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Principle and operation method of the handwheel for pneumatic control valves – Kle-Valve, Wuxi Clariant Fluid Control Pneumatic Control Valves. In nitric acid production facilities, many control valves are equipped with manual operation handwheels. This is necessary in cases where the control valve cannot be operated remotely from the control room due to non-process-related issues such as problems with the instrument air supply, damage or leakage in the valve diaphragm, faults in the positioner, issues with the 4–20mA signal lines, or problems with the DCS output. Yet it is not desirable for the production process to be interrupted as a result. In certain special operating conditions, the handwheel can be used to operate the control valve locally, allowing for manual adjustment of the process flow or ensuring the continuous operation of the equipment. Additionally, in emergency situations, the handwheel can be used to operate the control valve manually in order to prevent equipment failures or to stop the escalation of such failures under abnormal conditions. There are a wide variety of handwheels; the handwheels fitted to different types and models of control valves vary in terms of working principle and method of operation. For a long time, process engineers have rarely used hand wheels and know very little about them. As the number of failure factors associated with control valves increases over time due to their length of use, and in order to meet the goal of achieving long-term safe operation, there is an increasing need to operate these control valves manually. To enable process technicians to use the adjustment handwheel correctly and skillfully for manual operation of control valves, this article provides an introduction to the principle and usage method of the control valve handwheel mechanism. 1. Types of control valve actuators: In the concentrated nitric acid workshop, the more important control valves equipped with handwheel mechanisms include the following types: .11 Diaphragm linear-acting actuator control valves – FV-206 (ammonia oxidation-reduction and ammonia addition control valve); FV-205I (nitric acid secondary air control valve); FV-202I (air control valve for nitric acid entering the oxidation furnace); SV-101II (nitric acid rotation speed control valve). .2 Gear and rack rotary-acting actuator control valves – 782’s M2; remote controller for the primary oxidation furnace – HV-207; ammonia oxidation-reduction bypass valve – 781; spray water control valves (4 in total); 661; PV-102 (air control valve for entering the distillation tower). It should be noted that although FV-205, FV-202, and SV-101 have linear-acting diaphragm actuators, their operation mode is rotary, with the transition between these modes being achieved through the use of cranks. 2 Types and working principles of handwheel mechanisms 2.1 Handwheels used for diaphragm-type linear actuators 2.1.1 Top-mounted handwheel The handwheel is located on the top of the actuator (i.e., above the diaphragm housing). PV-206 belongs to this type, and its working principle is shown in Figure 1. . When the handwheel is turned clockwise, its shaft drives the diaphragm core and the valve stem upward, causing the valve element to move away from the valve seat and thus opening the control valve. (For a directly-acting pneumatic-operated control valve, the handwheel must be turned counterclockwise to cause the valve element to move downward and close the control valve.) #178# Luzhou Tianhua Technology, Issue 2, 2008 At the connection point between the handwheel and the solid core, there is a clearance larger than the stroke of the valve stem; when the handwheel shaft moves down to its lowest position, the handwheel exerts no force on the valve stem, and the position of the valve stem corresponds solely to the magnitude of the input signal. At this point, the control valve is in remote control mode. 2.1.2 Side-mounted handwheel: The handwheel is mounted on the side of the actuator. SV-101, FV-202, and FV-205 belong to this type. There are also two types of side-mounted handwheels. The working principle of this type of handwheel is based on the lever principle, and there are two types. 2.1.2.1 The L-type lever wheel SV-101 belongs to this type (there are 3 more of them in air separation units); its working principle is shown in Figure 2. Both the handwheel shaft and slider a have trapezoidal threads; when the handwheel is turned, slider a moves horizontally left and right along the handwheel handle, causing the end b of the L-shaped lever c to move vertically up and down around the pivot point d, thereby driving the valve stem to move up and down as well. At point a of the L-shaped lever c, there is a long hole whose length is greater than the stroke of the valve stem. When slider a is at the center of the long hole (i.e., when the position pointer indicates the /00 mark), the handwheel exerts no force on the valve stem throughout its full range of motion; in this case, the control valve is in a remote control mode. The position indication window of this handwheel mechanism is perpendicular to the valve stem. 2.1.2.2 The straight-plate lever wheels FV-202 and FV-205 belong to this type. Its working principle is shown in Figure 3. The handwheel is in mesh with the screw that drives slider a through sector gears; when the handwheel is rotated, the screw causes slider a to move up and down vertically, which in turn causes end b of lever c to move up and down vertically around the pivot point d, thereby driving the valve stem to move up and down as well. Similarly, at point a of lever c, there is a long slot whose length is greater than the stroke of the valve stem. When slider a is at the center of the long hole (i.e., when the position pointer indicates the /00 mark), the handwheel exerts no force on the valve stem throughout its full range of motion; in this case, the control valve is in a remote control mode. The position indication window of this handwheel mechanism is parallel to the valve stem. The locking element of the above two types of handwheel mechanisms is a U0-shaped fork. /When the U0-type fork locks down on the handwheel, the handwheel cannot rotate. When operating using a handwheel, first lift the U0-type fork, then turn the handwheel in the direction indicated on it to manually control the control valve. 2.2 Handwheel for gear-rack angular travel actuators. Gear-rack angular travel actuators are an improved version of piston actuators; they are commonly used in angular travel control valves, with the kinetic energy that causes the valve stem to rotate still coming from the piston mechanism. The working principle of such actuators is as follows: when the rack driven by a piston inside the actuator moves horizontally, a gear on the valve stem that meshes with the rack causes the valve stem to undergo angular displacement. There are a total of 7 control valves of this type: HV-207, M2, 781 spray water (4 units), and air separation PV-102. The working principle of the handwheel mechanism in these types of control valves is the same. The handwheel is located below the actuator, on the right side of the valve stem. The handwheel shaft and the valve stem are driven by a worm-gear mechanism; their engagement or disengagement is controlled through a clutch. When the handwheel shaft engages with the valve stem and the handwheel is rotated, the handwheel rotates in the X-Y direction, which in turn drives the valve stem to undergo an angular displacement in the Z-X direction, thereby opening or closing the control valve. When the handwheel is separated from the valve stem, the handwheel no longer exerts any force on the valve stem, and the control valve is in a remote-controlled state. The clutches of such control valves vary considerably in terms of installation position and usage methods depending on the model and specifications; currently, there are 3 types available. (1) The clutch is equipped with a locking device, and the clutch handle is located on the left side of the handwheel shaft. Four control valves for the spray water in No. 781 belong to this type, as shown in Figure 4. When it is necessary to manually operate the control valve using a handwheel, first pull the handle of the locking device outward with the right hand, while simultaneously rotating the clutch handle clockwise upward to about 90 degrees with the left hand. Then, rotate the handwheel back and forth while also rotating the clutch handle back and forth; once a noticeable force is felt on the handwheel, it indicates that the worm driven by the handwheel has engaged with the worm gear on the valve stem. At this point, the handle of the locking device will return to its original position, and the control valve can now be operated using the handwheel. To exit the handwheel operation mode, first pull the handle of the locking device outward with the right hand, while simultaneously rotating the clutch handle counterclockwise and downward by about 90 degrees with the left hand. When the handle of the locking device returns to its original position, it indicates that the handwheel has been separated from the valve stem; at this point, it can be felt that the handwheel is no longer under any force, and it can be rotated freely without having any effect on the valve stem. (2) The clutch is equipped with a locking device, and the clutch handle is located below the handwheel shaft. The 782M2 and air separation PV-102 control valves belong to this type, as shown in Figure 5. The method of manually operating the control valve using a handwheel is exactly the same as in (1), except that the clutch handle is rotated counterclockwise upward by about 90 degrees. (3) The clutch has no locking mechanism. The handwheel of the HV-207 control valve does not have a locking mechanism, as shown in Figure 6. Figure 6 shows the structural diagram of a clutch without a locking mechanism. This type of control valve does not have a clutch locking device, and instead of rotating the clutch handle, it is sufficient to pull it outward in order to engage the worm gear with the worm. 3 Additional functions of the handwheel: The handwheel used in linear actuators for films can not only be used to manually operate the control valve but also has a function to limit the valve position; whereas the handwheel used in rotary actuators with gear-rack mechanisms does not have such a function for limiting the valve position. 3.1 Principle and method of limit control for top-mounted hand wheels: As stated in 2.1.1, the principle behind using a top-mounted hand wheel to operate a control valve is to utilize the thrust generated by the hand wheel shaft when it is rotated; this thrust replaces the thrust produced by the diaphragm in response to an air signal, thereby overcoming the reaction force of the spring and enabling the valve stem to move. Once the handwheel exerts force on the valve stem and moves it to a certain position (other than its original position), the thrust generated by the diaphragm head under the action of the air signal can only combine with the thrust exerted by the handwheel, causing the valve stem to continue moving in the same direction; it cannot move in the opposite direction and pass beyond a certain position. Therefore, the top-mounted handwheel serves as a one-way limit device. To set a limit for the control valve equipped with a top-mounted handwheel, first remove the air signal from the diaphragm head, or adjust the current signal to 4 mA; then rotate the handwheel to move the valve stem to the desired limited position. 3.2 Principle and method of limitation for side-mounted handwheels: As stated in 2.1.2, the principle behind using side-mounted handwheels to operate control valves is that when the handwheel is rotated, the slider on the handwheel shaft pushes end a of lever c, causing it to move horizontally (in the case of an L-shaped lever) or vertically (in the case of a straight lever). Through the pivot point d, this thrust is transferred, resulting in vertical movement of end b of the lever, and thereby causing the valve stem to move up and down. The slider can exert force on the lever on both sides within the slotted hole; in other words, the control valve can force the valve stem to move in either direction, regardless of whether there is a signal or not. Therefore, the side-mounted handwheel has the function of providing bidirectional limit control for the regulating valve. However, it is not possible to set limits on a control valve in both directions at the same time; only one direction can be used to limit the control valve. When it is necessary to set a limit for the control valve, the direction of the limit must be determined first before proceeding with the operation. To limit the control valve in the same direction as the force exerted by the diaphragm head, first adjust the current signal to 20mA, then operate the handwheel to bring the valve stem to the desired position. To limit the control valve in the opposite direction to the force exerted by the diaphragm head, first adjust the current signal to 4mA, then operate the handwheel in the opposite direction to bring the valve stem to the desired position. 4 Special precautions: For handwheels used in gear-and-rack angular stroke actuators, after manual operation of the handwheel is completed, it is necessary to exit the manual operation mode. When remote control from the main control room is to be performed, the handwheel must first be turned so that the position of the valve stem matches the signal output by the main control room; alternatively, the signal output by the main control room must be adjusted to correspond to the position of the valve stem. Only then can the clutch be operated to disengage the handwheel from the valve stem. Otherwise, due to the mismatch between the position of the valve stem and the signal from the main control room, the resultant forces will make it very difficult to operate the clutch. Secondly, since during manual operation the handwheel determines the position of the valve stem, once the handwheel is detached, the valve stem immediately moves to the position corresponding to the signal output from the control room. As a result, the sudden change in the valve stem’s position may damage the clutch or even the control valve itself. Kle-Valve, Wuxi Clariant Fluid Control Equipment Co., Ltd. Further reading: Selection of control valves: Choosing the appropriate valve body type; How to maximize the effectiveness of control valves in power systems?