Rules for selection of valves in water works
The valves are indispensable for the conventional water production processes in water plants, such as raw water intake, flocculation and sedimentation in sedimentation tank, filtration in filter tank and water supply from the final outlet pump house.However, the selection requirements for valves vary greatly in different locations.The main differences are the diameter of the valve, the type of valve and the control method of the valve.
Valve Distribution of Conventional Water System in Waterworks
1. Process valves for raw water intake
The characteristics of the head raw water valve are that the valve diameter is large and it does not need to be operated frequently. Most of the valves need to be operated only once a year or even several years. The valves are normally open or closed for a long time.Therefore, in terms of control methods, more than 95% of the valves are manual valves, all of which are gate valves or butterfly valves. Because the head loss of gate valves is better than that of butterfly valves, the head valve is slightly more than that of butterfly valves.The diameter is centralized in 1200~1600mm, and 3000mm valve is also present in the head intake pipe.
2. Valve of sedimentation tank
The valves of sedimentation tank are mainly intake valve, outlet valve and isolation valve.Outlet valve is normally open, isolation valve is normally closed and operated once a year. Therefore, manual gate valves are used for both valves, and the specifications are distributed between 2200 x 1800 and 1200 x 840.The intake valves of sedimentation tank are responsible for water distribution, which requires frequent adjustment, reliable performance and scattered distribution in front of each sedimentation tank. Electric control valves with intelligent control system are used.It is a key link that determines the water quantity and system balance of sedimentation tank. It is particularly important for water-making line to be a system which is regulated dynamically according to the water output, and PID dynamic tracking adjustment is also required. At this time, the allowable starting and stopping frequency will have corresponding requirements for the full stroke time of the valve.
3. Filter valve
The most common type of filter is a four-valve filter, which is also the most basic configuration of the filter.That is, regardless of the filter type, there are at least four types of valves: intake valve, drain valve, flushing valve and clean water valve.Of course, now there are many 5-valve, 6-valve or even 7-valve filters, which are also equipped with air scour valve, primary filter valve or one more intake valve, etc.The butterfly valve is usually used to distinguish the water- cleaning valve from the flushing valve in terms of its structure. This is mainly due to the fact that both the water-cleaning valve and the flushing valve are installed in the pipeline in the pipe gallery and the frequent adjustment of the water- cleaning valve is required, while the flushing valve also has the need to regulate the flushing intensity.As water intake and drainage are usually carried out through channels, gate valves are basically used for both intake and drainage valves.The diameter of the flushing valve of the filter is slightly larger than that of the clean water valve. The flushing valve is basically between DN600 and DN800, while the clean water valve is between DN400 and DN600.Dimensions of intake and drain valves are related to channel dimensions.From the point of view of control method, all four types of valves are controlled electrically or pneumatically.However, the clean water valve must be an adjustable intelligent control mode in accordance with the requirements of the inlet valve of the sedimentation tank.In addition, considering the comprehensive wiring and modular design method, the control mode of bus type or PLC site type can be selected for the filter valve.The former has the advantage of saving wiring space, while the latter has the advantage of separating the cell filter for easy maintenance, management and transplantation.
4. Pump house valve
The valves in the pump house are mainly intake valve, outlet valve, overhaul valve and check valve on the pump unit.Butterfly valves are basically used for the first three valves, but the difference lies in the control mode.Intake and overhaul valves only need manual valves, because one is normally open and the other is normally closed, while the outlet valve needs to be opened and closed every time the pump is opened or stopped. If the factors of using the outlet valve to regulate the water flow and make the pump have different working conditions to meet the demand are considered, the outlet valve also needs to be controlled by an adjustable intelligent device.To prevent reverse flow and water hammer, silent check valves and slow-closing check valves with slight resistance are usually used.Sometimes, multi- function valves are selected to integrate the water outlet check function.
Valves for water plant dosing system
The characteristics of water plant dosing system determine that the valve used above has small diameter and small flow rate, but it requires high flow regulation, high accuracy and fast speed. Therefore, flow control valve is usually selected. This valve makes the flow capacity of medium only depend on the structure of the valve itself, so that the flow regulation can be completed once.In addition, the medium in the pipeline of the dosing system is ammonium sulfate, sodium hypochlorite and other solutions, which requires the valve to also have corrosion resistance.In addition, ball valves are used as switch valves and back pressure valves in the dosing system.
Steps for valve selection
Determine the process requirements, i.e. determine the pressure and flow rate in the field pipeline.
Select the type of valve according to the diameter of the pipeline and the on- site operating conditions, and combine the process parameters of the first step to select the valve. At the same time, decide whether to install the extension pipe according to the installation position of the valve.
Drive mechanism is selected according to the transmission type. Butterfly valves in water plants are mainly multi-turn rotary mechanism, while gate valves are straight-stroke driving mechanism.
To determine the control mode, the selection of cylinder and matching driving mechanism is for air control system, and electric actuator is for electric control system.For the selection of electric actuator, speed ratio and output torque of driving mechanism, interface type, speed and power of motor are determined according to the parameters such as full stroke time and torque of valve.
Considering the protection level and communication method of the actuator, whether to choose other configurations such as SCR or not.
Considering the protection level and communication method of the actuator, whether to choose other configurations such as SCR or not.With the improvement of automation level, the requirements for valves in water plants become higher and higher. However, in any case, the general selection is within the scope described in this paper.Main valves are mentioned in this paper, of course, water plants also use exhaust valves, solenoid valves, etc.