Selection and Setting of valves
How to choose a Valve Used in Water Supply Pipelines?
The valves used in water supply pipelines are generally selected according to the following principles:
- When the pipe diameter is not larger than 50mm, a stop valve should be used, and when the pipe diameter is larger than 50mm, a gate valve or a butterfly valve is used.
- When the flow and water pressure need to be adjusted, a regulating valve and a stop valve should be used.
- For parts requiring small resistance to water flow (such as on the suction pipe of a pump), a gate valve should be used.
- A gate valve or a butterfly valves should be used on the pipe sections where the water needs to flow in both directions, and stop valves must not be used.
- A butterfly valve or a ball valve should be used in small installation space.
- It is advisable to use a stop valve on the frequently opened and closed pipe sections.
- Large-diameter pumps should use multi-function valves on the outlet pipe.
Where to install Valves?
Valves shall be provided on the following parts of the water supply pipeline:
- The introduction section of water supply pipeline for the residential area from the municipal water supply pipeline
- The nodes of the outdoor circular pipe network in the residential area shall be set according to the requirements of separation. When the annular pipe section is too long, a segmented valve should be provided.
- The beginning of the branch pipe or the household pipe from the water supply main pipe for the residential district
- The household pipe, water meter and each branch riser (the bottom of the riser, the upper and lower ends of the vertical ring network riser)
- The branch pipe of the annular pipe network and the connecting pipe that runs through the branched pipe network
- The starting point of the water distribution pipe for indoor water supply pipes to residents, public toilets, etc. and when there are 3 or more water distribution points on the water distribution branch pipe
- The outlet pipe of the water pump and the suction pump of the self-filling water pump
- The inlet and outlet pipes and drain pipes of the water tank
- Water inlet and water supply pipes of equipment (such as heaters, cooling towers, etc.)
- Water pipes for sanitary appliances (such as large and small urinals, washbasins, showers, etc.)
- The front and back of the pressure reducing valve and backflow preventer of some accessories like the front of the automatic exhaust valve, pressure relief valve, water hammer eliminator, pressure gauge, sprinkler, etc.
- A drain valve should be set at the lowest point of the water supply pipe network.
How to choose the Right Check Valve Types?
The check valve should generally be selected according to factors such as its installation location, water pressure before the valve, the performance requirements after closing, and the size of the water hammer caused when closing.
- When the water pressure before the valve is small, swing check, ball and shuttle check valves should be used.
- When the tight performance is required after closing, a check valve with a closing spring should be used.
- When it is required to weaken and close the water hammer, a quick-closing muffler check valve or a slow-closing check valve with a damping device should be used.
- The check valve or spool of the check valve should be able to close by itself under the action of gravity or spring force.
Where to Install Check Valves?
Check valves shall be provided on the following sections of the water supply pipeline:
- On the introduction section.
- Closed water heater or water inlet pipe of water equipment.
- On the water outlet pipe of the pump.
- The outlet pipe of the water tank, water tower and highland pool that use a same pipeline for inlet and outlet pipe
Note: It is not necessary to install check valve in the pipeline section equipped with pipeline backflow preventer.
Where to Install Air Valves?
The following parts of the water supply pipeline shall be provided with exhaust devices:
- The end and the highest point of the network pipe which is not frequently used should be provided with automatic exhaust valves.
- The water supply pipe network which has obvious undulations and accumulated air sections should be set automatic exhaust valve or manual valve at the peak of this section.
- When the air-pressure water supply device uses automatic air-supply water tank, the highest point of its water distribution network should be provided with an automatic exhaust valve.
Advantages and disadvantages of various valves
1. Gate valve
A gate valve is a valve that moves the closing piece (gate plate) in the vertical direction of the axis of the channel. It is mainly used as a cutting medium on the pipeline, that is, fully open or fully closed. Generally, the gate valve cannot be used for regulating flow. It can be used for low temperature and low pressure as well as high temperature and high pressure, and can be based on different materials of the valve. But the gate valve is generally not used in pipelines that transport mud and other media.
Advantages of Gate Valves
- Small fluid resistance;
- The opening and closing moment is small;
- It can be used on the ring network pipeline where the medium flows in both directions, which means that the flow direction of the medium is not restricted;
- When fully opened, the sealing surface is less eroded by the working medium than the stop valve;
- The shape structure is relatively simple, and the manufacturing process is good;
- The structure length is relatively short.
Disadvantages of Gate Valves
- The external dimensions and opening height are large, and the space required for installation is also large;
- In the process of opening and closing, the sealing surface is relatively frictional, and the abrasion is large, and it is easy to cause abrasion even at high temperature;
- Generally, the gate valve has two sealing surfaces, which adds some difficulties to processing, grinding and maintenance;
- Long opening and closing time.
2. Butterfly valve
The butterfly valve is a valve that uses a disc-type opening and closing member to revolve around 90° to open, close and adjust the fluid channel.
Advantages of Butterfly Valve
- It is simple in structure, small in size, light in weight, saves consumables, and is not used in large-caliber valves;
- Quick opening and closing, small flow resistance;
- It can be used for media with suspended solid particles, and it can also be used for powdery and granular media depending on the strength of the sealing surface. It can be used for two-direction opening and closing and adjustment of ventilation and dust removal pipelines, and is widely used in gas pipelines and waterways of metallurgy, light industry, power, petrochemical systems, etc.
Disadvantages of Butterfly Valve
- The flow adjustment range is not large. When it is turned on to 30%, the flow will be over 95%;
- Due to the structure and sealing material of butterfly valve, it is not suitable for high temperature and high pressure pipeline system. The general working temperature is below 300 ° C and below PN40;
- Compared with ball valves and globe valves, the sealing performance is poor, so it is used in places where the sealing requirements are not very high.
3. Ball valve
It is evolved from a plug valve. Its opening and closing member is a ball. The ball is rotated 90 ° around the axis of the valve stem to achieve the purpose of opening and closing. The ball valve is mainly used to cut off, distribute and change the medium flow direction on the pipeline. The ball valve designed as a V-shaped opening also has a good flow adjustment function.
Advantages of Ball Valve
- It has the lowest flow resistance (actually 0);
- Because it will not get stuck during work (without lubricant), it can be reliably used in corrosive mediu and low boiling point liquids;
- In a wide range of pressure and temperature, complete sealing can be achieved;
- Quick opening and closing can be realized, and the opening and closing time of some structures is only 0.05 ~ 0.1s, so as to ensure that it can be used in the automation system of the test bench. When opening and closing the valve quickly, the operation is without impact;
- The spherical closure can be positioned automatically at the boundary position;
- The working medium is reliably sealed on both sides;
- 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;
- Its compact structure and light weight can be considered as the most reasonable valve structure for low temperature media systems;
- The valve body is symmetrical, especially the welded valve body structure, which can well withstand the stress from the pipeline;
- The closing member can withstand the high pressure difference when closing.
The fully welded ball valve can be directly buried in the ground, so that the internal parts of the valve are not eroded, and the maximum service life is up to 30 years. It is the most ideal valve for oil and gas pipelines.
Disadvantages of Ball Valve
- Because the material of the main sealing ring of valve seat is polytetrafluoroethylene, it is inert to almost all chemicals. And it has the comprehensive characteristics of small friction coefficient, stable performance, not easy to age, wide temperature range and excellent sealing performance. However, the physical characteristics of PTFE, including higher expansion coefficient, sensitivity to cold flow and poor thermal conductivity, require that the design of the valve seat seal must be based on these characteristics. Therefore, when the sealing material becomes hard, the reliability of the seal is impaired. Moreover, PTFE has a low temperature resistance grade and can only be used at temperatures below 180 ° C. Beyond this temperature, the sealing material will age. In the case of long-term use, it is generally not used at 120 ° C.
- Its regulating performance is worse than that of globe valves, especially pneumatic valves (or electric valves).
4. Globe valve
It refers to the valve in which the closing member (valve) moves along the center line of the valve seat. According to this type of movement of the valve disc, the change in the valve seat port is proportional to the valve stroke. Because this kind of valve has a relatively short opening or closing stroke, and has a very reliable shut-off function, and because the change in the valve seat port is proportional to the stroke of the valve, it is very suitable for regulating flow. Therefore, this kind of valve is very suitable for shut-off or regulation and throttling.
Advantages of Globe Valve
- 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;
- The opening height is generally only 1/4 of the valve seat channel, so it is much smaller than the gate valve;
- Usually there is only one sealing surface on the valve body and valve disc, so the manufacturing process is better and it is easy to maintain;
- Because its filler is generally a mixture of asbestos and graphite, its temperature resistance level is high. Generally, the steam valve uses a globe valve.
Disadvantages of Globe Valve
- Because the flow direction of the medium through the valve has changed, the minimum flow resistance of the globe valve is also higher than most other types of valves;
- Because the stroke is longer, the opening speed is slower than the ball valve.
5. Plug 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 with or separated from the channel port on the valve body to achieve opening or closing. The shape of the valve plug can be cylindrical or conical. Its principle is basically similar to the ball valve. The ball valve was developed on the basis of a plug valve. It is mainly used for oilfield exploitation and also used in petrochemical industry.
6. Safety valve
It refers to a pressure vessel, equipment or pipeline as an overpressure protection device. When the pressure in the equipment, container or pipeline rises above the allowable value, the valve automatically opens, and then the full discharge is performed to prevent the equipment, container or pipeline and 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.
7. Steam trap
In the transport of steam, compressed air and other media, 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 time to ensure the device’s consumption and use. It has the following functions: ① it can quickly remove the condensed water generated; ② it prevents steam leakage; ③ it excludes 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 to keep the outlet pressure automatically stable.
9. Check valve
It is also called reverse flow 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, which belongs to an automatic valve. The check valve is used in pipeline system, and its main function is to prevent the medium from flowing backward, prevent the pump and drive motor from reversing, and release the container medium. The check valve can also be used in supply pipelines whose auxiliary pressure may rise above the system pressure. It can be divided into swing type (rotation according to the center of gravity) and elevation type (moving along the axis).
