Control of power boilers
- Learn the definition of power boilers.
- Understand the correct codes and standards that apply to boiler controls.
- Know the importance of controlling the three basic elements common to all boilers.
Boilers provide a critical role in providing energy to facility operations. Most large boilers provide steam, which can be used for heating of buildings and processes, and are also a useful motive power source. Hot water also can be provided to facilities using boilers.
Due to the high energy intensity available from large boilers, special precautions must be taken to ensure safe operation. A large user of energy boilers also must set up, control, and maintain the boilers properly to ensure the boilers operate at maximum efficiency.
Per the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code-2017 (BPVC), power boilers generate steam greater than 15 psig or water in excess of 160 psig with temperatures greater than 250°F. There are two types of fuel-fired power boilers: watertube and firetube. As the name suggests, a watertube boiler contains the heated fluid within the tubes and the combustion gases are external to the tubes; heat from the combustion gases passes through the metallic tube walls to the internal fluid. Watertube boilers have a wide range on pressure and capacity. The tubes in a watertube boiler generally are arranged in a vertical pattern so steam bubbles generated inside the tubes rise to the top and are collected in a steam drum.
Firetube boilers have larger-diameter tubes located in a large drum. The fire-combustion gases pass through these tubes, heating the water in the large drum. The firetubes are arranged horizontally with the steam formed on the outside of the tube and rising in the large drum and collected at the top of the drum. Firetube boilers generally are limited to 200,000 lb/hour of steam generation and 1,000 psig pressure. A close-up view of a low-fire dual-fuel burner with flame safeties is shown in Figure 1. The dual-fuel burner shown burns grade no. 2 oil with steam atomization and natural gas.
Combustion requires three components: fuel, oxygen, and heat. All three of these must be controlled to efficiently control a boiler, and oxygen is usually provided as a component of air. The rate of fuel being supplied is controlled to match the requirements of the process to maintain the boiler temperature and/or pressure. The rate of oxygen is controlled to provide complete stoichiometric combustion of the fuel. Too little air and not all the fuel is not burned, resulting in pollution including hazardous carbon monoxide and wasted fuel. When excess air is supplied energy is wasted, as the excess air robs heat from the flame. High flame temperatures also cause nitrogen to react with oxygen and this create oxides of nitrogen, commonly referred to as NOx. Heat is provided to initiate combustion via a spark and/or pilot flame. Once combustion is established it is usually self-sustaining.
All boilers require control of three basic elements to operate safely and efficiently. The three elements are water flow, airflow, and fuel flow. For steam boilers, water flow is used to maintain the drum level. Because the tubes are the part of the boiler used to transfer heat from the flue gases to the water, they are exposed to the greatest temperatures. Ensuring that they are submerged in water is vital to preventing them from overheating and failing. Fuel and air are controlled in concert, but the quantity of fuel delivered is a function of the thermal input required to control either the boiler temperature or steam pressure. The air is then controlled to optimize the combustion process.
Boilers producing saturated steam need only control pressure because the temperature is a thermodynamic property of saturated steam. Figure 2 shows a photo of a burner retrofit for a 50,000-lb/hour, 150-psi saturated steam boiler with a new burner assembly with combustion safety controls. In this retrofit, a grade no. 6 residual oil/natural gas burner was replaced with a grade no. 2 distillate oil/natural gas burner with flue gas recirculation (FGR) for improved NOx emissions.
Superheated steam boilers are a special case, as both pressure and temperature need to be controlled. The boiler controls the three basic functions, the saturated steam then passes through coils in the hot-gas stream to further increase the temperature of the steam. The temperature of the final steam is controlled by either controlling the amount of flue gas passing through the superheater coils or slightly overheating the steam, then cooling to the desired temperature by either adding water into the steam-using attemperators-allowing the latent heat of vaporization to cool the steam to the desired temperature-or bypassing saturated steam and mixing it with the superheating steam to achieve the desired temperature. In many cases, it is desirable to have both automatic control and manual overrides of these control functions. Figure 3 shows a picture of a replacement flue gas economizer with a new draft damper.
The critical control elements must be interlocked to shut down the firing of the boiler in certain conditions. These interlocks are for personnel safety and equipment protection. The equipment must be shut down on loss of combustion airflow, abnormal furnace pressure, low water level, loss of flame, high or low fuel pressure, abnormal atomizing fluid conditions, and high temperature and/or steam pressure. The interlock must shut down the equipment regardless of whether the primary control function is in automatic or manual control. A safety interlock must be manually reset after the cause of the failure has been determined and corrected.
Codes and requirements
The primary codes governing boiler controls are ASME CSD-1-2015, Controls & Safety Devices for Automatically Fired Boilers, and NFPA 85: Boiler and Combustion Systems Hazard Code, currently the 2015 edition. CSD-1 applies to boilers up to 12.5 million Btu/h. The lower limit of the scope of power boilers is 400,000 Btu/h; the discussion of controls in CSD for this article is limited to this range. For boilers 12.5 million Btu/h and greater, NFPA-85 is the governing code. Many local and international codes incorporate parts of these two codes into their wording.
The following discussion highlights many of the requirements found in the codes. Refer to the codes for exceptions and, in some cases, more stringent requirements than those described in this article.
Often overlooked requirements of boiler controls are wiring of external devices not provided with the manufacturer-provided controls. One is the requirement for interlocking the boilers with the source of combustion air. The intake dampers or combustion air fans are not integral components of the controls often supplied by the burner manufacturer. Another external control element is the remote shutdown switch. This requirement is defined in CSD-1 CE-110 (b). These elements external to the burner/boiler controls require the specifying engineer to include these interface requirements in the design documents.
Low-water shutdown is so critical for steam boilers that two separately piped devices are required. One of the devices can be automatic, allowing the boiler to restart if the water level is regained. This device functions at a higher water level. The device set for the minimum water level is a manual reset device, requiring the operator to confirm sufficient water is in the boiler to reset the device manually. The water level is also visually indicated, usually with a glass water-column gauge. The devices must be piped without shut-off valves.
For forced-circulation boilers, one or more methods of determining adequate water flow through the boiler is required. The flow can be sensed with a flow-measuring device or derived from the difference between the entering and leaving water temperatures of the boiler.
Steam boilers also require a high-pressure switch. This switch shall interrupt the flow of fuel to the boiler in case of high pressure. This pressure should be below the maximum allowed working pressure and the pressure-relief valve setting. This device is manually reset. The device is installed without valves that could isolate it from the boiler’s internal pressure. Boiler-room audio and visual alarms are recommended for high-pressure cutout controls for boilers covered under ASME CSD-1.
Hot-water boilers also require a high-temperature safety switch. This switch shall interrupt the flow of fuel to the boiler in case of high temperature. This high-temperature limit is below the maximum allowed temperature of the equipment. Each boiler in the system requires a separate switch. This device is manually reset. The device is installed without valves that could isolate it from the boiler water representing the operating temperature.
Fuel trains must meet the requirements of ASME CSD-1 and NFPA 85. States and insurance underwriters may have further requirements for the fuel trains, which exceed the ASME and NFPA codes. Some of these requirements include double-block and vent valves on the gas trains. If air is mixed with the gaseous fuel (such as mixtures of propane/air by a local utility, landfill, or digester gas), a flame arrester may be required in a gas train.
The purpose of the double-block and vent valves are purely for safety of operation. When the burner is not in operation, the vent valve is open and the two block (shut-off) valves are closed. When the burner is firing, the vent valve is closed and the two block (shut-off) valves are open. This design prevents gas from entering the combustion chamber if the upstream gas-valve leaks. Where multiple boilers are operated in parallel, the fuel trains on each individual boiler must have manual isolation from the common fuel source. Multiple boiler installations that have more than one boiler in operation are recommended to have a master boiler control panel (MBCP). This MBCP will provide automatic transition for lead-lag operation and selection of the operating boiler(s). It also is recommended that individual boilers have their own boiler control panels with a manual switch for local and remote control (via the MBCP).
The gas train and all its components require approval of the insurance underwriter, which is usually included with the boiler and/or burner by its manufacturer. The make and model of these components must be replicated in case of replacement to maintain the underwriter’s approval. Custom-designed and -built gas trains may be used for specific applications. However, the components and controls must be reviewed and approved by the underwriting authority. The installed system also must be inspected by an inspector from the underwriting authority before the fuel train is put in service.
All these additional accessories create added pressure drop in the train. This may require designing for higher pressure at the inlet to the gas train or specifying a larger size pipe and gas train to achieve the rated boiler capacity. Gas regulators usually are required to throttle the incoming gas pressure to acceptable limits. These increase in size with oversized gas trains and lower pressure gas. The vent valves and, usually, gas regulators require vent piping to the outside of the building. Some boiler manufacturers are offering “ventless” gas regulators with natural gas trains as an option for specific applications. This reduces the labor and material cost for regulator venting. Figure 4 shows a new gas-train installation for a burner retrofit.
Bidding documents should specify and/or show these items in a manner for contractors to have realistic sizes, quantities of materials and devices, and lengths of vent piping. The specifier also must be careful to not overspecify the details of these accessories with proprietary language in a publicly bid contract. The specifier should confirm the accessories required for a specific gas train with the owner’s insurance underwriter and the specified boiler manufacturers to strike this balance of information.
It is important to note that the requirements of the International Fuel Gas Code-2018 (IFGC) end at the inlet to the gas train. The fuel-gas piping, which includes the pilot fuel, may be provided by a separate contractor than the contractor who provided the boiler. If this is the case, the demarcation of scope should be shown clearly on the bid documents.
Prior to introducing fuel into a power boiler, the furnace must be purged. Most boilers require proof of combustion air to start the boiler and to allow main firing. Then, depending on the type of pilot used, fuel is introduced. For most ignition types, a flame must be established within 4 seconds-if the flame is lost for more than 4 seconds, the fuel source is interrupted. The allowable time, from 1 to 5 seconds, for the fuel valve to close is determined by the heating capacity of the burner.
Boilers greater than 2.5 million Btu/h require lockout with manual reset for flame failure and loss of combustion air. Upon loss of control power, manual reset is required. Most boiler/burner manufacturers require a post-purge cycle and a pre-purge cycle to be used in case of a loss of electrical power, interruption of fuel, or when changing over to a secondary or tertiary fuel source. A boiler control panel that contains all the boiler control, with the exception of the combustion safety controls, is shown in Figure 5.
Oil-fired burners must comply with UL-296 Standard for Oil Burners, UL-726 Standard for Oil-Fired Boiler Assemblies, or UL-2096 Standard for Commercial/Industrial Gas and/or Oil-Burning Assemblies With Emission Reduction Equipment. Oil burners requiring atomizing air or steam require these mediums to be monitored for pressure. Boilers requiring preheated oil also require the temperature to be monitored.
Flame proving for oil burners also is required, though the allowable time for flame initiation and failure is slightly longer than for gas boilers, depending on ignition type and firing rate. Combustion-air interlocks are similar to those of gas-fired units.
For large boilers over 12.5 million Btu/h, alarms are to be generated to notify operators of hazardous conditions and equipment failures or operational failures. The alarms shall be audible and visual. The audible alarm may be silenced while the upset condition still exists, but the visual indication shall remain until the upset condition has been restored to normal.
Emission improvements to an existing boiler often alter the characteristics of the flame. The burner-flame safeties must be retested to ensure the limits of the modified flame. For boilers burning multiple fuel types, the flame detector(s) must correctly sense the flame for the fuel being used.
Another important organization governing boiler controls in the National Board of Boiler and Pressure Vessel Inspectors (NBBI). This important board oversees standards for boiler construction, installation, repair, and boiler-plant accessory equipment. Their standards are used by all boiler inspectors, both government and private, for inspection of boilers and boiler plants.
The NBBI oversees the construction, testing, and stamping of all ASME boilers and pressure vessels in the United States. Safety and pressure-relief valves are tested and certified with the NBBI tags when they are manufactured and when they are rebuilt.
They continually update many of their standards to keep up with new technologies. Boiler-plant items, such as deaerators and boiler feed-water pumps, must be selected to meet their pressure requirements for construction and selected delivery pressures, respectively. Boilers we are addressing in this article require NBBI inspections at least every 2 years, and some yearly, to receive a certificate to operate. These inspections may be completed by government agencies or an insurance underwriter, which are usually insurance companies. All these inspectors are required to be certified by the NBBI.
If these inspections are not completed within the allotted time or ignored, the owner has no insurance if a failure should occur. Simply losing the operation of a boiler may be financially crippling to the owner’s operation. A catastrophic failure is a financial liability to the owners and a hazard to public safety for all damages and injuries caused by the failure. The specifier and designer must be aware of these inspection and code requirements to minimize the client/owner’s liability with a responsible and thorough design.