CENTAC Inlet and Bypass Valve Positioners

Transcription

1 CENTAC Inlet and Bypass Valve Positioners INGERSOLL-RAND AIR COMPRESSORS

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3 INLET AND BYPASS VALVE POSITIONERS Copyright Notice Copyright 1992, 1999 Ingersoll-Rand Company THIS CONTENTS OF THIS MANUAL ARE PROVIDED AS IS AND WITHOUT ANY IMPLIED WARRANTIES WHATSOEVER. Ingersoll-Rand air compressors are not designed, intended, or approved for breathing air applications. Ingersoll-Rand does not approve specialized equipment for breathing air applications and assumes no responsibility or liability for compressors used for breathing air service. Disclaimer The information contained in this manual is general in nature and is meant to be used for widespread application in training on Centac products. The Operation Manual, diagrams, blueprints and other information developed for individual Centac machines must be consulted for specific or detailed information, specifications, and data applicable to specific machines. The application of any information contained in this manual carries no implied or expressed warranties. The user assumes the entire risk of usage. Trademark Acknowledgments Centac is a trademark of Ingersoll-Rand Company BELLOFRAM Type 80 is a trademark of Rexnard Company BRAY Series 64 SR/CCK is a trademark of Bray International, Inc. TP0011B

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5 INLET AND BYPASS VALVE POSITIONERS Table of Contents Introduction... 1 Part I: The BELLOFRAM Positioner... 1 I/P, positioner, actuator and valve relationship... 1 BELLOFRAM TYPE 80 Principles of Operation... 2 Feedback Spring... 3 Zero and Span Adjustment... 4 BELLOFRAM Positioner Stroking Procedure... 5 BELLOFRAM Positioner Output Pressure Balance Setting Procedure... 6 Maintenance of the BELLOFRAM Positioner... 6 Part II: The BRAY Positioner... 7 I/P, Positioner, Actuator and Valve Relationship... 7 BRAY SERIES 64, SR/CCK Principle of Operation... 8 Zero and Span Adjustment...10 Maintenance of the BRAY Positioner...12 Part III: Actuators...13 Bray Actuator...13 Vane Type Rotary Actuator...14 TP0011B

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7 INLET AND BYPASS VALVE POSITIONERS 1 Introduction Prior to the introduction of the Ingersoll-Rand standard Inlet and Bypass valve assembly, the BELLOFRAM Type 80 Universal Valve positioner and the BRAY series 64 SR/CCK type positioner were used extensively in the Centac inlet and bypass valve control system. The purpose of this manual is to provide a technical information reference source for service personnel who may encounter the BELLOFRAM or BRAY Positioners when servicing Centacs. Information in this manual has been taken from the BELLOFRAM Installation instruction dated 12/84 and provided by Rexnord Company under form R Information concerning the BRAY positioner is taken from the BRAY International Inc., BRAY control accessory modules, Series 64 for rotary actuator's publication. Information and data relative to how these devices are used in the Centac control system are provided in this manual. Part I: The BELLOFRAM Positioner I/P, positioner, actuator and valve relationship The purpose of the valve positioner is to receive a control signal, amplify it, and send a Air in Transducer (I/P) Butterfly valve Actuator Air in BELLOFRAM positioner Figure 1. Valve control with BELLOFRAM positioner power air-operating signal to a valve actuator. The actuator is the device that physically drives the control valve open or closed. The control signal provided to the positioner is a small air signal. This control signal is, normally, a variable 3 to 15 PSIG. This control signal is varied by a milliamp (ma) to air pressure transducer (I/P). The transducer receives its ma signal from the MP3 or other process system controller. Figure 1 illustrates the relationship between the I/P, the BELLOFRAM positioner, the actuator, and the valve being controlled. The actuator illustrated in Figure 1 is a BRAY device. Centac has used BRAY actuators in combination with BELLOFRAM and BRAY positioners. Other type actuators are also used. TP0011B

8 2 INLET AND BYPASS VALVE POSITIONERS Normal supply air pressure to the BELLOFRAM positioner is 80 to 100 PSIG. The supply air by specification is instrument grade air. At pressures less than 80 PSIG there may be insufficient power available to move the valves being controlled. BELLOFRAM TYPE 80 Principles of Operation The Type 80 double acting positioner controls the position of the valve actuator and thereby the valve by providing air pressure in the correct direction to move the valve. Use Figure 2 to follow the description of operation. A chain and stem connector arm are linked to a feedback (Range) spring. The feedback spring system causes the valve motion to stop at a specific control signal dictated position. A force balance condition is initially established at set-up between the feedback spring and the force developed by the signal piston assembly for the range of instrument signal pressures. In the case of Centac, signal pressures between 3 and 15 PSIG are nominally used. Any change in this force balance condition either because of a change in control signal pressure or a change in valve stem position caused by external forces, immediately produces a change in the clearance between the signal piston and the pilot nozzle. Signal pressure Balance pressure adjusting screw Servo diaphram No. 2 Servo chamber No. 2 Stem connector arm Range adjustment spring seat Zero adjustment Locking seat Range spring Signal piston chamber Pilot nozzle Signal pressure Servo diaphram No. 1 Servo chamber No. 1 Output chamber No. 2 Output chamber No. 1 Output pressure Output pressure 1 OUTPUT No. 2 OUTPUT No. 1 Output valve No. 2 Output valve No. 1 Instrument air supply Restriction Figure 2. Functional cutaway diagram of BELLOFRAM Type 80 positioner

9 INLET AND BYPASS VALVE POSITIONERS 3 Air from output chamber 1 is fed to the servo chamber through a restriction with the pilot nozzle serving to control the servo chamber exhaust rate. Any change in clearance between the nozzle and signal piston produces a corresponding change in servo chamber pressure. The servo diaphragms 1 and 2 are designed to produce 1:1 inverse operation of output valves 1 and 2. An increase in servo pressure opens valve 1 and closes valve 2 and exhausts output chamber 2. A decrease in servo pressure closes valve 1, exhausts output chamber 1, and opens valve 2. The combined and simultaneous action of increasing pressure from valve 1 and decreasing pressure from valve 2, entering the actuator from opposite ends causes a "push-pull" effect and moves the actuator shaft, stem connector arm and feedback spring to the new position that satisfies the previously described force balance condition. The working or balance pressure in the actuator can be adjusted by means of the balance pressure adjustment screw. Turning the screw clockwise increases the spring force acting to open valve 2. This produces an increase in output pressure of valve 2 that is immediately balanced by an increase in output pressure of valve 1 in satisfying the original force balance condition. A decrease in balance pressure is accommodated by turning the adjustment screw counterclockwise. Feedback Spring The feedback (Range) spring adjustment disks are named spring seats. These seats have a threaded center hole. The zero adjustment is screwed into the center hole after the seats are positioned on the spring. To assemble the range spring to the positioner, first assemble the two spring seats to the spring by inserting the spring wire (end opposite the hook) through the hole in the top of the disk and then screw the seat onto the spring. Figure 3. Spring seat Figure 3 shows the top (closed) side of the seat. This top side is where the spring wire is inserted. The closed side of the spring seat should face the hook end of the spring. There are two spring seats. The first spring seat should be screwed on approximately 3 turns, the second is screwed on 2 turns. See Figure 4. After the spring seats are installed, attach the feedback spring hook to the positioner clevis. The next step is to screw the zero adjustment screw through both seats. The chain is attached to the valve stem collar as illustrated in Figure 5. Figure 4. Feedback spring and attachments

10 4 INLET AND BYPASS VALVE POSITIONERS To achieve proper operation, the positioner output from the feedback spring side of the positioner (Output chamber 1) must be attached to the actuator in such a way that, when output pressure from chamber 1 increases, the valve stem will move in the direction that stretches the feedback spring. Zero and Span Adjustment Adjustment of the positioner to the stroke zero or valve stem starting position, for the corresponding desired minimum input signal, is made by means of the zero adjustment screw. The zero stroke position of the inlet valve is just fully closed when a 3 PSIG signal is fed to the positioner from the I/P. The zero stroke position of the bypass valve is just fully open with a 3 PSIG signal. In either case a 4 ma signal is being fed to the I/P at the zero stroke position. Adjustment of the positioner for signal span or full valve stem travel for the corresponding desired maximum input signal is made by changing the spring rate of the feedback spring. This rate change is accomplished by changing the number of spring coils between the spring seat and the positioner. The full travel position of the inlet valve is just fully open when a 15 PSIG signal is sent to the positioner from the I/P. The full travel position of the bypass valve is just full closed when a 15 PSIG signal is fed to the positioner. Regardless of which compressor control valve is being stroked, the first step in the stroke adjustment procedure is to properly stroke the I/P. Valve stem collar Zero adjust screw Positioner clevis Spring attachment TYPE Figure 5. Attachment of spring Valve Stroke Specifications MP3 output I/P input signal I/P output signal Positioner input 4 ma 20 ma 3 PSIG 15 PSIG Inlet valve position Just full CLOSED Just full OPEN Bypass valve position Just full OPEN Just full CLOSED

11 INLET AND BYPASS VALVE POSITIONERS 5 NOTE The Centac standard electro-pneumatic control system uses a common pressure control signal from a single pneumatic controller for both inlet and bypass valves. The procedure for stroking the positioner is the same except that there are different values for the zero and full travel (span) pressures. The nominal signal stroking values are: Inlet Valve: OPEN = 15 PSIG CLOSED = 7 PSIG Bypass Valve: CLOSED = *10 PSIG OPEN = *3 PSIG * These values will vary with ambient conditions. BELLOFRAM Positioner Stroking Procedure 1. Check the inlet valve and assure it is fully in the mechanically closed position. The bypass valve is checked in the fully open position. Check the spring seats and assure they are separated (unlocked) by approximately 1 spring wire turn. Use of the I/P output is recommended for setting the signal pressure to assure compatibility between the ma control signal and resultant valve motion. 2. With full supply pressure (instrument air) to the positioner input (80 to 100 PSIG), set the input signal pressure at the low end of the span (3 PSIG). 3. Adjust the zero adjusting screw until the desired valve stem position is obtained. I.e. Inlet = Closed, Bypass = Open. 4. Set the input signal pressure to the maximum span pressure; 15 PSIG. 5. Hold the feedback spring with one hand and with the other hand grasp and turn both spring seats together until the desired valve stem position is reached. i.e. Inlet = Open, Bypass = Closed. 6. Recheck and adjust the low end span as per step 1 and Repeat steps 3 through 5 until the desired stroke versus signal is obtained. Span is correct when a very light touch on the feedback spring in either the zero or full stroke signal condition causes the valve to start to respond. Response is detected by valve motion and air exhaust sound from the positioner. 8. Lock the spring seats in position by holding the spring seat nearest to the positioner and turning the other spring seat clockwise until both seats are locked together.

12 6 INLET AND BYPASS VALVE POSITIONERS BELLOFRAM Positioner Output Pressure Balance Setting Procedure 1. Ensure no external forces are being exerted on the valve or valve stem. 2. With full supply pressure (instrument air) to the positioner input (80 to 100 PSIG), set the input signal pressure at the mid point of the span (9 PSIG, 12 ma to I/P). 3. Adjust the balance adjusting screw so that the output pressures are approximately equal and the output pressure is 50% to 60% of the supply pressure. As an example, the balance pressure with 80 PSIG supply should be between 40 and 48 PSIG. Balance pressure at 100 PSIG should be between 50 and 60 PSIG. For proper operation, a minimum of 10 psi differential should be allowed between supply pressure and the balance pressure setting. Maintenance of the BELLOFRAM Positioner Maintenance other than calibration is required if the positioner is supplied with contaminated instrument air. Routine maintenance consists of cleaning the two installed 40 micron filters. Access to these filters is gained through the chrome plated hex bolts on the back side of the mounting plate. Remove filters and clean or replace as required by on site conditions. Contaminated supply air can damage diaphragms and clog internal passages. Corrective maintenance (overhaul or replacement) is required in the event of damage or contamination. Ruptured diaphragms are indicated by an excess amount of air exiting from the exhaust ports.

13 INLET AND BYPASS VALVE POSITIONERS 7 Part II: The BRAY Positioner I/P, Positioner, Actuator and Valve Relationship Figure 6 illustrates the relationship between the I/P, BRAY positioner, and actuator for the inlet valve. The configuration for the bypass valve is the same except for cam and power hose connection. The initial valve position control signal originates from the MP3 Microcontroller or other process controller. This signal is in the control span of 4 to 20 ma. The ma signal activates the I/P where the ma signal is converted to an air signal pressure. The air signal pressure provides the control feed to the valve positioner. The process of valve control when using the BRAY positioner is the same as when the BELLOFRAM positioner is used. Figure 6. Inlet Valve control configuration (BRAY) Normal supply air pressure to the BRAY positioner is 80 to 100 PSIG (max. 150 PSIG). Supply air by specification is instrument grade. At pressures less than 80 PSIG there may be insufficient power available to move the valve actuator under some conditions.

14 8 INLET AND BYPASS VALVE POSITIONERS BRAY SERIES 64, SR/CCK Principle of Operation Use Figure 7 in following the description of the principle of operation of the BRAY positioner. The SR/CCK positioner consists of a diaphragm (1), that is activated by the air signal pressure from the I/P, a feedback spring (2), that acts in opposition to the signal pressure, a double action spool valve (3), a stem (4), that connects the spool valve to the diaphragm and cam (7), that moves with the controlled valve to provide valve position feedback. An increase in air signal pressure on the diaphragm (1) causes a movement of the spool valve (3), the spool valve motion allows supply air to pressurize output (F) and exhaust output (E). The output and exhaust functions power the controlled valve motion. Valve shaft rotation is transmitted to the feedback spring (2) through the cam (7), cam roller (6), and the lever (5). The force of the feedback spring (2) balances the signal pressure on the diaphragm(1). Exhaust Supply pressure (Instrument air) Exhaust 8 9 Range adjustment ring nut Zero adjustment ring nut 5 Lever 3 Spool valve 4 Stem Spring 6 Cam roller 7 Cam R Pointer 2 Feedback spring 1 Diaphragm Signal from I/P Output to actuator (F) Output to actuator (E) Spool valve cap Spring cap Figure 7. Representative functional cutaway diagram of the BRAY SR/CCK positioner When the feedback spring and the signal forces acting on the diaphragm are in the balanced state the spool valve is in the neutral position. The controlled valve will remain in a stationary position as long as the feedback spring's force is equal to the signal air force on the diaphragm.

15 INLET AND BYPASS VALVE POSITIONERS 9 A decrease in signal air pressure upsets the balance and the feedback spring causes the spool valve to move in the opposite direction. Output (F) will exhaust and output (E) will pressurize. The controlled valve will move, the cam will move in response to the valve motion and the feedback spring will balance the signal air pressure. Under balanced conditions, the spool valve returns to the neutral position. The cam has three lobes. Each lobe is designed to serve a specific signal pressure and linear range. One side of the cam is used for a direct action valve and the other for a reverse action valve. The inlet valve is direct acting since it opens as the signal rises. The bypass is a reverse acting valve since it closes as the signal rises. The cam is marked D for direct use and R for reverse use. The lobes are also lettered with their linearity and signal pressures. The lobe that is used for both inlet and bypass is the one marked 0-90, 3-15 psi. The inlet valve uses the side of the cam marked D and the bypass uses the side marked R. Figure 8 illustrates how the cam is configured for inlet or bypass valve use. Inlet valve cam configuration Bypass valve cam configuration open Valve rotation Valve rotation Close 3-15 psi, 0-90 D R 3-15 psi, 0-90 Cam shown in inlet valve closed position Cam shown in Bypass valve open position Figure 8. Positioning of cam for Inlet or Bypass valve configuration Proper connection of the power air hoses to the actuator is required for correct valve rotation. In the case of the inlet valve (direct acting), view the positioner from the side, cam facing up with the plastic cover to the left. The power outlet fitting in the positioner body is the right hand fitting and the left fitting is for the gauge. The power hose goes from the power outlet to the left inlet connection on the actuator. Figure 6 illustrates the inlet valve connections. In the case of the bypass valve, the power connection from the positioner is made to the right inlet connection on the actuator.

16 10 INLET AND BYPASS VALVE POSITIONERS Zero and Span Adjustment The zero and span adjustments are made in a manner similar to the BELLOFRAM. The coils of the feedback spring are tensioned to achieve zero and the number of active coils is adjusted to achieve the range. An Allen wrench is provided to make the zero and span adjustments. It is stored on clips within the covered area. The zero adjustment nut is enclosed by the spring. The Allen wrench is positioned between spring coils and inserted in a hole provided in the nut to turn the zero adjustment ring. The range adjustment nut clamps to the spring with an Allen screw in the handle of the range adjustment nut. The range adjustment nut is made out of a plastic material. The tab on the range adjusting nut where the Allen screw is placed can easily be broken off if excessive tightening force is applied to the screw. Do not attempt to take an extra tightening turn on this screw because the tab will crack or break away. Figure 9 illustrates the major components used in setting the zero and span of the BRAY positioner. Pointer Allen wrench Pointer hold down screw Allen set screw Range adjusting ring D Zero adjust ring nut Wrench attachment holes Active coils Figure 9. Zero and span adjustments

17 INLET AND BYPASS VALVE POSITIONERS 11 Inlet Valve Calibration 1. The inlet valve must be in the fully closed position and the cam positioned with the D side up, on the 3-15 PSIG, 0-90 lobe with the cam roller at the lowest point on the lobe (bottom of the stroke). See the inlet valve configuration in Figure 7. If it is necessary to move the cam, loosen the recessed screws around the cam and the pointer hold down screw from the center of the cam. Rotate the cam and align with the "0" indication with the contact point of the roller. Then align the pointer along the direction of the zero indication mark that is on the see-through cover. 2. Instrument air must be turned on. Use a 4-20 ma source and apply a 4 ma signal to the I/P. Hold the spring and range adjust nut to prevent them from turning and rotate the zero adjust ring-nut until the inlet valve is just at the fully closed point. Rotation of the zero adjust ring-nut towards the roller support (clockwise rotation) decreases the zero start point. Rotation in the counterclockwise direction zero adjust away from the roller support increases the start point. 3. Apply a 20 ma signal to the I/P. Loosen the Allen screw in the range adjust nut and hold the spring to prevent it from turning while the range adjustment is being made. Rotate the range adjustment nut without turning the spring until the inlet valve is just fully open. Gently retighten the Allen hold down screw. Motion of the range nut toward the roller support decreases the range span. Movement away from the roller support increases the span. 4. Apply a 4 ma signal. If the valve goes to the just at the closed point, the valve is zeroed. If the valve is not at the just closed point, repeat steps 2 and 3 until both the zero and span conditions are met. 5. Replace the Allen wrench in its holder and reinstall the see-through cover. The pointer should be aligned to the 0 mark when the inlet valve is closed and to the 90 mark on the cover when it is open. NOTE The positioner exhausts into the area covered by the see-through cover. Holes are provided in the bottom of the cam housing to vent off this exhaust to the atmosphere. If these vent holes are blocked by a bracket or other structure, a provision must be made to vent the exhausting air to the atmosphere; i.e., notch the cover, drill small hole, etc. A tight cover with vents blocked will cause a back pressure in the positioner and it will not function properly. The positioner must be able to exhaust.

18 12 INLET AND BYPASS VALVE POSITIONERS Bypass Valve Calibration 1. The R (Reverse) side of the cam is used in the bypass valve configuration. See Figure 7 for cam configuration. The bypass valve is just full open at 4 ma and just fully closed at 20 ma. Zero and span are set accordingly using the same general procedure as for the inlet valve. Refer to the inlet valve procedure if details are necessary. 2. Adjust zero: 4 ma, just open. 3. Adjust span: 20 ma, just closed. 4. Repeat steps 2 and 3 until bypass valve position matches requirement. 5. Pointer at 0 when full open, 90 when closed. Maintenance of the BRAY Positioner Maintenance other than calibration is required if the positioner is supplied with contaminated instrument air. Routine maintenance for cleaning consists of cleaning the spool valve. To clean the spool valve, remove the hex head bolt (spool valve cap) on the back side body of the positioner. Remove the valve and clean with solvent. The spool valve is inserted long end first. Use care not to drop the spring when the cap is removed. CAUTION A constant rush of air, in excess of normal exhaust venting, from under the see-through cover indicates a diaphragm rupture. Overhaul or replacement is required in the event of diaphragm damage.

19 INLET AND BYPASS VALVE POSITIONERS 13 Part III: Actuators Bray Actuator Figure 10 is a functional diagram of the BRAY rotary actuator. Power air from the positioner is supplied to move the pistons in the CCW or CW direction. No routine maintenance is required. Travel limit is set with the travel adjusting screws. Both travel screws are adjusted to limit travel of the pistons. The pinion is connected to the valve stem with a collar. O rings provide for air tight sealing of caps and pinion. Piston (2) Pinion (1) Stop nut (2) Travel adjusting stop screw (2) Cap (2) Cap hold down screw (8) * Air in to move CCW Air exhaust * * Reverse flows cause CW rotation Upper bearing with O ring Lower bearing with O ring Figure 10. BRAY rotary actuator functional diagram

20 14 INLET AND BYPASS VALVE POSITIONERS Vane Type Rotary Actuator Figure 11 is a functional diagram of the vane type of actuator. No routine maintenance is required. A thin coat of silicone grease on the vane seal may release an actuator that is sticking due to contaminated air supply. Keep instrument air clean and free of moisture. Air exhaust * Actuator body TOP VIEW Shaft Vane Vane seal Air in to move CW * * Reverse flows cause CCW rotation Self lubricating bushing Travel stop pads Positioner coupling end Shaft SIDE VIEW SIDE VIEW Vane Vane seal Valve drive coupling end Figure 11. Typical vane operated rotary actuator