[Abstract] This article briefly compares the performance, installation and working principles of various common valves
1. Selection and setting position of valve:
(1) Valves used on water supply pipelines are generally selected according to the following principles:
1. When the pipe diameter is not more than 50mm, a stop valve should be used. When the pipe diameter is more than 50mm, gate valves and butterfly valves should be used.
2. Use regulating valves and stop valves when adjusting flow and water pressure
3. Gate valves should be used for parts requiring small water flow resistance (such as on the suction pipe of a water pump)
4. Gate valves and butterfly valves should be used on pipe sections where water flow needs to flow in both directions, and stop valves should not be used.
5. Butterfly valves and ball valves should be used in areas with small installation space
6. On pipe sections that are frequently opened and closed, stop valves should be used
7. Multi-function valves should be used on the water pump outlet pipe with larger diameter
(2) Valves shall be provided at the following parts of the water supply pipeline:
1. The water supply pipeline of the residential community is introduced from the pipe section of the municipal water supply pipeline.
2. The nodes of the outdoor annular pipe network in residential areas should be set up according to separation requirements. When the annular pipe section is too long, segmented valves should be set up
3. Starting from the branch pipe connected from the main water supply pipe of the residential area or from the household pipe connected to the household
4. Household pipes, water meters and branch risers (bottom of riser pipes, upper and lower ends of vertical annular pipe network risers)
5. The branch pipes of the annular pipe network and the connecting pipes that run through the branch pipe network
6. The starting end of the water distribution pipe connecting the indoor water supply pipe to residents, public toilets, etc., and when there are 3 or more water distribution points on the water distribution branch pipe
7. Water pump outlet pipe, suction pump of self-filling water pump
8. Water inlet, outlet pipe and drain pipe of water tank
9. Water inlet and makeup pipes for equipment (such as heaters, cooling towers, etc.)
10. Water distribution pipes for sanitary utensils (such as toilets, urals, washbasins, showers, etc.)
11. Certain accessories, such as automatic exhaust valves, pressure relief valves, water hammer arresters, pressure gauges, sprinkler plugs, etc., before and after pressure reducing valves and backflow preventers, etc.
12. Drain valves should be set 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, tightness requirements after closing, and the size of water hammer caused during closing.
1. If the water pressure in front of the valve is small, swing type, ball type and shuttle type check valves should be used.
2. When the tightness performance after closing is required to be strict, a check valve with a closing spring should be used.
3. When it is required to weaken and close the water hammer, it is advisable to use a quick-closing muffler check valve or a slow-closing check valve with damping device.
4. The valve break or spool of the check valve should be able to close by itself under the action of gravity or spring force.
(4) Check valves shall be provided on the following sections of the water supply pipeline:
On the inlet pipe; on the inlet pipe of a closed water heater or water-consuming equipment; on the outlet pipe of a water pump; on the outlet pipe section of a water tank, water tower, and highland pool where the inlet and outlet pipes share the same pipe.
Note: Check valves are not required for pipe sections equipped with pipeline backflow preventers.
(5) Exhaust devices shall be installed in the following parts of the water supply pipeline:
1. Automatic exhaust valves should be installed at the end and highest point of the water supply pipe network used intermittently
2. For pipe sections with obvious undulations and accumulated air in the water supply network, automatic exhaust valves or manual valves have been set up at the peak points of the section to exhaust air.
3. For pneumatic water supply devices, when automatic air supply type pneumatic water tanks are used, an automatic exhaust valve should be installed at the highest point of the water distribution pipe network.
2. Advantages and disadvantages of various valves:
1. Gate valve:Gate valve refers to a valve whose closing member (gate) moves along the vertical direction of the channel axis. It is mainly used as a cut-off medium in the pipeline, that is, it is fully opened or fully closed. In general, gate valves cannot be used to regulate flow. It can be suitable for low temperature pressure or high temperature and high pressure, and can be used according to the different materials of the valve. However, gate valves are generally not used in pipelines transporting media such as mud
Advantages:
① Low fluid resistance;
② The torque required for opening and closing is small;
③ It can be used on ring networks where the medium flows in both directions, which means that the flow direction of the medium is not restricted;
④ When fully opened, the erosion of the sealing surface by the working medium is less than that of the stop valve;
③ The body structure is relatively simple and the manufacturing craftsmanship is good;
④ The structure length is relatively short.
Disadvantages:
① The overall size and opening height are large, and the installation space required is also large;
② During the opening and closing process, people rub against the sealing surface, causing large friction, and even easily cause scratches at high temperatures;
③ Generally, gate valves have two sealing surfaces, which adds some difficulties to processing, grinding and maintenance;
④ Long opening and closing time.
2, butterfly valve:Butterfly valve is a valve that uses a disk-type opening and closing member to reciprocate by about 90° to open, close and adjust fluid channels.
Advantages:
① Simple structure, small size, light weight, and saving consumables. Do not use it in large-diameter valves;
② Rapid opening and closing, small flow resistance;
③ Can be used for media with suspended solid particles, and can also be used for powdered and granular media depending on the strength of the sealing surface. It can be suitable for two-way opening, closing and adjustment of ventilation and dust removal pipelines, and is widely used in gas pipelines and waterways in metallurgical, light industry, electric power, petrochemical systems, etc.
Disadvantages:
① The flow adjustment range is not large. When the opening reaches 30%, the flow will reach more than 95%;
② Due to the limitations of the structure and sealing materials of the butterfly valve, it is not suitable for use in high-temperature and high-pressure pipeline systems. The general working temperature is below 300℃ and below PN40;
③ The sealing performance is poor compared to ball valves and globe valves, so it is used in places where sealing requirements are not very high.
3, ball valve:It evolved from a plug valve. Its opening and closing part is a ball, which uses the ball to rotate 90° around the axis of the valve stem to achieve opening and closing purposes. Ball valves are mainly used to cut off, distribute and change the flow direction of media on pipelines. Ball valves designed with V-shaped openings also have good flow regulation functions.
Advantages:
① has the lowest flow resistance (actually 0);
② Because it will not get stuck during work (when there is no lubricant), it can be reliably used in corrosive media and low-boiling liquids;
③ Complete sealing can be achieved within a large pressure and temperature range;
④ It can realize rapid 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 automated system of the test bench. When opening and closing the valve quickly, there is no impact in operation;
③ The spherical closing piece can be automatically positioned at the boundary position;
④ The working medium is reliably sealed on both sides;
③ When fully opened 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 surfaces;
Compact structure and light weight, it can be considered to be the most reasonable valve structure for cryogenic medium systems;
The valve body is symmetrical, especially the welded valve body structure, which can well withstand the stress from the pipeline;
The closing piece can withstand the high pressure difference during closing. 11. Ball valves with fully welded bodies can be buried directly in the ground to prevent the valve internals from being eroded. The maximum service life can reach 30 years. It is the most ideal valve for oil and natural gas pipelines.
Disadvantages:
① Because the main seat sealing ring material of ball valves is polytetrafluoroethylene, it is inert to almost all chemical substances, and has a small friction coefficient, stable performance, is not easy to age, wide temperature range and excellent sealing performance. Comprehensive characteristics. However, the physical characteristics of PTFE, including high expansion coefficient, sensitivity to cold flow and poor heat conductivity, require that seat seals be designed around these characteristics. Therefore, when the sealing material hardens, the reliability of the seal is destroyed. Moreover, polytetrafluoroethylene has a low temperature resistance level and can only be used below 180 ° C. Above this temperature, the sealing material will age. When considering long-term use, it is generally not used at 120 ° C.
② Its adjustment performance is worse than that of the stop valve, especially the pneumatic valve (or electric valve).
4, stop valve:Refers to a valve whose closure member (disc) moves along the centerline of the seat. According to this movement of the valve flap, the change in the valve seat opening is directly proportional to the stroke of the valve flap. Because the valve stem opening or closing stroke is relatively short and has a very reliable shut-off function, and because the change of the valve seat port is directly proportional to the stroke of the valve flap, it is very suitable for adjusting flow. Therefore, this type of valve is very useful for shut-off or regulation as well as throttling.
Advantages:
① During the opening and closing process, the friction between the valve disc and the sealing surface of the valve body 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 the gate valve;
③ There is usually only one sealing surface on the valve body and the valve disc, so the manufacturing process is relatively good and it is easy to maintain;
④ Since its filler is generally a mixture of asbestos and graphite, the temperature resistance level is high. General steam valves use stop valves.
Disadvantages:
① Due to the change in the flow direction of the medium through the valve, the minimum flow resistance of the stop valve is also higher than that of most other types of valves;
② Due to the long stroke, the opening speed is slower than that of the ball valve.
5, stopcock valve:It refers to a rotary valve with a plunger-shaped closing member. Through 90° rotation, the channel port on the valve plug is communicated or separated from the channel port on the valve body to realize opening or closing. 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 on the basis of plug valves. They are mainly used for oil field exploration and also used in petrochemical industries.
6, safety valve:Refers to being used as an overpressure protection device on pressurized vessels, equipment or pipelines. When the pressure in the equipment, container or pipeline rises beyond the allowable value, the valve will automatically open and then discharge the full amount to prevent the equipment, container or pipeline and the pressure from continuing to rise; when the pressure drops to the specified value, the valve should be automatically closed in time to protect the safe operation of the equipment, container or pipeline.
7Steam trap:In transporting steam, compressed air and other media, some condensed water will be formed. In order to ensure the working efficiency and safe operation of the device, these useless and harmful media should be discharged in time to ensure the consumption and use of the device. It has the following functions: ① it can quickly eliminate the generated condensed water;② prevents steam leakage;③ it eliminates air and other non-condensable gases.
8, 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 a stable outlet pressure.
9, check valve:Also known as backflow valve, check valve, back pressure 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 are an automatic valve. Check valves are used in pipeline systems. Their main role is to prevent backflow of media, reverse rotation of pumps and driving motors, and release of container media. Check valves can also be used on pipelines that supply auxiliary systems where the pressure may rise to exceed the system pressure. They can be mainly divided into swing type (rotating according to the center of gravity) and lifting type (moving along the axis).
