Back to basics: Boiler selection

Boiler codes and design standards provide the basic guidelines for boiler application, design, construction, and operations.

09/15/2017


This article has been peer-reviewed.Learning objectives:

  • Grasp the codes, standards, and guidelines that assist in specifying boilers and boiler systems.
  • Know the different types of boilers available and understand when to specify each type.
  • Understand how control systems play into the overall boiler design.

Boilers come in various sizes, shapes, and colors. They are used to generate hot water or steam for process or heating applications. This article will focus on steam generation, as requirements for both types are similar until steam is generated within the drum.

The American Society of Mechanical Engineers (ASME) Boiler Pressure Vessel Code (BPVC), Section I, governs the design, fabrication, installation, and operation of fired pressure vessels (vessels in which steam is generated). The BPVC is one of the two primary references that engineers will require when designing boiler systems. The other is NFPA 85: Code on Boiler Combustion Systems and Hazards. NFPA 85 provides the requirements for the fuel and combustion systems that are used to heat the boiler. Compliance with both codes is required for safe boiler operation. Materials will be briefly reviewed, as their selection is governed by the process fluid and its properties.

Figure 1: Pictured is the firing aisle of the new steam plant addition at Iowa State University (ISU) in Ames. ISU replaced three aging coal-fired boilers with three new 150,000 lb/hr natural gas-fired package boilers. Courtesy: Burns & McDonnellA great wealth of knowledge is provided within ASME BPVC Section I. Depending on the boiler type and components, the limits of jurisdiction are defined within Section I and refer to ASME B31.1: Power Piping Code for the remainder of the requirements. The common limits engineers should be aware of are the water inlet and the steam outlet. The incoming water limit for Section I is at the inlet connection of the second isolation valve from the nozzle of the first water component of the boiler. This can either be an economizer or the drum itself. Steam limits vary depending upon the component and ASME BPVC, Section 1 should be reviewed.

NFPA 85 is applicable to boilers with heat inputs of 12.5 MMBtu/h or greater. Another important code in the design and installation of boilers is ASME B31.1. This code is applicable to the piping connecting to the boiler proper that are not under the jurisdiction of ASME BPVC Section I.

The design of boiler systems first requires the engineer to understand the requirements for the unit by understanding the system requirements it will be serving. This would include steam flow, pressure, and temperature requirements necessary to account for system pressure and temperature losses including the anticipated losses up to the boiler connection at the non-return valve. Steam-flow requirements are based on end use, plus losses in the distribution system including steam traps and leaks.

Understanding the system losses and adding them to the actual steam usage plus a growth factor, typically 1% per year, will assist in developing the steam generation required. Working with the boiler supplier, the engineer can select the necessary guaranteed outlet conditions of the boiler.

Piping and materials

Materials of construction will be dependent upon the selected input and output steam/water properties as well as the fuel being consumed. The typical materials used in the industry are A53-B/C and A106-B/C carbon steel for feedwater, fuel gas, and steam. The tubes within the boiler itself are selected by the boiler manufacturer but typically will be carbon steel of seamless construction.

Figure 2: A new central plant was designed and built for Parkland Hospital in Texas. Courtesy: Burns & McDonnellHelical welded tubes are occasionally quoted as they are easier to fabricate. However, care must be taken with their use in environments where liquid or biomass fuels are used since the sulfur, sodium, and chloride contents, when combusted, can damage tubes if left unchecked. Should a superheater section be used or a compact boiler employed with a high heat-release rate, materials can be upgraded with carbon-molybdenum or chromium-molybdenum (chrome-moly). A typical chrome-moly used is P11, which contains ~1.25% chromium. Drum materials are selected by operating conditions, but for package boilers under 250,000 lb/hour operation, it is common to see A516-70 carbon steel.

Fuel supplies

Fuels in use today include natural gas, No. 2 fuel oil, ultra-low sulfur diesel, and solid fuels including coal, biomass, and other materials that can be mixed with those base fuels. Deciding the fuel early in the design is important because the boiler supplier will select the burner based on the selected fuel. In addition to the type of fuel, the quality of the fuel selected will affect burner selection, turndown, and output.

If natural gas is supplied, the incoming pressure, temperature, and higher heating value will need to be provided. If the gas is subject to varying conditions, such as water entrainment, high sulfur content, or other constituent changes, this must be included in the fuels report to the boiler/burner supplier. It will assist with the selection of a burner that meets the needs of the boiler and the need for upstream equipment including heaters, separators, and pressure-control equipment.

Boiler efficiency

Figure 3: The photo shows a natural gas pressure-reducing skid with a start-up train and redundant operational trains at Pennsylvania State University in State College. Courtesy: Burns & McDonnellIt is the responsibility of the engineer and owner to define the desired efficiency of the boiler. This is typically defined at around 80% at minimum boiler operating conditions. Efficiency is measured by how much heat is input to the boiler to generate the target output divided by the ideal heat input to convert the target mass of water into steam (or hot water). This is impacted by several factors that the engineer should include in the boiler specifications.

Primary factors that are often overlooked are ambient conditions. Average annual conditions are the common values provided, but to ensure operational efficiency is met throughout the year, the maximum and minimum ambient conditions also need to be provided. These include temperature, wet-bulb temperature, humidity, and any air-bound contaminants that would get pulled into an air intake.

Contaminants can include debris from harvesting of field crops, seeds from trees that float, excessive snow, or other solids that could be carried by the wind. These items, when known, can be accounted for and filtered. Debris in the airstream can plug filters and bearings or add fuel to the burner that is not accounted for, which would affect heat input. Some debris, especially field crops, contain sodium and other components that, when heated, become sticky and can plug burners, stick to tubes, create hot spots, plug gas paths, and limit overall boiler capacity.

Water supply

Water supply quality has a large impact on boiler performance. All boiler suppliers require supply water that meets ASME and/or American Boiler Manufacturers Association (ABMA) boiler feedwater-quality standards. These quality standards are dependent, in part, upon the boiler-outlet pressure and define the feedwater conditions that the owner must supply to meet the warranty requirements provided by the boiler manufacturer. 

Figure 4: The photo at Pennsylvania State University in State College shows the shop fabrication of both a condensate-polishing skid and a water-softening skid for boiler feedwater. Courtesy: Burns & McDonnellThe required boiler feedwater must also be deaerated to remove excess oxygen and noncondensable gases prior to admittance into the boiler. This process requires the use of low-pressure steam, commonly referred to as “pegging steam.” If this steam is being supplied by the boiler, then the parasitic load needs to be accounted for during design. The feedwater rate used for design should be determined based on the output of the boilerand the associated the boiler blowdown rate, typically 5% to 8% of the steam generated, plus the growth margin determined during the steam-consumption calculations.

Feedwater piping should be capable of supplying the boiler with the maximum flow rate as well as excess margin and minimal pressure loss. The outlet of the deaerator should discharge to the boiler feedwater pumps with as much suction head as possible. The net positive suction head (NPSH) available to the boiler feed pump should be maximized. This reduces the horsepower requirements of the pumps. It also provides a time buffer in the event of a water supply failure at the deaerator inlet and reduces the chance of operating the boiler feedwater pumps in a low-NPSH condition, which can lead to cavitation and impeller damage.

Boiler feedwater pumps should be capable of supplying the boiler with water at a pressure at least 3% higher than that of the drum operating pressure. An evaluation of the line routing from the pump outlet to the drum inlet should be performed and pressure losses calculated to determine the proper discharge pressure. In the case of an existing system, this is critical to determine if existing pumps can supply a new boiler in a remote location where frictional losses may be greater than expected.


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