1. The characteristics of rotary valves.
Valves are usually divided into two categories: linear and rotary. The straight-stroke valve allows or cuts off the flow of material through a linear moving part such as a gate. The straight-stroke valve includes globe valve, gate valve, diaphragm valve and so on. The rotary valve allows or cuts off the flow through a rotating part. A rotary valve whose rotating part can rotate 90 degrees is called a 1/4 rotary valve. Sometimes the classification of 1/4 rotary valves can be extended to rotary valves with a rotation angle of less than 270 degrees. It can be seen from the definition that a rotary valve is a rotary valve that changes the valve from an open position to a closed position by rotating a rotating part such as a rotating baffle by 90 degrees. Rotary valves generally have the following characteristics.
(1) Wide adjustable range The
adjustable range is the most critical in the process industry. The adjustable range of rotary valves is about 150:1 (the adjustable range of VALTEK ball valves can reach 300:1, butterfly valves and cam flex valves The adjustable range is 100:1). The corresponding cut-off valve has an adjustable range of 30:1, which means that the rotary valve can provide an adjustable range that is 5 times wider. For example, when the maximum flow rate is 1000 liters/min, the cut-off valve can effectively reduce the flow rate to 30 liters/min. At the same time, the rotary valve can adjust the flow rate to less than 10 liters/min. The more precise the flow control of key valves is in the process The better the repeatability in the medium, the direct reduction of unqualified products in the process industry.
The two characteristics of the eccentric rotary valve increase its adjustable range. The first is its orifice plate design. The eccentricity is an effective 1/4 circle. The eccentric valve core is V-shaped, while the traditional valve core is Round, when the eccentric rotary valve is closed, its small V-shape makes it very precise control at low flow rates. For large flow rates, it almost opens the valve to almost the same diameter as the pipe. The second feature is that there are two types of actuators, one with a certain stroke, and one with a cylinder. The stroke of the 1” stop valve is about 3/4” to 7/8” and the stroke of the rotary valve exceeds 2”. For any other size valve. The greater the stroke of the input end, the better the control at the working end.
The control valve with high adjustable ratio provides the precise ratio according to the requirements of the formula, regardless of the flow rate of each flow path. The traditional linear or regulating valve is no longer suitable for this task. The rotary valve can improve the performance of the device while reducing the control valve. The cost in the life cycle of the plant. The wide-adjustable rotary valve allows users to increase the standard of strict shut-off, thereby improving cost/benefit indicators. The metal valve seat and the village made of special materials can prolong the service life of the valve and improve the performance of the valve. The simple design of the rotating structure can minimize the maintenance workload. Reduce the production cost of the owner.
(2) Strictly close
the shaft of the straight-stroke valve is the stem enters and exits the valve body, and the product is brought out every cycle. It is difficult for the straight-stroke design to avoid leakage. It can bring undesired products into the production process system. And can produce a large unacceptable external leakage of factory volatility. The rotary design of the rotary valve essentially does not have this problem, because this valve uses a low hysteresis, high thrust cylinder type actuator, which can provide more accurate and higher stability than other types of actuators, and can provide long-term tightness. Cut-off metal seat.
(3) Large flow capacity and small size
The flow capacity of a straight-stroke valve is a fraction of the flow capacity of the pipeline where the valve is installed. The way to improve the flow capacity is to increase the size of the straight-stroke valve, which may usually be larger than the pipeline. The capacity of the rotary valve is about 2 to 3 times that of the straight-stroke valve of the same size. The rotary valve can be used to increase the blending volume without changing the size of the feed line, which can greatly reduce the cost. For this reason, some rotary valves have been Designed to replace the flange-to-flange connection form of the stop valve, and equipped with connecting bolts, this type of valve is even cheaper than the stop valve they replace.
In the process industry, throttling can cause a large amount of cavitation. Cavitation may damage the pipeline and produce unacceptable noise levels. At the same time, it can also disturb the metering system and cause unclear or unreliable readings. The rotary valve has a sturdy design, which greatly reduces valve failure due to water hammer. At the same time, its valve core can absorb part of the excess energy, which significantly reduces cavitation and related problems.
(5) No external leakage
The external leakage of the standard rotary valve is 100 times better than that of the standard shut-off valve. The movement of the stem of the shut-off valve pulls dust out and brings sand into the valve, which damages the graphite packing. In the rotary valve, the valve stem does not make any movement in and out, but rotates, so as not to damage the packing and avoid external leakage.
(6) Maintenance costs and spare parts The
metal seat rotary valve is much less than the spare parts of the globe valve, and the maintenance is usually easier.
2. Types of rotary valves
to the type classification There are three types of rotary valves according to the type classification: butterfly valve, ball valve and plunger valve.
The butterfly valve controls the direction of flow by rotating the baffle or blade at a certain angle with the shaft as the center. The baffle or blade looks a bit like a butterfly. The butterfly valve can be used for cutting off or adjusting, and is suitable for slurry or fluid with solid suspension.
The valve core of the ball valve is a spherical body with a round hole. When the valve is in the open position, the fluid is allowed to flow through, and the spherical body is turned 90 degrees to cut off the flow. The ball valve can cut off and adjust, and its pressure drop is small, suitable for viscous and corrosive fluids, low-temperature slurry fluids. Ball valves can also be used for high temperature and high pressure fluids.
The spool of the plunger valve is a conical or cylindrical plunger with a round hole in the middle. When the plunger hole is consistent with the flow direction, fluid can pass through, and the flow path is cut off when the plunger rotates 90 degrees in any direction. Plunger valves are usually used for on/off control.
2.2. Classification according to operation mode
Rotary valves are classified into the following types according to operation mode: manual valve, automatic valve and regulating valve.
The manual rotary valve is operated by hand, and is operated by manual actuators such as hand lever, gear, hand wheel, etc., including a component that can be rotated 90 degrees. The actuator opens and closes the valve by operating the valve stem and flow control element.
The automatic rotary valve includes an automatic actuator, which uses external energy to automatically control or remotely switch the valve. Automatic valves should generally be selected in the following situations: large valves (such as valves larger than 8″), valves that are difficult to manually operate, and it is impossible to use manually operated valves in toxic or other harsh environments. Valves must be required to handle emergency situations. Valves that close quickly; valves that match all flow paths of batch formulations, etc. The types of automatic actuators include hydraulic, pneumatic, and electric, etc. Most automatic rotary valves can be operated by electric and pneumatic actuators.
Rotary regulating valves can be operated in specific This type of valve includes an actuator that can generate a corresponding rotation angle according to the size of the electrical signal. Butterfly valves are often used in low-pressure, large-diameter, and less strict requirements on leakage. The
rotary regulating valve is a kind of control valve. Control valves also include some straight-stroke valves such as globe valves, gate valves, and diaphragm valves. The automatic rotary cut-off valve is not a control valve, because it can only be limited to open and close positions.
3. Application trend of rotary valves
(1) Automatic valves instead of manual valves Valves have become a trend in industrial applications.
With the increase in the automation of process industries worldwide, the demand for automatic and regulating rotary valves is increasing, while the demand for manual valves is decreasing. This makes the future development of valve application technology the focus of the valve Actuator and positioner, so that the valve can be connected to a personal computer, PLC, DCS system. For example, a digital positioner is used to improve the control performance of the valve by improving the accuracy of the control valve. Successful applications include if the ND800 digital positioner is used in Rotating the control valve can increase the variable range of the process parameters by 50%, and then further improve the control performance by matching the electronic characteristics of the valve positioner with the characteristics of the control valve. A typical application is the Field Browser software using HART network.
Compared with the shut-off valve, the control valve is more suitable for continuous production process. Through the adjustment function, the control valve can change the flow rate according to the input parameters to keep the set value unchanged. In many cases, both the shut-off valve and the control valve Both are required. For example, in an oil refinery, a 1/4 rotary shut-off valve is used as a safety shut-off valve. A control valve is usually used with two shut-off valves to isolate and cut off the flow when the control valve is disassembled.
Electric actuators will gradually replace pneumatic actuators, because electric actuators do not require installation and maintenance of supply lines and compressors, can be more closely connected with the positioner, and can avoid converting electrical signals into pneumatic signals.
Other development directions of rotary valves are to improve valve performance and reduce valve leakage by improving the design of the valve seat and the material of the valve seat. Smart valves with control and positioning functions will make the market more and more extensive as manufacturers reduce costs and manufacturers improve production efficiency.
(2) The trend of replacing linear valves
because rotary valves can replace linear valves (such as gate valves) in many cases. Compared with linear valves (such as gate valves), rotary valves have many advantages (see Part 1 of this article) . Especially with the implementation of the Clean Air Action (CAA) amendment, preventing leakage outside the valve becomes more and more important. The CAA amendment proposed in 1990 included 189 volatile toxic air pollution sources (VHAPs), of which 150 are volatile organic compounds (VOCs). The latest CAA amendment passed in April 1997 includes not only the Leak Detection and Repair (LDAR) program in the previous version of the amendment, but also a quality improvement program.
The CAA amendment increases the investment of chemical companies, oil refineries, and other valve manufacturers that use straight-stroke valves to prevent leakage outside the valve. The CAA amendment and the result of the control leakage caused by the manual valve, automatic valve, control valve suppliers to adopt new technology to design the valve to reduce the valve leakage. The key to reducing leakage outside the valve lies in the material of the valve packing system. Polytetrafluoroethylene (PTFE) and graphite are two commonly used materials in valve packing systems. In order to reduce valve leakage, many suppliers combine perfluoroelastomers with some polytetrafluoroethylene and graphite for valve packing. system. The compression force of the perfluorinated elastomer packing system is greater than that of the PTFE packing system, but the compression force of the graphite packing system is many times higher than that of the perfluorinated elastomer packing system. The compression force will cause wear and damage of the valve stem, which will cause leakage outside the valve. In general, the use of rotary valves is an effective measure to reduce external leakage, that is, rotary valves have a tendency to replace actuator valves.
According to the Valve Manufacturer Associate VMA (Valve Manufacturer Associate VMA) market share ranking can also illustrate this trend: according to the data provided by VMA, the United States 1996 ball valve, industrial butterfly valve and plunger valve shipments (VMA specifically recorded ball valves, In addition to the shipment volume of industrial butterfly valves and plunger valves, VMA tracks automatic valves as another category, and does not separately record the sales volume of 1/4 rotary control valves, but classifies them as automatic valves. One category. VMA provides data on shipments of US suppliers, including exports but not imports, totaling US$1.04 billion. This amount includes manual valves and automatic valves, but does not include regulating valves. Ball valves account for the largest percentage, accounting for 54.2% of the total.
Link to this article：Application of Rotary Valve in Refining and Chemical Equipment
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