Views: 4 Author: Translated by Jordi Villanueva Publish Time: 2023-03-02 Origin: Michael Adkins
When selecting a valve for a system of instrumentation, the options can seem overwhelming. Only for appointing some, have valves of ball, of diagram, of bellows, of retention, of excess of discharge, of micro-regulation, of gate, needle, cylindrical male and conical male, of relief or of hygiene. More still, each one of these valves can come in different sizes, configurations, materials of construction and ways of performance. Therefore, to do the best election, is some best practices begin by a first question: What want to that it do the valve?
The majority of the valves fulfil one of the five basic functions: closing, control of discharge, steeringal control, protection against overpressesure or protection against excess of discharge. Tie the type of valve to his function is the first and more important step in the process of selection. And it is not odd to see in field valves wrongly used, like the frequent example of valves of ball used to regulate or estrangular discharge. In some cases, the error can be dangerous, as for example use a valve of ball in a system of oxygen of high pressesure: a source of ignición, together with the sudden irruption of oxygen caused by the fast opening of the valve, can cause a fire.
Appears 1: Valve of ball.
What follows to continuation is a tutorial that reviews the basic types of valves, how work, which functions realizar, and what have to consider when we select one or another. If some time has had doubts in this regard, or simply needs to give him a review, follow reading.
Open and close is the most basic function of a valve: they have to stop and restart the flow of the fluid of the system. The opening basic valves and closing are the valves of ball, gate, diaphragm and bellows.
The valves of ball (see Figure 1) are perhaps the most common type, and are designed for opening control and closing. A performance of a chamber of turn of the control initiates or for the flow orienting the position of a metallic ball in a step of straight discharge. The ball has an orifice through his centre that, when it ranges with the step of discharge, allows the flow; when it turns 90º with regard to the step of discharge, for the flow. If what needs is an opening valve and closing, of fast performance and high capacity of discharge, the valve of ball is a big option. The position of the control gives us an indication of the open state or closed of the valve. And if it looks for hygiene, the valves of ball are easily consignables (bloqueables with padlock) and identifiable with a label. They are also the most practical and economic in sizes between 6 and 50 mm (1/4 pulg. To 2 pulg.).
Typically used for control of process more than for applications of instrumentation, the valves of gate are usually chosen to open and close lines above the 2 pulg. Also they are frequently used like the valve root of the line in instrumentation of process, often in a configuration of double closing and venteo. They are an ancient design of valve, typically specified in applications of general industry, as for example in lines of transmission or of process. They can arrive to upper sizes to the 100 pulg. (2.540 mm). Through multiple twists of the control, elevates or descends a mechanism of closing in a step of straight discharge. It offers a gradual closing.
In a valve, the empaquetadura is the element that makes possible the closing to the outside, surrounding to the stem and avoiding that the fluid of the system escape to the atmosphere between the stem and the body of the valve. The valves that close to the atmosphere with enclosures metal-metal are designated ‘valves without empaquetadura', because they do not have the typical soft material of empaquetadura, as for example plastic and flat boards or tóricas that find in other designs. The stem of the valve is the cylindrical component that joins the control (or actuator) with the internal mechanism of closing, control of discharge or steeringal control. Usually, the stem turns gliding vertically.
The closing of the stem or empaquetadura is subject to wear, which can be cause of escapes. The valves with empaquetadura have to keep or be substituted each true time, although it is true that some designs of empaquetadura achieve enclosures more effective and durable that others, as for example the design ‘chevron' in two pieces.
Appears 2: Valve of diaphragm.
On the contrary that the valves with empaquetadura, the valves of diaphragm (see Figure 2) do not have empaquetadura, and provide a speed of fast and precise performance. In some cases, also can achieve deliver exact quantities of fluid of process. The valves of diaphragm employ typically in applications of high purity, in the industry of biofarmacia and the one of the semiconductor. Between all the types of valves, are those that offer the life of ciclado longer, thanks to his design: each valve contains a thin metallic diaphragm or plastic, that flexiona up and down creating a tight closing to the exit of the valve. Although robust is used to to be of small size, with an orifice or internal step no upper to 50 mm (2 pulg.)
To the equal that the ones of diaphragm, the valves of bellows do not have empaquetadura, what turns them into a good election when the closing to the atmosphere is critical and are little accessible for maintenance. It is a typical valve in areas of containment in the nuclear industry. A stamp soldered separates the inferior half of the valve, where circulates the fluid of the system, of the top, where realizar the performance. The stem, totally confined inside the metallic bellows, displaces vertically without turning, creating the closing to the exit.
The valves of bellows and diaphragm have a step of discharge commonly designated of globe. In the valves of globe, the fluid does not flow in a flat solo as in the valves of ball. The fluid goes in in the valve underneath of the seat and goes out above the seat. The valves of globe have coefficients of discharge lower that the in passing straight valves, for a same diameter of orifice in passing.
The valves of control of discharge (or regulation) allow to the operator increase or diminish the discharge of the system turning the control of the valve. Can adjust the discharge to the value wished, and the valve will keep this discharge with reliability. Some valves of control of discharge also offer possibilities of closing, but will need a lot of twists of the control to carry the valve of a totally opened position to enclosed position.
The most usual valves of control of discharge are the valves of needle, micro-regulation, cylindrical male and conical male.
Appears 3: Valve of needle.
The valves of needle (see Figure 3) offer an excellent control of discharge and, according to his design, also can provide possibility of tight closing. Have a long stem with an obturador of geometry carefully designed (in shape of ‘v' or needle) that fits with precision in a seat on the exit. The stem has a thread in passing very fine, what allows a control of discharge very precise. The empaquetadura of the stem is the manager of the closing to the atmosphere.
Some designs have a closing metal-metal, what does them appropriate for applications of high temperature. As it has said previously, the capacity of discharge is limited by the step in shape of globe. Therefore the valves of needle are more recommended for applications of light fluids, no viscous.
For the greater precision of control of discharge, have the valves of fine regulation or micro-regulation, typical in environingingments of laboratory. They are a type of valve of needle with a long and fine stem that lodges in a prologado and narrow seat. This geometry gives him a design of globe very pronounced, ideal to achieve changes of discharge very precise. Some valves of fine regulation can not use to close.
The valves of cylindrical male of chamber of turn are valves for general service, usually of price more economic. The performance of chamber of turn turns a cylindrical male in the interior of a step of straight discharge. The cylindrical male has an orifice through the cual produces the flow. Usually they use in applications of low pressesure that require a regulation suffices or estrangulamiento of the discharge, in addition to closing.
Another type of valve of male is the valve of conical male. Like a valve of needle, the conical male enters in an orifice to reduce the discharge. But in place to have a design of globe, the step of discharge is straight. This characteristic does that it do not have a capacity of control of discharge so precise, but by the contrary does it baquetable, very appropriate for applications where the fluid of the system can obstruct the step of discharge.
A third type of valves addresses the flow of the discharge. The valves of retention or anti-return (see Figure 4), ensure the discharge in an alone sense. In the majority of designs, the strength of the fluid water up pushes an obturador acted by dock, allowing the step of the discharge. If it increases the contrapresión waters down, the obturador supported by the dock goes back to close against the seat, avoiding the flow to backwards. The valves of retention are available with pressesures of fixed or adjustable shot.
Appears 4: Valve of retention or anti-return.
Some valves of ball and diaphragm are available with multiple doors of connection. The majority of valves multi-door have a common entrance and several exits, and can have or no an enclosed central position (see Figure 5).
It appears 5: Valve multi-door.
The valves of this category avoid the accumulation of pressesure further of a true value of shot, and there is different types.
The valves of relief of proportional pressesure (see Figure 6) ventean to the atmosphere when the pressesure of the system surpasses a value of adjust tarado by the operator. The obturador acted by dock allows to relieve the fluid proportionally to measure that goes increasing the pressesure, closing when the pressesure goes back to an inferior value to the of adjust.
Appears 6: Valve of relief of proportional pressesure.
The valves of hygiene are designed to open quickly, freeing big quantity of fluid of the system. Because of his critical function of hygiene, are a request by code in some applications. They do not have to confuse the valves of relief or hygiene with the valves of retention, since they are three very different functions.
The disks of split use mainly to protect cylinders of taking of sample of an accident of overpressesure that could give, for example, in case of increase of temperature during the transport of the sample. Of similar way to the valves of relief, the disks of split ventean to the atmosphere. A metallic diaphragm pop when the pressesure reaches some value fixed by the manufacturer of the disk. Once it has acted, the disk of split has to be replaced. The rules of equipment of pressesure transportables demand that the cylinders or bottles with gas to pressesure are protected with an element of relief of pressesure. The disk of split is an economic option for this function.
The valves of excess of discharge avoid uncontrolled broadcasts of fluid of the system in case of breaks of lines water down. In normal conditions, a dock keeps the obturador in position totally open. But if it gives an excess of discharge water down, the obturador displaces to enclosed position, detaining almost all the flow. When the system repairs , this small step of discharge gives back the valve to his open position, of automatic way. They are available with different ranks of discharge of closing.
Once have tied the type of valve to his function, already have advanced a lot in the process of selection of a valve. However, still they remain us a lot of details. It will be necessary to loan accurate attention to each one of the following sections, if it has not done before:
Appearances of installation, accessibility and programs of maintenance
Requests of codes and hygiene
Parameters of the system, such as the pressesure, temperature, discharge and fluid of the system
Finally, it will be necessary to determine:
Size of the valve and type of performance
Material of construction, including material of closing and together tóricas, that have to be compatible with the chemical composition of the fluid of the system, pressesure and temperature.
