Installation requirements for safety valves on boilers

A boiler is a thermal device that uses the heat generated by combustion to heat water or turn it into steam. Although there are many types of boilers, they all consist of three main parts: the boiler itself, the furnace, and the accessories, instruments, and auxiliary equipment necessary to ensure the proper operation of the boiler and furnace. “The \"vessel\" refers to the sealed, pressurized part of a boiler in which water and steam are contained; it is the heat-absorbing section of the boiler and mainly includes the drum, convection tubes, water wall, header, superheater, economizer, etc. “The “furnace” refers to the part of a boiler where fuel burns to release heat energy; it is the heat-releasing component of the boiler, and mainly includes combustion equipment, furnace walls, furnace arches, steel frames, flues, as well as smoke exhaust and dust removal equipment. Boilers are equipped with numerous accessories and instruments, such as safety valves, pressure gauges, water level gauges, and high/low water level alarms. The auxiliary equipment includes devices for the water supply system, systems for fuel supply and preparation, ventilation systems, and systems for ash and slag removal. I. Boiler safety accessories Boiler safety accessories are essential components in the operation of boilers; they mainly include instruments such as pressure gauges, water level gauges, safety valves, steam-water valves, and drain valves. These accessories are extremely important for the safe operation of boilers, especially pressure gauges, water level gauges, and safety valves. They serve as the means through which boiler operators can monitor operations properly, and they are essential components for ensuring the safe operation of boilers; therefore, they are commonly referred to as the three key safety accessories of boilers. 1. Safety valve: The safety valve is one of the important safety accessories of a boiler. It automatically prevents the steam pressure in the boiler from exceeding the predetermined allowable limit, thus ensuring the safe operation of the boiler. Function of the safety valve: (1) When the pressure inside the boiler reaches the specified limit, the safety valve opens automatically to release steam and emit an alarm, thereby alerting the operators to take action. (2) After the safety valve opens, it can discharge enough steam to reduce the pressure inside the boiler until it drops to a level lower than that when the valve first opened, at which point the valve closes automatically. In this way, the pressure in the boiler is kept within safe limits, preventing explosion accidents. The safety valves used in industrial boilers are generally of two types: spring-loaded safety valves and lever-type safety valves. Safety valves installed on boilers shall meet the following requirements: (1) Boilers with an evaporation capacity of >0.5 t/h shall be equipped with at least one safety valve. Safety valves must be installed at the outlet of the steam superheater, as well as at the outlet (or inlet) of the split economizer, at the inlet and outlet of the reheater, and at the start-up separator of a once-through boiler. (2) The safety valve shall be installed vertically at the highest point of the boiler drum (or header). No steam extraction pipes or valves shall be installed between the safety valve and the boiler drum (or header). (3) The safety valve must be equipped with the following devices: A lever-type safety valve must have a device to prevent the weight from moving on its own, as well as guides to restrict the lever from extending beyond certain limits ； Spring-loaded safety valves should be equipped with a lifting handle and mechanisms to prevent the adjustment screws from being turned arbitrarily ； Gravity-type safety valves must be equipped with a device to prevent the weight plate from flying off. The total steam discharge capacity of the safety valve must be greater than the boiler’s maximum continuous evaporation rate. It is also ensured that after all the safety valves on the boiler drum and superheater open, the rate of increase in steam pressure inside the boiler does not exceed 30% of the highest opening pressure of those safety valves, and the steam pressure in the boiler must not exceed 1.1 times the design pressure. The steam discharge capacity of the safety valves at the outlets of the superheater and reheater should be such that sufficient cooling is provided to these components, preventing them from being damaged ； The cross-sectional area of the economizer safety valve is determined by the design unit. If several safety valves are installed together on a short pipe directly connected to the boiler drum, the cross-sectional area of the passage in that short pipe should be not less than 1.25 times the total cross-sectional area of all the safety valves. A vent pipe should generally be installed on the safety valve to prevent injury during steam release. The exhaust area must be at least twice the total area of the safety valve. Below the safety valve exhaust pipe, there should be a drain pipe leading to a safe location. No valves are allowed to be installed on either the exhaust pipe or the drain pipe. If the sound of steam discharge from the safety valve cannot be heard by the stoker at his work location, a signaling device (such as a whistle) should be installed. The safety valve of the economizer should be equipped with a drain pipe leading to a safe location, and no valves shall be installed on this drain pipe. To prevent the valve core of the safety valve from sticking to the valve seat, it is necessary to regularly perform manual or automatic venting or water discharge tests on the safety valve. After being calibrated, the safety valve should be locked or sealed with lead, and the calibration results should be recorded in the boiler’s technical documentation. 2. Pressure gauge: The pressure gauge is used to measure the pressure inside the boiler during its operation. With a pressure gauge, staff can operate the boiler correctly to ensure its safe operation. The pressure gauge commonly used in industrial boilers is the Bourdon tube type, which offers advantages such as a simple structure, ease of use, and high accuracy and reliability. The pressure gauge should be installed in a location where it is easy to view and where the temperature is low. If the pressure gauge is installed in a location near high temperatures, the deformation of its transmission mechanism due to heat will affect the accuracy of the indicated pressure. Therefore, the pressure gauge should be installed as far away as possible from steam paths or areas prone to radiation, and it must have adequate lighting so that boiler operators can always see the steam pressure it indicates. A trap (U-shaped or ring-shaped) should be installed below the pressure gauge to allow steam to condense within the trap. In this way, it is condensed water rather than high-temperature steam that acts on the pressure gauge. If a trap is not installed, steam will flow directly into the spring-loaded bend inside the gauge, which can damage the components within it or result in inaccurate pressure readings. Each boiler must be equipped with a pressure gauge directly connected to the steam space of the drum. The size of the pressure gauge dial must ensure that the boiler operator can clearly see the pressure reading; the diameter of the dial should be no less than 100 mm. The calibration of pressure gauge installations shall comply with the regulations of the **metrology authorities. After installation, it should be calibrated at least once every six months. The pressure gauge should be sealed with lead after calibration. Pressure gauges must be accurate, sensitive, and reliable, in compliance with the requirements of the **Boiler Safety Inspection Regulations**, and must be closely monitored to prevent overpressure incidents. 3. Water level gauge: The water level gauge is an important safety device used to monitor the water level inside the boiler drum. The water level indicated on the water level gauge represents the water level of the water in the boiler drum. The stokers follow the proper procedures to ensure the safe operation of the boiler. The water level is determined based on the principle that the water surface heights in connected containers are equal. Therefore, the operating water level gauge must remain unobstructed in relation to the boiler. Each boiler is equipped with at least two independently operating water level gauges. The water level gauge should be installed in a location where it is easy to observe. The water level gauge should have distinct markers indicating the highest and lowest safe water levels. When the water level gauge is more than 6 m above the operating ground, a remote water level display device should be installed. The water level gauge must comply with the relevant provisions of the supervision regulations; an accurate and reliable water level gauge should be installed, and the normal water level in the boiler must be closely monitored to prevent water shortage incidents. Level gauges come in two types: glass tube and flat plate. A glass tube level gauge is a type of level gauge that mainly consists of a steam stopcock, a drain stopcock, and a glass tube. The glass tubes are made of heat-resistant glass, available in two inner diameter sizes: 15mm and 20mm. An excessively small inner diameter can cause capillary action, affecting the accuracy of water level indication. Steam cocks, water cocks, and drain cocks are made of cast iron, cast steel, or cast copper. The plug comes in two connection types: threaded and flanged. When a higher pressure is used, flange connection is preferred. At lower pressures, threaded connections can be used, but care must be taken to prevent thread leakage and corrosion. The gauges’ cocks should be parallel to each other, with their end faces lying in the same plane, to ensure that the glass tube is not damaged due to twisting. Glass tube water level gauges have a simple structure, are easy to manufacture and install as well as to replace, but the displayed water level is not clear enough and the glass tubes can easily break. They are suitable for small boilers with a working pressure of no more than 1.6 MPa. Flat-type level gauges come in two types: those with a single glass plate and those with a double glass plate. It is mainly composed of components such as glass panels, metal frames, steam valves, water valves, and drain valves. A single-sided glass plate water gauge features a flat glass plate mounted in front of a metal frame box; the contact surface is lined with asbestos paper, and then the cover is pressed onto the frame box using screws, ensuring a tight fit between the cover, the frame box, the lining, and the glass plate. When tightening the screws of the frame box, make sure that each screw is tightened to approximately the same degree to prevent leaks. Triangular prismatic grooves are engraved on the inner surface of the glass plate; with the light source located at the front, the light is refracted as it passes through these grooves, resulting in the vapor portion of the water level gauge appearing brighter while the water portion appears darker, thus giving a very clear boundary between the vapor and water. A double-glass plate water gauge has flat glass panels fitted on both the front and back sides of a metal frame box. The light source is usually placed at the back; after refraction, the steam portion in the water level gauge appears darker, while the water-filled portion appears brighter, making it easy to determine the water level. Although the structure of flat-panel water level gauges is relatively complex, they are safe and reliable, and provide clear readings of water levels, which is why they are widely used.

II. Boiler Water Treatment 1. The Importance of Boiler Feed Water Treatment Natural water usually contains three types of impurities: suspended solids (silt, oil, etc.), colloidal impurities (hydroxides of iron, aluminum, silicon, etc.), and dissolved impurities (dissolved gases and dissolved salts, etc.). These impurities can cause scale and sludge to form in the boiler, corrode the metal surfaces of the boiler, lead to foaming in the boiler water, abnormal boiling, steam containing water, and contamination of the steam. Sometimes they can also cause salt deposition and scaling in the superheater, resulting in superheater tube explosions. In particular, the formation of scale not only wastes fuel during combustion but also damages the heating surfaces ； It can also disrupt the water cycle and shorten the service life of boilers. The reason for scale formation in boilers is that, during the heating process of water, certain calcium and magnesium salts undergo chemical reactions that result in the precipitation of insoluble substances ； Moreover, the solubility of these calcium and magnesium salts decreases as the water temperature rises, and they precipitate once a saturated concentration is reached ； As the water in the pot continues to evaporate and concentrate, insoluble salts precipitate out. 2. Water quality standards: Water quality indicators are technical parameters that indicate the quality of water, and they are established based on the requirements for water use and the characteristics of impurities. The main water quality parameters for boiler water are as follows: Suspended solids: the amount of water-insoluble substances obtained by filtering the water under specified test conditions. Salt content: Total amount of all salts dissolved in water ； The content of dissolved solids is commonly used as a substitute. Hardness: The total amount of calcium and magnesium salts dissolved in water. Hardness is further divided into temporary hardness and permanent hardness. Temporary hardness is the bicarbonate content of calcium and magnesium in water. Permanent hardness is the non-carbonate hardness in water, including sulfates and chlorides of calcium and magnesium. pH value: the negative logarithm of the hydrogen ion concentration in water. Alkalinity: The total amount of substances in water that increase the concentration of hydroxide ions due to dissociation or hydrolysis. Water quality standards represent the acceptable ranges for various water quality parameters, and they are determined based on factors such as the boiler’s evaporation rate, operating pressure, steam temperature, and water treatment processes. The requirements for water quality parameters as specified in China’s \"Standards for Water Quality in Low-Pressure Boilers\" are shown in the table below: Requirements for water quality parameters per the \"Standards for Water Quality in Low-Pressure Boilers\": Water treatment methods – Chemical treatment added inside the boiler; External water treatment or chemical treatment. Suspended solids content

In principle, no isolation valve should be installed between the boiler safety valve and the boiler itself; however, without such a valve, normal maintenance and use are not possible, which is quite contradictory

Are there any regulations that explicitly require it not to be added?