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Valves are mechanical devices that control the pressure and flow within a system or process. They are an integral part of a piping system that conveys liquid, gas, vapor, etc. There are many types of valves which are distinctively utilized in different applications, each of which has its unique features and functionality. Some of the common types include: gate valve, butterfly valve, ball valve, pressure relief valve, etc., and today we are going to specifically address the slide valve.
Slide valves are rectilinear valves used to control the admission of steam. This includes the steam going into the cylinder of a steam engine from which the emission is exhausted as well. In steam, hydraulic and pneumatic engineering, a slide valve slides over and upon its seat without lifting in opening or closing a port, or multiple ports formed in the seat. It slides back and forth over those ports, especially ones in the cylindrical wall of a steam that enables the intake and outflow of steam to move the piston.
The components of a slide valve include the valve body, disc assembly, bonnet, stem and top works. Each side of the slide valve is designed to withstand full differential pressure. A combination of spring force and internal pressure create the replaceable double disc sealing with double bleed and block (DBB). When the middle chamber is under pressure, pressure is relieved by the floating seat automatically.
Also, when the pressure is greater than the pressure in the channel, the cavity pressure will be funneled into the channel. And when the upstream pressure of the channel is greater than the pressure in downstream when the valve is closed, the pressure in the middle chamber will be released to the channel of the upstream side. When both the upstream pressure and downstream (the valve is completely open) pressure in the channel are the same, the pressure in the middle chamber can carry out the release of bilateral channels. The valve seat then resets automatically after the pressure is relieved.
As mentioned earlier, the fundamental purpose of a slide valve is to control the steam going in and out. In the 19th century, slide valves had already been used by many steam locomotives to control the steam flowing in and out of the cylinders. And in the 20th century, slide valves had started to get replaced by, for instance, piston valves that involve the use of superheated steam in the engines. The reason for this is that the steam passages are too long with slide valves, and they often seem too resistant to steam flow, thus affecting the efficiency. Also, it is difficult to maintain a slide valve as opposed to a piston valve especially when superheated steam is present.
Despite the above described, slide valves have improved over the years with advanced technology, and they have some great benefits. First of all, slide valves have extremely low probability of leakage thanks to the self-lapping design of the moving disc. And by leakage we are referring to steam, not liquid! Slide valves are not designed to block off liquids as we’ll address this in the next section.
Following the above, slide valves have superb leak tightness thanks to the pressure from the media acting against the sealing disc. When the surrounding temperature is high, a surface seal is used instead of an annular seal by the slide valve.
Moreover, due to the recent technological improvement, slide valves have very short strokes. Closure is transverse not against the direction of flow but towards it. In other words, because of the relatively short strokes, little energy is required for the actuation force. This allows users to opt for smaller actuators, which greatly minimizes the energy consumption and the energy costs.
With the above said, a slide valve’s biggest limitation is that it is not designed to seal liquids. The slide plate of the valve seats into a polymer seal that has no cavity when it closes. Thus, it allows water to trick through the openings with least resistance. To stress once again, slide valves are designed to control steam not liquids. Slide valves are not designed for full shut, thus liquids will easily find their way through as they are being conveyed.
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