Valve knowledge

1、 What is a valve:

A mechanical product with a movable mechanism used to control the flow of pipeline media is called a valve.

2、 What is a universal valve

The valves commonly used on pipelines in various industrial enterprises are called universal valves.

3、 Classification of valves (there are mainly three classification methods)

（1） Classified by purpose and function:

1. Cut off valves: mainly used to cut off or connect medium flow, including gate valves, globe valves, diaphragm valves, plug valves, ball valves, butterfly valves, etc.

2. Regulating valves: mainly used to regulate the flow rate, pressure, etc. of media, including regulating valves, throttle valves, pressure reducing valves, etc.

3. Check valves: used to prevent medium backflow, including various structures of check valves.

4. Diverter valves: used for distributing, separating, or mixing media, including various structures of distribution valves and drain valves.

5. Safety valves: used for overpressure safety protection, including various types of safety valves.

（2） Classification of main parameters:

1. Classified by pressure:

(1) Vacuum valve: a valve with a working pressure lower than standard atmospheric pressure

(2) Low pressure valve: valve with nominal pressure PN ≤ 1.6Mpa

(3) Medium pressure valve: valve with nominal pressure PN2.5-6.4Mpa

(4) High pressure valve: a valve with a nominal pressure of PN10-80Mpa

(5) Ultra high pressure valve: nominal pressure PN ≥ 100Mpa

2. Classified by working temperature of the medium

(1) High temperature valve: valves with t>450 ℃

(2) Medium temperature valve: valves with a temperature range of 120 ℃ ≤ t ≤ 450 ℃

(3) Normal temperature valve: Valve with -40 ℃ ≤ t ≤ 120 ℃

(4) Low temperature valve: Valve with -100 ℃ ≤ t ≤ -40 ℃

3. Classified by valve body material:

(1) Non metallic material valves: such as valves made of ceramics, fiberglass, or plastics.

(2) Metal material valves: such as copper alloy, aluminum alloy, lead alloy, titanium alloy, Monel alloy, cast iron, carbon steel, low alloy steel, high alloy steel valves.

(3) Metal valve body lined valves: such as lead lined valves, plastic lined valves, and enamel lined valves.

（3） Universal classification method: (This classification method is currently the most commonly used classification method both domestically and internationally, consisting of 12 categories, based on both principle and structure.)

1. Gate valve 2, globe valve 3, plug valve 4, ball valve 5, butterfly valve 6, diaphragm valve 7, check valve 8, throttle valve 9, safety valve 10, pressure reducing valve 11, drain valve 12, regulating valve

How to choose valves
　

1、 Select valves based on the on-off properties of the medium

1. Gate valve

Gate valves are used as shut-off media, and when fully opened, the entire flow is straight through, resulting in minimal pressure loss during the operation of the medium. Gate valves are usually suitable for working conditions that do not require frequent opening and closing, and keep the gate fully open or closed. Not suitable for use as a regulator or throttle. For high-speed flowing media, when the gate is partially opened, it can cause vibration of the gate, which may damage the sealing surface of the gate and valve seat. Throttling can cause erosion of the gate by the medium. The main difference in structural form is the form of the sealing element used. According to the form of sealing components, gate valves are often divided into several different types, such as wedge gate valves, parallel gate valves, parallel double gate valves, wedge double gate valves, etc. The most commonly used forms are wedge gate valves and parallel gate valves.

2. Globe valve

The axis of the stem of the globe valve is perpendicular to the sealing surface of the valve seat. The valve stem has a relatively short opening or closing stroke and a very reliable cutting action, making it very suitable for use as a medium for cutting, adjusting, and throttling.

Once the valve disc of the globe valve is in the open position, there is no longer contact between its valve seat and the sealing surface of the valve disc, and it has a very reliable cutting action. Therefore, this type of valve is very suitable for cutting off, regulating, and throttling media.

Once the globe valve is in the open state, there is no longer contact between its valve seat and valve disc sealing surface, so its sealing surface has less mechanical wear. As most globe valves have valve seats and discs that are relatively easy to repair or replace, there is no need to remove the entire valve from the pipeline when replacing sealing components. This is very suitable for situations where the valve and pipeline are welded together. When the medium passes through this type of valve, the flow direction changes, so the flow resistance of the shut-off valve is higher than that of other valves.

There are several commonly used globe valves:

(1) Angle globe valve; In a corner globe valve, the fluid only needs to change direction once, so that the pressure drop through this valve is smaller than that of a conventional structure globe valve.

(2) DC globe valve; In a DC or Y-shaped globe valve, the flow path of the valve body is inclined with the main flow path, which reduces the degree of damage to the flow state compared to conventional globe valves, and therefore reduces the pressure loss through the valve.

(3) Piston type globe valve: This type of globe valve is a variation of conventional globe valves. In this valve, the valve disc and seat are usually designed based on the plunger principle. The valve disc is polished into a plunger and connected to the valve stem, and the sealing is achieved by two elastic sealing rings fitted on the plunger. Two elastic sealing rings are separated by a ring, and the sealing ring around the plunger is pressed firmly by the load applied to the valve cover by the valve cover nut. The elastic sealing ring can be replaced and made of various materials. This valve is mainly used for "opening" or "closing", but it is equipped with a specially designed plunger or special ring, which can also be used to adjust the flow rate.

3. Butterfly valve

The butterfly plate of the butterfly valve is installed in the diameter direction of the pipeline. In the cylindrical passage of the butterfly valve body, the disc-shaped butterfly plate rotates around the axis at an angle between 0 ° and 90 °. When it rotates to 90 °, the valve is fully open.

Butterfly valves have a simple structure, small size, and light weight, consisting of only a few components. And it can be quickly opened and closed by simply rotating 90 °, with simple operation, and the valve has good fluid control characteristics. When the butterfly valve is in the fully open position, the thickness of the butterfly plate is the only resistance for the medium to flow through the valve body. Therefore, the pressure drop generated by this valve is very small, and it has good flow control characteristics. Butterfly valves have two sealing types: elastic sealing and metal sealing. Elastic sealing valve, the sealing ring can be embedded on the valve body or attached around the butterfly plate.

Valves with metal seals generally have a longer lifespan than those with elastic seals, but it is difficult to achieve complete sealing. Metal seals can adapt to higher operating temperatures, while elastic seals have the drawback of being limited by temperature.

If butterfly valves are required for flow control, the main thing is to choose the correct size and type of valve. The structural principle of butterfly valves is particularly suitable for making large-diameter valves. Butterfly valves are not only widely used in general industries such as petroleum, gas, chemical, and water treatment, but also in the cooling water system of thermal power plants.

There are two commonly used butterfly valves: clamp type butterfly valve and flange type butterfly valve. The clamp type butterfly valve is connected between two pipeline flanges with double headed bolts. The flange type butterfly valve has a flange on the valve, and the flanges on both ends of the valve are connected to the pipeline flange with bolts.

4. Ball valve

Ball valves evolved from plug valves. It has the same 90 degree rotation lifting action, but the difference is that the plug body is a sphere with a circular through-hole or channel passing through its axis. The ratio of the sphere to the channel opening should be such that when the sphere rotates 90 degrees, it should all appear spherical at the inlet and outlet, thereby intercepting the flow.

Ball valves can be tightly closed with only a 90 degree rotation and a small rotational torque. The completely equal valve body cavity provides a low resistance, straight through flow channel for the medium. It is generally believed that ball valves are most suitable for direct opening and closing, but recent developments have designed ball valves to have throttling and flow control functions. The main characteristics of ball valves are their compact structure, easy operation and maintenance, suitable for general working media such as water, solvents, acids, and natural gas, as well as for media with harsh working conditions such as oxygen, hydrogen peroxide, methane, and ethylene. The valve body of a ball valve can be integral or modular.

2、 Select valves to prevent medium backflow

The function of this type of valve is to only allow the medium to flow in one direction and prevent directional flow. Usually, this type of valve works automatically, and under the pressure of fluid flowing in one direction, the valve disc opens; When the fluid flows in the opposite direction, the pressure of the fluid and the self weight of the valve disc act on the valve seat, thereby cutting off the flow. Check valves belong to this type of valve, which includes swing check valves and lift check valves. The swing check valve has a hinge mechanism and a valve disc like a door that freely rests on the inclined valve seat surface. In order to ensure that the valve disc can reach the appropriate position on the valve seat surface every time, the valve disc is designed in a hinge mechanism so that the valve disc has sufficient swing space and truly and fully contacts the valve seat. The valve disc can be made entirely of metal, or it can be embedded with leather, rubber, or a synthetic cover on top of the metal, depending on the performance requirements. When the swing check valve is fully open, the fluid pressure is almost unobstructed, so the pressure drop through the valve is relatively small. The valve disc of the lift check valve is located on the sealing surface of the valve seat on the valve body. Except for the valve disc, which can be freely raised and lowered, the rest of this valve is like a globe valve. Fluid pressure lifts the valve disc from the sealing surface of the valve seat, causing the medium to reflux and causing the valve disc to fall back onto the valve seat, cutting off the flow. According to the usage conditions, the valve disc can be made of all metal structure or embedded with rubber pads or rubber rings on the valve disc frame. Like globe valves, the passage of fluid through lift check valves is also narrow, so the pressure drop through lift check valves is greater than that through swing check valves, and the flow of swing check valves is less restricted.

3、 Select valves according to the parameters of the regulating medium