In water systems, engineering, such as urban pipeline network, seawater desalination, sewage treatment, water pipeline, and pumping station, gate valves and butterfly valves are used in large quantities as devices to stop the flow of media. In the system, the selection and use of valves are very important. If the valve selection is unreasonable, it will not only malfunction, affect production, but also may cause accidents. The selection of the valve should be determined according to the requirements of the actual working conditions and the structural characteristics of the valve.
Basic parameters of gate valve and butterfly valve
The basic parameters should be selected according to the corresponding national standards.
(1) The nominal diameter is in accordance with GB1047-70 <Nominal diameter of pipes and piping accessories”. The nominal diameter refers to the nominal diameter of the passage where the valve and the pipeline connect. It is indicated by the word DN followed by a number.
(2) The nominal pressure is in accordance with the GB1048-90 “Nominal Pressure and Test Pressure of Pipes and Piping Accessories”. Nominal pressure is the maximum allowable working pressure of the valve at the reference temperature. It is represented by the code PN.
It should be noted that the valve can not be used under its nominal pressure under any working conditions, and the operating pressure changes with temperature. The general metal material is heating and reducing pressure, that is, when the working temperature rises, the allowable use pressure of the valve is correspondingly reduced, and its value refers to the relevant standards.
(3) In the water network, the corrosiveness of different water quality is also different. It is necessary to select the correct valve according to the PH value.
How to choose gate valve:
(1) The characteristics of the gate valve:
The flow resistance coefficient of the gate valve is small, the opening and closing are labor-saving, and the medium flow direction is not restricted. Compared with other valves, the gate valve has a large structure, along with opening and closing time, easy damage to the sealing surface, and a complex structure.
(2) The choice of open-stem gate valve and dark-stem gate valve:
The threaded part and nut of the stem of the open-stem gate valve are not corroded by the medium outside the body cavity, but the valve installation space is large. Because it can display the degree of opening and closing more intuitively. So small and medium diameters have been widely used for many years. Under normal circumstances, DN≤800mm chooses the rising stem gate valve. The threaded part of the valve stem of the dark-stem gate valve and the nut is in contact with the medium in the valve cavity, so they cannot be lubricated and are easily corroded by the medium. Concealed stem gate valve has a small structure height (about three-fifths of each open stem gate valve), so large diameter gate valves mostly use dark stem gate valves.
(3) The choice of double gate valve and single gate valve:
From the structural analysis of the gate valve, the single gate structure is simple, especially since the connection between the single gate and the valve stem is not prone to failure. The connection of the two gates of the double gate and the connection between the valve stem and the gate are complicated, but the double gate has better sealing performance and convenient processing.
The gate valve cannot be used to adjust the pressure or flow of the medium, but can only be used as a cut-off medium. In addition, the gate valve cannot be used at the end of the pipeline, that is, the outlet of the pipeline. The flow rate should be less than 3m/s. Under normal circumstances, the rising stem series single gate valve should be selected first.
How to choose a butterfly valve
(1) The characteristics of the spider valve:
The butterfly valve is small in size, light in weight, and simple in structure. The opening and closing time of the butterfly valve is short, and the sealing performance is good. The flow resistance of small and medium-sized butterfly valves is larger than that of gate valves. The use temperature of non-metallic sealing ring materials is low. Butterfly valves usually have structures such as lever type (double rocker), centrally symmetrical gate type, offset plate type, and inclined plate type.
(2) The structure of the butterfly valve:
There are currently more than 10 types of sealing between the butterfly plate and the sealing surface of the valve body, and its sealing performance should meet the requirements of GB/T13927-92 “General Valve Pressure Test”. The flow resistance of the centrally symmetric gate butterfly valve is small, but its upper and lower shaft ends are also part of the sealing surface, which is often in a friction state, which is easy to wear and causes leakage. The offset plate butterfly valve overcomes this weakness, but the flow resistance is relatively large. The offset plate butterfly valve has two series, long and short, that is, the structural length of the flange at both ends of the valve body is different. The design standard of the long series butterfly valve is GB12238-89, the structural length dimension standard is GB12221-89, and the design and structural length dimension standard of the short series butterfly valve is the American Waterway Association AWWA C5O4. The butterfly valve with a pressure of 1.6~2.5MPa should be constructed according to the long series, and other pressure grades can be selected for both.
(3) Selection of butterfly valve:
For butterfly valves with a nominal diameter of DN<800mm, a centrally symmetrical gate type (wafer type) should be selected. The butterfly valve with nominal diameter DN≥800mm should choose the offset plate type, that is, the butterfly valve with double eccentric or triple eccentric structure. The butterfly valve with a nominal pressure of PN1.6~2.5MPa should choose a long series offset rattan valve. There are two types of butterfly valves, vertical and horizontal. If it is not restricted by the installation space and location, the large-diameter butterfly valve should choose the horizontal type, because the vertical butterfly valve’s lower valve shaft is easy to deposit mud and sand, which affects the sealing of Gaining. Moreover, due to the weight of the butterfly plate and the valve shaft, the axial string movement of the butterfly plate is easy to occur, which increases the opening and closing torque of the valve and leakage of the sealing surface. This kind of structure needs to adjust the valve shaft from time to time (the lower valve shaft end is equipped with an adjustment mechanism). Lever-type butterfly valves are not suitable for high-pressure water media due to their structure. Due to their complex structure and technology, lever-type butterfly valves have developed slowly in recent years and are less used in engineering.
Comparison and application of gate valve and butterfly valve
Compared with the butterfly valve, the gate valve is mainly different in its fluid resistance coefficient, external dimensions, and sealing performance. The fluid resistance coefficient of the gate valve is relatively small, β =0.06~0.1. The fluid resistance coefficient of the butterfly valve is relatively large, generally β=0.15~0.4, the smaller the caliber, the larger the fluid resistance. Therefore, gate valves are usually used when the nominal diameter is DN≤800mm. At this time, the fluid resistance coefficient is much smaller than that of the spy valve of the same diameter, and the size of the valve is not much larger than that of the butterfly valve. The butterfly valve is selected when the nominal diameter is DN>800mm. The fluid resistance coefficient of the butterfly valve decreases sharply with the increase of its diameter. Although it is still larger than the gate valve of the same diameter, its overall size and weight will be much smaller than that of the gate valve. If the working conditions require complete sealing, a soft sealing butterfly valve should be selected. The opening and closing time of the butterfly valve is much less than that of the gate valve.
There is no very strict difference between the selection of gate valves and butterfly valves. Users can choose the corresponding valve according to the actual working conditions and the respective characteristics of the gate valve and butterfly valve. But under normal circumstances, it can be selected according to the following principles.
(1) For the valves used in the process in the system, when the fluid resistance is not a key control index, butterfly valves should be selected.
(2) Small and medium-sized valves on the pipe network. Considering their fluid resistance requirements, gate valves should be selected.
(3) Valves with larger diameters in the pipeline. Butterfly valves should be selected in consideration of their dimensions and weight.
(4) Butterfly valves should be used for valves requiring strict sealing.
Author: Sun Dianyu