Selection and setting location of valves
1. Valves used on water supply pipelines are generally selected according to the following principles
（1）When the pipe diameter is not greater than 50mm, globe valves should be used, and gate valves and butterfly valves should be used when the pipe diameter is greater than 50mm.
（2）When adjusting flow and water pressure, it is advisable to use regulating valves and shut-off valves.
（3）For areas with low water flow resistance (such as on the suction pipe of a water pump), gate valves should be used.
（4）Gate valves and butterfly valves should be used on pipe sections where water flow needs to flow in both directions, and globe valves should not be used.
（5）Butterfly valves and ball valves should be used in areas with small installation space.
（6） Globe valves should be used on frequently opened and closed pipe sections.
（7） It is advisable to use a multifunctional valve on the outlet pipe of water pumps with larger diameters.

2. Valves should be installed in the following parts of the water supply pipeline
（1）The water supply pipeline in residential areas is led in from the municipal water supply pipeline.
（2） The nodes of the outdoor circular pipe network in residential areas should be set up according to the separation requirements. When the circular pipe section is too long, segmented valves should be installed.
  (3) The starting point of the branch pipe or household pipe connected from the main water supply pipe in the residential area.
  (4) Household pipes, water meters, and various branch risers (bottom of riser, upper and lower ends of vertical circular pipe network riser).
  (5) Branch pipes of circular pipe network and connecting pipes of branched pipe network.
  (6) The starting point of the indoor water supply pipeline leading to households, public bathrooms, etc. shall be set when there are three or more water distribution points on the water distribution branch pipe.
  (7) The outlet pipe of the water pump and the suction pump of the self filling water pump.
  (8) The inlet and outlet pipes and drain pipes of the water tank.
  (9)The inlet water supply pipe of equipment (such as heaters, cooling towers, etc.).
  (10)Distribution pipes for sanitary appliances (such as large and small urinals, washbasins, showers, etc.).
  (11) Some accessories, such as automatic exhaust valves, pressure relief valves, water hammer eliminators, pressure gauges, sprinkler hydrants, and the front and back of pressure reducing valves and backflow preventers.
(12) A drain valve should be installed at the lowest point of the water supply network.

3.  Check valves should generally be selected based on factors such as their installation location, water pressure in front of the valve, sealing performance requirements after closure, and the size of water hammer caused during closure
When the water pressure in front of the valve is low, it is advisable to use swing, ball, and shuttle check valves.
When the sealing performance after closure is required to be tight, it is advisable to choose a check valve with a closing spring.

When it is required to weaken the closing water hammer, it is advisable to choose a quick closing sound-absorbing check valve or a slow closing check valve with a damping device.

4. The valve stem or core of the check valve should be able to close on its own under the action of gravity or spring force.

（1） Check valves should be installed on the following sections of the water supply pipeline
Introduce onto the pipe; On the inlet pipe of a sealed water heater or water equipment; On the water pump outlet pipe; On the outlet pipe section of the water tank, water tower, and high ground water tank that share the same inlet and outlet pipe.
Note: Pipe sections equipped with backflow preventers do not require the installation of check valves.

（2） The following parts of the water supply pipeline should be equipped with exhaust devices
a. Intermittent use of water supply networks should have automatic exhaust valves installed at the end and highest point of the network.
b. For sections of the water supply network with obvious undulations and accumulation of air, automatic exhaust valves or manual valves have been installed at the peak points of this section for exhaust.
c. When using an automatic pneumatic water tank for pneumatic water supply, the highest point of the water distribution network should be equipped with an automatic exhaust valve.

5、 Advantages and disadvantages of various valves
(1) Gate valve
Gate valve refers to a valve that moves the closing element (gate) vertically along the axis of the channel. It is mainly used to cut off the medium in the pipeline, that is, fully open or fully closed. Generally, gate valves cannot be used to regulate flow. It can be suitable for low temperature and high pressure as well as high temperature and high pressure, and can be made of different materials according to the valve. But gate valves are generally not used in pipelines that transport media such as mud.
Advantages: ① Low fluid resistance; ② The required torque for opening and closing is relatively small; ③ It can be used on ring network pipelines where the medium flows in two directions, which means that the flow direction of the medium is not restricted; ④ When fully open, the sealing surface is less susceptible to erosion by the working medium than the shut-off valve; ⑤ The body structure is relatively simple and the manufacturing process is good; ⑥ The structural length is relatively short.
Disadvantages: ① The external dimensions and opening height are relatively large, and the required installation space is also large; ② During the opening and closing process, the sealing surface is subject to relative friction and significant wear, which can even cause scratches at high temperatures Generally, gate valves have two sealing surfaces, which adds some difficulties to processing, grinding, and maintenance; ④ Long opening and closing time.

6. Butterfly valve
Butterfly valve is a type of valve that uses a disc type opening and closing element to rotate back and forth about 90 ° to open, close, and regulate the fluid channel.
Advantages: ① Simple structure, small size, light weight, low consumables, not suitable for use in large-diameter valves; ② Quick opening and closing, low flow resistance; ③ Can be used for media with suspended solid particles, and can also be used for powdered and granular media based on the strength of the sealing surface. Suitable for bi-directional opening and closing and adjustment of ventilation and dust removal pipelines, widely used in gas pipelines and waterways in metallurgical, light industry, power, petrochemical systems, etc.
Disadvantages: ① The flow regulation range is not large, and when it reaches 30%, the flow will increase by more than 95%. ② Due to the limitations of the structure and sealing materials of butterfly valves, they are not suitable for use in high-temperature and high-pressure pipeline systems. The general working temperature is below 300 ℃ and below PN40 The sealing performance is relatively poor compared to ball valves and globe valves, so it is used in places where the sealing requirements are not very high.

7. Ball valve
It evolved from a plug valve, and its opening and closing components are a sphere that rotates 90 ° around the axis of the valve stem to achieve the purpose of opening and closing. Ball valves are mainly used in pipelines to cut off, distribute, and change the direction of medium flow. Ball valves designed with V-shaped openings also have good flow regulation functions.
Advantages: ① It has the lowest flow resistance (actually 0); ② Because it does not get stuck during operation (in the absence of lubricants), it can be reliably applied to corrosive media and low boiling liquids; ③ Can achieve complete sealing within a large range of pressure and temperature; ④ It can achieve fast opening and closing, and the opening and closing time of some structures is only 0.05-0.1s to ensure that it can be used in the automation system of the test bench. When quickly opening and closing the valve, there is no impact during operation. ⑤ The spherical closure component can automatically locate at the boundary position; ⑥ The working medium is reliably sealed on both sides; ⑦ When fully open and fully closed, the sealing surfaces of the ball and valve seat are isolated from the medium, so the medium passing through the valve at high speed will not cause erosion of the sealing surface; ⑧ Compact in structure and light in weight, it can be considered the most reasonable valve structure for low-temperature medium systems; ⑨ The valve body is symmetrical, especially the welded valve body structure, which can withstand the stress from the pipeline well The closing component can withstand the high pressure difference when closed The ball valve with a fully welded valve body can be directly buried underground, ensuring that the valve components are not corroded. The maximum service life can reach 30 years, making it the most ideal valve for oil and natural gas pipelines.
Disadvantages: ① Because the main seat sealing ring material for ball valves is polytetrafluoroethylene, which is inert to almost all chemicals, and has comprehensive characteristics such as low friction coefficient, stable performance, not easy to age, wide temperature range, and excellent sealing performance. However, the physical properties of polytetrafluoroethylene, including its high coefficient of expansion, sensitivity to cold flow, and poor thermal conductivity, require the design of valve seat seals to revolve around these characteristics. So, when the sealing material becomes hard, the reliability of the seal is compromised. Moreover, the temperature resistance level of polytetrafluoroethylene is relatively low and can only be used at temperatures below 180 ℃. Beyond this temperature, the sealing material will age. When considering long-term use, it is generally only not used at 120 ℃ Its regulating performance is relatively inferior to that of a shut-off valve, especially for pneumatic valves (or electric valves).

8. Globe valve
It refers to a valve where the closing part (disc) moves along the centerline of the valve seat. According to the movement of the valve disc, the change in the valve seat opening is directly proportional to the valve disc stroke. Due to the relatively short opening or closing stroke of the valve stem and the highly reliable cut-off function, as well as the proportional relationship between the change of the valve seat opening and the stroke of the valve disc, this type of valve is very suitable for regulating flow. Therefore, this type of valve is very suitable for cutting or regulating as well as throttling purposes.
Advantages: ① During the opening and closing process, the friction between the valve disc and the valve body sealing surface is smaller than that of the gate valve, so it is wear-resistant. ② The opening height is generally only 1/4 of the valve seat channel, so it is much smaller than a gate valve; ③ Usually, there is only one sealing surface on the valve body and disc, so the manufacturing process is relatively good and easy to maintain Due to the fact that its filler is generally a mixture of asbestos and graphite, it has a higher temperature resistance level. Generally, steam valves use globe valves.
Disadvantages: ① Due to the change in the flow direction of the medium through the valve, the minimum flow resistance of the globe valve is also higher than most other types of valves; ② Due to the long stroke, the opening speed is slower than that of the ball valve.

9. Plug valve
It refers to a rotary valve with a plunger shaped closure, which opens or closes by rotating 90 ° to connect or separate the channel port on the valve plug from the channel port on the valve body. The shape of the valve plug can be cylindrical or conical. Its principle is basically similar to that of ball valves. Ball valves are developed based on plug valves and are mainly used for oilfield development, as well as in petrochemical industry.

10. Safety valve
It refers to the use of overpressure protection devices on pressure vessels, equipment, or pipelines. When the pressure inside the equipment, container, or pipeline increases beyond the allowable value, the valve automatically opens and then fully discharges to prevent the equipment, container, or pipeline and pressure from continuing to rise; When the pressure drops to the specified value, the valve should automatically close in a timely manner to protect the safe operation of equipment, containers, or pipelines.

11. Steam trap valve
In the transportation of media such as steam and compressed air, some condensate will form. In order to ensure the efficiency and safe operation of the device, these useless and harmful media should be discharged in a timely manner to ensure the consumption and use of the device. It has the following functions: ① it can quickly eliminate the generated condensate water; ② Prevent steam leakage; ③ Remove air and other non condensable gases.

12. Pressure reducing valve
It is a valve that adjusts the inlet pressure to a certain required outlet pressure, and relies on the energy of the medium itself to automatically maintain stable outlet pressure.

13. Check valve
Also known as reverse flow valve, check valve, backpressure valve, and one-way valve. These valves are automatically opened and closed by the force generated by the flow of the medium itself in the pipeline, and belong to a type of automatic valve. Check valves are used in pipeline systems, and their main functions are to prevent medium backflow, prevent pump and drive motor reversal, and prevent the discharge of container medium. Check valves can also be used on pipelines that provide supply to auxiliary systems whose pressure may rise above the system pressure. It can be mainly divided into swing type (rotating according to the center of gravity) and lift type (moving along the axis).