Being aware of the key characteristics helps in using valves successfully and safely. Here is a brief overview:
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Nominal size
The nominal size (code DN from the French "diamètre nominal") in building services refers to the inside diameter of a pipe. It is composed of the letters DN and a number linked directly with the physical size of the connections (in mm). Example: DN 32
Pressure
Pressure (code p) is defined as the force exerted per unit area.
Nominal pressure
The nominal pressure (code PN from the French "pression nominale") is a reference value. It indicates the design pressure in bar at room temperature (20 °C). It is composed of the letters PN and the highest permissible pressure. Example: PN 10
Volume flow rate
The volume flow rate (code Q) refers to the quantity of liquid or gas that flows through a pipe in a specified time unit.
Resistance coefficient
The resistance coefficient (code ζ, zeta) indicates the resistance of valves or other fittings in pipes acting in the direction opposite to the flow direction. The larger ζ, the higher the pressure loss. Good to know: The ζ value applies to the valve in fully open condition.
Pigging
Pigging is the cleaning of a pipe with a pig or another cleaning device travelling through the pipe.
Flow coefficient
The Kv value and Kvs value are also known as the flow factor or flow coefficient. They are used for comparing, selecting and dimensioning valves. The value is specified in m³/h.
The Kv value corresponds to the water flow through a valve at:
There is an associated Kv value for every degree of opening (actuator stroke, actuator angle).
Plotting these Kv values over the stroke provides the characteristic curve of the valve. The Kv value is calculated as follows, where r [kg/dm³] is the density factor of the fluid, Q [m³/h] is the volume flow rate and [bar] is the differential pressure.
Kv = Q ∙ √ r/∆p
→ for water with r = 1, the following results:
Kv = Q / √∆p
Kvs value → Flow through a fully opened valve (100 % degree of opening)
→ A valve has only got one Kvs value but several Kv values.
The best way of finding a suitable valve that is also the most efficient choice is by employing a systematic approach. Follow these step-by-step instructions to find a valve meeting your requirements. You will automatically eliminate the valves that are unsuitable for your project. This is illustrated at the example of a cooling circuit for supplying a heat exchanger in a data centre. The volume flow rate in the example is 200 m3/h, which leads to the recommended nominal size of DN 150.
1. What function is to be fulfilled?
We require a valve for the shut-off task (OPEN/CLOSED). Examples of other functions are measuring, throttling or controlling.
Suitable valves:
Gate valve / globe valve / ball valve / butterfly valve / diaphragm valve
2. What is the fluid that will flow through our valve?
What chemical properties has the fluid got? Is the fluid corrosive and/or abrasive? Does the fluid contain solids or could it also be explosive? It is important that the valve material is resistant to the fluid handled. The fluid in our example is cooling water used in a closed circuit.
Suitable valves:
Gate valve / globe valve / ball valve / butterfly valve / diaphragm valve
3. What are the temperature requirements?
The fluid temperature and ambient temperature also play a major part in selecting the right valve. Examples of some key temperature limits are -50 °C / -30 °C / -10 °C / +60 °C / +120 °C / +350 °C. These are values at which, usually, a different material has to be chosen. In our example, the fluid temperature is +6 °C and the ambient temperature is +20 °C. We are in an indoor environment.
Suitable valves:
Gate valve / globe valve / ball valve / butterfly valve / diaphragm valve
4. What are the pressure requirements?
How much pressure does the valve need to seal to atmosphere? What is the pressure or differential pressure to be decreased by our valve between the inlet and outlet?
In our example, the system pressure is approx. 8 bar. The planned nominal pressure class of the piping is PN 16.
Suitable valves:
Gate valve / globe valve / ball valve / butterfly valve / diaphragm valve
5. What standards, regulations and acceptance criteria need to be met?
In our example, the safety requirements of Annex I of the European Pressure Equipment Directive /68/EU (PED) for fluids in Groups 1 and 2 apply. In addition, the valve is to be maintenance-free.
Suitable valves:
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Gate valve / globe valve / ball valve / butterfly valve / diaphragm valve
6. Resistance coefficient ζ, zeta
Since we would like our system to be as efficient as possible, we are looking for a valve with an extreme (very low) resistance coefficient. This is where one of the valve types is eliminated in our example.
Suitable valves:
Gate valve / globe valve/ ball valve / butterfly valve / diaphragm valve
7. How much space is available for the valve?
For us, space is very limited. The more compact the valve installation, the better. In the case of a 1:1 replacement for existing valves, identical design is important. In our example, this means two further valves are no longer in the race.
Suitable valves:
Gate valve / globe valve/ ball valve / butterfly valve / diaphragm valve
8. What is the free passage required?
This is linked with pigging and with whether solids have to pass through the valve. This is not the case in our example.
Suitable valves:
Gate valve / globe valve/ ball valve / butterfly valve / diaphragm valve
9. Are there any hygienic requirements to be met by the valve?
This is particularly important for drinking water installations and in the foodstuffs industry. Our example has not got any such requirements.
Suitable valves:
Gate valve / globe valve/ ball valve / butterfly valve / diaphragm valve
10. The price/performance ratio
Projects are governed by budget planning and the investors' desired security. What counts is: as cost-efficient as possible while meeting the requirements. Availability and interchangeability are further key criteria. A ball valve in DN 32 or DN 50 would be cheaper than a butterfly valve that has to be flanged into the piping. However, this option is not suitable for the flow rate of 200 m3/h (DN 150) in our cooling circuit example. We require a valve meeting DN 150. This eliminates the last option in the selection.
Suitable valves:
Gate valve / globe valve / ball valve / butterfly valve / diaphragm valve
CONCLUSION:
The systematic approach has led us to the right valve for our project: a butterfly valve.
Globe valves are a type of control valve used to stop, start and regulate the flow of media through a pipe. From the outside, they have a spherical body design; inside, a plug-like disc sits on the end of a threaded rotating stem, which raises and lowers to control media flow. Globe valves are part of the linear motion valve family, which means that the valve mechanism moves along a straight line.
Globe valves provide a tight seal with low chances of leakage, which is why they are a common choice in high-pressure, industrial piping systems.
Globe valves are designed so that the media flowing through them does not travel in a straight line. Rather than encountering a perpendicular blockage at the valve, the media must take a slight vertical detour up through the valve cavity, where it meets the plug disc. As the valve stem is turned via wheel and raises to allow media through, the disc moves parallel to the flow. The position of the stem tells operators whether the valve is open or closed.
This vertical movement is what enables the globe valve to regulate media flow, rather than simply stopping or starting it. The distance between the disc and the seat determines the flow rate: the farther apart the disc and seat are, the higher the volume of media passing through. Globe valves are ideal for throttling, or regulating the flow of media while in a partially open position, and make starting and stopping flow less abrupt which can help prevent water hammer.
Because their design forces media to change direction (horizontal to vertical), globe valves create a high-pressure drop. They are best used in applications that require only unidirectional flow, where massive changes in pressure are not an issue and where safety and leakage are primary concerns. Globe valves are often found in piping systems that transport corrosive, viscous, highly pressurized or extremely hot or cold media.
Most of the working parts of a globe valve lie inside the cavity, and access to the internal components is through the top; this is known as top-entry design.
Gate valves are linear motion valves that are used only to start or stop media flowing through a pipe. They are considered shutoff valves instead of control valves, since they are not ideal for throttling.
Gate valves are designed with a flat or wedge-shaped disc on the end of a threaded stem, which is turned via a wheel and raises and lowers to operate the valve. Gate valves provide a tight shutoff, and as such are commonly used for isolating media. Because they do not force media to change course (as in a globe valve) gate valves cause little pressure drop.
Gate and globe valves often work in tandem inside a piping system, and are the two most common types of valves. Gate valves can be designed with rising or non-rising stems, which helps when space optimization is critical.
When a gate valve is activated, the stem will turn clockwise or counter clockwise to raise or lower the disc inside the pipe. In its closed position, the disc lodges between the seats in the base of the valve, providing a full stop at a right angle to the media flow. The disc can be shaped like a wedge, knife or parallel panel.
Most of a gate valve’s internal components are located in the top part of the valve body, through which they can be accessed for maintenance.
Gate valves have lower pressure drops than globe valves since the media is not diverted on its way through. Once open, the flow space is as large as the valve cavity and offers little resistance to flow. Gate valves are best for applications in which low-pressure drops are critical, and their multidirectionality makes them more versatile than unidirectional valves.
A downside to the gate valve is the thinness of the disc, which can fall out of alignment with the seat if vibrations from the media become too strong. Because of this, it’s important that the gate stay in either the fully open or fully closed position — too much vibration from pressurized media can cause damage and shorten the life of the valve. Due to the abruptness of their shutoff, gate valves can also trigger water hammer, which can cause major damage in industrial settings.
Gate valves are best in applications where tight sealing or isolation of media is required. Gate valves cannot handle a strong flow of media while in a partially open position, and thus are not used for regulating flow. They can be found in industrial oil and gas piping systems, irrigation networks and marine industries; for example, gate valves with non-rising stems are popular on ships since they take up less vertical space.
Compared to globe or ball valves, gate valves are less durable over long periods of time and more prone to leaks.
Unlike gate or globe valves, ball valves are a type of quarter-turn valve that utilizes a bead shaped ball with a hole in the middle to control the flow of media. Called a bore or port, the hole in the middle of the ball allows media to pass through the valve body when opened, and is rotated 90 degrees to stop flow. The port is aligned perpendicular to the valve body in the closed position.
Ball valves are multidirectional quarter-turn valves, since the rotating ball regulates the flow of media through the port. The stem that controls the rotation of the ball is attached to a perpendicular lever, or actuator. When the lever is parallel to the pipe, the valve is open; when the lever is at a right angle to the pipe, the valve is closed.
Ball valves can also be controlled by actuators: attachments that activate to open or close a valve. Actuators can be hydraulic, electric or pneumatic.
One characteristic of ball valves is their exponential flow rate, which means that the volume of media flowing through the valve increases or decreases at an exponential rate when the valve is opened or closed. This makes it more difficult to precisely calibrate the flow rate, and is why ball valves are generally not the first choice for throttling applications.
Ball valves are favored for their compact design, fast cycle speeds and long service life, which make them the choice for applications that require durable, reliable on/off control such as in the oil, gas, automotive, chemical processing and manufacturing industries. Since such little motion is required to operate ball valves — no multi-turn rotating stem as in globe or gate valves — ball valves tend to last much longer and are less prone to corrosion or damage. If it’s necessary to keep a valve closed for long periods of time, a ball valve will provide the most secure seal.
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