CSA B51 Boiler, Pressure Vessel, and Piping Code PDF

Objective 2 Describe four important Biogeochemical Cycles that Operate within the environment. CSA B51 Boiler, PRESSURE VESSEL, AND PIPING CODE In Canada, the CSA B51 Boiler, Pressure Vessel, and Pressure Piping Code takes precedence over the ASME codes, unless specifically stated otherwise in the...

Objective 2 Describe four important Biogeochemical Cycles that Operate within the environment. CSA B51 Boiler, PRESSURE VESSEL, AND PIPING CODE In Canada, the CSA B51 Boiler, Pressure Vessel, and Pressure Piping Code takes precedence over the ASME codes, unless specifically stated otherwise in the relevant jurisdictional legislation. With regard to steam boilers, CSA-B51 6.3.2.1 states: Steam boilers not continuously attended by a certified operator shall be equipped with at least two low-water fuel cut-off devices, each of which shall be independent of the other or others. These devices shall be installed so that they cannot be rendered inoperative. The installation shall be such that the devices can be tested under operational conditions. The words “cannot be rendered inoperative” mean that no valves may be installed on the low water cut-off piping, because they interfere with the operation of the control. As well, no switches may be installed that render the cut-off inoperable. Large capacity boilers may be required to run continuously. A boiler trip may have negative consequences for plant operations. In order to blow down the low water cut-off without tripping the boiler, a momentary contact switch may be installed in parallel to the low water cut-off switch. The operator must maintain pressure on the bypass switch during a blowdown. When the blowdown is over, and the water level in the cut-off chamber is normal, the operator can release the pushbutton to restore the operation of the low water cut-off control. With regard to hot water boilers, CSA-B51 6.3.2.2 states: Automatically fired hot water boilers not continuously attended by a certified operator shall be equipped with a low-water fuel cut-off device, except as specified in Clause 6.3.2.3. This device shall be installed so that it cannot be rendered inoperative. The installation shall be such that it can be tested under operational conditions. Page 13 of 45 It may be difficult to test the low water cut-off for hot water boilers without draining significant amounts of water from the system. Therefore, special “test and check” fittings may be installed to prevent the system from draining while testing the low water cut-off. In normal operation, these valves permit free circulation of water through the connecting piping and float chamber. A sudden onrush of water, such as that caused by opening the low water cut-off drain valve, causes dampers located in each valve to restrict water flow to the float chamber. The water level in the float chamber quickly falls, and trips the low water cut-off with a minimum loss of boiler water. When the float chamber drain is closed, the dampers return to their normally open position. The CSA B51 Code exempts some hot water boilers from the low water cut-off requirements of 6.3.2.2. Regarding these exemptions, 6.2.2.3 states: In lieu of a low-water fuel cut-off device, automatically fired hot water boilers requiring forced circulation to prevent overheating (e.g., coil or fin-tube type boilers) shall be equipped with a flow-sensing device to automatically cut off the fuel supply to the burner if the flow rate is reduced to a point where it is inadequate to protect the boiler against overheating. The device shall be installed on the boiler outlet piping… and installed so that it cannot be rendered inoperative. The installation shall be such that it can be tested under operational conditions. ASME CODES ASME BPVC I Rules for Construction of Power Boilers ASME BPVC I does not require the installation of low water fuel cut-off devices on fuel-fired or waste heat fired boilers. However, low water cut-offs are required on some electric boilers. Part PEB-13.2 states: Electric boilers of the resistance element type shall… be equipped with an automatic low-water cutoff on each boiler pressure vessel so located as to automatically cut off the power supply to the heating elements before the surface of the water falls below the visible level in the gage glass. Page 14 of 45 Part PEB-16 goes on to say: Electric boilers shall be provided with pressure and/or temperature controls and an automatic low-water fuel cutoff. No low-water cutoff is required for electrode type boilers. ASME BPVC I, Part PG-61.1 requires “two means of feeding water” for boilers with more than 47 m2 (500 ft2) of water heating surface. This is to reduce the likelihood of a low water condition occurring. PG-61.2 permits the use of a low water cut-off as an alternative to a secondary means of feeding water, but only for boilers fired by gaseous, liquid, or solid fuels in suspension. PG-61.2 stipulates the LWCO must function prior to the boiler water reaching the lowest permissible water level. ASME BPVC VII Recommended Guidelines for the Care of Power Boilers ASME BPVC VII Part C5.100 recommends that each automatically fired boiler have two independent low water cut-offs. Note that this is only a recommendation, and not a requirement. ASME VII Parts C2.423 and C9.220 state: Where water columns are equipped with automatic low water boiler trips, suitable operating procedures should be developed to preclude tripping the boiler while determining that the water column is functional and in good operating condition. For this reason, larger capacity power boilers often have momentary contact bypass switches to prevent them from tripping when their low water cut-off float chambers are blown down. ASME BPVC IV Rules for Construction of Heating Boilers Unlike ASME I, ASME IV has extensive and specific requirements. Part HG604 states the installation requirements for steam heating boilers: If the water column, gage glass, low-water fuel cutoff, or other water level control device is connected to the boiler by pipe and fittings, no shutoff valves of any type shall be placed in such pipe, and a cross or equivalent fitting to which a drain Page 15 of 45 valve and piping may be attached shall be placed in the water piping connection at every right angle turn to facilitate cleaning. Note that ASME also prohibits the installation of valves in the interconnecting piping of the LWCO. Valves would render the LWCO inoperative, which could be extremely hazardous. Part HG-606 states the LWCO requirements for steam heating boilers: Each automatically fired steam or vapor-system boiler shall have an automatic low-water fuel cutoff... so located as to automatically cut off the fuel supply before the surface of the water falls below the lowest visible part of the water gage glass. Part HG-607 refers to the rules for modular steam heating boilers: Each module of a modular steam heating boiler shall be equipped with… (a) low water cutoff. Part HG-614 refers to the LWCO requirements for hot water heating boilers: Each automatically fired hot water boiler with heat input greater than 400,000 Btu/hr (117 kW) shall have an automatic low-water fuel cutoff. This device shall be so located as to automatically cut off the fuel supply… above the lowest safe permissible water level established by the boiler manufacturer. Note that the ASME BPVC always defers to the boiler manufacturer LPWL determination. Part HG-614 goes on to state rules for forced circulation coil-tube boilers: A coil-type boiler or a watertube boiler with heat input greater than 400,000 Btu/hr (117 kW) requiring forced circulation to prevent overheating of the coils or tubes shall have a flow-sensing device installed in lieu of the low-water fuel cutoff… to automatically cut off the fuel supply when the circulating flow is interrupted. Note that this is similar to the requirements of CSA B51. B51, though, requires the flow-sensing device to be installed at the hot water outlet. As well, the CSA B51 does not specify a minimum boiler capacity. Rather, it defines a boiler as “a vessel as defined in the Act.” This permits jurisdictions to define what capacity of hot water boiler the flow switch requirement applies to. Page 16 of 45 Part HG-614 also requires a means of testing the low water cut-offs of hot water boilers: A means shall be provided for testing the operation of the external low-water fuel cutoff without resorting to draining the entire system. Such means shall not render the device inoperable except as described as follows. If the means temporarily isolates the device from the boiler during this testing, it shall automatically return to its normal position. The connection may be so arranged that the device cannot be shut off from the boiler except by a cock placed at the device and provided with a tee or lever-handle arranged to be parallel to the pipe in which it is located when the cock is open. The “test and check” valves discussed previously comply with this code requirement. ASME CSD-1 Controls and Automatically Fired Boilers Safety Devices for CSD-1 is a code that addresses the unique hazards of boilers, in capacities up to 300 boiler horsepower output, that are automatically fired. Due to the variations in jurisdictional requirements, many of these boilers may be legally exempt from supervision requirements. For this reason, CSD-1 has fairly stringent and specific control requirements that are not found in the ASME BPVC. Low Pressure Steam Boilers Regarding LWCO requirements for steam heating boilers, ASME CSD-1 Part CW120 states: Each automatically fired, low-pressure steam… boiler shall have at least two automatic low-water fuel cutoffs, one of which may be a combined feeder/cut-off device. When installed external to the boiler, each device shall be installed in individual chambers (water columns), which shall be attached to the boiler by separate pipe connections below the waterline. A common steam connection is permissible. Each cutoff device shall be installed to prevent startup and to cut off the boiler fuel or energy supply automatically, prior to the fall of the surface of the water below the level of the lowest visible part of the gage glass. Note that the two cut-offs can share a steam piping connection, but cannot share a water connection. This is because water connections are prone to plugging, but Page 17 of 45 steam connections are not. If the cut-off devices shared a common water connection, and the water connection became plugged, both cut-offs would fail. Independent water connections provides contingency, due to the unlikely occurrence that both water connections would plug simultaneously. CW-120 goes on to state: One cut-off control shall be set to function ahead of the other… the lower of the two cut-off controls shall cause safety shutdown and lockout. The lowest cut-off must be situated above the LPWL, and in no cases lower than the lowest visible part of the gauge glass. This is so that when a cut-off occurs, the boiler water level can be identified and determined to be high enough to prevent boiler damage. Hot Water Boilers ASME CSD-1 Part CW-130 states the rules for hot water boilers. • Each automatically fired, hot-water heating boiler… shall be protected by a lowwater fuel cutoff intended for hot-water service • The low-water fuel cutoff can be located any place above the lowest safe permissible water level established by the boiler manufacturer • If the low-water fuel cutoff is connected to the boiler by pipe or fittings, no shutoff valves of any type shall be placed in such piping. A cross, or equivalent fitting, shall be placed in the water piping connection at every right angle to facilitate cleaning and inspection. • Functioning of the low-water fuel cutoff due to a low-water condition shall cause safety shutdown and lockout. • A means shall be provided for testing the operation of the low-water fuel cutoff without resorting to draining the entire system. Note that these rules are similar to those in the CSA and ASME BPV codes. Page 18 of 45 High-Pressure Steam Boilers The rules in ASME CSD-1 Part CW-140 for high-pressure steam boilers are similar to those in CW-120: • Each automatically fired, high-pressure steam boiler… shall have at least two automatic low-water fuel cut-off devices. • Each device shall be installed in individual chambers (water columns), which shall be attached to the boiler by separate pipe connections below the waterline. A common steam connection impermissible. • Each cut-off device shall be installed to prevent startup and cut off the boiler fuel or energy supply automatically when the surface of the water falls to a level not lower than the lowest visible part of the gage glass. • One control shall be set to function ahead of the other. • Functioning of the lower of the two controls shall cause safety shutdown and lockout. • If the low-water fuel cutoff is connected to the boiler by pipe or fittings, no shutoff valves of any type shall be placed in such piping. • The steam and water connections to a water column shall be readily accessible for internal inspection and cleaning. CSD-1 also states that the lowest of the cut-off devices must cause safety shutdown and lockout. This will prevent the boiler from restarting if the water level is restored to normal. In other words, a low water condition requires manual operator intervention to reset the cut-off, and restart the boiler. The assumption is that the operator will determine the cause of the low water condition, and take steps to correct the root cause before restarting the boiler. Page 19 of 45 FREQUENCY OF TESTING ASME BPVC VII and CSD-1 recommend testing frequencies and test procedures for low water cut-offs in heating boiler and power boiler service. Three common low water cut-off tests are recommended: 1. Rapidly draining the float chamber, to see if the boiler shuts off. 2. Isolating the feedwater, and blowing off the boiler (a slow drain test). 3. Isolating the feedwater, and allowing the boiler to steam off water. These test methods and frequencies are covered in Part B, Unit 4, Chapter 4 Operational Checks. Page 20 of 45