Types and Selection of Valves for Air-conditioning Water System

In heating and air conditioning water systems, valves are widely used to control the pressure, flow and direction of water. 1. Types, structures and characteristics of valves in heating and air-conditioning water systems Commonly used valves in heating and air-conditioning water systems can be divided into clarification valves, butterfly valves, globe valves, ball valves, plug valves, check valves, Pressure reducing valve, safety valve, steam trap, balance valve, etc. According to the driving mode, it is divided into four modes: manual, electric, hydraulic and pneumatic. According to the nominal pressure, it is divided into three categories: high pressure, medium pressure and low pressure. The working principle and characteristics of valves commonly used in heating and air-conditioning water systems are as follows:

1. The valve: The valve refers to the valve in which the closing member (the plate) moves in the vertical direction of the axis of the medium channel. The advantages are that the flow resistance coefficient is small, the torque required for opening and closing is small, and the flow direction of the medium is not restricted. The disadvantage is that the size of the structure is large, the opening and closing time is long, the sealing surface is easily damaged, and the structure is complex. The valve is divided into different types, the most common form is the parallel type and the wedge type valve, according to the structure of the valve stem, it can also be divided into a clear stem valve. (1) Three-precision parallel valve refers to the valve whose two sealing surfaces are parallel to each other. Suitable for low pressure, medium and small diameter (DN50-400mm) pipelines. (2) Wedge valve refers to the valve whose two sealing surfaces are wedge-shaped. Divided into double-analyzed board, single-analyzed board and elastic analytic board. (3) Three-shrewd stem clarification valve Since it can more intuitively display its opening and closing degree, small and medium diameters have been widely used for many years. Usually, the DN is less than or equal to 80mm. (4) Three-precision dark stem valve The stem nut is in direct contact with the medium in the valve body. Suitable for large-diameter valves and pipelines with limited installation space, such as underground pipelines.

2. Sanjing butterfly valve Its name comes from the butterfly plate of the wing-like structure. It is mainly used for cutting off and throttling on the pipeline. When the butterfly valve is used for cutting off, the elastic seal is mostly used, and the material is selected from rubber, plastic, etc. When it is used for throttling, the metal hard seal is mostly used. The advantages of butterfly valves are small size, light weight, simple structure, rapid opening and closing, good adjustment and sealing performance, and small fluid resistance and operating torque. Butterfly valve can be divided into lever type (double rocker), center symmetrical door type, offset plate type and inclined plate type according to the structure. Offset plate type should be selected for butterfly valve with nominal diameter DN<800mm.

3. Sanjing globe valve refers to the valve whose closing body (valve) moves along the centerline of the valve seat. It is generally only used for cutting off in pipelines, not for throttling, and usually the nominal diameter is limited to below DN250mm. The disadvantage is that the pressure loss is large. There are many types of globe valves, which are generally divided into straight-through type, positive type and direct-current type according to the structure. Angle globe valves are often used in refrigeration systems. The inlet channel is at a right angle of 90 degrees, which will cause pressure drop. The biggest advantage is that it is installed at the corner of the pipeline system, which not only saves 90 degree elbows, but also facilitates operation.

4. Sanjing ball valve The ball valve is evolved from the plug valve. It is mainly used to cut off, distribute and change the flow direction of the medium on the pipeline. It is characterized by the smallest fluid resistance, its resistance coefficient is equal to that of the pipe section of the same length, fast opening and closing, reliable sealing, compact structure, easy operation and maintenance, so it is widely used in many occasions. The ball valve can be divided into the following three types according to the structure of the ball: (1) The floating ball valve has a simple structure and good sealing performance. Since the load of the working medium on the ball is all transferred to the valve seat seal ring at the outlet end, this structure It is only suitable for medium and low pressure occasions, and its disadvantage is that it is difficult to assemble, requires high manufacturing precision, and has a large operating torque. (2) Sanjing fixed ball valve The ball of the ball valve is fixed and supported by two fixed shafts connected with the ball. Under the action of the medium pressure, the ball will not be displaced. It is suitable for high pressure and large diameter pipes. (3) Elastic ball valve is suitable for high temperature and high pressure medium. There is an elastic groove on the sphere, this structure can reduce the friction between the two sealing surfaces when opening and closing, and also reduce the operating torque.

5. Plug valve The important feature of the plug valve is that it is easy to adapt to the multi-channel structure, so that one valve can obtain two, three or even four different flow channels. There are many types of cocks, which can be generally divided into: (1) Tightening type is generally used in occasions where PN is less than or equal to 0.6MPa. (2) Packing type It is widely used in the occasions where the nominal pressure PN>1.0MPa. (3) Self-sealing type It is mainly realized by the pressure of the medium itself, and it is mostly used for air medium. The spring under the self-sealing structure can play the role of preload and compression. In addition, there are three-way and four-way plug valves, which are mostly used for distribution and reversing.

6. Check valve Also known as reverse flow valve, check valve, back pressure valve and one-way valve. It is a valve used to prevent the backflow of the medium in the pipeline and equipment. It is automatically opened and closed by the force generated by the flow of the medium itself in the pipeline. The disc of the swing return valve rotates around the pin shaft outside the valve seat, and is divided into single disc and multi-lobe. The former is generally used for nominal diameter DN50-500mm, and the latter is generally used for nominal diameter DN greater than or equal to 600mm. There is also an empty discharge check valve, which is used for the outlet of the boiler feed pump to prevent the backflow of the medium and play the role of emptying; the new slow-closing check valve has the effect of eliminating water hammer; the diaphragm check valve has good water hammer prevention performance; The ball check valve is an ideal product to prevent the backflow of the medium.

7. The pressure reducing valve reduces the pressure of the medium through the throttling and adjustment of the valve opening, and through the direct action of the medium pressure behind the valve, the pressure behind the valve automatically meets the predetermined requirements. Usually, the pressure P2 after the pressure reducing valve should be less than 0.5 times the pressure P1 before the valve, that is, P2>0.5P1. According to the structure, the pressure reducing valve is divided into piston type, film type, bellows type, spring film type, lever spring type, etc.

8. Safety valve It is used as a safety protection device on equipment, devices and pipelines to prevent the pressure of the medium from exceeding the specified value. There are three main structures of safety valve: lever type, spring type and pilot type (pulse type). Its advantages are small size and light weight. The sensitivity is high, and the installation position is not limited; the disadvantage is that the force acting on the valve stem changes with the deformation of the spring. Spring-type safety valve is divided into micro-opening and full-opening, with and without a spanner, closed and open and other different types.

9. Traps Traps are suitable for steam heating equipment and pipelines to automatically remove condensed water, air and other non-condensable gases, and prevent the loss of steam, that is, to prevent steam and drainage. When selecting, first select the type according to the maximum displacement of condensate and the pressure difference between the inlet and outlet, and then correct it. The correction coefficient is 1.5-4; secondly, it must be installed reasonably. Steam traps can be divided into the following categories according to the opening and closing forms: (1) Mechanical steam traps: the opening and closing parts are driven by the level of condensate. (2) Thermostatic steam trap: the valve opening is driven by the change of condensate temperature. (3) Thermodynamic steam trap: the valve opening is driven by the dynamic characteristics of the condensate.

10. Balance valve Balance valve is a valve used to measure and adjust to the design flow in the hydraulic pipe network, and has the necessary measurement accuracy. 2. Several references for valve selection in heating and air-conditioning water systems is widely used. (2) Globe valve The valve body is long, and the dial-type adjusting rod has good adjustment performance. It is suitable for the occasions where the site is spacious and the diameter of the pipe is small (generally DN is less than or equal to 150mm). (3) Butterfly valve The valve body is short, and the handle-type adjustment rod has slightly poor adjustment performance and higher price, but the adjustment operation is easy. It is suitable for occasions with small site and large pipe diameter (generally DN>150mm). 2. For the adjustment of the chiller, the inlet and outlet of the heat exchanger, and the main pipeline, gate valve, globe valve or butterfly valve can be selected according to the situation. 3. On the sub and water collector, because the main function is to adjust, generally choose a globe valve or a gate valve. 4. One valve is installed at the inlet of the water pump, and two valves are installed at the outlet. Among them, the outlet end is close to the valve on one side of the pump, and the other two valves can choose gate valve, globe valve or butterfly valve. 5. Stop valve or ball valve can be selected for the small diameter pipeline at the inlet and outlet of the heating and air conditioning terminal equipment. 6. Balance valves can be installed on the horizontal pipes of each layer of multi-layer and high-rise buildings to balance the flow of each layer. 7. A drain valve is installed at the lowest point of the water tank, pipeline and equipment. Since it is not used for adjustment, it is advisable to select a valve that can be shut off tightly, such as a gate valve and a globe valve. 8. In steam-condensed water piping systems, such as steam heating systems, boiler water systems, steam lithium bromide chillers, and steam-water heat exchanger systems, a pressure reducing valve is generally installed at the steam inlet; Install a safety valve; install a trap at the place where the condensate is drained. 9. The exhaust valve on the heating and air-conditioning water system generally adopts a plug valve.