Valve Purchasing Notes

Lined Valve What should be paid attention to in purchasing
(1) Requirements for valve use
① According to their structural characteristics, ordinary gate valves, ball valves, and stop valves are strictly prohibited from being adjusted. However, in process design, they are generally used for adjustment. Due to the use of adjustment, the valve seals are in a throttling state for a long time, and impurities in the oil product wash the seals, damaging the sealing surface, causing loose closure or the operator to seal the damaged sealing surface, causing the valve to be over-closed or over-opened.
② The valve installation position is unreasonable. When the medium contains impurities, no filter or filter screen is installed at the front end, allowing impurities to enter the valve, causing damage to the sealing surface, or impurities are deposited at the bottom of the valve, causing the valve to be loosely closed and leaking.
(2) Consider from the perspective of 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, lined valves can be used to save precious metals. When selecting non-metallic valves, economic rationality should still be considered; for media with high viscosity, smaller flow resistance is required, and valves with small flow resistance such as direct-flow stop valves, gate valves, ball valves, and plug valves should be used. Valves with small flow resistance consume less energy; when the medium is a special medium such as oxygen or ammonia, corresponding oxygen-specific valves or ammonia valves should be selected.
② Directional valves should not be used for dual-flow pipelines, and non-directional valves should be used. For example, after the heavy oil pipeline of a refinery stops running, steam should be used to reversely purge the pipeline to prevent the heavy oil from solidifying and blocking the pipeline. Here, stop valves should not be used, because when the stop valve flows in reverse, it is easy to erode the sealing surface of the stop valve and affect the efficiency of the valve. Instead, gate valves should be used.
③ For some media with crystallization or containing sediments, stop valves and gate valves should not be used, because their sealing surfaces are easily worn by crystallization or sediments. Therefore, ball valves or plug valves should be used; flat gate valves can also be used, but jacket valves are best used.
④ In the selection of gate valves, single gate with rising stem and double gate with concealed stem are more suitable for corrosive media; single gate is suitable for media with high viscosity; wedge-type double gate is better than single gate in adaptability to high temperature and deformation of sealing surface, and will not cause blocking due to temperature change, especially better than rigid single gate.
⑤ For valves on general water and steam pipelines, cast iron valves can be used, but if the steam pipeline is stopped outdoors, condensed water will freeze, thus freezing the valve. Therefore, in cold areas, it is advisable to use cast steel, low-temperature steel or effective insulation measures for valves.
⑥ For highly toxic media or other harmful media with great danger, bellows structure valves should be used to prevent the media from leaking from the packing. ⑦ Gate valves, stop valves and ball valves are the most used valves in valves, and comprehensive considerations should be made when selecting. Gate valves have strong flow capacity and consume less energy to transport the medium, but the installation space is large. Globe valves have a simple structure and are easy to maintain, but the flow resistance is large. Ball valves have the characteristics of low flow resistance and fast opening and closing, but the operating temperature is limited. For media with high viscosity such as petroleum products, gate valves are mostly used because of their strong flow capacity. On water and steam pipelines, globe valves are used, and the pressure drop is not large. Therefore, globe valves are more commonly used in water, steam and other medium pipelines, while ball valves can be used under conditions where the operating conditions allow.
(3) Consideration from the perspective of ease of operation
① For large-diameter valves and long-distance, high-altitude, high-temperature and 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 open and close quickly, butterfly valves, ball valves, plug valves or quick-opening gate valves should be used according to needs. General gate valves and globe valves should not be used. In situations where the operating space is limited, it is not appropriate to use a rising stem gate valve. Instead, a concealed stem gate valve should be used. However, a butterfly valve is preferred.
(4) Considering the accuracy of flow regulation When the flow needs to be accurately regulated, a regulating valve should be used. When the accuracy of small flow needs to be regulated, a needle valve or a throttle valve should be used. When the pressure behind the valve needs to be reduced, a pressure reducing valve should be used. When the pressure behind the valve needs to be kept stable, a pressure stabilizing valve should be used.
(5) Considering the temperature and pressure resistance High temperature and high pressure media often use chromium-molybdenum steel and chromium-molybdenum-vanadium steel of castings. For ultra-high temperature and high pressure media, the corresponding castings should be considered. The comprehensive performance of forgings is better than that of castings, and the temperature and pressure resistance is also superior to that of castings.
(6) Considering cleanliness In the production and transportation of food and bioengineering, the requirements for valves on process pipelines need to consider the cleanliness of the medium, which cannot be guaranteed by general gate valves and stop valves. In terms of cleanliness, no valve can compare with diaphragm valves. ① The diaphragm valve has a simple structure, and the valve body and diaphragm materials are diverse. It can be widely used in the fields of food and bioengineering, and is also suitable for some difficult-to-transport and dangerous media.
a. Only the valve body and diaphragm are in contact with the logistics, and the other parts are completely isolated. The valve can be thoroughly sterilized with steam.
b. It has its own drainage ability.
c. It can be repaired online. Therefore, the diaphragm valve has become the most widely used valve in the fields of food and bioengineering.
② Bottom valve. Under strict sterilization requirements, there is almost no choice for the discharge valve at the bottom of the tank. The bottom valve is directly welded to the bottom head of the tank during equipment manufacturing, which is very different from the commonly used method of making a pipe mouth at the bottom of the tank and then connecting the valve to the pipe mouth. When the valve is closed, its valve core is flush with the inner bottom of the tank, so it effectively avoids the dead corner in the tank, so that all the liquids in the tank can be fully mixed during the fermentation process. In addition, the unique steam sealing system greatly reduces the possibility of product contamination.