4AE3 U3 C3 Obj 8 PDF

Unit A-3 • Introduction to Power Engineering and its Governance in Canada Objective 8 Explain the purpose, intent, and limitation of ASME CSD-1 (Controls and Safety Devices) Standard. Introduction In 1977, the ASME published a code to address the main hazards that are related with the operation of...

Unit A-3 • Introduction to Power Engineering and its Governance in Canada Objective 8 Explain the purpose, intent, and limitation of ASME CSD-1 (Controls and Safety Devices) Standard. Introduction In 1977, the ASME published a code to address the main hazards that are related with the operation of smaller, automatically fired boilers: • Low water • Overpressure • Over-temperature • Furnace explosion The code is called Controls and Safety Devices for Automatically Fired Boilers (CSD-1). CSD-1 has been adopted by numerous jurisdictions across North America. The ASME intended to address the following facts: a) Small boilers (under 300 boiler horsepower) are the most commonly installed boilers. They are found in dry cleaners, apartment heating, plants, schools, shopping malls, recreation facilities and so on. b) They are automatically fired; therefore, these boilers may not receive attention by licensed or trained operators and maintenance personnel. c) Because of the two facts above, small automatically fired boilers pose a greater risk to the public than larger boilers. ASME realized that rules were necessary for the assembly, installation, maintenance, and operation of the controls and safety devices of automatically fired boilers. This provides additional safety when licensed and trained operators do not continuously attend the equipment. In the words of CSD-1: “It is believed that improved instrumentation, controls and safety devices, proper operating procedures, and a clearer understanding of installation requirements by the manufacturers, installers, and operators can greatly reduce the chances of personal injury, damage to property, and loss of equipment from accidents.” In its scope, CSD-1 covers automatically fired: a) Hot water and low pressure steam heating boilers b) High-pressure steam (“power”) boilers CSD-1 only applies to: a) Boilers that operate on gas, oil, or electric power b) Boilers under 300 boiler horsepower Note: CSD-1 does not apply to hot water heaters or direct gas-fired swimming pool heaters. Larger boilers fall under other codes, such as NFPA 85. Boilers under 422 022 kJ per hour input (400,000 Btu per hour input) that are labelled by a certified testing agency may also be recognized as meeting CSD-1. 3-26 4th Class Edition 3 • Part A Codes and Standards for Power Engineers and Pressure Vessels • Chapter 3 Low Water CSD-1 Part CW requires all automatically fired boilers to undergo a safety shutdown (shut off their fuel/heat input) if the water within the boiler falls to below what the boiler manufacturer deems safe. A “low water cut-off ” is the device that reacts to the boiler water level, and causes a safety shutdown. The low water cut-off may also cause a lockout. This means that the boiler cannot restart after the water level is restored, unless an operator is present to reset the control system. The low water cut-off has an important role. It must never be bypassed either mechanically (i.e., using isolation valves) or electrically (i.e., with a bypass switch or jumper wires). CW-120: Low Pressure Heating Boilers Each automatically fired, low-pressure steam or vapour system boiler shall have at least two automatic low-water fuel cut-offs. One of these may be a combined feeder/cut-off device. A low water condition shall cause a safety shutdown. CW-130: Hot Water Heating Boilers Each automatically fired, hot-water heating boiler shall be protected by a low-water fuel cutoff. This should be located any place above the lowest permissible water level established by the boiler manufacturer. A low water condition shall cause a safety shutdown and lockout, requiring manual reset. CW-140: High-Pressure Steam Boilers Each automatically fired, high-pressure steam boiler shall have at least two automatic low water fuel cut-off devices. The lower of the two controls shall cause safety shutdown and lockout, requiring manual reset. CW-200: Forced Circulation Boilers Some small hot water boilers rely on forced circulation to prevent them from overheating. CW210 permits these coil-type tubular boilers to have a flow switch at the boiler outlet. If the forced circulation pump fails, or if the heat exchanger is blocked or ruptured, the reduction in outlet flow will cause a safety shutdown. This will prevent the boiler from restarting until adequate water flow is restored. However, many jurisdictions also require a low water cut-off in addition to the flow switch. Over Pressure To paraphrase CSD-1 Part CW-300, steam boilers require pressure limit controls as follows. “Each automatically fired steam boiler shall have at least one steam pressure control device that will shut off the fuel supply to each boiler when the steam pressure reaches a preset maximum operating pressure. In addition, each individual automatically fired steam boiler shall have a high steam pressure limit control that will prevent generation of steam pressure greater than the maximum allowable working pressure. Functioning of this control shall cause safety shutdown and lockout.” A boiler may have additional pressure controls that are used to modulate the boiler firing rate during routine operation. The pressure controls must be protected from the effects of live steam with a water-seal, such as that provided by a “pigtail” siphon. Like the low-water cut off, CSD-1 does not permit any mechanical or electrical means whereby the high-pressure cut-off can be bypassed or defeated. Part CW-500 requires that all boilers have pressure relief valves. The pressure relief valve is the last line of defense against over-pressurization. For a steam boiler, the pressure relief valves operates if the high pressure cut-off fails to cause a safety shutdown and lockout of the boiler. This situation could also occur to a cold hot water heating boiler, if fired to its temperature set point with its supply and return valves shut. CW-510 directs the reader to the applicable ASME code for determining pressure relief valve requirements (ASME I or IV). 4th Class Edition 3 • Part A 3-27 Unit A-3 • Introduction to Power Engineering and its Governance in Canada Over Temperature To paraphrase CSD-1 Part CW-400, hot water heating boilers require temperature limit controls as follows: “Each automatically fired hot-water boiler shall have at least one temperature-actuated control to shutoff the fuel supply when the system water reaches a preset operating temperature. In addition, each individual automatically fired hot-water boiler unit shall have a high temperature limit control that will prevent the water temperature from exceeding the maximum allowable temperature. The upper set point limit of the selected control shall not exceed the maximum allowable temperature. This control shall cause safety shutdown and lockout.” A boiler may have additional temperature controls that are used to modulate the boiler firing rate during routine operation. Like the low-water cut-off, CSD-1 does not permit any mechanical or electrical means for the high- temperature cut-off to be bypassed or defeated. Furnace Explosion If a burner control system fails – either electrically or mechanically - gas and oil fired boilers may accumulate explosive mixtures of fuel and air if an ignition source is absent. When an ignition source is introduced, the mixture will ignite causing an explosion. CSD-1 Part CF is focused on automatic burner operation and controls, to prevent this explosive “delayed ignition.” The requirements of Part CF depend on the input rating of the burner, the type of fuel being burned, and the combustion air delivery method (mechanical or natural draft). The largest burners require the most stringent safety requirements, because they present greater hazard. Tables CF-2 and CF-3 outline the burner safety control requirements for mechanical and natural draft boilers firing gaseous fuels. Table CF-4 outlines the burner safety control requirements for oil-fired boilers. These tables outline permissible: a) Ignition systems and their application • Continuous, Intermittent or Interrupted pilots • Direct ignition • Hot surface ignition b) Combustion air proving c) Furnace pre-purge and post-purge requirements d) Flame failure response Part CF also covers fuel train components needed for safe automatically fired boiler operation. These components include: • Pressure gauges • Manual shut-off valves • Pressure regulators • Fuel pressure relief valves • Vent lines • Test ports • Bleed valves • Control valves • Safety shut-off (SSOV) valves CSD-1 has several figures to illustrate fuel train configurations for burners of various capacities, fuels, ignition, and draft systems. CSD-1 may require a fuel piping system to have switches that shut down the burner in the event of high or low fuel pressure. 3-28 4th Class Edition 3 • Part A Codes and Standards for Power Engineers and Pressure Vessels • Chapter 3 CF-310 describes the primary safety control. It is designed to monitor for the existence of flame and to commence safety shutdown and lockout if one of the following applies. a) A flame exists when the SSOVs are closed (indicating mechanical failure of the SSOV). b) No flame exists, when the SSOVs are open (indicating “flame failure”). These primary controls with extra functionality are commonly called “combustion programmers” or “programmed combustion controls.” A programmed combustion control satisfies the primary safety control requirements of CSD-1. Lastly, in Non-Mandatory Appendix D, CSD-1 recommends daily, weekly, monthly, semi annual, and annual preventative maintenance. Some of the maintenance activities are listed below. a) b) c) d) e) f) g) h) i) j) k) l) m) n) o) Check gauges, monitors, and indicators. Test low-water fuel cut-off, including a “slow drain test.” Test low draft, fan air pressure, and damper position interlocks. Check low-fire start interlock. Test high and low fuel pressure and fuel temperature interlocks. Check: • Burner flame • Igniter • Flame signal strength • Firing rate control • Flame failure detection system Check flue, vent, stack, or outlet dampers. Inspect burner components. Test high-limit and operating temperature or steam pressure controls. Replace vacuum tubes, scanners, or flame rods in accordance with manufacturer’s instructions. Conduct a combustion analysis test. Perform leakage test on pilot and main gas and/or oil fuel valves. Test purge air switch in accordance with manufacturer’s instructions. Clean oil-fired burners, atomizers, and oil strainers. Test safety / safety relief valves in accordance with ASME Boiler and Pressure Vessel Code, Sections VI and VII. Summary ASME CSD-1 is a valuable reference for Power Engineers when ordering new or replacement boilers, or seeking guidance on maintenance or control system repair. For example, a combustion programmer may fail, requiring replacement. CSD-1 will guide the boiler owner on the: • Ignition style • Purge timing • Low fire start requirements • Post-purge requirements • Other important operating parameters A replacement programmer can then be selected with the necessary features. A certified trade person must configure the programmer, in order to operate the boiler safely and in accordance with CSD-1. 4th Class Edition 3 • Part A 3-29 Unit A-3 • Introduction to Power Engineering and its Governance in Canada However, it is important for the Power Engineer to remember the limitations of CSD-1. Many jurisdictions have not adopted CSD-1 as law. Because of this, many boilers currently in-service were never equipped with controls that meet the requirements of CSD-1. If properly inspected and maintained, few if any of these boilers operate unsafely. However, a Power Engineer may want to upgrade the controls and safety devices on an existing gas-fired boiler to meet CSD-1. In Canada, if any modifications are made to a gas burner control system, the CSA B149.3 “Code for the Field Approval of Fuel-related Components on Appliances and Equipment” takes precedence over CSD-1 requirements, and must be consulted first. Finally, CSD-1 does not apply to boilers larger in capacity than 300 boiler horsepower. For larger boilers, different codes need to be consulted. 3-30 4th Class Edition 3 • Part A