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Max Differential Pressure Across An On/off Valve Prior To Open It
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(Chemical)
(OP)
26 Sep 18 10:49Gents,
Disclaimer: Process Eng background here with only superficial knowledge of intimate details of valve design: be gentle! ;)
It seems to be common practice amongst my Process Eng colleagues that a full bore, ball on/off valve shall not be open before ensuring that the differential pressure between the upstream and downstream side is lower than 15-30 psi, so a smaller bypass valve is normally provided to allow the pressurisation of the downstream side. Some colleagues argue that this holds for liquid, but some of them consider this should be followed even for gas at, say, 300 psig (i.e. valve should not be open until the downstream side is pressurised to 270-285 psig).
What is the rationale behind this? Is it only aimed to prevent oversizing the actuator? Let's assume the actuator is sized for full pressure rating. Does opening the valve across a large differential pressure (i.e. larger than 15-30 psi) creates high vibration, noise or any other operational upset? We are concerned about erosion of the ball at lower opening (high velocity, high erosion) or damage to the seats.
Any idea where this assumption of low DP across the valve may come from? Is it still valid?
Any feedback on this would be deeply appreciated.
Regards.
Replies continue below
(Petroleum)
26 Sep 18 11:54Actuator design, in the vast majority of cases, is sized for opening and closing under max DP as stated in the data sheet, plus a safety factor, sometimes of 2 or 1.5.However in practice this rarely happens because of the things you mention.Normally the issue is damage to the seats, not the ball itself, but both can occur.Especially if your seat has a "soft seat" insert, this can get chunks of it ripped out when opening under high differential pressures.The other issue with gas is whether the large DP leads to low temperatures / hydrates or excessive velocity in the downstream equipment.You can go overboard with these bypasses and usually there is some rationalization to work out when this large DP might appear and how to start up the plant using the least number of bypasses.Then you need also to look at the volume of fluid needed to pressure up the downstream section. If the volume is large then your little 2" bypass on a 30" valve might take hours for a gas system but seconds for a liquid system...Is it still valid - very much so.Do the operators use all these little bypasses.. Hmmmm
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
(Chemical)
27 Sep 18 02:23That criterion is valid for soft seated isolation valves >DN50. Metal seated isolation / safety critical valves can tolerate a much higher dp of up to 14bar, per operational guidelines in my former OpCo. Pressure equalisation bypasses are not required for main line valves DN50 and less. Actuators are sized for the full differential nevertheless.
(Mechanical)
3 Oct 18 18:59There may be situations where the valve seat design is not robust enough to withstand the large pressure difference when operating, but that is not the primary reason for reducing pressure or mandating slow operation. Reason is primarily to avoid water hammer, or a large temperature increase in the down stream pipe. Ball valves can be fast opening and create a large pressure spike (incompressible fluid) or temperature increase (with compressible media) in the downstream pipe that blows out gaskets or melts all the soft parts in the next downstream component. So slowly decrease the pressure before opening, or slowly open the valve. Either way usually works.
(Mechanical)
4 Oct 18 23:37Added to the comments above, the dp limitation across the ball valve may be due to the ball valve types. Per one of the design guidelines I used, the floating type ball valve is for the size of =<2" or the dp of <75psig, and the trunnion type ball valve for >2" or =>75 psig.
I believe the guideline is the best practice of the plant operation & reliability, and the similar design is also seen for the globe valves but in the larger dp value than that for the ball valve.
(Mechanical)
11 Oct 18 20:30All above is essential bits of a complicated picture, stating that one should always accurate describe all possible working conditions before and selecting a valve.If, for instance, you seek a ball valve with full opening and larger size suited for large pressure difference, you could look into double eccentric ball valves. (High price limited liquids)The point is that every valve problem has a best possible solution if all procession details are known.In addition to answers regarding sealing damage, cavitation could be a problem if the valve is left open with a smaller opening.
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Ball valves are one of the valve types most widely used in different industries. The demand for the ball valve is still growing. Have you ever wondered how ball valves make impact to your applications.In this article, you will learn about the common components of a ball valve and their functions. What&#;s more, we will show you how a ball valve works to help you better understand it before you have one for your applications.
As its name suggests, the ball valve has a ball-like disc that acts as a barrier when the valve is closed. Ball valve manufacturing companies often design the ball valve to be a quarter-turn valve but it can also be a rotatory kind when it controls or diverts the flow of media.
There is also a growing need for ball valves as the world seeks more alternative sources of energy. Apart from China, ball valves can also be found in India. There is no denying the importance of such valves in any industrial piping systems. But, much is to be learned about ball valves, and you should know it before you use it. This article will help you understand ball valves more so you&#;d learn whether these are suitable for your applications.
One of the most commonly used industrial valves, ball valves are often employed in tight shut-off applications. The ball valve got its name from the hollowed-out sphere component that allows media passage when it opens or blocks it when closed. These are members of the quarter-turn family of industrial valves.
The ball valve is often used in many industries so it&#;s not surprising to find that its demand is high. Nowadays, you can find high quality made in China ball valves or ball valves manufactured in India.
Many ball valve types share the same features as mentioned below:
# Swing check &#; this prevents backflow of media
# Valve stops &#; this allows only a 90-degree turn
# Anti-static &#; this prevents static electricity buildup that can cause sparks
# Fire-safe &#; a secondary metal seat is built to act as supplementary seats in high-temperature applications.
Ball valves are great to use when the system needs a quick opening and closing. These are also advantageous in applications where it needs a tight seal without having to consider high internal pressure.
However, ball valves have limited throttling capacities. In fact, these are not recommended for regulating media flow. Ball valves have partially exposed seats, which can erode quickly when slurries are used. These are also hard to open quickly and manually when pressure is high.
Ball valves come in different materials. Depending on the nature of the application, ball valves are often forged or cast using iron, stainless steel, and other steel alloys. Ball valve seats can be made of an elastomeric material such as PTFE or metal, often stainless steel.
Are you interested in learning more about high pressure on-off ball valve? Contact us today to secure an expert consultation!
Although there are several variants of the ball valve, there are five common components present in all ball valves as seen in the diagram below:
# Body
The body holds all the components together
# Seat
The seat seals the valve during shut-off
# Ball
The ball allows or blocks the passage of media.
# Actuator
The actuator or lever moves the ball so the latter can open or close.
# Stem
The stem connects the level to the ball.
Typically, ball valves have two ports. But with the advent of new services, ball valves can have up to four ports. These are often branded as two-way ball valve, three-way or four-way ball valves. A three-way valve can have an L-configuration or a T-configuration.
The ball disc is opened or closed by turning the actuator a quarter turn or 90-degrees. When the lever is parallel to the flow of media, the valve allows the latter to pass through. When the lever becomes perpendicular to the flow of media, the valve blocks the flow of the latter.
Ball valves are actually classified in several ways. You might encounter valve groups based on the number of components or the type of ball valves have.
You can classify ball valves depending on the number of components their bodies have. The cheapest among the three, the one-piece ball valve is made of a single block forged metal. This can&#;t be disassembled for cleaning or maintenance. One-piece ball valves are suitable for low-pressure applications.
On the other hand, the two-piece ball valve is made of two pieces connected by threads. This type should be completely removed from the pipeline while being cleaned or replaced. Lastly, the components of the three-piece ball valve are connected via bolts. Maintenance can be done on the valve even if it is still attached to the pipeline.
The design of the ball is a major classification for ball valves. Aptly named because the ball is suspended at the top of the stem, the floating ball valve is the most common design of this category. As it closes, the ball moves towards the downstream opening. The pressure load helps seal the valve tightly.
On the other hand, the trunnion mounted ball design is held steady by trunnions located at the bottom of the ball. The most suitable application for the trunnion mounted ball valves are those that have large openings and high-pressure ranges, typically more than 30 bar.
Ball valves can also be categorized based on the size of the connection in relation to the diameter of the pipes. A reduced bore ball valve means that the diameter of the valve is one size smaller than that of the pipes. This causes a minimal pressure loss. One-piece ball valves often have the reduce bore type.
The full bore type types have the same diameter as that of the pipes. The advantages of this type include no pressure loss and easier cleaning. Full bore ball valve are more expensive because of the size of the valve. Lastly, The V-shaped type has a V-shaped hole which enables precise fluid control whenever the valve is open.
Ball valves are often found in a wide variety of applications. Most often, you would find them in flowing systems on ships, corrosive services and fire safe protection services. These are not used in applications where contamination is an issue such as those in the food processing services. Ball valves are difficult to clean.
Ball valves are often used in applications that require tight sealing. They are known to have low-pressure drops. Its 90-degree turn makes it easy to operate even if the media has high volume, pressure or temperature. They are quite economical because of their long service life.
Ball valves are ideal for gases or liquids with little particles. These valves don&#;t work well with slurries as the latter easily damages the soft elastomeric seats. While they have throttling capacities, ball valves are not used as such because friction from the throttling can easily damage the seats too.
There are many ways to classify valves. Be as it may, there are seven valve components common to all valves.
The body is the framework of the entire ball valve. It acts as a barrier to the pressure load from the media so there is no transfer of pressure to the pipes. It holds all the components together. The body is connected to the piping via threaded, bolted or welded joints. Ball valves can be classified according to the type of body, often cast or forged.
Source: http://valve-tech.blogspot.com/
The opening or closing of the valve is provided by the stem. This is also what connects the ball disc to the lever, handle or actuator. The stem is the one that rotates the ball disc to open or close it.
This is the gasket that helps seal the bonnet and the stem. Many of the issues happen in this area so proper installation is important. Too loose, leakage happens. Too tight, movement of the stem is restricted.
The bonnet is the covering of the valve opening. This acts as the secondary barrier for pressure. The bonnet is what holds all the internal components together after these are inserted inside the valve body. Often made from the same material as the valve body, the bonnet can either be forged or cast.
This is the disc of the ball valve. Being the third most important pressure boundary, the pressure of the media acts against the disc when it is in the closed position. Ball discs are often made of forged steel or any durable material. Ball disc can either be suspended like in case of the floating ball valve, or it can be mounted like that of the trunnion-mounted ball valve.
Sometimes called the seal rings, this is where the ball disc rests. Depending on the design of the ball disc, the seat is either attached or not to the ball.
Actuators are devices that create the rotation needed by the ball valve to open the disc. Oftentimes, these have a power source. Some actuators can be remotely controlled so valves still work even if these are located in remote or hard to reach areas.
Actuators can come as handwheels for manually operated ball valves. Some other types of actuators include solenoid types, pneumatic types, hydraulic types, and gears.
Generally speaking, the ball valve working mechanism works this way. Whether it is manually or actuator operated, some force moves the lever or handle to a quarter turn to open the valve. This force is transferred to the stem, moving the disc to open.
The ball disc turns and its hollowed side faces the flow of media. At this point, the lever is in the perpendicular position and the port to parallel in relation to the flow of media. There is a handle stop near the connection between the stem and bonnet to only allow a quarter-turn.
To close the valve, the lever moves back a quarter turn. The stem moves to turn the ball disc in the opposite direction, blocking the flow of media. The lever is in the parallel position and the port, perpendicular.
However, take note that there are three kinds of ball disc movement. Each of these has different working operations.
There are many variants of the ball valves, like 3-way ball valve and ball valves in different materials. In fact, 3-way ball valve working mechanism is also different from common ball valve.
The floating ball valve has its ball disc suspended on the stem. There is no support at the bottom part of the ball so the ball disc partially relies on the internal pressure for the tight seal ball valves are known for.
As the valve closes, the upstream linear pressure from the media pushes the ball towards the cupped downstream seat. This provides a positive valve tightness, adding to its sealing factor. The downstream seat of the floating ball valve design carries the load of the internal pressure when the valve is closed.
The other kind of ball disc design is the trunnion mounted ball valve. This has a set of trunnions at the bottom of the ball disc, making the ball disc stationary. These trunnions also absorb the force from the pressure load when the valve closes so there is less friction between the ball disc and the seat. Sealing pressure is performed in both upstream and downstream ports.
When the valve closes, spring-loaded seats move against the ball which only rotates in its own axis. These springs push the seat tightly to the ball. Contrary to floating ball valves, trunnion-mounted ball types are suitable for applications that don&#;t need high pressure to move the ball to the downstream seat.
Lastly, the rising stem ball valve utilizes the tilt-and-turn mechanism. The ball disc wedges to the seat when the valve closes. When it opens, the disc tilts to remove itself from the seat and allow media flow.
# Oil valve
# Chlorine Manufacturing
# Cryogenic
# Cooling water and feed water system
# Steam
# Ship flowing systems
# fire safe ball valve for Fire-safe systems
# Water filtration system
Understanding how the ball valve works means you can make intelligent decisions whether these valves are suitable for your needs. If you need to know more about ball valves, connect with XHVAL Industiral Co. Or you can also get the great manufacturers in this complete valve manufacturers guide in China.
Ball valves are evolving together with the industries these are associated with. Being buyers, educating oneself about what ball valve is important. Or you can also get the great manufacturers in this complete valve manufacturers guide in China.
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