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Working principle of various valves of heating air conditioning water system

Release Date: October 24, 2015

Valves are widely used in heating air-conditioning water systems to control the pressure, flow and direction of water. Types and working principles of valves for heating and air conditioning water systems: Valves commonly used in heating and air conditioning water systems can be divided into gate valves, butterfly valves, globe valves, ball valves, plug valves, check valves, and pressure reducing valves according to the structure and function of the valve body , Safety valves, traps, balance valves, etc. According to the driving mode, it is divided into four modes: manual, electric, hydraulic and pneumatic. According to the nominal pressure, it can be divided into high pressure, medium pressure and low pressure. The working principle and characteristics of commonly used valves for heating and air conditioning water systems are as follows:

gate

A gate valve is a valve in which the closing member (panel) moves vertically along the axis of the media channel. The advantage is that the flow resistance coefficient is small, the torque required for opening and closing is small, and the medium flow direction is not restricted. The disadvantages are large structure size, long opening and closing time, easy sealing surface damage, and complicated structure. The gate valves are divided into different types. The most common types are parallel and wedge gate valves. Depending on the structure of the valve stem, it can also be divided into open stem gate valves. The gate valve can be divided into the following four types according to the structure:

(1) Parallel gate valve: refers to a gate valve whose two sealing surfaces are parallel to each other. Suitable for low pressure, medium and small diameter (DN50-400mm) pipelines.

(2) Wedge gate valve: refers to a gate valve whose two sealing surfaces are wedge-shaped. Divided into dual, single and flexible.

(3) Open pole gate valve: Because it can show its opening and closing degree more intuitively, medium and small diameters have been widely used for many years. Generally, open pole gate valve is used for DN less than or equal to 80mm.

(4) Dark-rod gate valve: its stem nut is in direct contact with the medium in the valve body. Suitable for large-caliber valves and pipelines with limited installation space, such as underground pipelines.

Butterfly valve

Its name comes from the wing-shaped disc. On the pipeline, it is mainly used for cutting and throttling. When the butterfly valve is used for cutting, it is mostly used for elastic sealing. The material is rubber, plastic, etc. When it is used for throttling, it is mostly used for metal hard sealing. The butterfly valve has the advantages of 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 gate type, offset plate type and inclined plate type according to structure. For butterfly valves with nominal diameter DN <800mm, the offset plate type should be selected.

Shut-off valve

Refers to a valve in which the closing body (valve) moves along the centerline of the valve seat. It is generally only used for cutting in the pipeline, not for throttling. Generally, the nominal diameter is limited to DN250mm. The disadvantage is the large pressure loss. There are many types of globe valves, which are generally divided into straight-through type, positive type and DC type according to the structure. Angle shut-off valves are more commonly used in refrigeration systems. Their inlet channels are at 90 degrees at right angles, which will produce pressure drops. The biggest advantage is that they are installed at the corners of the piping system, which saves 90-degree elbows and facilitates operation.

Ball valve

The ball valve is evolved from the plug valve, which is mainly used to cut off, distribute and change the medium flow direction on the pipeline. It is characterized by the smallest fluid resistance, its resistance coefficient is equal to the same length of pipe section, 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) Floating ball valve: Its structure is simple and its sealing performance is good. Because the working medium load on the ball is all transmitted to the valve seat seal at the outlet end, this structure is only suitable for medium and low pressure applications. The disadvantage is assembly Difficult, high precision required for production, and large operating torque.

(2) Fixed ball valve: The ball of the ball valve is fixed and supported by two fixed shafts connected to 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: suitable for high temperature and high pressure medium. There are elastic grooves on the sphere. This structure can reduce the friction between the two sealing surfaces during opening and closing, and also reduces the operating torque.

Stopcock

The important feature of the plug valve is that it is easy to adapt to a 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 generally be divided into:

(1) Tight-type stopcock: Generally used for occasions with PN less than or equal to 0.6MPa.

(2) Packing type plug valve: It is widely used in the place with nominal pressure PN> 1.0MPa.

(3) Self-sealing plug valve: 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 pre-tensioning and compression. In addition, there are three-way and four-way plug valves, which are mostly used for distribution and reversing.

Check valve

Also known as reverse flow valve, check valve, back pressure valve and check valve. It is a valve used to prevent the medium from flowing backward in pipes and equipment. It automatically opens and closes by the force generated by the medium flowing in the pipeline. The flap of the swing check valve rotates around the pin outside the valve seat, and is divided into single and multi-lobes. 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 air check valve, which is used to prevent the medium from flowing back and evacuate the outlet of the boiler feed pump; a new type of slow-closing check valve has the function of eliminating water hammer; a diaphragm check valve has a good performance of preventing water hammer; Ball check valve is the ideal product to prevent the medium from flowing backward.

Pressure reducing valve

It is through the throttling and adjustment of the opening valve to reduce the medium pressure, and through the direct action of the medium pressure behind the valve, the pressure behind the valve automatically meets the predetermined requirements. Generally, 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. Pressure reducing valve is divided into piston type, film type, bellows type, spring film type, lever spring type and so on.

Safety valve

It acts as a safety protection device on equipment, devices and pipelines to prevent the pressure of the medium from exceeding the specified value. The main structure of the safety valve is three types: lever type, spring type and pilot type (pulse type). Its advantages are small size and light weight. High sensitivity and unlimited installation position; the disadvantage is that the force acting on the valve stem changes with the deformation of the spring. The spring-loaded safety valve is divided into micro-opening and full-opening, with and without wrench, closed and open types.

Steam trap

Traps are suitable for steam heating equipment and pipelines, used to automatically exclude condensate, air and other non-condensable gases, and to prevent the loss of steam, that is, to block steam drainage. When selecting, first select the model according to the maximum displacement of the condensate and the pressure difference between the inlet and outlet, and also modify it. The correction factor 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 trap: The opening and closing parts are driven by the condensate level becoming public.

(2) Thermostatic steam trap: The valve opening device is driven by the temperature change of the condensed water.

(3) Thermodynamic steam trap: The valve opening is driven by the dynamic characteristics of the condensate.

Balancing valve

The balance valve is a valve used in the hydraulic pipe network to measure and adjust to the design flow rate, and has the necessary measurement accuracy. Balance valve is a special function valve, which has good flow characteristics, with valve opening degree indication, opening degree locking device and small pressure measuring valve for flow measurement. Use the special intelligent instrument, input the valve type and opening value, and directly display the flow value flowing through the balancing valve according to the measured pressure difference signal. As long as the appropriate specifications of the balancing valve are installed in each branch and the user inlet And using a dedicated smart meter for one-time debugging, the user's traffic can reach the set value.