What should I pay attention to when purchasing valves?

(1) Valve use requirements

1 Ordinary gate valve, ball valve and globe valve are strictly prohibited for adjustment according to their structural characteristics. However, in process design, it is generally used for mediation. Due to the adjustment and use, the valve seal is in a throttling state for a long time, the impurities in the oil scouring the seal, damaging the sealing surface, causing the closure to be unsatisfactory or because the operator has caused the seal to seal the seal surface, causing the valve to pass and open. .
2 The valve installation position is unreasonable. When the medium contains impurities, no filter or filter is installed at the front end to allow impurities to enter the valve interior, causing damage to the sealing surface, or impurities deposit on the bottom of the valve, causing the valve to be closed tightly. There is a leak.
(2) Considering process requirements
1 For corrosive media, if the temperature and pressure are not high, non-metallic valves should be used as much as possible. If the temperature and pressure are high, a lining valve can be used to save precious metals. When selecting non-metallic valves, economic rationality should still be considered; for mediums with higher viscosity, smaller flow resistance is required. DC-type shut-off valves, gate valves, ball valves, plug valves and other valves with small flow resistance should be used. Valves with small flow resistance have low energy consumption; when the medium is special medium such as oxygen or ammonia, the corresponding oxygen special valve or ammonia valve should be used.
2 Double-flow pipelines should not use directional valves, and non-directional valves should be used. For example, after the heavy oil pipeline of the refinery stops running, the steam should be reversely purged to prevent the heavy oil from solidifying and block the pipeline. It is not appropriate to use the shut-off valve because it is easy to erode the sealing surface of the shut-off valve when it flows backward. The effect of the valve is affected, and the gate valve should be selected.
3 For some media with crystallization or sedimentation, it is not advisable to use shut-off valves and gate valves because their sealing faces are susceptible to crystallization or deposit wear. Therefore, it is appropriate to use a ball valve or a plug valve; a slab gate valve is also available, but a jacket valve is preferred.
4 In the selection of gate valve, the single-gate and the dark-rod double-gate are more suitable for corrosive medium; the single-gate is suitable for medium with high viscosity; the double-shield is suitable for high temperature and deformation of sealing surface It is better than the wedge type single gate, and there is no jamming due to temperature change, especially superior to the rigid single gate.
5 Generally, the valves on the water and steam pipes can be cast iron valves. However, if the steam is stopped in the outdoor steam pipes, the condensed water will freeze and the valves will be frozen. Therefore, in cold regions, the valve is made of cast steel, low-temperature steel or effective insulation measures.
6 For highly toxic highly toxic media or other harmful media, bellows-structured valves should be used to prevent media from leaking from the packing.
7 gate valve, globe valve and ball valve are the most used valves in the valve, should be considered comprehensively. The gate valve has strong circulation capacity, and the energy consumption of the conveying medium is small, but the installation space is large. The structure of the shut-off valve is simple, the maintenance is convenient, but the flow resistance is large. The ball valve has the characteristics of low flow resistance and quick opening and closing, but the use temperature is limited. In the medium with high viscosity such as petroleum products, the gate valve is mostly used in consideration of the strong flow capacity of the gate valve. On the water and steam pipelines, the shut-off valve is applied, and the pressure drop is not large, so the shut-off valve is in the medium pipeline such as water and steam. There are many applications, and the ball valve can be used under the conditions of the working conditions.
(3) Considering the convenience of operation
1 For large diameter valves and long-distance, high-altitude, high-temperature, high-pressure occasions, electric and pneumatic valves should be used. For flammable and explosive occasions, explosion-proof devices should be used. For safety and reliability, hydraulic and pneumatic devices should be used.
2 For valves that need to be opened and closed quickly, butterfly valves, ball valves, plug valves or quick-opening gate valves should be used according to requirements. It is not advisable to use general gate valves and globe valves. In the case where the operating space is limited, it is not advisable to use a bright gate valve. It is advisable to use a dark rod gate valve, but it is better to use a butterfly valve.
(4) Considering the accuracy of regulating flow
When it is necessary to accurately adjust the flow rate, the regulating valve should be used. When the accuracy of the small flow rate needs to be adjusted, a needle valve or a throttle valve should be used. When it is necessary to reduce the pressure behind the valve, a pressure reducing valve should be used. To maintain the stability of the pressure behind the valve, a pressure reducing valve should be used.
(5) Considering temperature and pressure resistance
High-temperature and high-pressure medium often adopts chrome-molybdenum steel and chrome-molybdenum-vanadium steel for casting. For ultra-high temperature and high-pressure medium, the corresponding castings should be considered. The comprehensive performance of forgings is better than that of castings, and the temperature and pressure resistance are superior to castings.
(6) Considering cleanliness
In food and bioengineering production and transportation, the requirements of the valve on the process pipeline need to consider the cleanliness of the medium, and the general gate valve and globe valve cannot be guaranteed. From the standpoint of cleanliness, no valve can be compared to a diaphragm valve.
1 Diaphragm valve has a simple structure, a variety of valve body and diaphragm materials, can be widely used in food and bioengineering, but also suitable for some difficult and transportable and dangerous media.
a. Only the valve body and diaphragm are in contact with the stream, and the other parts are completely isolated, and the valve can be completely sterilized by steam.
b. Have their own ability to drain.
c. Online repair is available. Diaphragm valves have therefore become the most widely used valves in the food and bioengineering field.
2 bottom valve. In the case of strict sterilization requirements, there is almost no room for choice in the discharge valve at the bottom of the tank. The bottom valve is directly welded to the bottom head of the tank during the manufacture of the equipment. It is very different from the commonly used method of making a nozzle at the bottom of the tank, and then connecting the valve to the nozzle. When the valve is closed, the spool is closed. It is level with the inner bottom of the tank, so it effectively meets the dead angle in the tank, so that all the liquid in the tank can be fully mixed during the fermentation process. With the unique steam sealing system, the product is greatly reduced. The possibility.