2017년 1월 31일 화요일
2016년 12월 26일 월요일
The Advantages of Using Butterfly Valves
What Is A Butterfly Valve? A butterfly valve is a quarter-turn valve used to regulate flow. A metal disc in the body of the valve is positioned perpendicular to the flow in the closed position, and rotated one quarter of a turn to be parallel to the flow in the fully opened position. Intermediate rotations allow regulation of liquid flow. They are often used in agricultural and water or wastewater treatment applications and are one of the most common and well-known valve types.
Advantages of a Butterfly Valve
Butterfly valves are similar to ball valves but have several advantages. They are small and, when actuated pneumatically, open and close very quickly. The disc is lighter than a ball, and the valve requires less structural support than a ball valve of comparable diameter. Butterfly valves are very precise, which makes them advantageous in industrial applications. They are quite reliable and require very little maintenance.
Disadvantages of a Butterfly Valve
One disadvantage of butterfly valves is that some portion of the disc is always presented to the flow, even when fully opened. The use of a butterfly valve therefore always results in a pressure switch across the valve, regardless of the setting.
Butterfly Valves Operating Electronically, Pneumatically or Manually
Butterfly valves can be configured to operate manually, electronically or pneumatically. Pneumatic valves operate most rapidly. Electronic valves require a signal to the gearbox to open or close, while pneumatic valves can be either single or double actuated. A single-actuated valve is typically set up to require a signal to open with a failsafe, meaning that when power is lost the valve springs back to a fully closed position. Double-actuated pneumatic valves are not spring loaded and require a signal both to open and to close.
Automated pneumatic butterfly valves are both reliable and durable. Reduced wear improves the valve life cycle, which reduces operating costs otherwise lost in working hours to maintain the valves.
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2016년 12월 19일 월요일
How does it work: Butterfly Valves
A butterfly valve is a flow regulating disc valve. According to the BVAA, a butterfly valve is “a valve in which the obturator rotates about an axis at right angles to the direction of flow and, in the open position flow passes around the obturator.”
In other words, a butterfly valve consists of a circular disc or plate built with a stem through the middle or attached offset. When opened, the disc pivots 90 degrees in the valve bore, aligning with the flow, creating a nearly unrestricted flow path. Butterfly valves operate similar to ball valves in their 90 degree rotation and allow for quick shutoff.
Butterfly valves have gained popularity over the years due to their thin profile between flanges, making them lighter in weight and creating a smaller install footprint. This compact design also usually makes the initial cost for these valves lower when compared to other valve designs.
There are different types of butterfly valves and each type has unique design features and applications. Each type is defined by the orientation of the stem to the disc and the seating angle that the disc closes on.
The most basic butterfly valve design is commonly called a concentric or resilient-seated butterfly valve. In this type of valve, the stem is centered in the middle of the valve disc, which is centered in the pipe bore. This valve typically has a rubber (or resilient) seat and relies on the disc having a high level of contact with the seat to effect a seal. The disc will contact the seat the earliest (85+/- degrees) during the 90 degree rotation.
DEMCO butterfly valves are durable resilient-seated valves that feature a one-piece body for reduced weight and increased strength. The unique stem hole design in the disc ensures a dry stem journal, and the hard-backed seat allows ease of installation, reliable operation, and is field-replaceable without special tools. DEMCO butterfly valves are field-proven to be an excellent choice for industrial and commercial applications.
In a single-offset butterfly valve, the stem is located behind the disc. There are few, if any, valves of this type left on the market today due to the development of the double-offset, or high-performance, valve. The single-offset of the stem causes the disc to contact the seat with three to four degrees left to travel with the idea that less seat contact will enable the valve to last longer.
The double-offset or high-performance butterfly valve has a disc with two offsets and can be rated up to 1480 psi. Similar to a single-offset design, the stem is located behind the disc. With the high-performance butterfly valve, the second offset’s stem is moved once more off the center of the disc to one side. This geometry of offsets enables the 90 degree disc rotation to “rub” over the seat for only one to three degrees of the 90 degree rotation.
The double-offset butterfly valve is used in systems requiring higher pressure resistance. The disc is positioned in the center of the pipe bore and arranged to increase sealing ability and decrease wear to the valve. WKM high-performance butterfly valve is ideal for handling water, oil, steam, and gas in a cost effective, light weight design. The WKM butterfly valve’s corrosion-resistant, single-component thrust bearing/disc spacer reduces body wear and helps ensure positive centering of disc in the valve bore.
Both resilient and high-performance butterfly valves can be operated by handles, gears, or actuators. These devices move the valve disc to the optimal position for complete shutoff or fully open the valve. Resilient and high-performance butterfly valves are used mainly in the water, chemical and petrochemical industries, but can also be used in fuel handling systems, power, and many other applications.
The last type of butterfly valve is the triple-offset butterfly valve (a.k.a. TOV or TOBV). This type of butterfly valve is applied in difficult services where reliable performance under harsh conditions is needed. Exactly like a double-offset valve, the stem is located behind the disc and offset to one side (double-offset). The third offset is the geometry of the seating surface. This seating surface creates a sort of cone-shape of the disc and seat, which wedges into the seat with very minimal contact between the sealing surfaces until fully closed. By reducing the amount of contact that the seat has with the disc, sealing becomes more efficient and the life of the valve is extended.
TOVs are typically metal–seated, unlike single and double-offset butterfly valves. The Cameron WKM TOV has a metal seat design, which allows it to achieve zero leakage* in either direction for extreme services. The true triple-offset geometry of the WKM TOV allows for bubble-tight* sealing to create a fully bi-directional zero-leakage* shut-off valve.
Triple offset butterfly valves are commonly used in steam, chemical, refining, and offshore oil and gas applications up to 1400° F (760° C). The rugged metal seats can also withstand a high degree of abrasion and shock and, with appropriate material selection, can also be utilized in cryogenic services such as LNG and NPG facilities.
*Per API 598
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2016년 12월 12일 월요일
What is Steam and what are its principal applications?
Steam Control Valve
This article is not focused to any specific aspe
we will focus mostly on technicalities and industrial applications of steam control valve. We will try to make this article interesting for all readers, and navigate from basics to complex technology-oriented topics.
Let’s hit at the root of the subject, which is steam in this article.
What is Steam and what are its principal applications?
Water is converted to steam during transition from liquid to gas. Hydrogen bonds keep the H2O molecules together, and it breaks free to generate steam. In the liquid form, the molecules are continuously broken and joined with each other. The bond of the molecules becomes loose with application of heat, and eventually some molecules will break free at higher heat and create steam or dry steam. Both dry and wet steam is used in industries. Wet steam is formed when some water molecules release its’ latent heat and form tiny water droplets.
Steam is used in a large number of industrial applications. The most common applications are the process heating and driving the steam turbines to generate electricity. Apart from these, steam is used for atomization, cleaning, moisturize, and humidification. However, most of the applications will require some parameter control, and thus a steam control valve becomes a part of process control engineering.
Why a Steam Control Valve is used in process control applications?
Technically, a valve is a device which will control flow of fluid or vapor in any system. In case of steam, the major applications are to reduce the pressure of the inlet steam for process application. However, in addition to controlling pressure, a steam control valve will also control temperature.
Boilers will typically work at high pressures, as low pressure operation will result in carryover of water. High pressure steam has lower specific volume, which will allow pipes to carry less weight. In effect, distribution of steam becomes easier and cheaper due to the cost reduction in piping and insulation. Industrial process applications use steam at a lower pressure. The reason for this is that the steam at a low pressure has higher latent heat, which increases energy efficiency to a large extent. Steam pressure and temperature are related, and hence temperature is automatically controlled through the control of steam pressure. The reduction of steam pressure is also related to the required plant safety. Steam pressure can be controlled by using Steam Control Valve.
What is the principle of operation of a Steam Control Valve?
The old method of throttling flow or using an orifice plate will result in fluctuations of flow rate. The ideal solution is to adjust the downstream pressure automatically through proportional adjustment of valve opening. This technique will keep the steam pressure unaltered even during the fluctuations of flow rate.
The schematic in Fig-1 illustrates the use of a Steam Control Valve to reduce steam pressure to control heating inside the jacketed vessel. This system will ideally demand for manual interventions. A fully self-contained automatic system will comprise of an actuated Steam Control Valve, a pressure sensor and an actuator. This kind of system senses the fluctuations in the downstream pressure and adjusts it through a quick response actuator.
The typical mechanism to adjust the outlet pressure is through a balance of force adjustment between the steam pressure and the adjustment spring in the valve. The steam control valve is manufactured either by Non-piloted Acting Valve or Pilot-operated Acting Valve.
The non-piloted adjustment spring applies downward force directly on the main valve. The Piloted type adjustment spring executes downward pressure on a smaller pilot valve, which is not the same as the main valve.
The advantages of direct acting type are compact, economic, and straightforward installation. However, the controlling factor have larger variations from the set point, and mostly used for low loads.
The schematic in Fig-2 above is self-explanatory of the Piloted type Steam Control Valve arrangement normally used for a Heat Load along with a Heat Exchanger. The piloted type is used for heavy loads to achieve a fast response in a much broader range of flow range than the non-piloted types. However, these are larger in size and costlier.
What are the industrial applications of Steam Control Valves?
Steam is the mostly used medium for heat transfer in process control applications in industries. Steam Control Vales are largely used in steam driven turbines in electric power plants. The range of applications is quite large. However, Food-processing plants, Refineries and chemical plants are the users of different types of control valves driven by steam.
Here is a list of some typical applications of this valve:
- Fluid heat exchangers
- Re-boilers
- Reactors
- Combustion air pre-heaters
- Sterilizers
- Unit Heaters
- Humidifiers
- Direct Steam Injection Sanitary Heater
- Ironers
- Acid baths
- Small storage calorifiers
- Water treatment plants
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2016년 11월 30일 수요일
WHAT IS A BALL VALVE?
Valves are found in our everyday living and may go unnoticed. For example, there are plumbing valves which are at our sinks in our spigots. There are valves inside our washing machines, dishwashers, gas fireplaces, outside water spigots, refrigerators and more. Various types of industries utilize valves for their productions. Such industries include electronics, power, automotive, printing, plastics, textiles, metal, medical, chemical and food, to name a few. The industries that utilize ball valves typically have a need for supporting high pressure and temps exceeding 480 degrees. Ball valves are simplistic to operate and repairs are made easily without being removed from their pipeline.
Being made of steel, brass, iron, bronze or PVC, ball valves can range in sizes from .2 to 11.81 inches. More complex control systems using valves that need to regulate flow through a pipe will need an actuator. The actuator controls the valve pneumatically or is motor-operated and keeps it positioned appropriately so that the flow of the liquid is precise to the changing pressures and flow levels.
The basic types of ball valves include Full Port, Reduced Port, V-port, Multi-port, Standard Port and Cavity Filler ball valve. There are 3-way and 4-way ball valves. Depending on the application would determine the type of ball valve being used. The specifications to be considered for determining the appropriate ball valve would be the temperatures and pressure, number of ports, valve size, type of body material, end connectors and configurations.
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2016년 11월 23일 수요일
Soft Seated or Metal Seated Ball Valves?
There are two types of seat in a ball valves, soft seated and metal seated. Soft seated ball valves use a thermoplastic material such as PTFE, NBR etc. Metal seated ball valves use metal as seat material such as 316 SS, monel, etc. Each ball valve seat type has its own characteristic and thus having its own advantages and disadvantages. So how to select the best ball valves seat type?
In general application the soft seated ball valves is suitable for a clean fluid and not recommended for a dirty fluid. This is reasonable since some particulate may damage the soft seat material and causing a leakage. In dirty fluid application, the metal seated ball valve is the best choice.
The metal seated ball valve is also suitable for application that has a high erosion or corrosion rate. The metal ball valve will stay much longer than soft seated ball valve in such application. In term of the shut-off capability, the soft seated ball valve is the best. It can achieve the highest level of the seat leakage class while a metal seated ball valve can achieve only up to ANSI/FCI Class V or ISO 5208 Rate D. Thus if the tight shut-off or bubble tight shut-off is required, then there is no choice except using the soft seated ball valves.
In term of the required torque to operate the ball valves, the soft seateThe metal seated ball valve is also suitable for applicationd ball valve will have a lower torque rather than metal seated ball valves. The high torque of the metal seated ball valve may lead the selected actuator model is bigger than the soft seated ball valve thus it increase space consumption and add additional weight.
In general practice, a metal seated ball valve is only used in the severe service condition such as high temperature, high erosion or abbrasion, slurry, etc. In general application of oil and gas, the
soft seated ball valve is used. This type of seat will provide an excellent sealing capability with a fair life time in clean service application. While the metal seated ball valve is used when the soft seated ball valve can’t handle anymore the process condition.
Some valve manufacturers use a pure soft material as their soft seat material and there are also manufactures that use a metal oring (metal seat ring) and soft material as its seat insert for size 2 inch and above. This type of seat even though it use a metal seat ring but it is categorized as soft seated ball valve due to it use a
soft material as final part that in contact with the ball.
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