Aug. 12, 2024
Mechanical Parts & Fabrication Services
The wedge is the sealing part of a gate valve and is therefore crucial. Consider the following:
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The wedge nut connects the wedge to the stem. There are two basic wedge nut designs; A loose wedge nut design where the brass nut slides in a slot in the wedge core, and a fixed wedge nut design where the nut is expanded in the wedge core. With a fixed wedge nut design the number of movable parts is reduced, thus eliminating the risk of corrosion as a result of moving parts damaging the rubber surface of the wedge core. A fixed wedge nut design is therefore recommended.
The wedge is exposed to friction and stress forces when the valve is opened and closed during operation of the pipeline. Guides in the wedge fitting to corresponding grooves in the body help stabilizing the wedge position during operation and ensure that the stem does not bend downstream due to the flow velocity. Wedge shoes help ensuring that the rubber on the wedge surface is not worn through as a result of the friction between the wedge and the guiderail in the body. Make sure that the wedge shoes are fixed to the wedge and that the rubber layer underneath is sufficient to prevent corrosion of the wedge core.
It is vital for the tightness of the valve that the wedge is fully vulcanized with rubber and that the rubber volume on the sealing area of the wedge is sufficient to absorb impurities in the seat. A strong bonding between the rubber and the wedge core is important to ensure a correct seal even when the rubber is compressed, and to prevent creeping corrosion even if a sharp object penetrates the rubber during closing of the valve.
The rubber quality is critical for the durability as well as for the valve function. The rubber must be able to withstand continuous impact from impurities and chemicals without being damaged and it must be able to absorb small impurities in the seat to close tight. Consider the following:
The compression set means the rubber&#;s ability to regain its original shape after having been compressed. The EN 681-1 standard states the minimum requirements for the compression set value, but the better the compression set, the better is the rubber&#;s ability to regain its shape and close 100% tight year after year.
Organic substances migrate from the rubber compound and act as nutrients for microorganisms, which will then start forming biofilm causing contamination of the drinking water. Select valves with a wedge rubber that ensures minimum formation of biofilm.
Chlorine and other chemicals are commonly used to clean new pipelines or disinfect old ones. Ozone and chlorine may also be added in low concentrations to make the water drinkable. The rubber compound must not degrade or crack as a result of chemical treatment of the drinking water, as it would cause corrosion of the wedge core.
All rubber components in contact with the drinking water should carry a drinking water approval. If no local approvals are required, the rubber in direct contact with the drinking water should hold one of the major approvals like DVGW/KTW, KIWA or NF.
The external corrosion protection is critical for the service life of the valve. A uniform and even epoxy coating in compliance with DIN part 1, EN and GSK* requirements is recommended and involves the following:
According to ISO -4.
Min. 250 μm on all areas.
The curing of the epoxy coating is to be checked in a cross linkage test (MIBK test). One drop of methyl isobutyl ketone is put on a test piece. After 30 seconds the test area is wiped with a clean white cloth. The test surface may not become matt or smeared, and the cloth must remain clean.
A stainless steel cylinder is dropped on the coated surface through a one meter long tube. After each impact the component is to be electrically tested, and no electrical breakthrough shall occur.
A 3kV detector with a brush electrode is used to reveal and locate any pinholes in the coating.
There are two important design issues:
The sealing placed in the bonnet around the stem retaining the pressure inside the valve/pipeline. Stem sealings should always be designed to be maintenance-free and should last the service life of the valve or at least fulfil the service life demands according to EN -2. The main seal retaining the inside pressure should preferably be designed as a hydraulic seal giving tighter seal with increased internal pressure. Backup seals should be placed around the stem. To protect the sealings against contamination from outside, a sealing should be placed around the stem on the top. For safety and health reasons a drinking water approved high quality EPDM rubber compound must be used where direct contact to drinking water occurs.
Tightness between the bonnet and the body can be obtained by using a gasket embedded in a recess in the valve. This design ensures that the gasket will remain correctly positioned and not be blown out as a result of pressure surges. To protect the bonnet bolts against corrosion the bonnet gasket should encircle the bolts, and the bolts should be embedded in the valve in such a way that no threads are exposed to the surroundings.
When operating a gate valve either by handwheel or by means of an electric actuator it is important to pay attention to the operating and closing torque.
The torque needed to operate the valve from the open position to the closed position, should be between 5 Nm and 30 Nm depending on the valve size. It is important to consider that valves having an operating torque less than 5 Nm encourages the operator of the valve to close the valve to fast thus risking water hammer and pressure surges in the pipeline.
The torque needed to close the valve to a drop tight position. This torque should for handwheel operated valves be balanced against the handwheel diameter in such a way that it does not present the operator with a rim-force in excess of 30-40 kg. When operating the valve with an electric actuator or manual gearbox the torque should be within the limits of a standard range actuator. It is important to notice that the actuators normally have a torque range that is quite wide, and often it is the ISO flange connection between valve and actuator that determines the actuator choice. As a main rule valves with ISO flange connection should have max. closing torques as stated below:
To enable the use of pipe cleaning devices the inside diameter of the valves should correspond to the nominal size of the valve.
* GSK stands for Gütegemeinshaft Schwerer Korrosionsschutz, and is an independent quality association with about 30 members, all leading European valve and fittings manufacturers. GSK outlines requirements for the coating itself and for the control procedures of the finished coating.
Choosing a suitable valve for specific applications is essential in designing a process system. This article will dive into the properties and characteristics of the gate, ball, and butterfly valves and the differences between each type. Knowing how to set these valves apart and how they will interact with your system will help you decide which one best suits your needs.
Because valves are an integral part of piping systems that convey liquids and gases, they are one of the most common components found across most industrial sectors. They regulate liquid or gas flow by allowing it to start and stop, and they must function adequately to prevent leaks and unwanted discharges. They are also effective means to route flows or isolate parts of the piping process for maintenance or monitoring.
Valves come in various sizes and configurations, but their sole purpose is to regulate and control the gas or liquid passing by the pipe. In this article, we will compare the three most common types of valves: gate valves, ball valves, and butterfly valves. We will start by defining each component, exploring what each can do, and what makes them effective, reliable tools.
Here's a closer look at the distinctions between each valve device.
As the name implies, gate valves act as gatekeepers. The term originated from the closing component of the valve sliding into the flowing fluid to induce shut-off, thereby functioning as a gate. They have a flat- or wedge-shaped gate on a threaded stem, and the gate plates move in a straight line parallel to it.
When closed, the gate moves into the flow path and seats into a cavity opposite the stem, creating a solid seal. It is entirely out of the flow path when opened, resulting in zero-flow obstruction or pressure drop. Gate valves work best in high-pressure systems compared to low-pressure systems, where a tight seal is critical and might need special accommodations. A low-pressure operation might lead to seeping at the valve while closed.
These linear motion isolation valves should be kept either completely open or closed as partial opening can cause damage; not used to control medium flow. Gate valves isolate specific areas of the supply network for repair, maintenance, new installation activities, and reroute flow across the pipeline.
Its simple design and versatility in low-pressure drop applications make the gate valve one of today's most commonly used valves. It is designed to be a full-port valve, meaning the valve port is the same size as the inner diameter of a connecting pipe. A full-bore gate valve allows fluid to pass through the pipeline without obstructing or producing a pressure drop. This also makes cleaning the line easier.
Gate valves are widely utilized with bigger pipe diameters, ranging from 2" to the largest pipelines, because they are less complex to design compared to other large-sized valves.
As the name suggests, ball valves are quarter-turn valves that use a rotatable ball with a bore to regulate gas and fluid flow from one opening to the next. The opening is in line with the pipe, allowing the medium to flow freely when turned on. When turned off, the gap is perpendicular to the flow of the medium, stopping it completely.
Also called shut-off valves, ball valves are the only way to shut off medium flow immediately. They are incapable of immediate redirection. Ball valves are best used with gases because they have a robust seal. They can withstand temperatures of up to 350°C and up to 700 bar pressures. Because of their simple structure and compact sizes, typically ranging from 0.5 cm to 30 cm, these valves are easy to use and repair.
Ball valves are known to be durable and reliable, closing securely even after prolonged rest periods and functioning well even after many cycles. Ball valves have a higher resistance to contaminated media than other valve types and provide a good seal despite the unclean medium. These features make them an excellent choice for control and shut-off applications, where gate valves are frequently chosen, but they lack the power of those alternatives in throttling applications.
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Butterfly valves are named after the rotating disc opening and closing the valve. They are quarter-turn rotational motion valves frequently used to shut off flow in pipelines. Also known as flap valves, butterfly valves are composed of a disc fixed on the valve's stem, and it rotates around the stem to control the flow of the medium in the pipeline. A butterfly valve is fast and easy to actuate, requiring only 90 degrees of movement to move from fully open to close.
Because the disc sits perpendicular to the flow when open, a minor pressure drop and slight flow turbulence are unavoidable. This means butterfly valves are not generally suited for use in processes containing solids, grit, and other abrasive material present in them, particularly in waterworks, as they might collect on the disc and prevent a tight seal when attempting to close the valve. Unlike gate valves, the lack of flow restriction elevates the risk of fluid hammering. Fluid hammering can cause a sudden increase in pressure and damage the instrumentation.
These valves are applied in various processes and industries, including water supply, distribution, collection, and pumping stations. They also have a wide range of uses, especially in flow isolation. Easy fabrication and compactness are two of their primary advantages over other valve types, resulting in lightweight, cost-efficient products with smaller installation footprints and faster actuation rates.
Gate Valves
The plate moves in a straight line parallel to the stem in a gate valve. These are lightweight valves with a flat bottom seat gate, precision casting valve body, and integral glue coating. Gate valves are shut-off valves designed to fully open and close to allow unrestricted flow.
Ball Valves
Ball valves contain a stem and ball which turns horizontally. They are best used in applications requiring on and off control without pressure drop. These valves are characterized by a long service life and provide reliable sealing during their lifespan, even when they are not in use for a long time. As a result, they are more popular shut-off valves than gate valves.
Butterfly Valves
Butterfly valves are small and simple compared to gate and ball valves. Their lightweight and compact design make them ideal for installation in tight spaces. In addition, the disc has a 90-degree reciprocating rotation capacity which makes regulating the flow and facilitating a complete, leak-proof shut-off more convenient.
Gate Valves
The gate valve is a complete on-and-off valve. The gate plate, which regulates the media, can only fully open or close. As a result, the pressure drop is minimal, and the flow is bi-directional. The design of the gate is primarily wedge-shaped and does not facilitate flow regulation. The ends of the valve are either threaded or flanged.
Ball Valves
The ball valve drives the valve handle to rotate by a transmission, which in turn causes the ball to pivot perpendicular to the media flow. It opens when the ball's hole is in line with the flow and closes when it is spun 90 degrees by the valve handle. While mainly used for non-slurry applications, ball valves are also ideal for applications that need a tight shut-off. The immediate opening and closing of the ball make it essential in some applications that require media isolation.
Butterfly Valves
The disc moves around its axis in a butterfly valve, enabling complete or partial opening and closing. This makes the butterfly valve suitable for partial isolation and flows that need to be regulated. The primary characteristic of this valve is the change in the deflection of the butterfly disc, making it an appropriate choice for large-diameter applications.
Gate Valves
Thanks to its robust open and close mechanism, the gate valve has the most minimum pressure drop among other valves. These valves offer no resistance to the flow of fluid, making them unfit for controlled applications. The straight-through flow path provides minimum erosion and turbulence. Media accumulation is almost negligible in gate valves.
Ball Valves
Ball valves have an uncomplicated structure and a good sealing performance. They are lightweight, easy to operate, have low material consumption, small driving torque, trim installation size, and offer minimal pressure drop. The ball valve is best used for general working fluids such as water, acids, solvents, and natural gas and media with extreme working conditions, such as oxygen, hydrogen peroxide, methane, and ethylene - making it a versatile option. These valves are commonly used in low-pressure applications.
Butterfly Valves
The butterfly valve's speedy opening and closing speed make it suitable for throttling, flow regulation, and adjustment control. However, the pressure loss is more considerable than the ball and gate valve. Hence, the pressure loss and limitations in operating temperatures should be considered when choosing a butterfly valve.
Lockout valves are devices used by authorized employees to isolate or shut off operating valves to prevent life-threatening accidents from occurring during repair or maintenance work. These lockout devices are different from the valves themselves. They are devices fitted over the operational section of the valve and secured with lockout hasps or padlocks to prevent unauthorized access.
There are different types of valve lockout, each tailored to a certain type of industrial valve. We'll discuss each of these in detail below:
Gate valve lockout mechanisms are classified into two types: rotating and hinged. Rotating gate valve lockout devices have an inward/outward rotating feature, while hinged gate valve lockouts are made up of two half-moons that are hinged on one side.
While their mechanisms differ, both are designed to enclose and cover the gate valve handle in order to prevent unauthorized or accidental valve opening.
Ball valve lockouts come in standard and adjustable designs. The standard ball valve lockout has a simple and efficient single-piece design. To use this, simply clamp the lockout/tagout valve device onto the ball valve lever as far as possible and make sure it sits over the angled portion of the handle. Squeeze the valve handle and the LOTO device together, then secure it with a LOTO lock at its closest fit.
On the other hand, the adjustable valve lockout device comes with a two-piece design. To use this, place the base of the lockout device over the ball valve lever, then slip the sliding piece over the end as far as possible to secure the entire device. To complete the lockout, insert the LOTO lock at its closest fit.
Butterfly valve lockouts have a two-piece design that&#;s quick and easy to install with no extra tools needed. Simply position the main part over the top of the valve lever, then slide the outer sleeve into the handle to block the release trigger.
Using gate valves, ball valves, and butterfly valves have their advantages and disadvantages, largely dependent on the requirements of the application where they are going to be installed. Generally, gate valve devices are ideal for strict sealing applications and do not require a frequent operation. On the other hand, ball valves are best suited for systems with periodic shut-off requirements. Lastly, butterfly valves are most suited for throttling purposes, occupying less space for massive systems.
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