Please share some precautions for the electric door and pneumatic door tuning competition

What company? Which model of mechanical positioner do you use from Fisher? It seems like you have quite a few devices in use

That’s all I can help you with; take your time learning, young man. Valve positioners can be classified by structure into pneumatic valve positioners, electrical valve positioners, and intelligent valve positioners. They are essential accessories for control valves, usually used in conjunction with pneumatic control valves. They receive the output signal from the controller and use this signal to control the pneumatic control valve. When the control valve moves, the displacement of its stem is fed back to the valve positioner through mechanical mechanisms, and the valve’s position is transmitted to the higher-level system via electrical signals. Based on their structural design and working principles, valve positioners can be divided into pneumatic valve positioners, electro-pneumatic valve positioners, and intelligent valve positioners. Valve positioners can increase the output capacity of control valves, reduce delays in the transmission of control signals, speed up the movement of the valve stem, improve the linearity of the valve, overcome frictional forces on the valve stem and eliminate the effects of unbalanced forces, thereby ensuring the accurate positioning of the control valve. 01 Classification of valve positioners 1. Valve positioners can be classified into pneumatic valve positioners, electrical valve positioners, and intelligent valve positioners based on their input signals. (1) The input signal of a pneumatic valve positioner is a standard pneumatic signal, such as a 20–100 kPa pneumatic signal, and its output signal is also a standard pneumatic signal. (2) The input signal for electrical valve positioners is a standard current or voltage signal, such as a 4–20 mA current signal or a 1–5 V voltage signal. Inside the electrical valve positioner, this electrical signal is converted into electromagnetic force, which is then used to generate a pneumatic signal that operates the control valve. (3) Intelligent electric valve positioner: It converts the current signal output from the control room into a pneumatic signal that drives the control valve. Based on the friction of the valve stem during operation, it counteracts any unbalanced forces caused by fluctuations in medium pressure, ensuring that the valve opening corresponds to the current signal output from the control room. Furthermore, intelligent configuration can be used to set the corresponding parameters, thereby improving the performance of the control valve. 2. Based on the direction of movement, they can be divided into single-direction valve positioners and double-direction valve positioners. When a one-way valve positioner is used with a piston actuator, the valve positioner operates in only one direction; whereas a two-way valve positioner acts on both sides of the cylinder of the piston actuator, enabling operation in two directions. 3. Valve positioners are classified into positive-action valve positioners and reverse-action valve positioners based on the sign of the gain of their output and input signals. As the input signal to a positive-action valve positioner increases, the output signal also increases; therefore, the gain is positive. As the input signal to the reaction valve positioner increases, the output signal decreases; therefore, the gain is negative. 4. Depending on whether the input signal to the valve positioner is an analog signal or a digital signal, they can be divided into conventional valve positioners and fieldbus-based electrical valve positioners. The input signal for a conventional valve positioner is an analog pressure or current/voltage signal, while the input signal for a fieldbus-based electrical valve positioner is a digital signal from the fieldbus. 5. Based on whether the valve positioner is equipped with a CPU, they can be divided into conventional electrical valve positioners and intelligent electrical valve positioners. Ordinary electrical valve positioners do not have a CPU; therefore, they lack intelligence and are unable to perform any intelligent calculations. Intelligent electrical valve positioners come equipped with a CPU that can handle various intelligent calculations; for example, they can perform nonlinear compensation for the forward channel. Fieldbus electrical valve positioners may also include functional modules such as PID to carry out corresponding calculations. 6. It can also be classified according to the detection method of the feedback signal. For example, valve positioners that detect the valve position signal using mechanical linkages; valve positioners that detect the displacement of the valve stem by means of the Hall effect; valve positioners that detect the displacement of the valve stem through electromagnetic induction; etc. 02 Principle of operation of the positioner: A valve positioner is a key accessory for control valves. It uses the signal indicating the displacement of the valve stem as an input feedback signal, and takes the output signal from the controller as the setpoint signal; by comparing these two signals, it adjusts the output signal sent to the actuator when there is a discrepancy, thereby causing the actuator to move. This establishes a one-to-one relationship between the displacement of the valve stem and the output signal from the controller. Therefore, the valve positioner constitutes a feedback control system that uses the valve stem displacement as the measurement signal and the controller output as the setpoint signal. The control variable of this control system is the output signal from the valve positioner to the actuator. (1) Used for important control systems with high requirements for regulation quality, to improve the positioning accuracy and reliability of control valves. (2) Used in applications where the pressure difference across the valve is high (△p>1MPa). By increasing the gas supply pressure to boost the output force of the actuator, it is possible to overcome the unbalanced force exerted by the liquid on the valve core and reduce stroke error. (3) When the medium being regulated is at high temperature, high pressure, low temperature, or is toxic, flammable, or explosive, packing is often compressed tightly to prevent external leakage. As a result, the friction between the valve stem and the packing becomes relatively large. In such cases, a positioner can be used to overcome this time lag. (4) When the medium being handled is a viscous fluid or contains solid suspensions, a positioner can overcome the resistance exerted by the medium on the movement of the valve stem. (5) Used for control valves with large diameters (Dg>100mm) to increase the output thrust of the actuator. (6) When the distance between the regulator and the actuator is over 60 m, a positioner can overcome the delay in the transmission of control signals and improve the response speed of the valve. (7) Used to improve the flow characteristics of control valves. (8) When a regulator controls two actuators for step control, two positioners can be used to receive the low and high input signals respectively; thus one actuator operates at the low level while the other operates at the high level, thereby achieving step regulation. 03 Collection of Common Positioner Calibration Methods I. ABB Positioner Calibration Steps: 1. Panel settings for the positioner; 2. Internal wiring (4 wires): feedback and command wires. 3. Method of switching important parameters before commissioning: ? (1) Switch between local and remote modes. Hold down the MODE key without releasing it, then press the ↑ ↓ keys to switch. ?（2）Enter 1.1 (remote control) or 1.2 (local control) in the manner described in (1).?（3）To achieve quick operation, press the ↑ key first and then the ↓ key ； To achieve quick shutdown, first press the ↓ key and then hold down the ↑ key to complete the operation. ?（4）Enter 1.3 in the same way as in ?（1); the word SENS-POS will appear, indicating that after adjusting the positioner, the angle between the linkage and the rear knob remains within a symmetrical range. 4. Debugging steps: (1) For P1.0: Press the ↑ and ↓ keys simultaneously, then click the “ENTER” key; the word “LINEAR” will appear, allowing adjustment of the angular and linear travel distances. (2) P1.1: Hold down the MODE button and press the ↑ ↓ buttons to enter the P1.1 menu. Press the ENTER key for 3 seconds, then release it once the countdown display on the panel shows 0; this will initiate self-tuning until the word “COMPIETE” appears. (3) P1.4: When exiting (E*T), options for “Save” and “Do not save” will be displayed. Hold down “ENTER” for 3 seconds to save the debugging settings; if you do not want to save them, press the ↑ key to go back to the “Cancel” option, and then hold down “ENTER” for 3 seconds to exit. (4) The word REVERSE appears on P2.3, indicating the forward and reverse actions of the control valve and actuator. (5) When the CW/CCW words appear on P3.2, it adjusts the forward and reverse action of the DCS and local positioner commands. (6) The word E*T appears on P3.3, meaning to exit. (7) If the word DIGEET appears on P8.2, it indicates that the forward and reverse actions of the feedback from the DCS and the local locator are being adjusted. The parameters listed above are important for the debugging process; please refer to the manual for details! II. Adjustment steps for the FISHER valve positioner DVC6000: Turn on the 275/375 controller, select the HART Application from the Main Menu, and locate the positioner under On line. Enter Setup&Diag ——Detailed Setup——Mode——Instrument Mode in sequence (or go directly there using the shortcut key on the controller) ——Warning! When the \"In Service\" mode is selected and sent to the instrument, the valve may move (WARNING! The valve may move when In Service mode is sent to the instrument.). Press OK to enter Instrument Mode; select \"Out of Service\" and then press ENTER. A message stating \"Instrument mode is already out of service!\" will appear, after which you return to the main interface and proceed to Setup&Diag – Basic Setup – Auto setup – Setup Wizard: 1. Select the pressure unit you want to use (Pressure units (psi)), then press ENTER ； 2. Enter the maximum supply pressure (Max Supply Press (psi)) and then press ENTER ； 3. After selecting the actuator manufacturer, press ENTER; if none of the above brands are available, choose OTHER ； 4. For single-acting actuators with springs, select 3; for double-acting actuators with springs, select 4; for double-acting actuators without springs, select 2 ； 5. Select ROTARY. Next, if the valve is at zero signal and it should be in the open position, choose 2; otherwise choose 1. Then select 1 again, and finally choose YES to automatically determine the valve’s orientation and gain level ； 6. Choose whether an accelerator or quick release valve is present (Is a Volume Booster or Quick Release Present?) ； 7. Note: Actuator information is being sent to the instrument. Please wait. “Actuator information is being sent to the instrument. Please wait”…” ； 8. Choose whether to use the factory default settings: Use Factory defaults for setup? (Yes is recommended for initial setup). Enter after selecting Yes ； 9. Prompt: The factory default settings are being sent to the instrument. Please wait. “Factory defaults are being sent to the instrument. Please wait.”… ” ； 10. Continue Auto Setup: Select Amplifier Adjustment. “To continue Auto Setup, select Relay Adjustment” ” ； 11. Do you wish to adjust the amplifier now? (Do you wish to run Relay Adjustment Calibration now? ) Select Yes for double-acting and No for single-acting, then press ENTER ； 12. Select “Auto Calib Travel” (To continue, select Auto Calib Travel.) and press OK ； 13. Do you want to perform automatic trajectory calibration now? (Do you wish to run Auto Travel Calib now? Select YES if initial setup) Choose Yes and then press ENTER ； 14. Warning! Calibration will cause a sudden change in the instrument’s output. Press OK ； 15. Selection of crossover adjustment (Select Crossover Adjust): Manual/Last Value/Default. Under normal circumstances, select Default and then press ENTER; only if the precision after the default adjustment is poor should Manual be chosen ； 16. Next, we proceed with the automatic stroke calibration process. Upon completion, the instrument mode will revert to “In service”! III. Debugging steps for the Moisture-Land SVI-II positioner: 1. Check whether the wiring and valve installation are complete ； 2. Switch to manual mode (Manual) ； 3. Check and adjust all structural parameters (CONFIGuration) ； 4. Automatically search for full open and full close points (STOPS). 5. Run the autoTUNE function to obtain new dynamic response parameters ； 6. Check for any error messages ； 7. Perform manual operation to check if it can run properly ； Exit manual mode and return to automatic control mode. Menu procedure: After connecting the signal source, adjust the input signal to 12mA. The system will perform a self-check, and then alternate between displaying the valve position and the value of the input signal. At this point, the system may be in manual or automatic mode, depending on its state before the power outage during the last run. If (POS-M) is used when the valve position is displayed on the screen, it indicates that the system is in manual mode ； If (POS) is used to display the valve position, it indicates that the system is in automatic mode. Then follow the flowchart to complete the setup. IV. Adjustment steps for the Korean Yongtai YTC positioner: 1. Conditions: ① A stable air supply with pressure ≥ 0.3 MPa (for angle-type positioners, the pressure should be ≥ 0.5 MPa); ② A device for inputting 4-20 mA signals. 2. Adjustment: ① Open the front cover of the positioner and the power connection box. ②Connect a stable gas source to the filter pressure regulator and seal it tightly. ③Connect the signal input device properly to the positive and negative input terminals. ④Turn on the gas supply switch, adjust the “0” knob downward until the pointer on the exhaust pressure gauge is not at 0, then slowly rotate the “0” knob in the opposite direction until the gauge pointer is exactly at 0. ⑤Apply 25% (8mA), 50% (12mA), 75% (16mA), and 100% (20mA) signals, and observe the downward (upward) travel of the pointer: A. If a single stroke exceeds 1/4 of the dial, it indicates that the range of the positioner is too large; in this case, adjust the range knob (first loosen the screw, then adjust the knob below in the “—” direction). Determine the degree of adjustment based on the actual situation; make slight adjustments one by one, adjusting while testing). B. If a single stroke is too small or less than 1/4 of the dial, it indicates that the range of the positioner is too small; the procedure is the same as in A, but in the opposite direction. ⑥After adjusting the range knob each time, repeat the test; if the actual travel distance still differs significantly from the theoretical value, adjust the range knob again ； If the actual path differs little from the theoretical path, adjust the \"0\" knob to move the pointer to the corresponding scale, then input a signal to check whether the pointer moves correctly. Note: The range and stroke are adjusted by combining the range adjustment knob with the \"0\" adjustment knob, thereby achieving the purpose of calibration. Precautions: (1) When the input signal is 20 mA, the exhaust pressure of the locator must be > 3 kg (5 kg for ball valves); generally, it is 3.2–4 kg. If the pressure is insufficient, the control valve does not open/close properly and cannot function correctly. (2) Air release valve: When the input signal is 0, the exhaust pressure must be 0; otherwise, the valve is not fully closed. V. Steps for debugging Siemens positioners: 1. Preparatory work before debugging: (1) Connect the steam supply first, then the power supply, and set the current to above 4mA ； (2) If the positioner has not been calibrated, the text “P” should appear on the display to indicate that it is time to enter configuration mode; press “+” first and then “–” at the same time. The same procedure applies in reverse, in order to determine the maximum or minimum value for the valve ； (3) Ensure that the minimum value is between 5 and 9; do not swap the black gears of the positioner. Make sure the maximum value does not exceed 95, and adjust the minimum value to be as close as possible to 5 ； (4) Use the “+” and “–” buttons to adjust the valve stroke to 50%; all preparatory work before testing is completed ； Note: If the locator has been tuned, it must be reset. The reset procedure is as follows: Press the button to enter (the value is 50 for new models and 55 for the original ones); then press “+” for 5 seconds until OCAY appears; press the button again for 5 seconds, after which C4 will appear and P will be displayed. Once in configuration mode, the tuning steps are the same as those in points 2, 3, and 4 above. 2. Initialization adjustment steps A: Automatic initialization of the actuator. Note: It is essential to set the opening/closing direction of the valve correctly before performing automatic initialization! Otherwise, initialization cannot be performed! (1) Move the actuator correctly away from the center position to begin initialization. Linear travel selection: Angular travel selection: Use “+” and “–” keys to switch ； (2) Press the function key briefly to switch to the second parameter: Display; or use the “+” and “–” keys to switch ； Note: This parameter must match the setting of the leverage ratio switch. (3) Use the function key to switch to Parameter 3; the following will be displayed: If you want the total stroke length calculated to be expressed in mm after the initialization is complete, this setting is necessary. To do this, you need to select on the display screen a value that is the same as the set value of the driving pin on the scale rod. (4) Use the function key to switch to Parameter 4; the following will be displayed: (5) Press the “+” key for more than 5 seconds to start initialization. During initialization, “RUN1” through “RUN5” will appear one after another at the bottom of the display screen. Note: The initialization process depends on the actuator and can take up to 15 minutes. Initialization is complete when the following is displayed. After you press the function key briefly, the following message appears: Press the function key for more than 5 seconds to exit configuration mode. After about 5 seconds, the soft key display will appear. Once the function key is released, the device operates in Manual mode; pressing the function key switches it to AUTO mode, allowing remote control operation at this point. B. Manual actuator initialization: With this function, initialization can be carried out without having to physically drive the actuator to its end position. The start and end positions of the rod can be set manually. The remaining steps of initialization (control parameter optimization) are carried out automatically, just like in automatic initialization. Sequence steps for manual initialization of a linear actuator. (1) Initialize the linear actuator. By manually driving to ensure coverage of the entire stroke, that is, by ensuring the potentiometer setting is within the allowable range between P5.0 and P95.0, pressing the function key for more than 5 seconds will bring you into the configuration mode. Linear travel selection: Angular travel selection: Use “+” and “–” keys to switch ； (3) Press the function key briefly to switch to the second parameter: Display; or use the “+” and “–” keys to switch ； Note: This value must correspond to the setting of the transfer rate selector. (33° or 90°) (4) Use the function key to switch to parameter three; the following will be displayed: If you want the total stroke measured at the end of the initialization process to be shown in mm, you need to select on the display a value that is equal to the one set for the drive pin on the rod scale, or the next higher value for medium adjustment. (5) Use the down function to select Parameter 5: ① First hold down the “—” key and simultaneously press the “+” key to quickly close the valve (the value shown is around 6.5); otherwise, adjust it using the black knob to keep it within the specified range ； Note: If the value displayed by following these steps decreases, first adjust the direction of the actuator’s switch ； ②Then press the “+” key first, and at the same time press the “—” key to quickly open the valve. Observations after activation show that the value should be within 95; otherwise, adjust the black knob to bring it within the normal range, and then press the function key to confirm ； ③First press the “—” key, then hold it down while pressing the “+” key to quickly close the valve; the display value should be between 5 and 9, after which press the function key to confirm ； ④Initialization starts automatically. ⑤The stop of initialization occurs automatically. RUN1 to RUN5 appear in sequence at the bottom of the display. When initialization is fully complete, the following display appears: Press the function key for more than 5 seconds to exit the rod configuration mode. After nearly 5 seconds, the soft key display will appear. Once the function key is released, the device operates in Manual mode; pressing the function key switches it to AUTO mode, allowing remote control operation at this point. Note: To change the switching direction of the adjustment valve, it is necessary to adjust items 7 and 38 of the regulator; both values must be changed using the “+” and “–” keys, and their settings must be identical. During manual initialization, press the “+” key for more than 5 seconds to start the initialization process and see the display. VI. Adjustment steps for the Yamaku positioner: 1. Automatic adjustment is a unique program that can be used to automatically carry out various adjustments to the positioner. Automatic setting is carried out using the opening switch; when performing automatic setting and zero-to-range adjustment, it is necessary to observe the positioner. The opening button is used to initiate automatic setting and perform manual zero-to-range calibration. Steps: (1) Set the input signal of the positioner to DC 18±1mA ； (2) Open the front cover of the SCP and press the opening button to the “UP” position (“DOWN” for Flowing Rotary VFR valves) ； (3) Hold this button until the valve starts to move (about 3 seconds); this will initiate the automatic setting procedure. Then release this button. (4) The valve will move back and forth from fully closed to fully open twice. After that, the valve was opened to 50% and held there for 3 minutes ； (5) Confirm that the automatic setting procedure is complete by changing the input signal. The entire automatic setup process takes about 3 minutes ； Note: During the automatic setting process, do not set the input model to below 4mA. (As long as the signal is within the 4-20mA range, changing the input signal during automatic setting will not affect the execution of the program.) ) If the input signal drops below 4mA, the automatic setting will become invalid, and it must be restarted. After automatic setting is completed, the signal remains at a level of at least 4 mA for at least 30 seconds to ensure that the data and parameters are saved in the SVP memory. After the operation is complete, check the valve’s operation by changing the input signal to confirm whether the valve moves to the correct position corresponding to the signal. If the full-scale position is offset, perform full-scale adjustment again. 2. After the zero-to-range adjustment is automatically set, the locator calibrates itself to the fully closed (zero) and fully open (range) positions of the valve. If the valve cannot achieve the correct relationship between its opening degree and the controller signal, manually adjust the zero-point and range following the steps below. Note: The position switch will only function when the closed and full-open input signals (e.g., 4-20) are identical to those stored in the positioner, or to those set for closed and full-open in the factory. (1) Steps to adjust the valve to the closed position (zero point): a. Send an electrical signal corresponding to the fully closed position of the valve from the controller (e.g., 4mA) ； b. Adjust the valve to its fully closed position by pressing the opening buttons “UP” or “DOWN”. The default value for the forced shutdown feature is set at 0.5%. (2) Steps to adjust the valve to the fully open position (full scale): a. Send an electrical signal corresponding to the fully closed position of the valve from the controller (e.g., 20mA) ； b. Adjust the valve to its fully closed position by pressing the opening buttons “UP” or “DOWN”. Until the valve position is adjusted to the correct place. Note: After completing the zero-to-range adjustment, change the input signal to confirm whether the valve is operating accurately. 3. Maintenance: (1) Filter replacement and throttle nozzle repair allow for the removal of instrument air contaminants that have accumulated in the positioner’s throttle nozzle during the maintenance process. Steps: 1) Cut off the air supply to the positioner ； 2) Unscrew the fixing screw from the label area of the A/M switch (Note: When unscrewing the screw, be careful not to lose the A/M switch cover washer and the lock washer) ； 3) Switch from A/M to MAN (manual) position ； 4) Use forceps or other tools to remove the clamp and take out the old filter screen ； 5) Use wire (with a diameter of 0.3 μm) to remove contaminants from the throttle nozzle (be careful not to allow oil or grease to contaminate the throttle nozzle while removing contaminants) ； 6) Wrap the new filter screen around the A/M switch and press it into place with a clip ； 7) Turn the A/M switch all the way down ； 8) Use set screws to fix the nameplate of the A/M switch section to the A/M switch cover plate. (2) Cleaning the baffle: If contaminants in the instrument air accumulate on the baffle (Note: Applying pressure to the actuator will change the back pressure of the cleaning baffle and nozzle, causing a sudden change in the valve position), 1) Remove the cover ； 2) Unscrew the four screws on the cover plate ； 3) Slide the cover plate to the left, then remove it ； 4) Prepare paper sheets with a thickness of 0.2 mm; ordinary business cards will do ； 5) Use a piece of paper to clean the dirt from the gap between the EPM nozzle and the baffle ； 6) After cleaning the gaps, reinstall the cover plate and the lid. 4. Troubleshooting (1) The locator does not function (no output air pressure). a. Verify that the rotation angle of the locator’s feedback rod does not exceed 20°. If this angle is exceeded, add an extension rod to the feedback rod to obtain a sufficient feedback length ； b. Check for leaks in the gas supply ； c. Check the electrical input signal ； d. Check whether the automatic/manual switch is in the automatic position ； e. Check the cleanliness of the baffle and filter screen. (2) If the full range cannot be achieved or the speed is affected and is slow, a: Check whether the adjustments for the zero point (full open) and the range (full open) are correct ； b. Check the cleanliness of the filter screen and baffles. (3) Improper adjustment or exceeding the limit range a. Check the allowable rotation angle of the feedback rod. VII. Adjustment steps for the Yokogawa positioner: 1. Check whether the pneumatic and electrical circuits meet the requirements for the operation of the positioner ； 2. With a 12mA signal applied, adjust the feedback rod to a horizontal position and tighten it ； 3. With a 8mA signal, adjust the zero point to the corresponding value using the zero adjustment nut ； 4. With a 16mA signal, adjust the range to the corresponding value using the range adjustment nut ； 5. With a 4mA signal applied, check the fully closed position of the valve; make fine adjustments if necessary ； 6. Given a 20mA signal, check the fully open position of the valve ； Make fine adjustments if necessary ； 7. Perform calibration verification using 4mA (or 20mA), 8mA (or 16mA), 12mA, 4mA (or 20mA), 16mA (or 8mA), and 20mA (or 4mA), and make fine adjustments if necessary. Note: 1. The operating mode of the positioner can be changed by adjusting the range adjustment nut. 2. The 8mA and 12mA signals are used to adjust the zero point and range respectively, because both 8mA and 12mA have upper and lower scale values that allow for an accurate determination of the zero point and range. In contrast, there are no scale values below 4mA (and also no scale values above 20mA); therefore, 4mA and 20mA are not suitable for adjusting the zero point and range. 3. When calibrating the positioner, it is necessary to ensure that the valve can be completely closed. Sometimes, even when a 4 mA (or 20 mA) signal is applied, the valve still remains partially open. 4. Both pneumatic valve positioners and electro-pneumatic valves are mechanical valve positioners; therefore, their adjustment methods are similar, and no further details will be provided. 04 Common faults of positioners 1. The valve positioner has an input signal but no output signal. (1) The electromagnet assembly has malfunctioned; it is recommended to replace the electromagnet assembly. (2) The supply gas pressure is incorrect; it is recommended to check the gas source pressure. (3) The zero adjustment of the pneumatic amplifier baffle is set too high; the baffle is too far from the nozzle. (4) Blockage in the gas path. (5) Incorrect gas line connections (including the amplifier). (6) The positive and negative poles of the input signal wires for the electro/pneumatic positioner are connected reversely. 2. The valve positioner has no input signal, but the output signal remains at its maximum level. (1) The zero-point adjustment of the pneumatic amplifier damper is too low; the damper presses too tightly against the nozzle. (2) Nozzle clogging. (3) The output pressure is slow or abnormal. This can cause damage to the diaphragm of the control valve and lead to air leakage, resulting in a situation where a signal is present but the control valve responds slowly; as a result, it is unable to carry out timely adjustments. The solution is to inspect the diaphragm chamber and replace the diaphragm. 3. Poor linear performance of the positioner: (1) The feedback cam or spring is improperly selected or oriented. (2) The feedback linkage mechanism is not properly installed or gets stuck in certain positions. (3) There are foreign objects in the nozzle or baffle. (4) There is a slight leak in the back pressure