Thread Content
There are many types of valves, and they are used in a wide range of applications. In pipelines, it is sometimes the main device that plays a controlling role; sometimes it is a secondary device that serves an auxiliary function. If used improperly, it will cause problems such as leakage, vapor release, dripping, and seepage; in mild cases this affects production, while in severe cases it can lead to accidents. Therefore, understanding and using valves correctly is a very important matter. 1. Valve classification: There are many types of valves used in heating systems. Such as gate valves, globe valves, ball valves, butterfly valves, check valves, safety valves, control valves, balance valves, self-acting balance valves, and so on. I will introduce them one by one below. 1.1 Gate valves, also known as gate plate valves or gate valve types, are a widely used type of valve. Working principle: The sealing surface of the gate and the sealing surface of the valve seat are highly smooth, flat, and uniform, forming a very tight and secure sealing pair. The gate plate enables the flow of the medium to be opened or closed by lifting or pressing down the valve stem. It serves to shut off the flow in the pipeline. Advantages: Low fluid resistance; the sealing surface is not subject to erosion when fully open; it can be used with two-way flow of the medium, without any directionality requirement; durable and robust; suitable not only for small valves but also for large ones. Disadvantages: High height; long opening and closing time; bulky; difficult to repair; manual operation is laborious for large-diameter gate valves. Gate valves are classified into stem-type and hidden-stem type based on the difference in their valve stems; they are also divided into parallel-type and wedge-type according to the structure of the gate plate. Additionally, there are single-gate plate and double-gate plate types. In heating systems, the commonly used valves are the wedge-type single-disc gate valve with exposed stem (Z41H-16C) and the wedge-type single-disc gate valve with concealed stem (Z45T-10); the former is installed on the primary side of the heat station, while the latter is installed on the secondary side of the heat station. It generally serves two functions: it acts as a switch for the main device; and it is installed in front of or behind the main device as a device for maintenance purposes. When installing a gate valve, do not position the handwheel below the horizontal level (in an inverted position); otherwise, the medium will remain in the valve cover for an extended period, which can lead to corrosion of the valve stem. In heating systems, gate valves used to be the main type of valve. Now, with the widespread use of butterfly valves, gate valves have been replaced by them. 1.2 Globe valves are also a type of valve that is widely used. The diameter is generally below 100 mm. Its working principle is similar to that of a gate valve, except that the closing element (valve disc) moves along the center line of the valve seat. It acts as a shut-off valve in the pipeline and can also roughly regulate the flow rate. Advantages: Easy to manufacture, convenient to repair, strong and durable. Disadvantage: It only allows one-way flow of the medium, and there is a directionality requirement during installation. High flow resistance and poor sealing. Based on their structure, they are classified into straight-through type, right-angled type, DC type, and balanced type. In engineering, flange-through type (J41H) and internal thread-through type (J11H) are commonly used. Stop valves are directional and cannot be operated in the reverse direction. It’s also not advisable to pour it upside down. In our production and daily life, straight-through, small-bore globe valves were commonly used in the past, but they are now gradually being replaced by ball valves. 1.3 Ball valves: Compared to gate valves and globe valves, ball valves are a newer type of valve that are gradually being widely used. Its working principle is as follows: the valve core is a sphere with a passage, and the valve stem is used to control the 90° rotation of the valve core, thereby allowing the valve to be open or closed. It serves to shut off the flow in the pipeline. Advantages: In addition to the advantages of gate valves and globe valves, it also features a small size, excellent sealing (zero leakage), and ease of operation. It is currently widely used in industries such as petrochemicals, power generation, nuclear energy, aviation, and aerospace. Disadvantage: Difficult to repair. Ball valves come in two types: floating ball type and fixed ball type. In heating systems, at some key locations such as important branches and the inlet points of heat stations, with diameters below DN250, inlet ball valves are commonly used. It differs in structure from domestic ball valves: the valve bodies of domestic ball valves are generally two-piece or three-piece and connected by flanges; whereas the valve bodies of imported ball valves are unitary and welded together, resulting in fewer failure points. Its origin is in Northern Europe, countries such as Finland and Denmark where heating technologies are highly developed. Such as FINLAND’S NAVAL, VEXVE, and DENMARK’S DAFOSS, etc. Due to its excellent sealing performance and operational reliability, it has long been highly favored by users. Ball valves have no directionality and can be installed at any angle. When a welded ball valve is installed horizontally, it must be open to prevent electric sparks generated during welding from damaging the surface of the ball. When installed in a vertical pipe, if welding is carried out on the upper end, the valve must be open; if welding is done on the lower end, the valve must be closed to avoid damage to its interior caused by high temperatures. 1.4 Butterfly valves are currently the most widely used and varied type of valve in heating systems. Working principle: The valve disc is a disk that, through the rotation of the valve stem, rotates 90° within the valve seat to enable the opening and closing of the valve. It serves to shut off the flow in the pipeline. The flow rate can also be adjusted. Advantages: Simple structure, lightweight, easy to operate, and good sealing performance. Disadvantage: When fully open, the valve disc (seal ring) is eroded by the medium. In heating systems, the butterfly valves used include three-eccentric metal-sealed butterfly valves and rubber soft-sealed butterfly valves. 1.4.1 Three-eccentric metal-sealed butterfly valve: The term “three eccentricities” refers to the offsets in the relative positions of the valve shaft and the valve disc within the valve. Ordinary butterfly valves have one degree of eccentricity, meaning that the centerline of the valve shaft is offset from the centerline of the sealing surface (the centerline of the valve disc). High-performance versions have an additional degree of eccentricity, with the centerline of the valve shaft being offset from the centerline of the valve itself (the centerline of the pipeline). The purpose of this dual eccentricity is to allow the sealing surfaces to separate from each other once the valve disc is rotated 20°, thereby reducing friction (the cam effect). The triple-eccentric butterfly valve incorporates an additional unique eccentricity in addition to the aforementioned double eccentricity: an inclined cone, that is, the offset of the valve disc (the sealing surface being tilted at an angle relative to the vertical plane of the pipeline). This ensures that, within a 90° stroke range, the sealing surfaces are completely separated from each other, which enhances the cam effect and eliminates friction entirely. Meanwhile, when the valve is closed, as the sealing surfaces gradually come together, a \"wedge effect\" is generated, allowing for the tightest closure with the minimum amount of torque required. The so-called “metal seal” refers to the use of high-quality alloys that are resistant to wear, corrosion, and high temperatures for the valve seat and sealing rings. To prevent direct contact between the sealing ring and the valve seat, a flexible contact is designed between them, thereby creating an “elastic metal seal” that ensures a tight seal when closed and friction-free operation when opened. With a \"three-eccentric\" structure and supplemented by an \"elastic metal seal\", such valves are easy to operate, durable, and provide excellent sealing. Three-eccentric metal-sealed butterfly valves are generally used in the main lines and major branches of heating systems. The diameter is above DN300. Imported triple-eccentric metal-sealed butterfly valves are non-directional, but an installation direction is generally recommended and they should not be installed in the reverse orientation. Domestic versions are directional; installing them in the reverse direction results in a leakage level that is one grade worse, or a pressure rating that is one to two grades lower, than when installed in the correct direction, so they must not be installed reversely. When welding on horizontal pipes, the valve should be closed to protect the seal ring; when welding on vertical pipes, the valve should also be closed, and water should be poured onto the valve plate at the welding site to extinguish the weld spatter. When installing on horizontal pipes, it is recommended that the valve stem be horizontal or at a slight angle to the vertical direction, in order to maintain cleanliness at the bottom bearings. 1.4.2 Rubber soft-sealed butterfly valves: The butterfly disc is generally made of electroplated ductile iron, while the sealing ring is made of rubber material. Different sealing materials result in varying performance. Commonly used types include: Nitrile rubber, with a suitable operating temperature range of -12°C to +82°C; EPDM rubber, with a suitable operating temperature range of -45°C to +135°C; Heat-resistant EPDM rubber, with a suitable operating temperature range of -20°C to +150°C. Commonly used in heating systems are wafer-type (D371X) and flange-type (D341X). Models below DN125 can be driven by a handle (D71, D41X). Clamped butterfly valves are compact and lightweight, capable of quick opening and closing, easy to operate. They are simple to install and maintain, offer excellent sealing and adjustment performance, and provide good value for money; therefore, they should be widely adopted. Soft-sealed butterfly valves have no directionality and can be installed in any orientation. When storing butterfly valves, the valve disc should be opened at 4° to 5°. To prevent the sealing ring from deforming due to prolonged pressure, which could affect its sealing performance. 1.5 Check valve, also known as non-return valve or one-way valve. A commonly used valve for auxiliary purposes. Working principle: A valve that opens and closes automatically, relying on the force of the fluid itself and the weight of the valve disc. As the name implies, its function is to prevent the medium from flowing back. It is generally installed at the outlet of the water pump to prevent water hammer from damaging the pump. Commonly used in heating systems are the horizontal lift type (H41H), single-disc swing type (H44H), and double-disc butterfly type (H77H). Check valves have a directionality and must never be installed in the reverse direction. Different types of check valves have fixed installation methods based on their structure, and it is essential not to install them incorrectly. The horizontal lift type can only be installed on horizontal pipelines, with the valve disc kept in a vertical position; the single-disc swing type can also only be installed on horizontal pipelines, with the valve disc’s axis kept horizontal; the double-disc butterfly type can be installed in any orientation. 1.6 Control valve is also known as a throttle valve. It is a commonly used valve in the secondary network of heating systems. Working principle: Its appearance and structure are similar to those of a globe valve. It’s just that the sealing pair is different; the valve disc and seat of a control valve are similar to the plug and mouth of a thermos bottle, and the flow rate is adjusted by changing the flow area through the movement of the valve disc. There is a scale on the valve shaft indicating the corresponding flow rate. Function: Regulate the distribution of fluid flow between pipelines to achieve thermal balance. In heating systems, the straight-through type (T41H) was once used, but it has some disadvantages: high flow resistance and it is not suitable for vertical installation. Thus, with the advancement of technology, the balance valve (PH45F) replaced the control valve. 1.7 Balance valve: an improved type of control valve. The flow channel uses a direct-flow design, and the valve seat is made of polytetrafluoroethylene; this overcomes the problem of high flow resistance and simultaneously brings two advantages: better sealing performance and cut-off functionality. It is used in the secondary network of heat stations in heating systems, offering excellent flow regulation capabilities, and is particularly suitable for variable-flow systems. It is directional and can be installed horizontally or vertically. 1.8 Self-acting balance valve, also known as flow control valve. Its working principle is as follows: inside the valve, there is a mechanism composed of a spring and a rubber diaphragm, which is connected to the valve stem. If the flow rate increases, an unbalanced force is generated, causing the valve disc to move in the closing direction in order to reduce the flow area, lower the flow rate, and bring it back to the set value. The same is true in reverse. Thus, the flow rate behind the valve remains constant at all times, achieving the purpose of flow control. Installed at the hot water inlet of the heating system, a branching point. It automatically eliminates hydraulic imbalances, improves system efficiency, and enables economical operation. Self-acting balance valves have a directionality; do not install them in the reverse direction. 1.9 Safety valve: When the pressure of the medium exceeds a specified value, this valve opens automatically to release pressure; once the pressure returns to normal, it closes again automatically, thereby ensuring the proper operation of the system. A valve that functions in this way is called a safety valve. By structure, they are divided into: spring-type, lever-type, and pulse-type. Classified by degree of operation: a ratio of the valve disc opening height to the valve seat diameter of 10% or less is considered partial opening, while 20% to 30% is considered full opening. Classified by discharge method: when the medium is discharged through pipes, it is called a closed system; when it is discharged directly into the air, it is called an open system. Taking the spring-type type as an example for a brief explanation: The spring force balances the normal pressure exerted by the medium on the valve disc, thereby closing the sealing surface; when the medium pressure is too high, the spring is compressed and the valve disc opens, allowing the medium to flow through; once the pressure returns to normal levels, the spring force closes the valve again. Safety valves are widely used in pressure vessels. 2. Common problems in valve use a) Keep the inside of the valve clean. b) When lifting, do not tie the rope to the handwheel or valve stem. c) Ensure the valve is working properly before installation. d) During welding, the welder’s ground wire must be connected to the steel pipe at the same side of the weld joint, to prevent electric current from damaging the valve. e) Cooling measures should be applied to the valves during the welding process of medium and small diameter valves. f) Valves that are not opened and closed frequently in the pipelines should be rotated regularly. In addition, during use there are issues such as environmental corrosion and protection of the valve, corrosion and protection of the valve’s interior caused by the medium, temperature and pressure problems, as well as sealing and leakage problems. In short, although valves are small, there is a great deal of knowledge involved, which requires us to keep learning and summarizing. This post was last edited by ryn on 2009-3-15 11:32]