1. The function of the valve in the water supply pipeline
The water supply pipeline distributes the tap water from the water plant to thousands of households. Therefore, the water supply pipe is distributed in every corner of the city, and its length is from hundreds of kilometers to thousands of kilometers.
The irreplaceability of water and the necessity of people's survival determine the importance of the safe operation of the water supply pipeline. However, some failures often occur for pipelines due to various subjective and objective reasons. Pipelines need to be renewed and reconstructed. Therefore, it is inevitable to avoid cutting off the water in partial pipelines. In order to reduce the scope of the water cut, it is necessary to install control valves appropriately in the pipeline. There are thousands of valves in a city's water supply pipeline, which are randomly distributed under the city streets. The valve is required to be opened well to reduce water loss. The valve should be able to be closed quickly and cut the flow reliably.
2. The selection of the valve
(1) Gate valves and butterfly valves
In order to reduce the depth of soil covering in the water supply pipeline, butterfly valves are generally adopted for pipelines with large diameters. Gate valves should be used due to their little effect on the depth of soil covering, so that the boundary of the diameter should be considered according to the specific conditions of each place. From the perspective of use in the past ten years, the failure of butterfly valves is more serious than that of gate valves. Therefore, when conditions permit, expand the range of application of gate valves.
(2) Gate valves
In recent years, many valve manufacturers in China have developed and imitated soft sealed gate valves. Compared with traditional wedge gate valves or parallel gate valves with double plates, this kind of gate valve has the following characteristics:
a. The valve body and bonnet of the soft sealed gate valve are made by precision casting with molding only once, and there is no mechanical processing, no sealing copper rings, which saves non-ferrous metals.
b. There is no pit at the bottom of the soft sealed gate valve; no slag is accumulated, and the failure rate of the gate valve's opening and closing is low.
c. The size of the soft sealed rubber-lined valve plate is uniform, and the interchangeability is good.
Therefore, the soft sealed gate valve will be adopted for the water supply industry. At present, the domestically manufactured soft sealed gate valve has a diameter of 1500 mm, but soft sealed gate valves made by most manufacturers have diameters between 80 and 300 mm.
For soft sealed gate valves, the copper nut block is embedded in the rubber-lined valve plate to form a whole. However, the requirement for the concentricity of the valve bonnet and valve body is relatively high.
When opening and closing the soft sealed gate valve, don't over close it, as long as the water can be stopped. Otherwise. it will not be easy to open the gate valve or the rubber lining will peel off. A torque wrench is used to control the opening and closing of the soft sealed gate valve.
(3) Butterfly valves
The following matters should be paid attention to in the current selection of butterfly valves:
a. Centerline butterfly valves
Centerline butterfly valves with small diameters are widely used in China, and the effect is relatively good. There are two major differences in manufacturing methods: one is the support frame provided behind the rubber lining layer of the valve body; the other is the support frame not provided behind the rubber lining layer of the valve body. The former has less rubber lining, and the latter has a larger flow area. Some people suspect that the latter valve has a shorter service life, but this is not the case according to investigations.
b. A comparison of vertical and horizontal butterfly valves
For medium and large butterfly valves, there are vertical and horizontal valve shafts. Generally, vertical butterfly valves have a deep soil covering. Debris in the water is likely to wrap around the shaft ends and affects the opening and closing; the variable speed transmission box of horizontal butterfly valves is on the side. The valve well occupies a wider plane position on the road, which affects the arrangement of other pipelines. Therefore, the medium-sized butterfly valve is mostly vertical, and the large-diameter butterfly valve should be horizontal under the condition of the plane position. Regardless of the vertical or horizontal type, measures should be taken for the valve indicator to observe the valve opening and closing from the ground.
c. Eccentric butterfly valves
The closing seal of the centerline butterfly valve is essentially a rubber lining squeeze seal, especially the vicinity of the valve shaft is more squeezed, so that the service life of the valve is affected, and the opening and closing torque of the valve is too great. In order to alleviate the shortcomings in this respect, one-dimensional, two-dimensional and three-dimensional eccentric butterfly valves are designed. Among them, the three-dimensional eccentric butterfly valve is developed. The theoretical sealing state is a contact sealing state. Many manufacturers have developed from this aspect.
The eccentric butterfly valve is directional in bearing water pressure, especially the three-dimensional eccentric butterfly valve has a weak reverse pressure bearing capacity. Since the pipeline is ring-shaped, the valve is required to be able to bear the same pressure in both directions, which should be emphasized when placing an order.
d. Soft sealed and metal sealed butterfly valves
(4) Ball valves and plug valves
The main disadvantage of the butterfly valve is that the butterfly plate occupies a certain cross-section of the water, which increases a certain head loss; although the gate valve does not have this problem, the height of the large-diameter vertical gate valve affects the covering depth of the pipeline. The length of the large-diameter horizontal gate valve increases the horizontal area of the pipeline, which affects the arrangement of other pipelines; the ball valve and plug valve maintain the cross-section of the gate valve and overcome its shortcomings.
The ball valve has the advantages of simple structure, small flow resistance, reliable sealing, convenient maintenance and operation and being flexible. The plug valve also has similar advantages, except that the water passing area is not round.
Ball valves and plug valves are heavier than butterfly valves, which are difficult to cast and process, and are more expensive. They are generally suitable for pipelines with small and medium diameters. In the past ten years, using the lost foam or resin sand casting can avoid or reduce mechanical processing due to the improvement of casting technology, thereby reducing costs. Therefore, the feasibility of ball valves for pipelines with large diameters is worth exploring.
In recent years, a fan-shaped eccentric plug valve is developed. The fan-shaped valve plate is lined with rubber, which has good sealing performance, small opening and closing torque, and a large flow area. It has passed the valve inspection center of the Ministry of Machinery for valve opening and closing tests, durability tests and flow resistance tests, thus filling the gap of this type of valve in China. The current fan-shaped eccentric plug valve has a groove at the bottom, which is easy to accumulate slags and easily causes opening and closing failures, which should be improved.
3. The materials of the valve
(1) For the valves in the water supply pipeline, most of the valve body, valve bonnet, and gearbox were made of gray cast iron, and some were ductile iron in the past. In view of the popularization of ductile iron for pipes and pipe fittings, the material used for the valve should also be the same. In order to improve the quality of castings, and reduce the weight of the valve, ductile iron (QT405-10) should especially be used for the valve plate.
(2) It is recommended that 2CR13 stainless steel should be used for the valve stem.
(3) It is recommended to use cast aluminum brass or cast aluminum bronze, and the hardness and strength of the nut are greater than that of the valve stem.
