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The Importance of Pipeline Valves

Publication date:

Sep 07,2016

In the fluid pipeline system, the valve is the control element, and its main function is to isolate the equipment and pipeline system, adjust the flow, prevent backflow, adjust and discharge pressure. As the pipeline system to choose the most suitable valve is very important, so, understand the characteristics of the valve and the selection of the valve steps and basis has become crucial.

So far, the valve industry has been able to produce 12 categories, more than 3000 models and more than 4000 specifications of valve products, including gate valves, globe valves, throttle valves, plug valves, ball valves, electric valves, diaphragm valves, check valves, safety valves, pressure reducing valves, steam traps and emergency shut-off valves. The maximum working pressure is 600MPa, the maximum nominal diameter is 5350mm, the maximum working temperature is 1200 ℃, and the minimum working temperature is -196 ℃, applicable medium is water, steam, oil, natural gas, strong corrosive medium (such as concentrated nitric acid, medium concentration sulfuric acid, etc.), flammable medium (such as stupid, ethylene, etc.), toxic medium (such as hydrogen sulfide), explosive medium and radioactive medium (metal sodium,-loop pure water, etc.). Valve pressure parts material cast copper, cast iron, ductile iron, high silicon cast iron, cast steel, forged steel, high and low alloy steel, stainless acid resistant steel, Hastelloy, Inconel, Monel, duplex stainless steel, titanium alloy, etc. And can produce all kinds of electric, pneumatic, hydraulic valve drive device. In the face of so many varieties of valves and such a complex variety of conditions, to choose the most suitable for the installation of the pipeline system of valve products, I think, first of all, we should understand the characteristics of the valve; secondly, we should master the steps and basis of selecting the valve; then we should follow the principle of selecting the valve.

1. There are generally two characteristics of the valve, the use of characteristics and structural characteristics.
Use characteristics: It determines the main use performance and use scope of the valve, which belongs to the use characteristics of the valve: valve category (closed-circuit valve, regulating valve, safety valve, etc.); Product type (gate valve, globe valve, butterfly valve, ball valve, etc.); Material of main parts of the valve (valve body, valve cover, valve stem, valve disc, sealing surface); Valve transmission mode, etc.

Structural characteristics: It determines some structural characteristics of the valve installation, repair, maintenance and other methods. The structural characteristics include: the structural length and overall height of the valve, and the connection form with the pipeline (flange connection, threaded connection, clamp connection, External thread connection, welding end connection, etc.); The form of sealing surface (insert ring, thread ring, surfacing, spray welding, valve body body); stem structure (rotating rod, lifting rod), etc.

2. The steps and basis for selecting valves are generally as follows:
(1) Selection steps

① clear valve in the equipment or device, determine the working conditions of the valve: the applicable medium, working pressure, working temperature and so on.

② Determine the nominal diameter and connection mode of the pipe connected with the valve: flange, thread, welding, etc.

③ Determine the way to operate the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electric linkage or electro-hydraulic linkage.

④ According to the medium, working pressure and working temperature of the pipeline, the materials of the selected valve housing and internal parts are determined: gray cast iron, malleable cast iron, ductile iron, carbon steel, alloy steel, stainless acid-resistant steel, copper alloy, etc.

⑤ Select the type of valve: closed-circuit valve, regulating valve, safety valve, etc.

Determine the type of valve: gate valve, globe valve, ball valve, butterfly valve, throttle valve, safety valve, pressure reducing valve, steam trap, etc.

⑦ Determine the parameters of the valve: For automatic valves, according to different needs to determine the allowable flow resistance, discharge capacity, back pressure, etc., and then determine the nominal diameter of the pipeline and the diameter of the valve seat hole.

⑧ Determine the geometric parameters of the selected valve: structural length, flange connection form and size, opening and closing the valve height direction size, connecting bolt hole size and quantity, the entire valve appearance size, etc.

⑨ the use of existing information: valve product catalog, valve product samples to select the appropriate valve products.

⑵ Basis for selecting valves

In the understanding of the selection of valve steps at the same time, should further understand the basis for the selection of valves.

The use of the selected valve, the use of working conditions and control mode.

The nature of the working medium: working pressure, working temperature, corrosion performance, whether it contains solid particles, whether the medium is toxic, whether it is flammable, explosive medium, medium viscosity and so on.

③ Requirements for valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, sealing level, etc.

④ Requirements for installation dimension and overall dimension: nominal diameter, connection mode and connection dimension with pipeline, overall dimension or weight limitation, etc.

⑤ Additional requirements for the reliability, service life and explosion-proof performance of electric devices of valve products.

(When selecting parameters, note that if the valve is to be used for control purposes, the following additional parameters must be determined: operating method, maximum and minimum flow requirements, pressure drop for normal flow, pressure drop at closure, maximum and minimum inlet pressure of the valve.)

According to the above selection of the valve basis and steps, a reasonable and correct selection of the valve must also be on the various types of valves of the internal structure of a detailed understanding, so that the preferred valve to make the right choice.

The ultimate control of the pipeline is the valve. The valve opening and closing parts control the flow mode of the medium in the pipeline. The shape of the valve channel makes the valve have certain flow characteristics. This must be taken into account when selecting the most suitable valve for the installation of the pipeline system.

3. The following principles should be followed for the selection of valves:
(1) Valve for cut-off and open medium

The flow channel is a straight-through valve with small flow resistance, and is usually selected as a valve for blocking and opening media. Downward closed valve (cut-off valve, plunger valve) due to its flow twists and turns, flow resistance than other valves, it is less selected. In the case of allowing a higher flow resistance, closed valves can be used.

(2) Valve for flow control

Usually choose easy to adjust the flow of the valve as a control flow. A closed-down valve (such as a globe valve) is suitable for this purpose because its seat size is proportional to the stroke of the closing member. Rotary valves (plug valves, butterfly valves, ball valves) and flexible body valves (pinch valves, diaphragm valves) can also be used for throttle control, but usually only within a limited range of valve diameters. Gate valve is a disc-shaped gate to the circular seat mouth to do cross-cutting movement, it is only close to the closed position, in order to better control the flow, it is usually not used for flow control.

(3) Valve for reversing shunt

This valve can have three or more passages depending on the need for commutation diversion. Plug valves and ball valves are more suitable for this purpose, so most of the valves used for commutation shunt are selected from one of these valves. However, in some cases, other types of valves, as long as two or more valves are properly connected to each other, can also be used for commutation diversion.

Valve for medium with suspended particles

When the medium with suspended particles, the most suitable for its closure along the sealing surface of the sliding with the wiping effect of the valve. If the closure of the valve seat back and forth movement is vertical, it is possible to clamp particles, so this valve unless the sealing surface material can allow embedded particles, otherwise only suitable for basic clean media. Ball valve and plug valve in the opening and closing process of the sealing surface are wiped, it is suitable for use with suspended particles in the medium.

At present, whether in petroleum, chemical industry, or in other industries of the pipeline system, valve applications, operating frequency and services ever-changing, to control or eliminate even the low leakage, the most important, the most critical equipment is the number of valves. The ultimate control of the pipeline is the valve, which is unique in its service and reliable performance in all areas.

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