Types and selection of metal valves commonly used in chemical plant

With more and more imported devices into the country, API valves have been widely used, this paper only API ferrous metal valves as the analysis object.
 
Valve is an important part in pipeline system. Metal valve is the most widely used in chemical plant. The valve is mainly used for opening and closing, throttling and ensuring the safe operation of pipelines and equipment. Therefore, the correct and reasonable selection of metal valves plays an important role in factory safety and fluid control system.
 
1, valve type and use
 
There are many kinds of valves in engineering. Due to the different pressure, temperature and physical and chemical properties of fluid, the control requirements of fluid system are not the same, among which gate valve, globe valve (throttle valve, needle valve), check valve, plug valve, ball valve, butterfly valve and diaphragm valve are most widely used in chemical equipment.
 
 
 
1.1 gate valves
f1
Generally used to control the opening and closing of fluid, fluid resistance is small, sealing performance is good, the flow of medium is not restricted, the external force required for opening and closing is small, the structure length is short.
 
The valve stem is divided into open rod and dark rod, open rod gate valve is suitable for corrosive media, in chemical engineering, the basic use of open rod gate valve. Dark stem gate valves are mainly used in watercourses for low-pressure, non-corrosive media, such as some cast iron and copper valves. The structure of the gate includes wedge gate and parallel gate.
 
Wedge gate has single gate, double gate. Parallel RAMS are mostly used in oil and gas delivery systems and are not commonly used in chemical installations.
 
 
 
1.1stop valve
f2
Mainly used for cutting. Globe valve fluid resistance, large opening and closing torque, and flow requirements. Globe valve has the following advantages compared with gate valve:
 
(1) In the process of opening and closing the sealing surface friction is smaller than the gate valve, wear-resistant.
 
(2) the opening height is smaller than the gate valve.
 
(3) Globe valve usually only one sealing surface, good manufacturing process, easy maintenance.
 
Globe valves and gate valves also have open rod and hidden rod points, not described here. According to the body structure of different globe valve straight-through, Angle and Y type. Straight-through type is the most widely used, and Angle type is used when the flow direction changes by 90°.
 
In addition, throttle valve and needle valve is also a type of globe valve, better than ordinary globe valve regulation function.
 
 
 
1.1 check valve
f3
Check valves, also known as check valves, are used to prevent reverse flow of fluid, so when installing check valves, pay attention to the flow direction of the medium should be consistent with the direction of the arrow on the check valve. There are many types of check valves, and each manufacturer has different products, but the main structure is swing and lift. Swing check valves mainly include single and double valve type.
 
 
 
1.1 butterfly valve
f4
Butterfly valve can be used with suspended solid liquid medium opening and closing and throttling, fluid resistance is small, light weight, small size structure, opening and closing quickly, suitable for large caliber pipeline. Butterfly valve has a certain adjustment function, can transport slurry. Due to the backward processing technology in the past, butterfly valve has been used in the water system, but rarely used in the process system, with the improvement of materials, design and processing, butterfly valve has been more and more used in the process system.
 
There are two types of butterfly valve: soft seal and hard seal. The choice of soft seal and hard seal mainly depends on the temperature of the fluid medium. Relatively speaking, the soft seal is better than the hard seal.
 
Soft seal rubber and PTFE(PTFE) seat two. Rubber seated butterfly valves (rubber lined body) are used in water systems. They are of mid-line design. Gaskets can be eliminated in pipe installation because the flanging of the rubber liner can act as gaskets. PTFE seat butterfly valve is used in the process system, generally single eccentric or double eccentric structure.
 
There are many varieties of hard seal, such as hard fixed sealing ring, multi-layer seal (Laminated seal), etc., because the design of manufacturers is often different, the leakage rate is not the same. The structure of hard seal butterfly valve is better with three eccentricity, which solves the problem of thermal expansion compensation and wear compensation. Double eccentric or three eccentric structure hard seal butterfly valve also has two-way sealing function, its reverse (low pressure side to high pressure side) sealing pressure should not be less than 80% of the positive (high pressure side to low pressure side). Design selection should be in consultation with the manufacturer
Valve plays an important role in chemical system. Pipeline valve selection should be based on the pipeline to be transported in the fluid phase (liquid, steam), solid content, pressure, temperature, corrosion properties and many other aspects to consider. In addition, reliable operation without failure, economical cost and manufacturing cycle are also important considerations.
 
In the past, the selection of valve materials in engineering design generally only considers the shell material, and ignores the selection of materials such as inner parts. Improper selection of internal materials will often lead to the sealing failure of the valve internal seal, stem packing and valve cover gasket, affecting the life, so as to not achieve the original expected use effect, easy to cause accidents.
 
From the current situation, API valves do not have a unified identification code, and the national standard valve although there is a identification method, but can not be very clear to show the lining and other materials, as well as other special requirements. So in the engineering project should be needed to the valve through the preparation of the valve data table to carry out a detailed description. In this way, the selection, procurement, installation, debugging and spare parts of the valve are convenient, improve the work efficiency, and reduce the probability of error.


Post time: Oct-26-2021