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Boiler Types and Classifications

1.2.4 Pumps
1.2.6 Flue
1.2.9 Piping


A gas/oil central heating boiler (heat generator) is like the engine of a car, this provides the heat that the facility needs to warm itself up. The size of the boiler is matched to the size of the facility.

  • If the boiler is oversized, the fuel bills will be excessive.
  • If the boiler is undersized, it may not generate enough heat in winter.


The ideal size for a boiler is one that just copes adequately on the coldest day of the year. Most boilers are oversized by at least 30%. This is due to the way systems used to be calculated with a card calculator. These were always over-calculated "to be on the safe side." Today, the emphasis is on energy conservation, and the fact that heat loss calculations can be done very accurately, means there is no need to oversize. This allows smaller radiators and less water in the system, which in turn, means a smaller boiler and reduced costs for both installation and fuel bills.

The boiler does not directly govern the amount of radiators fitted to the system. It is the power of the pump and circulation of the water through adequately sized pipes that determines the number of radiators you can have. But the total output of all the radiators, pipes, and cylinders determines the size of the boiler.


The boiler is not the heating system; it is only one of the parts in the global heating system. As shown in '''Figure 1''', a heating system consists of four main parts:

  1. Boiler/burner combination (the part producing the heat)
  2. Piping with pumps and valves (the part distributing the heat)
  3. Radiators and convectors (the part emitting the heat to the room)
  4. Control equipment such as room thermostat and outside temperature control (the part controlling room and water temperature)

Different Parts of a Heating System

'''Figure 1: Different Parts of a Heating System'''

Boiler Types and Classifications

There are two general types of boilers: ''fire-tube'' and ''water-tube''. Boilers are classified as "high-pressure" or "low-pressure" and "steam boiler" or "hot water boiler." Boilers that operate higher than 15 psig are called "high-pressure" boilers.

A hot water boiler, strictly speaking, is not a boiler. It is a fuel-fired hot water heater. Because of its similarities in many ways to a steam boiler, the term ''hot water boiler'' is used.

  • Hotwater boilers that have temperatures above 250° Fahrenheit or pressures higher than 160 psig are called ''high temperature hot water boilers''.
  • Hotwater boilers that have temperatures not exceeding 250° Fahrenheit or pressures not exceeding 160 psig are called ''low temperature hot water boiler''s.


Heating boilers are also classified as to the method of manufacture, i.e., by casting (cast iron boilers) or fabrication (steel boilers). Those that are cast usually use iron, bronze, or brass in their construction. Those that are fabricated use steel, copper, or brass, with steel being the most common material.


Steel Boilers

'''Steel boilers''' are generally divided into two types: ''firetube'' and ''water-tube''.


Fire-tube Boilers

In fire-tube boilers, combustion gases pass through the inside of the tubes with water surrounding the outside of the tubes. The advantages of a fire-tube boiler are its simple construction and less rigid water treatment requirements.

The disadvantages are the excessive weight-per-pound of steam generated, excessive time required to raise steam pressure because of the relatively large volume of water, and inability to respond quickly to load changes, again, due to the large water volume.

The most common fire-tube boilers used in facility heating applications are often referred to as ''scotch'' or ''scotch marine'' boilers, as this boiler type was commonly used for marine service because of its compact size (fire-box integral with boiler section).

The name "fire-tube" is very descriptive. The fire, or hot flue gases from the burner, is channeled through tubes ('''Figure 2''') that are surrounded by the fluid to be heated. The body of the boiler is the pressure vessel and contains the fluid. In most cases, this fluid is water that will be circulated for heating purposes or converted to steam for process use.

Fire-tube Boiler Gas Flow

'''Figure 2: Fire-tube Boiler Gas Flow'''

Every set of tubes that the flue gas travels through, before it makes a turn, is considered a "pass." So, a three-pass boiler will have three sets of tubes with the stack outlet located on the rear of the boiler. A four-pass boiler will have four sets and the stack outlet at the front.

Fire-tube boilers are:

  • Relatively inexpensive
  • Easy to clean
  • Compact in size
  • Available in sizes from 600,000 btu/hr to 50,000,000 btu/hr
  • Easy to replace tubes
  • Well suited for space heating and industrial process applications


Disadvantages of fire-tube boilers include:

  • Not suitable for high pressure applications 250 psig and above
  • Limitation for high capacity steam generation

Water-tube Boilers

In a water-tube boiler ('''Figure 3'''), the water is inside the tubes and combustion gases pass around the outside of the tubes. The advantages of a water-tube boiler are a lower unit weight-per-pound of steam generated, less time required to raise steam pressure, a greater flexibility for responding to load changes, and a greater ability to operate at high rates of steam generation.

Water-tube Boiler

'''Figure 3: Water-tube Boiler'''

A water-tube design is the exact opposite of a fire-tube. Here, the water flows through the tubes and is encased in a furnace in which the burner fires. These tubes are connected to a steam drum and a mud drum. The water is heated and steam is produced in the upper drum.

Large steam users are better suited for the water-tube design. The industrial water-tube boiler typically produces steam or hot water primarily for industrial process applications, and is used less frequently for heating applications. The best gauge of which design to consider can be found in the duty in which the boiler is to perform.

Water-tube boilers:

  • Are available in sizes far greater than a fire-tube design , up to several million pounds-per-hour of steam
  • Are able to handle higher pressures up to 5,000 psig
  • Recover faster than their fire-tube cousin
  • Have the ability to reach very high temperatures

Disadvantages of the water-tube design include:

  • High initial capital cost
  • Cleaning is more difficult due to the design
  • No commonality between tubes
  • Physical size may be an issue

Cast Iron Boilers

Cast iron boilers ('''Figure 4''') are made in three general types: horizontal-sectional, vertical-sectional, and one-piece. Most of the sectional boilers are assembled with push nipples or grommet type seals, but some are assembled with external headers and screw nipples. Horizontal-sectional, cast iron boilers are made up of sections stacked one above the other, like pancakes, and assembled with push nipples. Vertical-sectional, cast iron boilers are made up of sections standing vertically, like slices in a loaf of bread. One-piece cast iron boilers are those in which the pressure vessel is made as a single casting.

Cast Iron Boiler

'''Figure 4: Cast Iron Boiler'''

Steam and Condensate Boiler System

Boilers are generally used to provide a source of steam or hot water for facility heating and process needs.

In steam and condensate systems ('''Figure 5'''), heat is added to water in a boiler causing the water to boil and form steam. The steam is piped to points requiring heat, and as the heat is transferred from the steam to the building area or process requiring heat, the steam condenses to form condensate. In some very low-pressure saturated steam heating applications, the steam distribution piping may be sized to slope back to the boiler so that the steam distribution piping also acts as the condensate return piping (single-pipe system).

Steam and Condensate Boiler System

'''Figure 5: Steam and Condensate Boiler System'''

In other low-pressure applications, there may be steam supply piping and condensate return piping (two-pipe system), although the condensate system is open to the steam system. In typical packaged steam boiler operations, the boiler system may generate steam at about 150 psig for distribution throughout the facility and may be lowered to the operating pressure of equipment supplied through point-of-use pressure reducing stations. As heat is transferred from the steam, condensate is formed which collects in discharge legs until enough condensate is present to operate a trap that isolates the steam distribution system from the condensate system. In common facility heating applications, the condensate system is at atmospheric pressure and the system is arranged to drain the condensate to a central condensate receiver, or into local smaller receivers that pump the condensate back to the central condensate receiver.

Hydronic Boiler System

A boiler is used to heat water that is circulated through a closed loop piping system for general facility and service water heating. Low-temperature systems generally operate below 200° Fahrenheit Medium-temperature systems generally operate at temperatures between 200 and 250° Fahrenheit.

A feature of hot water systems ('''Figure 6)''' is an expansion tank to accommodate the expansion of the water in the system as the water is heated. The expansion tank, when piped into the system on the suction side of the circulating pumps, also pressurizes the system to prevent flashing in the circulating pump, piping, and piping components. In many low- and medium-pressure systems, pressurization is maintained by flash steam in the expansion tank. In a few hot water systems, pressurization is maintained by maintaining a compressed gas blanket above the water level in the expansion tank.


'''Figure 6: Hydronic (Hot Water) Boiler System'''

High-temperature hot water systems, which operate above 250° Fahrenheit, are basically the same as hot water systems that operate below 250°F. High-temperature systems are generally installed when a process requires the higher temperature, a number of locations require small quantities of low-pressure steam that the high-temperature hot water can generate in a local converter, or high-temperature drop equipment can be used at end use points to minimize the size of water circulation piping required.

Most facility boiler systems are fired using a combustible gas (typically natural gas or propane) or fuel oil. In many facilities, the boilers are designed to fire both a combustible gas fuel and a fuel oil. In these facilities, the combustible gas fuel is generally natural gas that is considered the primary fuel, and fuel oil is considered to be the backup fuel.

Boiler System Major Components

Boiler systems are comprised of the major components described below and shown in '''Figure 7'''.

Boiler Room Schematic

'''Figure 7: Boiler Room Schematic'''

Feedwater Heaters

Feedwater heaters are energy recovery devices generally found only in large steam generating plants where all of the steam generated is not reduced to condensate by the steam user. This "waste steam" is reduced to condensate for return to the boiler in the feedwater heater. The boiler feedwater is used as a cooling medium to reduce the steam to condensate, which increases the temperature of the feedwater and, thereby, increases the thermal efficiency of the boiler.

Fuel Heater

Many boilers firing heavy fuel oil require fuel heaters to reduce the fuel viscosity, so the fuel can be atomized by the burner system for complete combustion.


A deaerator is a special case of feedwater heater that is designed to promote the removal of non-condensable gases from the boiler feedwater. The principal gases of concern are oxygen, carbon dioxide, and ammonia, which are major contributors to boilers, and steam and condensate piping corrosion problems. In small steam plants, a portion of the steam generated by the boiler is used to operate the deaerator if "waste steam" is not available. Failure to maintain and properly operate the deaerator can lead to early failure of the boiler, steam using equipment, and the steam and condensate piping.


In most hot water systems, the system circulating pumps are electric motor-driven, end suction centrifugal pumps. In steam systems, the condensate return pumps are typically electric motor-driven, end suction, centrifugal or turbine-type pumps. Feedwater pumps are generally electric motor-driven, multiple-stage, end suction centrifugal pumps. The shutoff head of the pump must be greater than the steam or hot water system operating pressure.

Combustion Air Blowers

In many packaged boiler installations, the combustion air fan is designed and provided by the boiler manufacturer and is integral with the boiler housing. In installations where a stand-alone fan is provided, low-pressure centrifugal blowers are commonly used. An important characteristic of the blower is the ability to maintain a relatively constant air pressure over a wide range of airflows.


Flues (boiler firebox exhaust duct or boiler discharge stack) must be large enough to conduct the products of combustion away from the boiler with a minimum of duct friction loss. Flues may be fabricated from any material suitable for the operating temperature and pressure. Common materials of construction associated with packaged boiler installations are carbon steel and stainless steel.


An economizer is an energy recovery device that uses the hot exhaust gases from the boiler (waste heat) to heat combustion air or feedwater.

Steam Traps

Steam traps are installed throughout steam systems to remove condensate (spent steam), air, and non-condensable gases from the steam system. There are five types of steam traps in general use today, as described below.

  1. The heart of a '''balanced pressure thermostatic trap''' is the flexible bellows that moves the valve head from its seat to discharge the condensate. The bellows is filled with a volatile fluid and hermetically sealed. The fluid has a pressure-temperature relationship that closely parallels, but is approximately 10 degrees Fahrenheit below that of steam.
  2. The '''liquid expansion steam trap''' has for its operating element a liquid-filled cartridge. Within this cartridge is a hermetically sealed bellows which is attached to the valve head and plunger.
  3. '''Float and thermostatic steam traps''' provide immediate and continuous discharge of condensate, air, and non-condensables from a steam system as soon as they reach the trap. The trap consists of a ball float connected by a lever assembly to the main valve head. As condensate reaches the trap, the ball float rises, positioning the valve to discharge the condensate at the same rate as it reaches the trap.
  4. The '''inverted bucket steam trap''' is a type of trap with an inverted bucket attached to the valve head by a lever mechanism and operates to open and close the trap. When condensate enters the trap, a water seal is formed around the bottom of the inverted bucket which, since it is filled with air, becomes buoyant and rises and closes the trap. A small hole in the top of the inverted bucket allows air to escape with condensate taking its place inside the bucket. The inverted bucket loses its buoyancy and sinks to the trap bottom, opening the valve to discharge the condensate.
  5. '''Thermodynamic steam traps''' are a type of steam trap that responds to differences in kinetic energy between steam and condensate to open and close the valve for discharging condensate.


Piping two-inches and smaller used in steam and hot water systems is typically Schedule 80, American Society for Testing and Materials (ASTM) A 106, Standard Specification for Seamless Carbon Steel Pipe for High-Temperature Service (1999), Grade B, steel pipe with threaded joints and carbon steel fittings. Piping larger than two-inches is typically standard weight, ASTM A 106, Grade B, steel pipe with flanged joints and carbon steel fittings.

Boiler Room Definition/Terminology

There are many terms used in a discussion of boilers, the following is a list of some of the most common terms. There is a glossary provided that covers some of the other terms that may be also used. This section also contains some of the basic valves that are used on boiler and boiler systems, along with some of the common HVAC and piping symbols.

Boiler and Heating Terms

'''BTU''' – British Thermal Unit; the amount of energy required to raise one pound of water one degree Fahrenheit.

1,000 BTU


1 lb. of steam

150 BTU


1 sq. ft. of hot water

34.5 lbs steam/hr


1 boiler horsepower

1 boiler hp


140 sq. ft. steam radiation

240 BTU


1 sq. ft. of steam

34,500 BTU


1 boiler horsepower

1 gallon of #2 oil


140,000 BTU

1 cu. ft. LP gas


2,550 BTU

1 cu. ft. natural gas


1,000 BTU



3,413 BTU

1 therm. natural gas


100,000 BTU


'''Boiler'''- An enclosed vessel in which water is heated and circulated, either as hot water or as steam, for heating or power. A container, such as a kettle, is used for boiling liquids. In our context, a boiler is "a piece of heating equipment that is used to heat water for use in a hot water-based heating system." Examples of hot water-based heating systems include under-floor radiant heat, baseboard hot water, and radiator-based systems. A ''furnace'' is a piece of heating equipment that is used in a hot air-based heating system to heat the air that is circulated through the ductwork.

'''Burner'''- One that burns, especially:

  • A device, as in a furnace, stove, or gas lamp, that is lighted to produce a flame
  • A device on a stovetop, such as a gas jet or electric element that produces heat
  • A unit, such as a furnace, in which something is burned such as an oil burner
  • An incinerator

'''AFUE'''- Annual Fuel Utilization Efficiency, a standard government rating for energy efficiency.

'''Air Conditioner'''- A device used to decrease the temperature and humidity of air that moves through it.

'''Anode Rod'''- A sacrificial metal used to protect against corrosion in a hot water heater.

'''Baseboard Heating'''- Heating elements around the perimeter of a room used to give off heat produced by hot water circulating through them.

'''Blower'''- A unit used with a furnace to circulate air through a network of ducts.

'''BTU/h'''- (British Thermal Units per hour) a standard rating for heat transfer capacity.

'''Cast Iron'''- A durable metal with an exceptional capability to hold and transfer heat.

'''Chimney Venting'''- A vertical vent used to transfer exhaust products from a boiler or furnace to the outdoors.

'''Combustion'''- The process of converting fuel into heat; requires oxygen.

'''Convective Heat'''- The natural circulation of air across a heat source to heat the air.

'''Direct Vent'''- A boiler design where all the air for combustion is taken from the outside atmosphere and all exhaust products are released to the outside atmosphere, also known as sealed combustion.

'''Draft Hood'''- A device that prevents a backdraft from entering the heating unit or excessive chimney draw from affecting the operation of the boiler or furnace.

'''Ductless Split A/C Systems'''- A system that cools and dehumidifies air without the use of conventional duct work. The equipment location is split, with the condenser and heat pump outside of the home and the air handler and controls inside.

'''Efficiency Rating'''- The ratio of heat actually generated versus the amount of heat. Theoretically possible from the amount of fuel inputted.

'''Flue'''- The passageway that takes combustion exhaust from the combustion chamber to the flue collector and venting system.

'''Forced Hot Air'''- A furnace system using a blower to circulate air from within the home through the furnace and back into the home (as opposed to gravity circulation).

'''Furnace'''- An enclosure in which energy in a non-thermal form is converted to heat, especially such an enclosure in which heat is generated by the combustion of a suitable fuel.

'''Heater'''- An apparatus that heats or provides heat.

'''Heat Exchanger'''- The part of the boiler or furnace used for transmitting heat from the flame to air or water for heating.

'''Heat Transfer'''- The transmission of heat from the source (flame) to air or water.

'''Heating Capacity'''- The amount of usable heat produced by a heating unit.

'''High-boy'''- A term used to describe a furnace which has a small "footprint" but is tall. The blower is under the heat exchanger.

'''Hot Water Boiler'''- A heating unit that uses water circulated throughout the home in a system of baseboard heating units, radiators, and/or in-floor radiant tubing.

'''Hot Water Heater'''- A unit with its own energy source that generates and stores hot water.

'''Hydronics'''- The science of heating or cooling with water.

'''Indirect Hot Water Storage Tank'''- A unit that works in conjunction with a boiler to generate and store domestic hot water, it does not require its own energy source.

'''In-Floor Radiant Tubing'''- Tubing, typically plastic or rubber, used in conjunction with heated boiler water to heat floors.

'''Low-boy'''- A term used to describe a furnace that has a low profile. The blower is located on the same level plane as the heat exchanger.

'''Low Water Cut-off'''- A device used to shut down a boiler in the event of a low water condition exists.

'''Natural Gas'''- Any gas found in the earth (e.g., methane gas) as opposed to gases which are manufactured.

'''Oil Heating'''- The production of heat by burning oil.

'''Propane'''- A manufactured gas typically used for cooking or heating.

'''Push Nipples'''- Metal sleeves used to join adjacent sections of a boiler.

'''Radiant Heating'''- The method of heating the walls, floors, or ceilings in order to transfer heat to the occupants of a room.

'''Radiator'''- A heating element, typically metal, used in conjunction with water or steam to give off heat.

'''Safety Shut-off Device'''- Any device used to shut down a heating appliance in the event an unsafe condition exists.

'''Sealed Combustion'''- A boiler design where all the air for combustion is taken from the outside atmosphere and all exhaust products are released to the outside atmosphere, also known as ''direct vent''.

'''Steam Boiler'''- A heating unit designed to heat by boiling water, producing steam, and circulating it to radiators or steam baseboard units throughout the home.

'''Stack Damper'''- A device installed in the venting system that will automatically close when the appliance shuts down.

'''Supply Tapping'''- Opening in a boiler by which hot water enters the heating system.

'''Tankless Heater'''- A copper coil submerged into the heated boiler water used to transfer heat to domestic water.

HVAC and Boiler Symbols

'''Figure 8''' and '''Figure 9''' show common HVAC and boiler system symbols.

Common HVAC and Boiler Symbols

'''Figure 8: Common HVAC and Boiler Symbols'''

Common Boiler System Symbols

'''Figure 9: Common Boiler System Symbols'''

Basic Boiler Room Valves and Identification

The following valves are the most common valves found on boilers and boiler systems. Each type is briefly discussed.


Butterfly Valves

A butterfly valve is composed of two semicircular plates hinged on a common spindle, used to permit flow in one direction only. Butterfly valves ('''Figure 10''') are of the quarter-turn family and are so designed because a 90-degree turn of the operator fully opens or closes the valve.

Butterfly Valve

'''Figure 10: Butterfly Valve'''

The valve uses elastomer seats and seals and their surge in popularity can be attributed to these advantages. In some cases, they may be used for non-critical throttling applications. They are lighter in weight than conventional valves. The position of the lever indicates whether they are wide open, partially open, or fully closed.

Butterfly valves are compact and space-saving and easily installed in new piping or as replacements in existing piping. They are easily adapted to lever, manual, gear, electric, or pneumatic operation.

Gate Valves

Gate valves ('''Figure 11''') are, by far, the most widely used in industrial piping. They are used as stop valves – to fully shut off or fully turn on flow – the only job for which gate valves are recommended'''. '''

Gate Valve

'''Figure 11: Gate Valve'''

Gate valves are inherently suited for full open service. Flow moves in a straight line and practically without resistance when the wedge is fully raised. There are two basic designs of gate valves: inside screw stem and outside screw stem.

Seating is perpendicular or at right angles to the line of flow – meets it head on. That is one reason why gate valves are impractical for throttling service. When throttling is necessary, globe valves should be used. Repeated movement of the wedge near the point of closure, under high velocity flow, may create a drag on the seating surfaces and cause galling or scoring on the downstream side. A slightly opened wedge may also cause turbulent flow with vibration and chattering of the wedge. A gate valve usually requires more turns to open it fully. Also, unlike many globe valves, the volume of flow through the valve is not in direct relation to the number of turns of the hand-wheel.

Globe Valves

Unlike the perpendicular seating in gate valves, globe valve ('''Figure 12''') seating is parallel to the line of flow.

Globe Valve

'''Figure 12: Globe Valve'''

All contact between seat and disc ends when flow begins. These are advantages for more efficient throttling of flow, with minimum wire drawing and seat erosion. Valve disks and seats in most globe valves can be conventionally repaired or replaced – often without removing the valve body from the line.

Shorter disk travel, with fewer turns required to operate globe valves, saves considerable time and work, also wear on valve parts.

Check Valves

Check valves ('''Figure 13''') are designed to automatically prevent the reversal of flow in a pipeline system. They control the direction of flow, rather than throttling or isolating flow as other valve designs do. Reverse flow may create problems or it could cause damage to equipment. Check valves are sometimes known as ''reflux'' valves.

Check Valve

'''Figure 13: Check Valve'''

There are several basic designs of check valves, a few of which are described below.

Swing Check Valves

This type of check valve uses a hinged mounted disc that swings open and closed with flow. They can be used in the horizontal and vertical (flow upwards) position. The swing check ('''Figure 14''') is the most commonly used design of check valve as it does not restrict flow.

Swing Check Valve

'''Figure 14: Swing Check Valve'''

Lift or Piston Check Valve

This type of check valve ('''Figure 15''') uses a piston rather than a hinge-mounted disc to prevent the reversal of flow. This provides a cushioning effect during the operation of the valve. They must only be used in a horizontal position. Lift check valves, like globe valves, are flow restricting; therefore, they are generally used as companions to globe valves.

Lift Check Valve

'''Figure 15: Lift Check Valve'''

Pressure Relief Valves

''Pressure relief'' is simply a dumping of excess fluid safely into the atmosphere. The excess fluid is that which would cause pressure to exceed the safety limit. The relief/safety valve is the most widely used piece of equipment in this category. However, liquid seals and rupture discs may also be used.

There are two basic kinds of relief valves: self-operated and pilot-operated. The spring-type relief valve is the most widely used. The pilot-operated type is also frequently used, and it offers more precise operation. The pilot-operated type is more frequently used as pressures become higher and capacities greater.

Pressure relief valves are designed to open automatically at a pre-determined set pressure level of system pressure and to achieve a rated relieving capacity at a specified pressure and temperature above the setpoint (overpressure) before re-closing at a pressure below the opening point (blowdown). The simplest and most reliable type of pressure relief valve, even some four-hundred years on from the first design, is the spring-loaded design ('''Figure 16''') where a spring force opposes the system pressure acting on the valve disc. When the system pressure rises above the level of the spring force, the valve opens. This valve type may also be fitted with a bellows ('''Figure 17''') for better emission control performance.

Spring-Type Pressure Relief Valve

'''Figure 16: Spring-Type Pressure Relief Valve'''

Bellow Spring-Type Relief Valve

'''Figure 17: Bellow Spring-Type Relief Valve'''

The significant elements of all spring-loaded pressure relief designs are the springs and the seats. The springs must provide the desired compression rate and a reasonable range of adjustment. They must also fit into the valve bonnet and stay within the design perimeters. The seats may be flat or angled, metal or soft. As the seat area usually defines the load transmitted to and from the spring, very high precision is essential to ensure proper valve operation.

A second type of valve, which is more sophisticated and offers operating advantages in selected applications, is the pilot-operated pressure relief valve ('''Figure 18''').

Pilot-Operated Relief Valve

'''Figure 18: Pilot-Operated Relief Valve'''

This type of valve consists of a main valve and a pilot valve. The pilot responds directly to system pressure and communicates with the main valve. As with the spring-loaded valve, many unique models exist. However, some common design features of pilot-operated pressure relief valves include the sensing line, pilot valve, and main valve.

The sensing line is either connected to the valve inlet or a remote location and conveys system pressure to the pilot.

The pilot valves senses and responds to the system pressure. The pilot is the controlling member of the valve system and determines all of the operating characteristics of the valve. It consists of many small parts and passages and usually relies on elastomer seals for operation.

The main valve operates in response to the pilot and provides the main rated flow capacity to reduce excess pressure.

Regulating Valves

Regulating valves are used in many systems on a boiler from feed flow to fuel flow. Regulators attempt to manage flow by adjusting the flow area available to the fluid. Most designs incorporate a plug or similar element that occupies a portion of a stationary orifice while throttling. In an action often called ''modulation'', the plug or similar element is shifted to increase or decrease available flow area. Flow through a particular size of open orifice area is primarily dependent upon the available pressure difference.

Where the managed parameter is pressure, obviously, flow and pressure are interdependent. While the pressure being maintained may be at a location remote from the valve, there must always be an immediate and inherent relationship maintained among the managed flow, the pressure to which the regulator is responding and the regulator’s response mode. Essentially, the regulator must be able to influence pressure at the measured point and must exercise that influence in the appropriate direction of response.

Where the managed pressure is upstream from the regulating valve, you normally would want the valve to open to permit flow as pressure begins to rise.

Therefore, the valve’s response disposition is "open on rise." The term normally applied to this mode of operation is inlet pressure regulator function. Conversely, if managed pressure is downstream from the valve, the valve’s disposition would be "open on drop," and the normal reference for function is outlet pressure regulator.

Finally, regulators often are called upon to manage operating conditions other than pressure, such as temperature or flow. A regulator will be either direct- or pilot-operated. Direct-operated regulators are simple, single-minded devices intended for a specific application within a particular range of pressures.

Direct-Operated Regulator

In a direct-operated pressure regulator ('''Figure 19'''), managed pressure is applied to some internal surface within the valve. The resulting force is transmitted to the modulating parts of the valve and balanced by a second force applied within the valve. In many cases, the second force is adjustable to establish the setpoint. Movement of the modulating parts inherently involves a change in the balanced forces and thus a change in the managed pressure. The more the demanded flow deviates from what the regulator was experiencing at the time it was first adjusted, the more the pressure must deviate from setpoint.

Direct-Operated Regulator

'''Figure 19: Direct-Operated Regulator'''

Pilot-Operated Regulator

A pilot-operated regulator ('''Figure 20''') consists of a pilot section and a slave section. Two flows are managed by the valve. The first, which is by far the greater proportion of the total flow, is managed by the plug and orifice in the slave section. The second, the pilot stream, is managed by the pilot section. The pilot section may have its inlet connected to the immediate upstream side of the main valve or may source its flow from a higher pressure space. The pilot section’s modulating portion is available in all dispositions and functions of direct-operating regulators.

Pilot-Operated Regulator

'''Figure 20: Pilot-Operated Regulator'''

Boiler Blowdown

Blowdown of steam boilers is very often a highly neglected or abused aspect of routine boiler room maintenance. The purpose of boiler blowdown is to control solids in the boiler water. Blowdown protects boiler surfaces from severe scaling or corrosion problems that can result otherwise.

There are two types of boiler blowdowns: ''continuous'' and ''manual''.

A continuous blowdown uses a calibrated valve and a blowdown tap near the boiler water surface. As the name implies, it continuously takes water from the top of the boiler at a predetermined rate. A continuous blowdown is an optional feature and may not be included on your steam boiler; however, all steam boilers should include a means for manual blowdown as standard equipment.

Manual blowdowns are accomplished through tapings at the bottom of the boiler. These openings allow for the removal of solids that settle at the bottom of the boiler. Manual blowdown is also used to keep water level control devices and cutoffs clean of any solids that would interfere with their operation. All steam boilers require manual blowdown whether or not they are supplied with continuous blowdowns.

Frequency of Manual Blowdown

When continuous blowdown is used, manual blowdown is primarily used to remove suspended solids or sludge. The continuous blowdown removes sediment and oil from the surface of the water along with a prescribed amount of dissolved solids.

When surface or continuous blowdown is not used, manual blowdown is used to control the dissolved or suspended solids in addition to the sludge.

In practice, the valves of the bottom blowdown are opened periodically in accordance with an operating schedule and/or chemical control tests. From the standpoint of control, economy, and results, frequent short blows are preferred to infrequent lengthy blows. The length and frequency of the blowdown is particularly important when the suspended solids content of the water is high. With the use of frequent short blows, a more uniform concentration of the pressure vessel water is maintained.

In cases where the feedwater is exceptionally pure, or where there is a high percentage of return condensate, blowdown may be employed less frequently since less sludge accumulates in the pressure vessel. When dissolved and/or suspended solids approach or exceed predetermined limits, manual blowdown to lower the concentrations is required.

It is generally recommended that a steam boiler be blown down at least once in every eight-hour period, but frequency may vary depending upon water and operating conditions. The blowdown amounts and schedule should be recommended by your local Cleaver-Brooks authorized representative.

A hot water boiler does not normally include openings for surface blowdown and bottom blowdown since blowdowns are seldom practiced. Always be alert to system water losses and corresponding amount of raw water makeup. A water meter is recommended for water makeup lines.

Proper blowdown is performed as follows:

  1. Blowdown should be done with the boiler under a light load.
  2. Open the blowdown valve nearest the boiler first. This should be a quick-opening valve.
  3. Crack open the downstream valve until the line is warm. Then open the valve at a steady rate to drop the water level in the sight glass 1/2 inch. Then close it quickly being sure that the handwheel is backed off slightly from full close to relieve strain on the valve packing.
  4. Close the valve nearest the boiler.


Repeat the above steps if the boiler has a second blowdown tapping. Water columns should be blown down at least once a shift to keep the bowls clean. Care should be taken to prevent low water shutdown if this will affect process load.

Please keep in mind that all blowdown piping should be checked once a year for obstructions.

Blow Down Purpose

The purpose of blow down is to control the amount of solids and sludge in the boiler water. The blow down process involves partially draining the boiler to remove sludge and to maintain pre-determined concentration levels of solids.

As the water is turned into steam, the solids remain behind. Unless there is 100% condensate return, the solid content tends to build up when the boiler takes on make-up water. On hot water systems, there is generally no make-up water. Therefore, the solid concentration remains constant and no blow down is needed.

The amount and frequency of blow down differs for each boiler application and should be determined by your water management consultant. Blow down is affected by the type of boiler, operating pressure, water treatment, and the amount and quality of make-up water.

Blow down piping should be at least the same size as the blow down tapping on the boiler. Blow down valves should be sized in accordance with the ASME code and piped to a safe point of discharge. There should be either two slow opening valves or one quick opening valve and one slow opening valve piped in series. A slow opening valve is defined as needing five complete 360 degree turns to go from fully closed to fully open. A quick opening valve goes from fully closed to fully open in one complete motion. In the case of one quick and one slow opening valve, the quick opening valve should be located closest to the boiler. If possible, the blow down valves should be piped on the same side of the boiler as the water column gauge glass.

To blow down the boiler:

  • Open the quick opening valve (valve closest to the boiler) first.
  • Open the slow opening valve last.
  • Blow down the boiler for the required amount of time, per your water management consultant, by opening and then closing the slow opening valve.

'''Remember''': Pay close attention to the water level in the gauge glass. Certain loads may require several blow down cycles of short duration to maintain proper water level in the boiler.

  • Close the slow opening valve first.
  • Close the quick opening valve (the valve closest to the boiler) last.
  • Open the slow opening valve again to drain the line between the quick and slow opening valve.
  • Close the slow opening valve again and double-check for tight shutoff after the valve has cooled off.

'''NEVER''' pump the quick opening valve to blow down the boiler! This may cause water hammer, which could damage piping and valves and may cause personal injury. Also, NEVER leave an open blow down valve unattended!

'''Remember: The quick opening valve (the valve closest to the boiler) is opened first and closed last, which ensures its protection from the wear associated with blow down. This will make this valve more reliable so maintenance and repair can be performed on the slow opening valve furthest from the boiler, without draining the boiler.

Boiler Water Quality Recommendations

Refer to '''Table 1''' for recommended boiler water quality for total dissolved solids (TDS), alkalinity, and hardness.

Table 1: Boiler Water Quality Recommendations at Increasing Pressures

Boiler Steam Pressure (psi)

Maximum TDS (ppm)

Maximum Alkalinity (ppm)

Maximum Hardness (ppm)

Low – 300




301 – 450




451 – 600




601 – 750




751 – 900




901 – 1000




1001 – 1500




1501 – 2000




2001 – 3000





'''Proper Feedwater Treatment is an absolute necessity!'''

Unless your boiler receives water of proper quality, the boiler’s life will be needlessly shortened. A steam plant’s water supply may originate from rivers, ponds, underground wells, etc. Each water supply source requires a specific analysis. Depending upon this analysis, various pretreatment methods may be employed to prepare makeup water for your boiler feedwater system.

General Information on Water Treatment

Suspended solids represent the undissolved matter in water, including dirt, silt, biological growth, vegetation, and insoluble organic matter. When minerals dissolve in water, ions are formed. The sum of all minerals or ions in the water in the total dissolved solids or the TDS.

Iron can be soluble or insoluble. Insoluble iron can clog valves and strainers and can cause excessive sludge build-up in low lying areas of a water system. It also leads to boiler deposits that can cause tube failure. Soluble iron can interfere in many processes, such as printing or the dying of cloth. In domestic water systems, porcelain fixtures can be stained by as little as 0.25 ppm of iron.

''Water hardness'' is "the measure of calcium and magnesium content as calcium carbonate equivalents." Water hardness is the primary source of scale in boiler equipment.

Silica in boiler feedwater can also cause hard dense scales with a high resistance to heat transfer.

''Alkalinity'' is "a measure of the capacity of water to neutralize strong acid." In natural waters, the capacity is attributable to bases, such as bicarbonates, carbonates, and hydroxides; as well as silicates, borates, ammonia, phosphates, and organic bases. These bases, especially bicarbonates and carbonates, break down to form carbon dioxide in steam, which is a major factor in the corrosion of condensate lines. Alkalinity also contributes to foaming and carryover in boilers.