make it possible to override the signal of the modulating control and control the firing rate manually at a number of stages between high and low.
High Limit Control (High Steam Pressure Fuel Cut-Off)
The Boiler Code requires that automatically fired steam heating boilers not under constant supervision must be equipped with a high limit pressure control that shuts off the fuel supply at a boiler pressure not to exceed 103 kPa (15 psi).
This control must be connected to the boiler without any shut-off valves and it shall be protected by a syphon or a similar means of maintaining a water seal to prevent steam from entering the control. The purpose of the high limit control is to shut off the firing equipment should the operating limit pressure control fail to shut off the burner when it reaches its cut-out point and pressure continues to rise. The high limit control thus acts as a safeguard to prevent excessive boiler pressure and will prevent the pressure from rising to the point at which the safety valves would open up and release the steam to atmosphere. In design and operation the high limit control is quite similar to the on-off pressure control but it is not equipped with a differential spring so that the cut-out and cut-in points of the control are very close together. Also, the control after having shut off the burner on reaching its cut-out point, locks out and does not automatically start the burner up again when the pressure drops. Instead, it has to be manually reset by a reset lever or button on the outside of the control before the boiler can be started again. The reason for this arrangement is to indicate to the boiler operator that the operating control is malfunctioning. The operating control must be repaired or replaced immediately. It is generally recommended to set the cut-out pressure of the high limit control sufficiently above the cut-out pressure of the operating limit control to avoid unnecessary lock-outs should the operating control be slightly out of calibration. Ideally the cut-out pressure of the high limit control should not exceed ninety percent of the safety valve setting.
Operating Controls for Hot Water Boilers
Hot water boilers are equipped with operating controls similar in construction and application to the operating controls used on steam boilers except that the bellows of these controls are not actuated by steam pressure but by pressure fluctuations caused by expansion and contraction of a liquid in a thermal element due to increases or decreases in the water temperature. An operating control commonly used on hot water boilers may be designed either for on- off or for modulating operation. The bottom of the bellows is connected to a sensing bulb by means of a capillary tube (a tube with a very slender or hair-like bore). Bellows, tube, and sensing bulb form a sealed unit (thermal element) that is filled with liquid. The sensing bulb is inserted in a thermowell mounted in the hot water outlet of the boiler. The control box is mounted at an easily accessible location away from the point of temperature sensing. The temperature of the liquid in the sensing bulb will closely follow the temperature of the water leaving the boiler. When the water temperature rises the liquid in the bulb will expand. The excess liquid passes through the capillary tube to the bellows which, in turn,
As on a low-pressure steam heating boiler, the Code requires a high limit control on hot water boilers. This control must cut off the fuel supply before the maximum allowable temperature of 121°C (250°F) is reached at the boiler outlet.
Objective Two
When you complete this objective you will be able to… Explain the operation of the common control switches found on a low-pressure heating boiler.
Learning Material
CONTROL SWITCHES
Depending on the type of operating control used on automatically fired heating boilers, the control panel is equipped with one or more of the following switches: burner switch, damper positioning switch, automatic-manual selector switch, flame control switch, low fire switch, and high fire switch.
Burner Switch (On-Off)
All boilers are equipped with a manually operated start-stop switch that is used to start and stop the operation of the boiler.
Damper Positioning Switch
Boilers equipped with a high-low fire burner or modulating burner require a damper motor which positions the air damper by means of a linkage system to provide the right amount of combustion air to maintain the proper fuel-air ratio for each specific firing rate.
For a high-low fire burner a two-position motor is used. This motor is controlled by the high-low fire control which also controls the fuel valves for the high and low fire burners. A modulating damper motor is used in conjunction with a modulating type burner. This motor simultaneously adjusts the air supply to the burner in relation to the amount of fuel supplied to the burner nozzle and is controlled by a modulating pressure control. A damper positioning switch is used on boilers with a high-low fire control. The switch has three positions: automatic, high, and low. When set on automatic, the high-low fire control controls the operation of burner and air damper. When set on either high or low, the switch overrides the signal of the high-low fire control and burner operation will be continuous at the rate selected.
Automatic-Manual Selector Switch
On boilers equipped with a modulating control, an automatic-manual switch is provided which overrides the signal of the modulating control and allows the firing rate to be controlled manually by means of a flame control switch.
Flame Control Switch
This switch is used in conjunction with the automatic-manual selector switch on boilers controlled by a modulating control. When the selector switch is set on manual, the firing rate of the burner and the corresponding setting of the air damper are manually set by means of the flame control switch at any rate varying from low, through a number of intermediate rates, to high.
Low Fire Switch
Boilers controlled by a high-low fire or modulating control equipped with a damper motor usually have a low fire switch attached to the damper motor shaft that is closed when the damper is in low air position. On these boilers the burner is lit on low fire and the air damper must be in low position to ensure proper lighting. The switch forms part of the starting circuit and will prevent start- up when contacts are open due to an improperly positioned damper.
High Fire Switch
On boilers with a high-low burner, the damper motor shaft also operates a high fire switch which is actuated when the damper is moving toward the high fire position in preparation for the lighting of the high fire nozzle. This switch acts as an interlock preventing the high fire fuel valve from being opened when the amount of combustion air is insufficient for firing at the high rate.
Objective Three
When the fan is started and comes up to speed, sufficient air should be forced into the furnace to build up enough pressure for the air entering the control to move the diaphragm against the force of the spring and to close the switch. This will allow the fuel valve to be energized and the burner to be lit. If during operation the amount of combustion air supplied would drop off below the safe minimum required due to a malfunction of the fan, the pressure of the air will drop as well. The cut-off switch will open, de-energizing the fuel valve, and the burner will cease firing.
Low Gas Pressure Fuel Cut-Off Switch
This safety control device works on the same principle as the low air pressure cut-off. If, for any reason, the pressure of the gas supplied to the burner drops below a minimum value, which indicates insufficient gas for safe combustion, the switch opens up, and the fuel supply is cut off. The switch in Fig. 7 consists of a diaphragm or bellows with pressure from the fuel acting on its lower side and the opposing force of the spring acting on its upper side. If the pressure of the fuel drops below a safe level, the leaf spring will drop, opening the switch to the solenoid operating the gas valve. The valve then closes, shutting down the burner, preventing an explosive condition in the boiler due to an unstable flame. Figure 7. Pressure Actuated Switch
High Gas Pressure Fuel Cut-Off Switch
This safety device works on the same principle as the low gas pressure fuel cut-off switch. If for any reason the gas pressure supplied to the burner increases beyond the maximum required for safe combustion, the switch will open up and the fuel supply will be cut off.
Objective Four
When you complete this objective you will be able to… Explaing the required testing and maintenance of heating boiler controls.
Learning Material
TESTING AND MAINTENANCE OF BOILER CONTROLS
The controls used on automatically operated boilers are reliable devices that play a vital role in safeguarding the boiler. They will do the job for which they are designed but they do require regular attention in order to keep them in good operating condition. The operator should check at regular intervals that the controls are working properly and, for this purpose, the following tests are strongly recommended.
Testing of Operating and High Limit Controls
The pressure and temperature controls which regulate the operation of the burner do not require any special testing. However, the operator should observe the cut-in and cut-out pressure or temperatures of these devices at regular intervals so any deviation from the control setting, which may be an indication of developing trouble, will be detected. The pressure and temperature high limit controls cannot be tested during normal boiler operation since they only operate above the cut-out point of the operating control. To test the high limit control, disconnect the power to the boiler controls and place a test lead across the terminals of the operating control. Check the setting of the high limit control. Restore power to the controls and start the boiler. Allow the boiler to fire until the steam pressure or water temperature reaches the setting of the high limit control. The control should operate at this point and shut down the firing equipment. If the test is O.K., disconnect the power and remove the test lead. Reset the high limit control and put the boiler back into operation. The high limit control of a low-pressure steam heating boiler should open at a pressure higher than the cut-out pressure of the operating control, but lower than 103 kPa (15 psi), the popping pressure of the safety valve. On low-pressure hot water boilers the cut-out temperature of the high limit control should be set above the cut-out setting of the
