The conventional water production process of the water plant takes water from raw water, flocculates and settles in the sedimentation tank, filters in the filter tank, and finally outputs water from the pump room, all of which cannot be separated from the use of valves. However, the selection requirements for the valve are very different in different parts. The correct and reasonable selection can obtain the most economical service life and the best performance of the valve. The main difference lies in the diameter of the valve, the material of the valve, the type of the valve and the control method of the valve. This article mainly analyzes the selection of valves in waterworks.

1.    The role of valves in urban and rural water supply networks

　　In the process of water distribution, it is indispensable. It plays a role as a transportation intermediary. Urban and rural water supply pipe networks are widely used, and construction projects are also very large. Different types of urban and rural water supply pipe networks can meet people's different needs. In ordinary management, valves are not always applied, but in emergency situations, a qualified valve must be able to achieve flexible control and control water flow. When selecting a valve, the main focus is on the type, material, manufacturer, production technology, etc., to ensure that the quality of the valve meets the standard from all aspects.

2. Valve distribution of conventional water production system in water plant

2.1 Process valve for raw water intake

The head raw water valve is characterized by a large valve diameter and does not require frequent operation, most of which only need to be operated once a year or even a few years. The valve is in a normally open or normally closed state for a long time. Therefore, from the control method, more than 95% of the manual valves are selected. In terms of valve types, they are gate valves or butterfly valves. Since the head loss of gate valves is better than that of butterfly valves, head valves have slightly more gate valves than butterfly valves. The caliber is concentrated in 1200～1600mm, and there are valves with a caliber of 3000mm or more on the head inlet pipe.

2.2 Sedimentation tank valve

The valves of the sedimentation tank are mainly inlet valves, outlet valves and isolation valves. The outlet valve is basically in a normally open state, and the isolation valve is basically in a normally closed state, and is operated once a year. Therefore, these two types of valves use manual gate valves, with specifications ranging from 2200×1800 to 1200×840. The inlet valve of the sedimentation tank bears the important responsibility of water distribution, requiring frequent adjustment, reliable performance, and distributed in front of each sedimentation tank, the distribution is relatively scattered, all use electric control valves with intelligent control systems. It is a key link that determines the water volume of the sedimentation tank and the balance of the system. It is especially important for a system where the water production line is dynamically adjusted according to the water output, and it also needs to be dynamically tracked and adjusted by PID. At this time, for the full stroke time of the valve, The allowable start-stop frequency will have corresponding requirements.

2.3 Filter valve

　　The most common filter tank is a four-valve filter tank, which is also the most basic configuration of the filter tank. That is to say, no matter what kind of filter tank, there are at least four types of valves: inlet valve, drain valve, flush valve and clean water valve. Of course, there are also many five-valve, six-valve (V-type filter) and even seven-valve filter tanks, which are also equipped with air flush valves, primary filter valves or more equipped with an inlet valve and so on. From the structure of the valve, the butterfly valve is usually used to distinguish the clear water valve and the flush valve. The main reason is that the clear water valve and the flush valve are installed on the pipeline in the pipe gallery, and the clear water valve needs to be adjusted frequently, and the flush valve will also adjust the flushing intensity. Demand. The water intake and drainage are usually carried out through channels, so basically gate valves are used for the water intake and drain valves. The diameter of the flush valve of the filter is slightly larger than that of the clean water valve. The flush valve is basically DN600-DN800 according to the requirements of filter area, flushing intensity and flushing time, while the clean water valve is DN400-DN600 according to the requirements of flow rate and filtration speed. The size of the inlet valve and drain valve is related to the design flow rate and channel size of the sedimentation tank.

2.4 Pump room valve

　　 The valves in the pump room are mainly the inlet valve, outlet valve, maintenance valve and check valve on the pump unit. The first three kinds of valves basically use butterfly valves, the difference lies in the control method. The inlet valve and maintenance valve only need manual valves, because they are normally open. The selection of the outlet valve of the water pump must first satisfy that the water pump can be started and stopped when the valve is closed. If the water outlet valve is used to adjust the water volume so that the water pump has different working conditions to meet the requirements, the water outlet valve also needs to be controlled by a regulating intelligent device; Secondly, it needs to be equipped with a check valve, which can quickly close the valve when the pump unit is unexpectedly shut down to prevent the pump from reversing for a long time, and to avoid the occurrence of water hammer when the pump is stopped.

2.5 Valves of water plant dosing system

The characteristics of the water plant dosing system determine that the valve used on it has a small caliber and a small flow, but has high requirements for flow adjustment, high precision and fast adjustment speed. Therefore, flow control valves are usually used, which make the medium flow capacity only depends on the structure of the valve itself, so that the flow adjustment can be completed at one time. The medium in the pipeline of the water plant dosing system is mainly aluminum sulfate, ammonium sulfate, sodium hypochlorite and other solutions, which requires the valve to also have the ability to resist corrosion. Ammonium sulfate has a corrosive effect on copper. Valves made of copper materials cannot be used. Stainless steel ball valves or UPVC ball valves are generally used. Sodium hypochlorite is corrosive to steel and iron. Valves made of steel and iron materials cannot be used. Generally, UPVC ball valves or valves with PTFE lined Hastelloy valve cores are used.

 In addition, the dosing system will also use stainless steel ball valves, UPVC ball valves or butterfly valves as on-off valves, switching valves, etc.

3 Basis for valve selection

3.1 The purpose, operating conditions and control methods of the selected valve.

3.2 Installation dimensions and overall dimensions, and connection methods with pipelines.

3.3 Requirements for valve fluid characteristics: head loss, flow characteristics, sealing grade, etc.

3.4 The nature of the working medium: working pressure, working temperature, corrosion performance, whether it contains solid particles.

3.5 If the valve is to be used for control, the following additional parameters must be determined: control method, maximum and minimum flow requirements, pressure drop for normal flow, pressure drop when closed, and maximum and minimum inlet pressure of the valve.

4 Valve selection procedure

4.1 The sealing performance of the valve must meet the requirements of the medium.

4.2 Determine the process requirements, that is, determine the pressure, flow and back pressure in the pipeline on site, and then determine the nominal diameter of the pipeline and the diameter of the valve seat hole to determine the form and type of the valve.

4.3 Choose the type of valve according to the diameter of the pipeline and the site conditions, and combine the process parameters in the second step to select the valve. At the same time, according to the installation position of the valve, determine the structure length, flange connection form and size, and opening of the selected valve. And the dimension of the height direction of the valve after closing, the size and number of bolt holes connected, the overall dimension of the valve, etc.

4.4 Determine the material of the housing and internal parts of the selected valve according to the medium, working pressure and working temperature conveyed by the pipeline.

4.5 The transmission mechanism is selected according to the transmission form. The butterfly valve of the water plant is mostly a multi-turn rotating mechanism, and the gate valve is a straight-stroke transmission mechanism.

4.6 Determine the control method. For the pneumatic control system, it is the selection of the cylinder and the supporting transmission mechanism. For the electronic control system, it is the selection of the electric actuator. The selection of the electric actuator is to determine the speed ratio and output torque of the transmission mechanism, the interface type, and the speed and power of the motor according to the required full stroke time of the valve, the torque of the valve and other parameters.

4.7 Consider the protection level of the actuator, the communication method, and whether to use other configurations such as thyristor.

4.8 Use existing information: valve product catalog, valve product samples, etc. to select appropriate valve products.

　　 In summary, with the improvement of automation level, water plants have higher and higher requirements for valves, and more and more, but no matter what changes, the conventional selection is roughly within the scope of this article. The main valves are mentioned in this article. Of course, the water plants also use exhaust valves, solenoid valves, back pressure valves, etc., which will not be described in detail here.