Valve is an important part of pipeline system, and metal valve is widely used in chemical plant. The valve is mainly used for opening and closing, throttling and ensuring the safe operation of pipeline and equipment. So it is very important to select metal valve correctly and reasonably for safety and fluid control system of factory. With more and more imported devices coming into China, API valves have been widely used. In this paper, only API ferrous metal valves are analyzed.


Type and application of valve



There are many kinds of valves in engineering. Due to the different pressure, temperature and physical and chemical properties of the fluid, the control requirements for the fluid system are also different. Among them, gate valve, stop valve (throttle valve, needle valve), check valve, plug valve, ball valve, butterfly valve and diaphragm valve are most widely used in the chemical equipment.


1.1 gate valve

It is generally used to control the opening and closing of fluid, with small fluid resistance, good sealing performance, unrestricted flow direction of medium, small external force required for opening and closing, and short structure length.

The valve stem can be divided into open stem and concealed stem. The open stem gate valve is suitable for corrosive medium. The open stem gate valve is basically used in chemical engineering. The concealed stem gate valve is mainly used in water channel, and it is mostly used in low-pressure and non corrosive medium occasions, such as some cast iron and copper valves. The structure of RAM includes wedge ram and parallel ram.

Wedge ram is divided into single ram and double ram. Parallel ram is mostly used in oil and gas transportation system, not commonly used in chemical plant.


1.2 stop valve

It is mainly used for cutting off. The stop valve has large fluid resistance, large opening and closing torque and flow direction requirements. Compared with gate valve, stop valve has the following advantages:


In the process of opening and closing, the friction of the sealing surface is smaller than that of the gate valve, which is wear-resistant.


(2) the opening height is smaller than the gate valve.

(3) the stop valve usually has only one sealing surface, which has good manufacturing process and is convenient for maintenance.

Stop valve and gate valve are also divided into open stem and concealed stem, which will not be discussed here. According to the structure of the valve body, there are straight, angle and Y-type stop valves. The straight through type is the most widely used, and the angle type is used in the situation where the flow direction changes by 90 °.

In addition, throttle valve and needle valve belong to stop valve, which have better regulating function than common stop valve.


1.3 check valve

Check valve, also known as one-way valve, is used to prevent reverse flow of fluid. Therefore, when installing the check valve, it should be noted that the flow direction of medium should be consistent with the direction of arrow on the check valve. There are many types of check valves, and each manufacturer has different products, but the structure is mainly divided into swing type and lift type. Swing check valve mainly includes single disc type and double disc type.


1.4 butterfly valve

Butterfly valve can be used for the opening and closing of liquid medium with suspended solids and throttling. It has small fluid resistance, light weight, small structure size, quick opening and closing, and is suitable for large diameter pipeline. Butterfly valve has a certain adjustment function, which can transport slurry. Due to the backward processing technology in the past, butterfly valve has been used in water system, but rarely in process system. With the improvement of material, design and processing, butterfly valve has been used in process system more and more.

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 sealing performance of soft seal is better than that of hard seal.

There are two kinds of soft seals: Rubber and PTFE seats. Rubber seat butterfly valve (rubber lined valve body) is mostly used in water system, which is of medium linear structure. This butterfly valve can be installed without gasket, because the flanging of rubber lining can act as gasket function. PTFE seat butterfly valve is mostly used in the process system, generally single eccentric or double eccentric structure.

There are many kinds of hard seal, such as hard fixed seal ring, multi-layer seal and so on. Because the manufacturer's design is often different, its leakage rate is also different. The structure of hard seal butterfly valve is three eccentricities, which solves the problems of thermal expansion compensation and wear compensation. Double eccentric or triple eccentric structure hard seal butterfly valve also has two-way sealing function, and its reverse (low pressure side to high pressure side) sealing pressure shall not be less than 80% of the positive (high pressure side to low pressure side). Design and type selection shall be negotiated with the manufacturer.


1.5 plug valve

Plug valve has small fluid resistance, good sealing performance, long service life, and can be sealed in two directions, so it is often used in high or extremely hazardous materials, but the opening and closing torque is relatively large, and the price is relatively high. There is no liquid in the body of the plug valve, especially the materials of the intermittent device will not cause pollution, so the plug valve must be used in some occasions.

When the plug valve and the ball valve are closed, there is liquid in the cavity. The flow passage of the plug valve can be divided into straight, three-way and four-way, which is suitable for multi-directional distribution of gas and liquid phase fluid.

Plug valve can be divided into two types: non lubrication type and lubrication type. The oil sealed plug valve with forced lubrication forms an oil film between the sealing surface of the plug and the plug due to forced lubrication. In this way, the sealing performance is better, the opening and closing are labor-saving, and the sealing surface is prevented from being damaged. However, it is necessary to consider whether the lubrication pollutes the materials, and the unlubricated type is preferred for regular maintenance.

The seat seal of the plug valve is continuous and surrounds the entire plug, so the fluid does not contact the shaft. In addition, the plug valve has a metal composite diaphragm as the second seal, so the plug valve can strictly control the external leakage. Plug valves generally have no packing. When there are special requirements (such as no external leakage, etc.), packing is required as the third seal.