4BE3.4 C4 Obj 3.pdf

OBJECTION 3 Describe the code requirements for the boiler connections and valves on steam boilers. BOILER OUTLET CONNECTIONS Outlet connections are tapings in a boiler drum or shell, for connecting external piping and fittings. They connect the boiler to various controls, instruments, or fittings,...

OBJECTION 3 Describe the code requirements for the boiler connections and valves on steam boilers. BOILER OUTLET CONNECTIONS Outlet connections are tapings in a boiler drum or shell, for connecting external piping and fittings. They connect the boiler to various controls, instruments, or fittings, such as: • Safety valve • Main steam stop valve • Drum vent • Water column and gauge glass • Feedwater inlet • Pressure controls • Bottom blowoff • Continuous blowdown • Surface blowoff • Chemical feed These fittings and piping stubs may be welded directly to the drum or shell. Fittings and piping can also be attached to the boiler with threaded fittings screwed directly into the drum or shell, and with flanged connections. ASME B 31.1 BOILER CONNECTIONS Outlets and External Piping The piping that leads to and from a boiler up to the first valve (or sometimes, the second valve) is boiler external piping, and falls under technical jurisdiction of ASME B31.1 Power Piping Code. However, ASME BPVC I also contains direction about the required configuration of boiler piping systems. Therefore, it is often necessary to consult both codes. Page 16 of 33 Steam Piping ASME B31.1 122.1.7 (A3) states: When two or more boilers are connected to a common header, or when a single boiler is connected to a header having another steam source, the connection from each boiler having a manhole opening shall be fitted with two stop valves having an ample free-blow drain between them. Figure 6 shows these requirements. Boiler #1 has a small steam drum, and is only equipped with a handhole. Boiler #2 has a larger shell, and requires a manhole for inspections and maintenance. Because both boilers are connected to a common steam header, the boiler with the manhole requires block and bleed valves in the steam line. If the small boiler also had a manhole, it too would require a block and bleed system. Figure 6 – ASME BPVC I Requirements for Steam Line Stop Valves and Drains The required steam shut-off valves must be equipped with a position indicator, to show from a distance whether the valve is open or closed. Outside screw and yoke, rising stem valves are preferred. Page 17 of 33 Feedwater Piping ASME B31.1 Power Piping Code Part 122.1.7 (B.1) covers feedwater piping requirements for power boilers. It states: The feedwater piping for all boilers… shall be provided with a check valve and a stop valve between the check valve and the boiler. The purpose of the check valve is to prevent the boiler from draining through the feedwater line when a boiler feedwater pump is not running. The stop valve permits isolation of the feedwater line for maintenance and repair without draining the boiler. CAUTION Feedwater components must not be repaired or replaced when the boiler is hot and under pressure. Follow plant lockout and tagout procedures. Part 122.1.7 (B.10) states: Wherever globe valves are used within feedwater piping for either isolation or regulation, the inlet shall be under the disk of the valve. In this way, if the valve disk becomes detached from the valve stem, it will not block the feedwater flow. Some boilers are equipped with multiple feedwater inlets. To address this, Part 122.1.7 (B.3) states: If a boiler is equipped with a duplicate feed arrangement, each such arrangement shall be equipped as required by these rules. Power plants with multiple boilers are often fed with a single feedwater pump. Each boiler has independent feedwater needs, depending on the capacities of the boilers and their firing rates. To address this, Part 122.1.7 (B.5) states: When two or more boilers are fed from a common source, there shall also be a globe or regulating valve in the branch to each boiler located between the check valve and the source of supply. Page 18 of 33 Figure 7 shows a typical feedwater arrangement that meets the criteria of B31.1. Figure 7 – Feedwater Valve Arrangement for Multiple Boilers Blowoff Piping ASME B31.1 Power Piping Code Part 122.1.7 (C.4) requires all boilers with allowable working pressures over 100 psig (690 kPag) to have two slow-opening blowoff valves, or one quick-opening valve at the boiler nozzle, followed by a slow-opening valve. A slowopening valve requires at least five 360-degree turns of the operating mechanism to change from fully closed to fully opened. The rationale for this rule is: • Slow-opening valves help the operator reduce thermal shock with a gradual, controlled warmup of the blowoff line, before subjecting it to high-pressure high-temperature liquid. • With a two-valve system, one valve operates in non-moving fluid, preserving its integrity for shut-off purposes. The other valve experiences the wear. Certain boilers are exempt from the two-valve rule: • Electric steam boilers with a normal water content of 380 L (100 US gallons) or less. • Boilers in traction service, including those used in steam traction engines and steam locomotives. • Portable steam boilers. Page 19 of 33 For these boilers, other rules apply: • According to Part 122.1.7 (C.11), traction and portable boilers require only one blowoff valve, which may be either slow-opening or quick-opening. • According to Part 122.1.7 (C.12), forced circulation and electric steam boilers with a normal water content of 380 L (100 US gallons) or less require only one blowoff valve, which must be of a slow-opening type. Ordinary globe valves have dams or pockets where sediment can collect. Because of this, Part 122.1.7 (C.1) prohibits them from blowoff service. Rather, 122.1.7 (C.2) requires Y-type globe valves, because they do not trap sediment. When the boiler MAWP is greater than 1725 kPa, the blowoff valves must be at least cast steel Class 300 valves. When the boiler MAWP is less than 1725 kPa, the blowoff valves may be Class 250 bronze, cast iron, ductile iron, or steel. Miscellaneous Piping ASME B31.1 Part 122.1.6 covers the miscellaneous piping, such as water level indicators, water columns, gauge cocks, and pressure gauges. ASME BPVC IV STEAM HEATING BOILER CONNECTIONS The various fittings attached to a low-pressure steam boiler include: • Steam outlet stop valve • Feedwater inlet stop and check valves • Blowoff valves • Vent valve • Water level controls • Water level indicators • Low water fuel cut-off • Pressure limit controls • Pressure gauge Pressure gauges, pressure limit controls, low water cut-offs, water level indicators, and water level controls have already been discussed. Page 20 of 33 Steam Outlet and Stop Valve The steam produced in the boiler leaves through the steam outlet, and flows through the main header to the branch lines, which distribute the steam to the various parts of the system. ASME BPVC IV Part HG-710.1 covers the steam stop valve requirements for installations with only a single steam boiler. Part HG-710.3 covers installations with multiple boilers. Part HG-710.1 When a stop valve is used in the supply pipe connection of a single steam boiler, there shall be one used in the return pipe connection. Part HG-710.3 A stop valve shall be used in each supply and return pipe connection of two or more boilers connected to a common system. The two types of valves commonly used on the steam outlet of a boiler are the gate valve and the globe valve. The connections may be threaded, flanged, welded, or brazed. The gate valve is more likely to be used since it offers the least resistance. It will be wide open during operation, and no throttling is involved. ASME BPVC IV Part HG-710.4(b) requires that all stop valves be suitably pressure and temperature rated. The minimum pressure rating of all valves or cocks shall be at least equal to the pressure stamped upon the boiler, and the temperature rating of such valves or cocks, including all internal components, shall be not less than 120°C. When the steam outlet is DN 50 (2” NPS) or larger, it is recommended to use outside screw and yoke valves with rising spindles. These valves enable the operator to see, even from a distance, whether the valve is open or closed. Also, since the threaded part of the spindle is outside the valve body, it is not exposed to corrosive action by steam or water, and can be easily lubricated. Page 21 of 33 Feedwater Connection and Check Valves The feedwater connection is the point at which the water enters the boiler. It should be arranged so that the water does not discharge directly against surfaces exposed to hot combustion gases or radiant heat from the fire. Frequently, an internal pipe is used to direct the feedwater to a suitable point within the boiler. ASME BPVC IV Part HG-705(a) states that: Feedwater shall not be introduced through openings or connections provided for inspection or cleaning, safety valve, water column, water gage glass, or pressure gage. Feedwater to the water column or safety valve connections could adversely affect the operation of feeders, cut-offs, or the safety valve. ASME BPVC IV Part HG-705(a) also states: The feedwater pipe shall be provided with a check valve near the boiler. A stop valve or cock shall be installed either upstream or downstream of the check valve. In pumped return systems, the check valve keeps the boiler from draining when the boiler feedwater pump is off. The stop valve boiler allows the feedwater system components to be isolated for repairs without draining the boiler. The check valve used in the feedwater line may be a swing, or a lift check type. In addition to the feedwater check valve, ASME BPVC IV Part HG-703.2 requires the installation of a return (or Hartford) loop. This piping configuration prevents boiler pressure from lowering the boiler water level to below the lowest permissible water level, if the feedwater check valve fails. Bottom Blowoff Valves ASME BPVC IV Part HG-715(a) requires all steam heating boilers to have bottom blowoff connections connected to the lowest water space practicable. This is to ensure: • The boiler can be completely drained. • Blowoff can discharge all the sediment that accumulates. The size of the blowoff line varies with the boiler steam production capacity. The sizes are shown in Table 5. The blowoff discharge piping must be full size to the point of discharge. Page 22 of 33 Table 5 – Size of Bottom Blowoff Piping and Valves, According to ASME BPVC IV HG-715 Minimum Required Safety Valve Capacity, kg/hr Blowoff Piping and Valve Size, DN (NPS) Up to 225 20 (3/4) 225 to 550 25 (1) 550 to 1200 32 (1 ¼) 1200 to 2700 40 (1 ½) 2700 and larger 50 (2) Packaged firetube boilers have the blowoff connection at the bottom of the shell close to the back end. Boilers with water legs, such as firebox boilers, have a blowoff connected at the lowest point of each water leg. The minimum pressure rating of valves used to blowoff or drain low-pressure steam boilers must be at least 200 kPa (30 psi). The temperature rating must not be less than 120°C (250°F). Blowdown valves are installed for the testing of, and intermittent removal of, sediment from the water column, gauge glass, and low water fuel cutoff. Again, these valves must be temperature and pressure rated. Page 23 of 33