Valves are usually made of metal or plastic and they have several different parts. The outer part is called the seat and it often has a solid metal outer casing and a soft inner rubber or plastic seal so the valve makes a closure that's absolutely tight. The inner part of the valve, which opens and closes, is called the body and fits into the seat when the valve is closed. There's also some form of mechanism for opening and closing the valve—either a manual lever or wheel (as in a faucet or a stop cock) or an automated mechanism (as in a car engine or steam engine).
Photo: Shutting off the water with a sturdy brass isolation valve. Turning this lever through ninety degrees closes a ball valve in the middle of the pipe, cutting off the water flowing through. Most homes have valves like this on the incoming cold water "feed" and the pipes leading into and out from the water tanks. Isolation valves are very useful during an emergency (such as a burst water pipe) or for carrying out routine maintenance. Once the valve is closed, you can safely carry out repairs without the fluid all flooding out.
It's often critically important for valves that are switched off to allow absolutely no escape of liquid or gas through a pipe to avoid accidents, explosions, pollution, or the loss of valuable chemicals (even a dripping faucet can be expensive if your water is metered). That's why the seal on a valve needs to be perfectly secure and a valve that's turned off must be tightly closed. Turning off a high-pressure flow of liquid or gas by obstructing it with a valve is physically hard work: in other words, you need to use a lot of force to do it. That's why some valves are operated by levers (like the one photographed here, but some can be much longer to give you more turning force) or large wheels (as in the top photo in this article). If really big valves require too much force for a human to supply, they're operated by hydraulic rams.
Not all valves are big, mighty, industrial-strength things made of metal. Look carefully at food containers in your kitchen and you'll find quite a few have valves in them. Water bottles (like the one I pictured above) often have poppet valves instead of screw caps. The food jar top I've photographed below is another really ingenious example of a valve, made from a springy elastomer (in practice, an elastic, synthetic, silicone rubber). It seals a food dispensing jar that normally sits upside down, so, in theory, the food could just dribble out onto the table beneath! This ingenious valve is what stops it. The rubbery material has four slits in it to let the food through, but it's also quite firm, so it opens only when you squeeze the jar. The pressure you supply when you squeeze forces the food through the four slits, which pop open. When you release the pressure, the elasticity of the valve makes the slits pop back down and the seal the jar up again. It's so simple and mundane that you've probably never even noticed it, yet it's an ingenious bit of engineering that relies on very careful selection of exactly the right material.
Photo: An elastomeric food-sealing valve. Left: Looking from below at the sealed valve. Middle: Looking from above at the same sealed valve. Right: Looking from above with my finger pushing up to reveal how the self-sealing slit mechanism works. (If you're interested, I believe this is a SimpliSqueeze® slit valve made by Aptar, and you can read all the technical details of how it works in their US10287066B2: Dispensing valve.)
And choosing materials for valves isn't just a matter of thinking how they'll function during their lifetime, but what happens to them after that. With food packaging, for example, recycling is an increasingly important consideration. Take the little valves you find on coffee bags. After coffee is roasted and stuffed into bags, it might have to sit on a store shelf for anything up to a year, during which time it continues to give off carbon dioxide gas. Without a valve on the bag, it would puff up and potentially burst in the shop (or your kitchen), hurling coffee all over the place. So coffee bags have these ingenious one-way "degassing" valves on them made of membranes that open when the pressure builds up inside. That's why you can "wake up and smell the coffee" without actually opening the bag. When air tries to push in from outside, it flattens the membrane and seals the bag tight. So far so good, but what about recycling? If you start putting complicated plastic valves on bags, it makes the bags far harder to recycle. What's the answer? Manufacturers are now making coffee bags and valves entirely from compostable bioplastics to eliminate the waste disposal problem.
Photo: How coffee valves work. Top row: Left: A typical valve on the inside of a bag of coffee. Middle: This compostable bioplastic valve (made by the Swiss Wipf company) has a fixed outer seat (black) and an inner body (red) with a gas vent. Right: Take it apart and you'll find it also has a plastic membrane inside (blue). The illustration below shows how these three parts work together. The membrane flexes up to let CO2 escape, then flattens back down to stop air and water vapor getting in.
Artwork: Eight common types of valves, greatly simplified. Color key: the grey part is the pipe through which fluid flows; the red part is the valve and its handle or control; the blue arrows show how the valve moves or swivels; and the yellow line shows which way the fluid moves when the valve is open.
The many different types of valves all have different names. The most common ones are the butterfly, cock or plug, gate, globe, needle, poppet, and spool:
Valves are often used to contain dangerous liquids or gases—maybe toxic chemicals, flammable petroleum, high-pressure steam, or compressed air—that mustn't be allowed to escape under any circumstances. In theory, a valve must be perfectly secure and, once closed, must never allow liquid or gas to get past it. In practice, that's not always true.
Sometimes it's better for a valve to fail, intentionally, to protect some other part of a system or machine. For example, if you have a steam engine powered by a water boiler in which steam is building up, but the pressure suddenly gets too high, you need a valve to blow open, let the steam escape, and release the pressure safely before the entire boiler explodes catastrophically.
Valves that work in this way are called safety valves. Normally they're held closed by very sturdy springs. They're designed to open automatically when the liquid or gas they contain reaches a certain pressure (though many systems and machines have safety valves that can be opened manually for the same purpose).
Photo: The spring-loaded safety valves on a steam locomotive are positioned on the roof, above the boiler. All steam engines have at least two safety valves; some have three or even four.
Animation: How a steam engine safety valve works. The valve (blue) is normally held shut by a sturdy spring (red) above it. When the steam (orange) builds up to too high a pressure, it pushes the valve up and escapes through the vent (along the yellow arrowed path).
Here's an example of a safety valve fitted into an ordinary hot-water faucet (tap):
Artwork: US Patent: 1,449,472: Safety Faucet by Paul B. Wesson and Hampden Brass Company, courtesy of US Patent and Trademark Office.
In a conventional faucet, you turn the orange handle at the top clockwise or counterclockwise to make the valve screw up or down. That allows water to flow from left to right through the horizontal pipe, around the bend (through the gap where the valve was), and out through the vertical pipe on the right.
You can turn the handle by different amounts to screw the valve open to a different height, letting different amounts of water through.
In this design by Paul Wesson, patented in 1923, there's an extra, safety valve at the bottom, colored green. It has a conical shape and is normally held tightly in place by the yellow spring coiled around it. However, if the water pressure builds up too much, it pushes against the cone, opens the valve, and the water escapes downward, releasing the pressure.
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@misc{woodford_valves, author = "Woodford, Chris", title = "Valves", publisher = "Explain that Stuff", year = "2008", url = "https://www.explainthatstuff.com/valves.html", urldate = "2023-08-04" }
Solenoid valves are used wherever fluid flow has to be controlled automatically. They are being used to an increasing degree in the most varied types of plants and equipment. The variety of different designs which are available enables a valve to be selected to specifically suit the application in question.
Solenoid valves are used wherever fluid flow has to be controlled automatically. They are being used to an increasing degree in the most varied types of plants and equipment. The variety of different designs which are available enables a valve to be selected to specifically suit the application in question.
Solenoid valves are control units which, when electrically energized or de-energized, either shut off or allow fluid flow. The actuator takes the form of an electromagnet. When energized, a magnetic field builds up which pulls a plunger or pivoted armature against the action of a spring. When de-energized, the plunger or pivoted armature is returned to its original position by the spring action.
According to the mode of actuation, a distinction is made between direct-acting valves, internally piloted valves, and externally piloted valves. A further distinguishing feature is the number of port connections or the number of flow paths ("ways").
With a direct-acting solenoid valve, the seat seal is attached to the solenoid core. In the de-energized condition, a seat orifice is closed, which opens when the valve is energized
Two-way valves are shut-off valves with one inlet port and one outlet port (Fig. 1). In the de-energized condition, the core spring, assisted by the fluid pressure, holds the valve seal on the valve seat to shut off the flow. When energized, the core and seal are pulled into the solenoid coil and the valve opens. The electro-magnetic force is greater than the combined spring force and the static and dynamic pressure forces of the medium.
figure 1
Three-way valves have three port connections and two valve seats. One valve seal always remains open and the other closed in the de-energized mode. When the coil is energized, the mode reverses. The 3-way valve shown in Fig. 2 is designed with a plunger type core. Various valve operations can be obtained according to how the fluid medium is connected to the working ports in Fig. 2. The fluid pressure builds up under the valve seat. With the coil de-energized, a conical spring holds the lower core seal tightly against the valve seat and shuts off the fluid flow. Port A is exhausted through R. When the coil is energized the core is pulled in, the valve seat at Port R is sealed off by the spring-loaded upper core seal. The fluid medium now flows from P to A.
figure 2
Unlike the versions with plunger-type cores, pivoted-armature valves have all port connections in the valve body. An isolating diaphragm ensures that the fluid medium does not come into contact with the coil chamber. Pivoted-armature valves can be used to obtain any 3-way valve operation. The basic design principle is shown in Fig. 3. Pivoted-armature valves are provided with manual override as a standard feature.
figure 3
With direct-acting valves, the static pressure forces increase with increasing orifice diameter which means that the magnetic forces, required to overcome the pressure forces, become correspondingly larger. Internally piloted solenoid valves are therefore employed for switching higher pressures in conjunction with larger orifice sizes; in this case, the differential fluid pressure performs the main work in opening and closing the valve.
Internally piloted solenoid valves are fitted with either a 2- or 3-way pilot solenoid valve. A diaphragm or a piston provides the seal for the main valve seat. The operation of such a valve is indicated in Fig. 4. When the pilot valve is closed, the fluid pressure builds up on both sides of the diaphragm via a bleed orifice. As long as there is a pressure differential between the inlet and outlet ports, a shut-off force is available by virtue of the larger effective area on the top of the diaphragm. When the pilot valve is opened, the pressure is relieved from the upper side of the diaphragm. The greater effective net pressure force from below now raises the diaphragm and opens the valve. In general, internally piloted valves require a minimum pressure differential to ensure satisfactory opening and closing. Omega also offers internally piloted valves, designed with a coupled core and diaphragm that operate at zero pressure differential (Fig. 5).
figure 4
Internally piloted 4-way solenoid valves are used mainly in hydraulic and pneumatic applications to actuate double-acting cylinders. These valves have four port connections: a pressure inlet P, two cylinder port connections A and B, and one exhaust port connection R. An internally piloted 4/2-way poppet valve is shown in Fig. 6. When de-energized, the pilot valve opens at the connection from the pressure inlet to the pilot channel. Both poppets in the main valve are now pressurized and switch over. Now port connection P is connected to A, and B can exhaust via a second restrictor through R.
figure 5
With these types an independent pilot medium is used to actuate the valve. Fig. 7 shows a piston-operated angle-seat valve with closure spring. In the unpressurized condition, the valve seat is closed. A 3-way solenoid valve, which can be mounted on the actuator, controls the independent pilot medium. When the solenoid valve is energized, the piston is raised against the action of the spring and the valve opens. A normally-open valve version can be obtained if the spring is placed on the opposite side of the actuator piston. In these cases, the independent pilot medium is connected to the top of the actuator. Double-acting versions controlled by 4/2-way valves do not contain any spring.
figure 6
All materials used in the construction of the valves are carefully selected according to the varying types of applications. Body material, seal material, and solenoid material are chosen to optimize functional reliability, fluid compatibility, service life and cost.
Neutral fluid valve bodies are made of brass and bronze. For fluids with high temperatures, e.g., steam, corrosion-resistant steel is available. In addition, polyamide material s used for economic reasons in various plastic valves.
All parts of the solenoid actuator which come into contact with the fluid are made of austenitic corrosion-resistant steel. In this way, resistance is guaranteed against corrosive attack by neutral or mildly aggressive media.
The particular mechanical, thermal and chemical conditions in an application factors in the selection of the seal material. the standard material for neutral fluids at temperatures up to 194°F is normally FKM. For higher temperatures EPDM and PTFE are employed. The PTFE material is universally resistant to practically all fluids of technical interest.
All pressure figures quoted in this section represent gauge pressures. Pressure ratings are quoted in PSI. The valves function reliably within the given pressure ranges. Our figures apply for the range 15% undervoltage to 10% overvoltage. If 3/2-way valves are used in a different operation, the permitted pressure range changes. Further details are contained in our data sheets.
In the case of vacuum operation, care has to be taken to ensure that the vacuum is on the outlet side (A or B) while the higher pressure, i.e. atmospheric pressure, is connected to the inlet port P.
The flow rate through a valve is determined by the nature of the design and by the type of flow. The size of valve required for a particular application is generally established by the Cv rating. This figure is evolved for standardized units and conditions, i.e. flowrate in GPM and using water at a temperature of between 40°F and 86°F at a pressure drop of 1 PSI. Cv ratings for each valve are quoted. A standardized system of flowrate values is also used for pneumatics. In this case the air flow in SCFM upstream and a pressure drop of 15 PSI at a temperature of 68°F.
A common feature of all Omega solenoid valves is the epoxy-encapsulated solenoid system. With this system, the whole magnetic circuit-coil, connections, yoke and core guide tube - are incorporated in one compact unit. This results in a high magnetic force being contained within the minimum of space, insuring first class electrical insulation and protection against vibration, as well as external corrosive effects.
The Omega coils are available in all the commonly used AC and DC voltages. The low power consumption, in particular with the smaller solenoid systems, means that control via solid state circuitry is possible.
figure 7
The magnetic force available increases as the air gap between the core and plug nut decreases, regardless of whether AC or DC is involved. An AC solenoid system has a larger magnetic force available at a greater stroke than a comparable DC solenoid system. The characteristic stroke vs. force graphs, indicated in Fig. 8, illustrate this relationship.
The current consumption of an AC solenoid is determined by the inductance. With increasing stroke the inductive resistance decreases and causes an increase in current consumption. This means that at the instant of de-energization, the current reaches its maximum value. The opposite situation applies to a DC solenoid where the current consumption is a function only of the resistance of the windings. A time-based comparison of the energization characteristics for AC and DC solenoids is shown in Fig. 9. At the moment of being energized, i.e. when the air gap is at its maximum, solenoid valves draw much higher currents than when the core is completely retracted, i.e., the air gap is closed. This results in a high output and increased pressure range. In DC systems, after switching on the current, flow increases relatively slowly until a constant holding current is reached. These valves are therefore, only able to control lower pressures than AC valves at the same orifice sizes. Higher pressures can only be obtained by reducing the orifice size and, thus, the flow capability.
A certain amount of heat is always generated when a solenoid coil is energized. The standard version of the solenoid valves has relatively low temperature rises. They are designed to reach a maximum temperature rise of 144°F under conditions of continuous operation (100%) and at 10% overvoltage. In addition, a maximum ambient temperature of 130°F is generally permissible. The maximum permissible fluid temperatures are dependent on the particular seal and body materials specified. These figures can be obtained from the technical data.
The small volumes and relatively high magnetic forces involved with solenoid valves enable rapid response times to be obtained. Valves with various response times are available for special applications. The response time is defined as the time between application of the switching signal and completion of mechanical opening or closing.
The on period is defined as the time between switching the solenoid current on and off.
The total time of the energized and de-energized periods is the cycle period. Preferred cycle period: 2, 5, 10 or 30 minutes.
The relative duty cycle (%) is the percentage ratio of the energized period to the total cycle period. Continuous operation (100% duty cycle) is defined as continuous operation until steady-state temperature is reached.
The coding for the valve operation always consists of a capital letter. The summary at left details the codes of the various valve operations and indicates the appropriate standard circuit symbols
The technical data is valid for viscosities up to the figure quoted. Higher viscosities are permissible, but in these cases the voltage tolerance range is reduced and the response times are extended.
Temperature limits for the fluid medium are always detailed. Various factors, e.g. ambient conditions, cycling, speed, voltage tolerance, installation details, etc., can, however, influence the temperature performance. The values quoted herein should, therefore, be used only as a general guide. In cases where operation at extremes of the temperature range are involved, you should seek advice from Omega's Engineering Department.
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