Control of power boilers

Proper control of power boilers is an essential element to assuring personnel safety, protection of property, control of environmental emissions, and efficient operations.

09/21/2018


Learning Objectives

  • 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.

Boiler operations

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.

Figure 1: The dual-fuel burner with flame safeties shown burns natural gas as the primary fuel, with grade no. 2 oil with steam atomization as a backup fuel. All graphics courtesy: CDM Smith 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.

Boiler controls

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.

Figure 2: Shown is 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 for improved NOx emissions.  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.

Figure 3: A replacement economizer (bottom right) is coupled with a new draft damper (middle right) and new flue gas recirculation duct (center). It’s in the final stages of construction.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.


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