The controls used on automatically operated boilers are reliable devices. Unfortunately, many operators believe that these devices do not require any attention when, in fact, the opposite is true. These controls require regular attention to keep them in good operating condition, especially since they play such a vital role in safeguarding the boiler.
The operator should regularly test the controls to ensure that they are in good functioning condition. The following tests are strongly recommended. They cover the key factors of operating controls. Note that what follows are general instructions. In the case of a specific make of control, the manufacturer’s recommendations for testing and maintenance should be followed.
Low-Water Fuel Cut-Off and Water Feeder Problems
First, a caution that the monthly test of a low-water fuel cut-off should not be applied to hot water heating boilers in a closed system because it requires a shutdown of the heating system (in a single boiler plant), the isolation of the boiler from the system, and drainage of all the water above the cut-off level.
The most common cause of feeder or low-water fuel cut-off trouble is the operator’s failure to observe blow-down instructions. The water in most boilers contains a certain amount of solids in suspension as a result of impurities in the water and the addition of certain water treatment chemicals. These solids will settle as sediment, especially in those parts of the boilers where circulation is sluggish, such as water columns, float chambers, and their connections. Even though float chambers are made large enough to accommodate a certain amount of sediment, failure to blow down regularly will cause the sediment to build up to the point where it renders the control unreliable or inoperative. Therefore, regular blowing down of the float chambers is a must.
Other parts of the controls that may cause trouble are the movable parts inside and outside the float chamber, the mercury switch, and the electrical wiring and terminals. The mechanical linkage should be able to move freely, and there should be no corrosion on any of the mechanical or electrical parts. If there is corrosion, the part or parts should be cleaned and the cause of the corrosion should, if possible, be corrected.
Testing the Low-Water Fuel Cut-Off
The low-water fuel cut-off should be tested on a regular basis, either weekly or monthly. The frequency of testing should be increased when large amounts of makeup water are required. This is certainly true in plants where a certain amount of steam is used for purposes other than heating, and not all condensate is returned. The larger amount of makeup water may require more chemicals for treatment, resulting in a larger amount of sediment.
Low-Water Fuel Cut-Off Weekly Test
While the burner is in operation, open the drain on the float chamber. The water level should drop sufficiently for the float to open the mercury switch and shut the boiler down. At the same time, any sediment collected in the chamber and connections will be flushed down the drain. The water level in the chamber should quickly return to normal once the valve is closed. The boiler will start up again automatically.
Low-Water Fuel Cut-Off Monthly Test
- Simulate a developing low-water condition in the boiler by shutting off the feed pump and feeder valve supply. Allow the boiler water level to drop. This can be sped up by opening the blow-off valve.
- Observe the gauge glass. The water level should never go lower than the lowest visible part of the glass. The burner should have shut off when the level is approximately one-half to one inch above the bottom of the glass.
- Restore the water level to normal and resume operation.
If the cut-off does not function properly during this test, it must be immediately overhauled and restored to proper operating condition.
The amount of makeup water in closed systems is usually negligible; the amount of sediment formed will be very small. Provided that the weekly test is performed, the chance of fouling the cut-off devices on a hot-water boiler is much smaller than on a steam boiler.
Feed Pump Control and Feeder Valve Testing
The operation of the feed pump control can be checked by observing the levels at which the pump starts and stops.
It is advisable to either mark these levels on the water gauge or to measure the distances of the levels above the bottom of the gauge glass and to write them down in a log book for future reference. A change in levels during later checks will be an indication of trouble developing in the control.
The operation of the feeder valve can be checked by shutting down the feed pump. The feeder valve should open up somewhat below the cut-in level of the feed pump, and it should be able to maintain this level at full load.
The tightness of the feeder valve can be observed when the boiler is shut down. With the stop valve in the pump discharge line closed, an increasing level indicates a leaking feeder valve.
Care of Low-Water Fuel Cut-Off and Feed Controls
- Blow down the float chamber at regular intervals.
- Periodically inspect the mercury switches. Burnout of these switches may occur, and in many instances the cause may be overloading due to either improper application or failure of wiring or connected equipment. The glass bulb of the mercury switch will sometimes crack, allowing air to enter the bulb. The mercury will acquire a dark scum as a result of oxidation. The scum will prevent the mercury from flowing properly and closing the circuit.
- A film on the inside of the glass tube is an indication of oxidation. When this occurs, the switch should be replaced.
- Check the electrical wires and terminals. Connections should be tight and free of corrosion. Wires with bare spots or brittle insulation must be replaced to prevent short circuits.
- Dismantle the low-water fuel cut-off of a heating boiler for a complete overhaul at least once a year. This should be done more often if boiler conditions are less than ideal. All the internal and external mechanisms should be cleaned and carefully checked for defects.
Testing Pressure and Temperature Controls
The pressure and temperature controls that regulate the operation of the burner do not require any special testing. However, the operator should, at regular intervals, observe and record in a log book the cut-on and cut-off pressures or temperatures of these devices. In this way, any deviations from the control settings will be detected.
Testing Operating and Limit Controls
Pressure and temperature high-limit controls cannot be tested during normal boiler operation since they operate only above the cut-off 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 fire 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 okay, disconnect the power and remove the test lead. Reset the high-limit control and put the boiler back into operation.
Testing Pressure and Temperature Limit Controls
The pressure limit control of low-pressure heating boilers should open the burner circuit at not more than 15 psig and the temperature limit control at a temperature not exceeding 250°F.
Maintenance of limiting controls generally requires only visual inspection of the device for evidence of wear, corrosion, and other deteriorations. If the switch is a mercury bulb, it should be checked as described in the instructions on the care of low-water fuel cut-off. If the limit control is defective, do not attempt to repair it unless the mercury bulb is cracked; otherwise, replace the entire device.
Because it is a sealed unit, the modulating control should not be serviced by the boiler operator. If either of the units in the control develop trouble, the operator should call the serviceperson, who will provide an exchange unit and return the defective unit to the factory for repair. At most, a boiler operator should clean off the modulating control with compressed air.
Testing Low Gas Pressure Cut-Off
The low gas pressure cut-off can be tested during the operation of the boiler by slowly closing the main gas cock. Observe the pressure gauge on the gas line. When the pressure in the fuel line drops to the cut-off point, the burner valve should shut automatically.
Testing Low Air Pressure Cut-Off
The low air pressure cut-off can be tested with the fan operating and the burner damper in a wide open position for maximum airflow. By restricting the air intake to the fan, the low air pressure cut-off should open the boiler control circuit, shutting the boiler down.
Testing Flame Safeguard Devices
To test the flame safeguard device, simulate a flame failure condition.
This article is adapted from BOMI International’s Boilers, Heating Systems, and Applied Mathematics. This is one of several BOMI International courses recently approved by the US Green Building Council for maintaining your LEED-AP credential. More information regarding this is available by calling 1-800-235-2664, or by visiting BOMI International’s website.