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Now that the basic structure of a boiler has been explained, boiler fittings (fig. 1-6) and the operation or function of various devices, such as controls, valves, and try cocks, must be presented. A sufficient number of essential boiler fittings and accessories are discussed in this section to provide a background for further study. As a reminder, and in case you should run across some unit or device not covered in this text, check the manufacturer's manual for information on details of its construction and method of operation.

Figure 1-6.—Boiler fittings.

The term fittings include various control devices on the boiler. Fittings are vitally important to the economy of operation and safety of personnel and equipment. You must understand fittings if you are to acquire skill in the installation, operation, and servicing of steam boilers.

All boilers require boiler fittings to operate safely. The American Society of Mechanical Engineers (ASME) requires all boiler fittings to be made of materials that withstand the pressure and temperatures that boilers are subject to. All of the boiler fittings discussed are important and must be operated and maintained properly to operate a boiler safely.


An air cock is located in the uppermost steam space of a boiler, as shown in item 7 in figure 1-6. This design allows for air to enter and escape during filling and draining of the boiler. Before firing a cold boiler with no steam pressure, the air cock is opened to allow air to escape during the heating of the water. When steam begins to come out of the air cock piping, close the valve.


Chimneys are necessary for discharging the products of combustion at an elevation high enough to comply with health requirements and to prevent a nuisance because of low-flying smoke, soot, and ash. A boiler needs a draft to mix air correctly with the fuel supply and to conduct the flue gases through the complete setting. The air necessary for combustion of fuel cannot be supplied normally by a natural draft. Therefore, draft fans may be used to ensure that the air requirements are properly attained. Two types of draft fans used on boilers are forced-draft and induced-draft fans. They are damper controlled and usually are driven by an electric motor.

The forced-draft fan forces air through the fuel bed, or fuel oil burner, and into the furnace to supply air for combustion. The induced-draft fan draws gases through the setting, thus facilitating their removal through the stack. Breechings (see item 1 in fig. 1-6) are used to connect the boiler to the stack. They are usually made of sheet steel with provision for expansion and contraction. The breaching may be carried over the boilers, in back of the setting, or even under the boiler room floor. Keep breechings as short as possible and free from sharp bends and abrupt changes in area. The cross-sectional area should be approximately 20 percent greater than that of the stack to keep draft loss to a minimum. A breaching with a circular cross section causes less draft loss than one with a rectangular or square cross section.


Blowdown valves on boilers are located on the water column and on the lowest point of the water spaces of the boiler (see items 2, 5, 10, and 11 in fig. 1-6). The blowdown valves on a boiler installed at the bottom of each water drum and header are used to remove scale and other foreign matter that have settled in the lowest part of the water spaces. Boilers are also blown down to control concentration ofdissolved and suspended solids in boiler water. The water column blowdown permits removal of scale and sediments from the water column. Additionally, some boilers have what is called a surface blowdown. The surface blowdown is located at the approximate water level so as to discharge partial steam and water. The surface blowdown removes foaming on the top of the water surface and any impurities that are on the surface of the water.


Fusable plugs are used on some boilers to provide added protection against low water. They are constructed of bronze or brass with a tapered hole drilled lengthwise through the plug. They have an even taper from end to end. This tapered hole is filled with a low-melting alloy. consisting mostly of tin. There are two types of fusible plugs—fire actuated and steam actuated.

The fire-activated plug is filled with an alloy of tin, copper. and lead with a melting point of 445°F to 450°F. It is screwed into the shell at the lowest permissible water level. One side of the plug is in contact with the tire or hot gases, and the other side is in contact with the water (see item 9). As long as the plug is covered with water, the tin does not melt. When the water level drops below the plug, the tin melts and blows out. Once the core is blown out, a whistling noise will warn the operator. The boiler then must be taken out of service to replace the plug.

The steam-actuated plug is installed on the end of a pipe outside the drum. The other end of the pipe. which is open, is at the lowest permissible water level in the steam drum. A valve is usually installed between the plug and the drum. The metal in the plug melts at a temperature below that of the steam in the boiler. The pipe is small enough to prevent water from circulating in it. The water around the plug is much cooler than the water in the boiler as long as the end of the pipe is below the water level. However, when the water level drops below the open end of the pipe, the cool water runs out of the pipe and steam heats the plug. The hot steam melts and blows the tin out, allowing steam to escape from the boiler warning the operator. This type of plug can be replaced by closing the valve in the piping. It is not necessary to take the boiler out of service to replace the plug.

Fusible plugs should be renewed regularly once a year. Do not refill old casings with new tin alloy and use again. Always use a new plug.


A water column (fig. 1-7) is a hollow vessel having two connections to the boiler. Water columns come in many more designs than the two shown in figure 1-7; however, they all operate to accomplish the same principle. The top connection enters the steam drum of the boiler through the top of the shell or drum. The water connection enters the shell or head at least 6 inches below the lowest permissible water level. The purpose of the water column is to steady the water level in the gauge glass through the reservoir capacity of the column. Also, the column may eliminate the obstruction on small diameter, gauge-glass connections by serving as a sediment chamber.

Figure 1-7.—Typical water columns.

The water columns shown are equipped with high- and low-water alarms that sound a whistle to warn the operator. The whistle is operated by either of the two floats or the solid weights shown in figure 1-7.

Water Level Control

The water level control not only automatically operates the boiler feed pump but also safeguards the boiler against low water by stopping the burner. Various types of water level controls are used on boilers. Boilers frequently are equipped with a float-operated type, a combination float and mercury switch type, or an electrode probe type ofautomatic water level control. Each of these types is described below.

The float-operated type of feedwater control, similar in design to the feedwater control shown in figure 1-8, is attached to the water column. This control uses a float, an arm, and a set of electrical contacts. As a low-water cutoff, the float rises or lowers with the water level in an enclosed chamber. The chamber is connected to the boiler by two lines, which allow the water and steam to have the same level in the float chamber as in the boiler. An arm and linkage connects the float to a set of electrical contacts that operate the feedwater pump when the water lowers the float. When the water supply fails or the pump becomes inoperative and allows the water level to continue to drop, another set of contacts operates an alarm bell, buzzer, or whistle, and secures the burners.

The combination float and mercury switch type of water level control shown in figure 1-8 reacts to changes made within a maintained water level by breaking or making a complete control circuit to the feedwater pump. It is a simple two-position type control, having no modulation or differential adjustment or setting. As all water level controllers should be, it is wired independently from the programmer. The control is mounted at steaming water level and consists of a pressurized float, a pivoted rocker arm, and a cradle-attached mercury switch. The combination float and mercury switch type of water level control functions as follows: As the water level within the boiler tends to drop, the float lowers. As the float lowers, the position of the mercury switch changes. Once the float drops to a predetermined point, the mercury within the tube runs to its opposite end. This end contains two wire leads, and when the mercury covers both contacts, a circuit is completed to energize the feedwater pump. The pump, being energized, admits water to the boiler. As the water level within the boiler rises, the float rises. As the float rises, the position of the mercury switch changes. Once the float rises to a predetermined point, the mercury runs to the opposite end of its tube, breaking the circuit between the wire leads and securing the feedwater pump. The feedwater pump remains off until the water level again drops low enough to trip the mercury switch.

Figure 1-8.—Combination float and mercury switch type of feedwater control.

The electrode probe type of feedwater control (fig. 1-9) and low-water cutoff consists of an electrode assembly and a water level relay. The electrode assembly contains three electrodes of different lengths correspondin g to high. low, and burner cutout in the boiler drum.

Figure 1-9.—Electrode type of water level control.

To understand the operation of a boiler circuit, refer to figures 1-9 and 1-10 as you read the information in table 1-1. Although this information is not complete. it is presented here to acquaint you with the operation of the electrode type of boiler water level control.

Try Cocks

Figure 1-6.—Boiler fittings.
(Repeated here for convenient reference)

The location of the try cocks is shown as item 6 in figure 1-6. The purpose of the try cocks is to prove the water level in the boiler. You may see water in the gauge glass. but that does not mean that the water level is at that position in the boiler. lf the gauge glass is clogged up. the water could stay in the glass giving a false reading. The try cocks, on the other hand, will blow water. steam. or a mixture of steam and water out of them when they are manually opened. When steam is discharged from the lowest try cock, you have a low-water condition.

Table 1-1.—Operation of a Boiler Circuit

Figure 1-10.—A typical boiler circuit.



When the water level is proved using the try cocks, personnel should stand off to the side of the try cocks away from the discharge. The discharged hot steam or very hot water can cause severe burns.

Gauge Glass

The gauge glass is located on the water column, as shown in figure 1-6, item 3. The gauge glass allows the boiler operator to see the water level in the boiler. Normally there are two valves associated with the gauge glass. One valve is located at the top and one is located at the bottom of the gauge glass. These two valves, named gauge cock valves (fig. 1-6, item 2). secure the boiler water and steam from the gauge glass. Another valve (fig. 1-6, item 4) located in line with the gauge glass, is used to blow the gauge glass down.

Figure 1-6.—Boiler fittings.
(Repeated here for convenient reference)


The safety valve shown in figure 1-11 is the most important of boiler fittings. It is designed to open automatically to prevent pressure in the boiler from increasing beyond the safe operating limit. The safety valve is installed in a vertical position and attached directly to the steam space of the boiler. The location can be seen in figure 1-6, item 8. Each boiler has at least one safety valve; when the boiler has more than 500 square feet of heating surface, two or more valves are required. There are several different types of safety valves in use but all are designed to open completely (POP) at a specific pressure and to remain open until a specified pressure drop (blowdown) has occurred. Safety valves must close tightly. without chattering, and must remain tightly closed after seating.

Figure 1-11.—A spring-loaded safety valve.

To understand the difference between boiler safety valves and ordinary relief valves is important, The amount of pressure required to lift a relief valve increases as the valve lifts, because the resistance of the spring increases in proportion to the amount of compression. When a relief valve is installed on a steam drum, it opens slightly when the specified pressure was exceeded; a small amount of steam is discharged; and then the valve closes again. Thus a relief valve on a steam drum is constantly opening and closing; this repeated action pounds the seat and disk and causes early failure of the valve. Safety valves are designed to open completely at a specified pressure to overcome this difficulty.

Several different types of safety valves are used on boilers; however, they all lift on the same general principle. In each case, the initial lift of the valve disk, or feather, is caused by static pressure of the steam acting upon the disk. or feather. As soon as the valve begins to open, however, a projecting lip, or ring, of the larger area is exposed for the steam pressure to act upon. The resulting increase in force overcomes the resistance of the spring. and the valve pops; that is, it opens quickly and completely. Because of the larger area now presented, the valve reseats at a lower pressure than that which caused it to lift originally.

Lifting levers are provided to lift the valve from its seat (when boiler pressure is at least 75 percent of that at which the valve is set to pop) to check the action and to blow away any dirt from the seat. When the lifting lever is used, raise the valve disk sufficiently to ensure that all foreign matter is blown from around the seat to prevent leakage after being closed.

The various types of safety valves differ chiefly as to the method of applying compression to the spring, the method of transmitting spring pressure to the feather, or disk, the shape of the feather. or disk, and the method of blowdown adjustment. Detailed information on the operation and maintenance of safety valves can be found in the instruction books furnished by the manufacturers of this equipment.


The steam injector (fig. 1- 12) is a boiler feed pump that uses the velocity and condensation of a jet of steam from the boiler to lift and force a jet of water into the boiler. This injection of water is many times the weight of the original jet of steam.

Figure 1-12.—A cross-sectional view of a steam injector.

The injector is used to some extent in boiler plants as an emergency or standby feed unit. It does not feed very hot water. Under the best conditions, it can lift a stream of water (that has a temperature of 120°F) about 14 feet.

The installation of an injector is not a difficult operation because the unit is mounted on the side of the boiler. The four connections (fig. 1-13) to the injector are as follows:

1. The discharge line to the boiler feedwater inlet
2. . The steam supply line from the boiler
3. . The water overflow line
4. The water supply line from the reservoir

The controls for the injector (fig. 1-13) include the following:

A. Steam supply valve
B. Water supply valve
C. Discharge valve to the boiler
D. Check valve in the discharge line

As you might expect. some degree of skill is needed to start the injector. After the injector begins to operate. however, it continues automatically until shutdown by the operator.

Figure 1-13.—Injector piping.

When starting the injector. first open the water supply valve (fig. 1-13B) about one full turn. Nest quickly turn the steam supply valve (fig. 1-13A) all the way open. At this point, steam rushes into the combining tube of the injector As the steam speeds past the water supply opening. it creates a suction that draws water through the opening into the combining tube. Water and steam are now mixed together inside the injector and the pressure opens a valve that leads to the boiler. Meanwhile. there is an excess of water in the injector; this excess is discharged through the overflow valve. As the next step of the procedure, slowly turn the water supply valve (fig. 1-13B) toward the closed position until the overflow stops. The overflow valve has now closed and all of the water being picked up from the supply line is going into the boiler. Remember, this feedwater system is used on boilers only as a standby method for feeding water.

The water supply should not be hotter than 120°F for the injector to operate. When several unsuccessful attempts are made to operate the injector. it will become very hot and cannot be made to prime. When you should encounter this problem. pour cold water over the injector until it is cool enough to draw water from the supply when the steam valve is opened.


Handholds and manholes provide maintenance personnel access into a boiler to inspect and clean it internally as needed. These handholds and manholes will be covered in depth when boiler maintenance is discussed later in this volume.


Figure 1-14 provides a graphic presentation of important boiler accessories. Refer to it as you study the table 1-2 which gives a brief description of each accessory, its location, and function.

Figure 1-14.—Boiler accessory equipment.

Table 1-2.—Boiler Accessories, Its Location, and Function

1 Boiler  Boiler room Generate steam or hot water in a closed vessel
2 Main steam stop On the steam outlet of a boiler Place the boiler on line or off line
3 Guard valve  On the steam outlet of a boiler directly following the main steam stop valve Guard or backup to the main steam- stop valve
4 Daylight (drain) valve Between the main steam-stop valve Open only when the main steam and and the guard valve guard valves are closed. Indicates if one of the valves is leaking through
5 Main steam line

The line that conveys steam from a boiler to all branch or distribution branches or lines. When a system is supplied by a bank of boilers connected into the same header, the line(s) conveying steam for the boiler(s) to the header

Carry steam from the boiler to the distribution lines
6 Root valve

Installed in branch or distribution lines just off of the main steam line

Isolate a branch or distribution line (serves as an emergency shutoff)
7 Pressure regulating valve (PRV)

Installed as close as practical (after a  reducing station) to the equipment or area it serves

Equipment that requires lower pressure than main steam line pressure (coppers,  dishwashers, steam chest, turbines, etc.)
8 Steam trap

Installed on the discharge side of all steam heating or cooking equipment, dead ends, low points, or at regular intervals throughout a steam system (automatic drip legs)

Automatically drains condensate and prevents the passage of steam  through equipment
9 Drip legs

Provided throughout a system where Remove condensate from a system condensation is most likely to occur, manually such as low spots, bottom of risers, and dead ends

Remove condensate from a system
10 Temperature regulating valve (TRV) Install in the steam supply line close to equipment needing temperature regulation (sensing element is installed at a point where the temperature is to be controlled, such as the hot-water discharge side of a heat exchanger Control steam flow through a vessel or heating equipment
11 Heat exchanger

Locate as close as practical to the source for which it is going to supply heated water or oil

An unfired pressure vessel that contains a tube nest or electrical element. Used to heat oil or water

12 Strainer

Install in steam and water lines just ahead of PRVs, TRVs, steam traps, and pumps

Prevent malfunctions or costly repairs to equipment and components by trapping foreign matter, such as rust, scale, and dirt

13 Condensate line

Return line extends from the discharge side of steam traps to the condensate/makeup feedwater tank

Carry condensed steam back through piping for reuse in the boiler or heating vessel

14 Condensate/makeup tank

Close to the boiler as practical and at a higher level than the boiler feed-pump suction line

Provide storage space for condensate and makeup/feedwater and vent non-condensable gases to the atmosphere

15 Feed pump Supplies water to the boiler as required

Installed between the condensate/makeup/feedwater tank and the boiler sheall or steam drum

16 Feedwater pipe

This line extends from the discharge side of the feedwater pump to the boiler shell or drum (installed below the steaming water level)

Provide feedwater to the boiler when required

17 Relief valve

Between the feed pump and the nearest shutoff valve in the external feed line

Relieve excessive pressure should the external feed line be secured and the feed pump started accidently. A ruptured line or serious damage to the feed pump could occur if there were no relief valve

18 Feed check valve

Between the feed pump and the stop valve in the feed-water pipe

Prevent backflow from the boiler through the feedwater line into the   condensate/feedwater tank during the off cycle of the pump

19 Feed stop valve

In the feedwater line as close to the boiler as possible between the boiler and feed check valve

Permit or prevent the flow of water to the boiler

Q9. Blowdown valves are installed at what location in a boiler?

Q10. How often should the fusible plugs in a boiler be renewed?

Q11. The two connections to the boiler of a water column are at what locations?

Q12. What are the three types of water level controls most often encountered?

Q13. The electrode probe type of feedwater control has what total number of electrode sensors?

Q14. What boiler fitting is considered the most important?

Q15. What is the function of the guard valve on a boiler?

David L. Heiserman, Editor

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Revised: June 06, 2015