pneumatic diaphragm valve

Introduction

The pneumatic diaphragm valve is a special form of shut-off valve that appeared in the 1920s. Its opening and closing part is a diaphragm made of soft material, which separates the inner cavity of the valve body from the inner cavity of the valve cover and the driving parts, so it is called a pneumatic diaphragm valve. The most prominent feature of the pneumatic diaphragm valve is that the diaphragm separates the inner cavity of the lower valve body from the inner cavity of the upper valve cover, so that the valve stem, valve disc and other parts located above the diaphragm are not corroded by the medium, eliminating the need for packing and sealing structure, and will not produce Medium leakage [1].

Classification 

Pneumatic diaphragm valves are usually divided into: normally closed pneumatic diaphragm valves, normally open pneumatic diaphragm valves and reciprocating pneumatic diaphragm valves.

Structural features 

The structural feature of the pneumatic diaphragm valve is that a group of thin hemispherical metal films are clamped in the valve body as a barrier between the valve stem and the fluid. Valve closing is achieved by pressing the valve disc against the valve seat by the stem tip and metal film. The valve is opened by an internal spring, and when the valve stem is raised and out of contact with the diaphragm, the valve disc is raised. Since the stroke of the valve is equal to the stroke of the diaphragm, the characteristic lift of this valve is much smaller than that of ordinary valves.

In order to overcome the resistance loss caused by the low lift, the valve seat and flow area are larger than the general valve, in order to obtain a larger flow. Due to the larger valve seat of this valve, the force is much higher than that of the usual valve in order to obtain sufficient sealing force on the valve disc because there is no direct contact between the valve stem and the valve disc. The valve disc will not pass the valve stem to make it off the valve seat, so this valve is suitable for one-way flow with differential pressure, and usually the medium is from "below the valve seat". However, a large amount of backflow should be avoided in use, so as not to affect the closing of the valve. Generally speaking, it is economical and effective to use metal diaphragm valves under high temperature and high pressure. The valve can also be selected in low temperature and low pressure occasions where synthetic rubber diaphragm valves are not expected.

The diaphragm separates the inner cavity of the lower valve body from the inner cavity of the upper valve cover, so that the valve stem, valve disc and other parts above the diaphragm are not corroded by the medium, the packing sealing structure is omitted, and the medium leakage will not occur.

1. The diaphragm made of soft seals such as rubber or plastic has better sealing performance. Since the diaphragm is a wearing part, it should be replaced regularly according to the characteristics of the medium.

2. Limited by the diaphragm material, the diaphragm valve is suitable for low pressure and relatively low temperature occasions.

3. Diaphragm valves can be divided into six types according to their structure: house type, DC type, cut-off type, straight-through type, gate type and right-angle type; the connection form is usually flange connection; according to the driving mode, it can be divided into manual, electric and pneumatic There are three types, among which the pneumatic drive is divided into three types: normally open, normally closed and reciprocating.

4. Generally, it should not be used in pipelines with temperatures higher than 60 degrees and conveying organic solvents and strong oxidizing media, and it is also not suitable to use pneumatic diaphragm valves in pipelines with higher pressure [1].

Advantage 

The pneumatic diaphragm valve has two main advantages: one is that the valve does not require a separate stem packing and sealing structure, the diaphragm plays a role in sealing the valve stem while cutting off the medium, and the other is that the highly flexible diaphragm closes reliably , and even the dirty liquid can be cut off well. Therefore, the operating mechanism and the medium channel are completely separated, so that the pneumatic diaphragm valve is not only suitable for the food industry and the medical and health industry, but also for some difficult-to-transport media and more dangerous media [1] .

Selection principle

Pneumatic diaphragm valve is a clamping valve, which is mainly used for opening and closing on the pipeline.

Usually, diaphragm valve is recommended for use conditions or when strict sealing performance, mud medium, wear, light structure, low pressure cut-off (small pressure difference) leakage to the atmosphere, and abrasive medium are required. Diaphragm valves can be used in the occasions of two-position adjustment, throttling, channel constriction, low noise, cavitation and vaporization, and small operating torque. Under the conditions of high temperature medium, high pressure medium, high pressure cut-off (large pressure difference), fast opening and closing action, and short structure length, diaphragm valves are not selected, except for metal diaphragm valves