An overview of the necessary fittings, accessories and controls for a boiler from nameplates and safety valves to gauge glasses and level controls.
Boiler Fittings and Mountings
Gauge glasses and fittings
All steam boilers are fitted with at least one water level indicator, but those with a rating of 100 kW or more should be fitted with two indicators. The indicators are usually referred to as gauge glasses complying with EN 12953.
A gauge glass shows the current level of water in the boiler, regardless of the boiler’s operating conditions. Gauge glasses should be installed so that their lowest reading will show the water level at 50 mm above the point where overheating will occur. They should also be fitted with a protector around them, but this should not hinder visibility of the water level. Figure 3.7.10 shows a typical gauge glass.
Gauge glasses are prone to damage from a number of sources, such as corrosion from the chemicals in boiler water, and erosion during blowdown, particularly at the steam end. Any sign of corrosion or erosion indicates that a new glass is required.
When testing the gauge glass steam connection, the water cock should be closed. When testing the gauge glass water connections, the steam cock pipe should be closed.
To test a gauge glass, the following procedure should be followed:
1. Close the water cock and open the drain cock for approximately 5 seconds.
2. Close the drain cock and open the water cock
Water should return to its normal working level relatively quickly. If this does not happen, then a blockage in the water cock could be the reason, and remedial action should be taken as soon as possible.
3. Close the steam cock and open the drain cock for approximately 5 seconds.
4. Close the drain cock and open the steam cock.
If the water does not return to its normal working level relatively quickly, a blockage may exist in the steam cock. Remedial action should be taken as soon as possible.
The authorised attendant should systematically test the water gauges at least once each day and should be provided with suitable protection for the face and hands, as a safeguard against scalding in the event of glass breakage.
Note: that all handles for the gauge glass cocks should point downwards when in the running condition.
Gauge glass guards
The gauge glass guard should be kept clean. When the guard is being cleaned in place, or removed for cleaning, the gauge should be temporarily shut-off.
Make sure there is a satisfactory water level before shutting off the gauge and take care not to touch or knock the gauge glass. After cleaning, and when the guard has been replaced, the gauge should be tested and the cocks set in the correct position.
The gauge glass should be thoroughly overhauled at each annual survey. Lack of maintenance can result in hardening of packing and seizure of cocks. If a cock handle becomes bent or distorted special care is necessary to ensure that the cock is set full open. A damaged fitting should be renewed or repaired immediately. Gauge glasses often become discoloured due to water conditions; they also become thin and worn due to erosion. Glasses, therefore, should be renewed at regular intervals.
A stock of spare glasses and cone packing should always be available in the boiler house. Remember:
• If steam passes are choked a false high water level may be given in the gauge glass. After the gauge has been tested a false high water level may still be indicated.
• If the water passages are choked an artificially high water level may be observed due to steam condensing in the glass. After testing, the glass will tend to remain empty unless the water level in the boiler is higher than the top connection, in which case water might flow into the glass from this connection.
• Gauge glass levels must be treated with the utmost respect, as they are the only visual indicator of water level conditions inside the boiler. Any water level perceived as abnormal must be investigated as soon as it is observed, with immediate action taken to shut down the boiler burner if necessary.
Water level controls
The maintenance of the correct water level in a steam boiler is essential to its safe and efficient operation. The methods of sensing the water level, and the subsequent control of water level is a complex topic that is covered by a number of regulations. The following few Sections will provide a brief overview, and the topic will be discussed in much greater detail later.
External level control chambers
Level control chambers are fitted externally to boilers for the installation of level controls or alarms, as shown in Figure 3.7.11.
The function of the level controls or alarms is checked daily using the sequencing purge valves. With the handwheel turned fully anticlockwise the valve is in the ‘normal working’ position and a back seating shuts off the drain connection. The handwheel dial may look similar to that shown in Figure 3.7.12. Some handwheels have no dial, but rely on a mechanism for correct operation.
The following is a typical procedure that may be used to test the controls when the boiler is under pressure, and the burner is firing:
• Slowly turn the handwheel clockwise until the indicating pointer is at the first ‘pause’ position. The float chamber connection is baffled, the drain connection is opened, and the water connection is blown through.
• Pause for 5 to 8 seconds.
• Slowly move the handwheel further clockwise to full travel. The water connection is shut-off, the drain valve remains open, and the float chamber and steam connections are blown through. The boiler controls should operate as for lowered water level in boiler i.e. pump running and / or audible alarm sounding and burner cut-out. Alternatively if the level control chamber is fitted with a second or extra low water alarm, the boiler should lock-out.
• Pause for 5 to 8 seconds.
• Slowly turn the handwheel fully anticlockwise to shut-off against the back seating in the ‘normal working’ position.
Sequencing purge valves are provided by a number of different manufacturers. Each may differ in operating procedure. It is essential that the manufacturer’s instructions be followed regarding this operation.
Internally mounted level controls
Level control systems with sensors (or probes) which fit inside the boiler shell (or steam drum) are also available. These provide a higher degree of safety than those fitted externally. The level alarm systems may also provide a self-checking function on system integrity.
Because they are mounted internally, they are not subject to the procedures required to blow down external chambers. System operation is tested by an evaporation test to ‘1st low’ position, followed by blowing down to ‘2nd low’ position.
Protection tubes are fitted to discourage the movement of water around the sensor.
Air vents and vacuum breakers
When a boiler is started from cold, the steam space is full of air. This air has no heat value, and will adversely affect steam plant performance due to its effect of blanketing heat exchange surfaces. The air can also give rise to corrosion in the condensate system, if not removed adequately.
The air may be purged from the steam space using a simple cock; normally this would be left open until a pressure of about 0.5 bar is showing on the pressure gauge. An alternative to the cock is a balanced pressure air vent which not only relieves the boiler operator of the task of manually purging air (and hence ensures that it is actually done), it is also much more accurate and will vent gases which may accumulate in the boiler. Typical air vents are shown in Figure 3.7.14.
When a boiler is taken off-line, the steam in the steam space condenses and leaves a vacuum. This vacuum causes pressure to be exerted on the boiler from the outside, and can result in boiler inspection doors leaking, damage to the boiler flat plates and the danger of overfilling a shutdown boiler. To avoid this, a vacuum breaker (see Figure 3.7.14) is required on the boiler shell.