Codes for boiler control systems and pressure vessels
The end user is ultimately responsible to use the correct code for pressure sensors and transmitters for boiler controls and pressurized vessels.
Pressure sensors and transmitters are extensively used in pressure vessels and boiler systems to monitor fuel, steam, water, and air pressure. These sensors perform safety and control functions within the plant to maintain a safe environment with maximum performance.
Sensors in these applications are typically connected to the process via a threaded connection that allows liquid or gas to be measured in a leak-free condition. To retain the pressure at operating and overload conditions, the pressure port must be able to handle the operating pressure and temperature conditions.
See Figure 1 for a typical pressure sensor with ¼-in. male national pipe thread (NPT) port, a very popular fitting used in boiler tubing and piping systems.
ASME B31 code
The ASME B31 code for pressure piping is a very popular boiler code; it covers power piping, fuel gas piping, process piping, pipeline transportation systems for liquid hydrocarbons and other liquids, and refrigeration piping, as well as heat transfer components and building services piping. Prior to ASME B31, this code was known as ANSI B31.
Within the B31 code are many subcodes that define the exact application and use of pressure sensor fittings and ports that are allowable under the code. The widely used codes include:
- B31.1- Power Piping for industrial plants and marine applications. This code prescribes minimum requirements for the design, materials, fabrication, erection, test, and inspection of power and auxiliary service piping systems for electric generation stations, industrial institutional plants, and central and district heating plants. It also covers boiler external piping for power boilers and high-temperature and high-pressure water boilers in which steam or vapor is generated at a pressure of more than 15 psig.
- B31.3 - Process Piping for use in chemical & petroleum plants, refineries processing chemicals and hydrocarbons, water and steam. The code contains rules for piping found in petroleum refineries; chemical, pharmaceutical, textile, paper, semiconductor, and cryogenic plants; and related processing plants and terminals. The code covers requirements for materials and components, design, fabrication, assembly, erection, examination, inspection, and testing of piping handling fluids, gases, steam, and air. Pressure sensors with either male or female process ports are covered under this code whenever the pressure is more than 15 psig.
Pressure port design considerations under B31.3
The B31.3 code is intended for manufacturers, users, constructors, designers, and others concerned with the design, fabrication, assembly, erection, examination, inspection, and testing of piping, plus all other potential governing entities.
All pressure sensors employ some form of a diaphragm that is either machined from one piece of metal or welded or O-ring clamped as a two-piece assembly. This section of the pressure sensor is the thinnest and most sensitive of all components.
Figure 2(a) shows a one-piece thick 0.022-in. diaphragm while Figure 2(b) depicts a welded thin 0.001-in. thick diaphragm. Together, with the thickness and the type of metal, the operating pressure and operating temperature range dictate the safety conditions under which the pressure sensor can operate.
Stresses and quality factors (for welded diaphragms only) are other important design considerations taken into account under B31. Metals such as strain hardened A-479-316L stainless steel level 2 need to be impact tested when used below -20 F. The impact tests must conform to B31.3 Table 323.2.2.
When sensors are constructed from metals such as N07718 (Inconel 718) and N10276 (Hastelloy C276) that offer temperature resistance from -325 F to +300 F, no impact testing is required.
Based on the calculations and thermal limits for the metals as required by B31.3 code, below are samples of calculations needed to determine the correct diaphragm thickness for a one-piece pressure sensor designed free of welds. Each sensor material has different containment capabilities based on the material properties and exposure to temperature.