Heating Boiler Safety and Safety Relief Valves PDF

Objective 2 Discuss the code requirements, construction, and operation of ASME Section IV Pressure Relief Valves and Devices. HEATING BOILER SAFETY and SAFETY RELIEF VALVES EATING BOILER SAFETY and SAFETY RELIEF VALVES ASME Code Requirements ASME BPVC, Section IV Rules for Construction of Heating...

Objective 2 Discuss the code requirements, construction, and operation of ASME Section IV Pressure Relief Valves and Devices. HEATING BOILER SAFETY and SAFETY RELIEF VALVES EATING BOILER SAFETY and SAFETY RELIEF VALVES ASME Code Requirements ASME BPVC, Section IV Rules for Construction of Heating Boilers contains the rules for safety valves and safety relief valves installed on heating boilers. Some of the requirements of Part HG-400.1 and Part HG-401 are stated below. HG-400.1 (a): Each steam boiler shall have one or more officially rated safety valves… of the spring pop type adjusted and sealed to discharge at a pressure not to exceed 15 psi (100 kPa). HG-400.1 (b): No safety valve for a steam boiler shall be smaller than NPS ½ (DN 15) or larger than NPS 4 (DN 100). HG-400.1 (e): The safety valve capacity for each steam boiler shall be such that with the fuel burning equipment installed, and operated at maximum capacity, the pressure cannot rise more than 5 psi (35 kPa) above the maximum allowable working pressure. HG-401.1 (h): A body drain below seat level shall be provided by the Manufacturer for all safety valves and safety relief valves… Body drain connections shall not be plugged during or after field installation. (Note: this drain will prevent the collection of condensate around the valve and seat. This could result in corrosion and subsequent sticking of the safety valve). Mounting Safety and Safety Relief Valves on Heating Boilers ASME BPVC IV, Part HG-700 Installation Requirements, All Boilers sets requirements for the mounting of safety and safety valves on heating boilers. The following points are covered. All Safety and Safety Relief Valves a) Safety valves and safety relief valves must be installed with their spindles vertical. b) Safety valves must be connected directly to one of the following: • A tapped or flanged opening in the boiler • A fitting connected to the boiler by a short nipple • A y‐base • A valveless header connecting steam or water outlets on the same boiler c) Safety valves and safety relief valves must be located in the highest practicable part of the boiler proper. d) Safety valves and safety relief valves must not be connected to an internal pipe in the boiler. e) The opening or connection between the boiler and any safety valve or safety relief valve must have at least the same cross-sectional flow area of the valve inlet. f) No shut-off valve of any description shall be placed between the safety or safety relief valve and the boiler, or on discharge pipes between these valves and the atmosphere. g) Discharge pipes must be used. The discharge pipe of the safety valve shall be: • As short and straight as possible, and arranged so as to avoid undue stress on the valve. • Properly drained to prevent collection of water. • Arranged so that there will be no danger of scalding the operator. h) The internal cross-sectional area of the discharge pipe must not be less than the full area of the valve outlet. i) If a long vertical discharge pipe is required, then it may be necessary to install a flexible joint near the safety valve, so that the expansion of the pipe will not place any stresses on the safety valve. Specific to Safety Valves (Steam Service) The safety valve must never be located below the normal operating water level, or below the lowest permissible water level. SAFETY VALVES Low-pressure steam heating boilers are designed to operate at no higher than 100 kPa. A safety valve especially designed for low-pressure steam heating boilers is shown in Figure 9. The valve housing consists of two main parts. The first is the valve body, which is directly connected to the boiler. The second is the bonnet, which is threaded and locked onto the valve body. The bonnet has an outlet opening to atmosphere. A valve disc Figure 9 – Cross-Section of Low-Pressure Steam Safety Valve closes the opening in the upper part of the valve body. The disc is tightly held down upon its seat by a heavy spring. The adjusting cap in the upper part of the bonnet compresses the spring. The cap is held in position by a locking screw. Note that unlike a safety valve for a power boiler, this valve does not have an adjustable blowdown ring. ASME BPVC IV, Part HG-401 does not require low-pressure steam safety valves to have adjustable blowdown. Figure 10 – Low-Pressure Safety Valve, Installed on a Steam Boiler Figure 10 shows a low-pressure safety valve installed on a steam boiler. Low-pressure safety valves are easily recognized, because they are shorter and wider than high-pressure safety valves of the same capacity. SAFETY RELIEF VALVES ON BOILERS ASME BPVC IV, Part HG-400.2 states: Each hot water heating or supply boiler shall have at least one officially rated safety relief valve, of the automatic reseating type… and set to relieve at or below the maximum allowable working pressure of the boiler. ASME BPVC IV, Part HG-401.1 states that safety relief valves shall have pop action when tested by steam. The design of the safety relief valve is quite similar to that of the safety valve used for steam boilers. It is spring loaded, has body drain openings, and has a lifting device (try lever). However, safety relief valves do not require blowdown adjustment rings (and seldom have them), do not have bottom guides for the disc, and their pressure is non-resettable. The safety relief valve opens partially when boiler pressure slightly exceeds the valve setting. However, the slight flow of the escaping water does not have the same lifting effect on the valve lip as steam has, so there is no immediate popping action. On further pressure rise, the valve pops wide open. Figure 11 shows a typical safety relief valve used with a hot water heating boiler or heating system use. Figure 11 – Small Safety Relief Valve Safety relief valves should not open frequently, or leak water, during normal plant operation. If they do, the cause must be found and remedied. Caution Never, under any circumstances, plug the outlet of a pressure relief valve. One common cause for frequent safety relief valve operation is a waterlogged heating system. Hot water heating systems are completely filled with water, except for the expansion tank, which contains a volume of compressed air or nitrogen. When the burners fire, the water expands, further compressing the air in the expansion tank. If the expansion tank becomes filled with water, the heating system pressure increases dramatically, because water is not compressible. This causes water to weep from the relief valve. When the burner shuts off, the water cools. Make-up water enters, and keeps the system waterlogged. In this situation, the expansion tank must be isolated, drained, and returned to service. Side Track Part B, Unit 4, Chapter 3 Boiler Operation explains how to inspect, drain, and recharge, both conventional and bladder-style expansion tanks, and return them to service. Other situations may cause safety relief valves to weep. These include: • Scale, corrosion products, or chemical residue accumulations on the valve seat, which will prevent it from closing tightly. • The installed safety relief valve is designed for the wrong pressure range. • The safety relief valve may have damaged seating surfaces or weakened compression springs from erosion, corrosion, and lengthy service duration. Each of these situations must be addressed in a timely manner.