Chapter 13 Control Loops PDF

Chapter 13: Control Loops Control Valves and regulators Objectives: 1. Given a drawing or actual device identify the components of a control valve system. 2. Define terms associated with valves and other final control elements and operating scenarios in which they are desirable. Readi...

Chapter 13: Control Loops Control Valves and regulators Objectives: 1. Given a drawing or actual device identify the components of a control valve system. 2. Define terms associated with valves and other final control elements and operating scenarios in which they are desirable. Reading Summary: I. Introduction i. Final control element last active device in control loop ii. Most common is control valve iii. Instrument air is used to actuate a valve a. Control Valves iv. Regulate flow of materials in process by producing a differential pressure drop across the valve v. Actuator changes the instrument signal into either a linear or rotatory motion that drives flow controlling mechanism vi. Sliding stem valve components 1. Body usually joined to the process by a threaded or flanged connection may also be welded 2. Bonnet connects the valve body to the actuator and can be removed 3. Valve plug includes the valve stem and moves to open or close the flow as the plug moves away from the seat the flow valve increases and decreases as it moves towards 4. Actuator device that provides motion to the valve using a spring diaphragm, spring piston or double acting piston 5. Regulator device used to control pressure f process fluid upstream of device location 6. Stem pushing and pulling rod that transfers the motion of the actuator to the valve plug 7. Seat stationary part of the valve tri, connected to the body that comes in contact with the valve plug 8. Spring provides energy to move the valve in the opposite direction to the diaphragm loading motions so that the valve can be opened in closed in proportion to the instrument signal 9. Diaphragm, flexible membrane that increases a force to move the stem 10. Valve positioner controller that is proportional only position of the stem is compared to value of instrument signal and then responds 11. Handwheel actuator accessory that is sued to override the actuator manually or to limit its motions 12. I/P transducer device that converts milliampere signal into a pneumatic pressure signal II. Control Valve failure conditions vii. If power to air failure they should move to safe position viii. Actuators without return mechanisms typically fail in last system to dive the control valve actuator to predestined intermediate position in event of power failure ix. Engineers will choose each control valve response according to safety and requirements x. Using P&IDS the control valve failure position should be indicated under the valve symbol b. Fail open xi. When air to close valve loses its instrument air supply the valve fails open because the return spring provides more opposing force than the diminished air applied 13. Ex during a failure you want pressure relieving valve o fail to open to reduce pressure buildup that could rupture the vessel c. Fail closed xii. When air to open valve fails the valve closes 14. Ex during a power failure a valve moving material into a tank would not be controlled and could overfill the tank d. Fail in place or fail last xiii. Naturally fail in direction of their spring unless they have a lockup relay attached xiv. Lockup relay seals in the existing signal applied at the point of power loss xv. Electric motor actuators natural fail in place III. Control Valve Actuators 15. Responsds to instument signal by creating a linear or rotational motion xvi. Actuator and valve actions for sliding stem valves 16. Actuators and valves are either direct acting or reverse acting 17. Direct acting when an increased application of pressure causes the stem to extend 18. Reverse acting when increased application cause the stem to retract 19. Valve considered direct acting when stem is pushed down to close and reverse acting when valve stem is pulled up to close 20. Four possible combinations a. Direct/ direct b. Direct/ reverse c. Reverse/ direct d. Reverse/ reverse 21. Best way to determine combination is to look at identification plate air to close (ATC/ atc) or air to open (ATO/ ato) xvii. Pneumatically driven actuators 22. Two major subcategories e. Spring and diaphragm, actuator f. Piston type actuators 23. Most common spring and diaphragm actuator has a single diaphragm supported by a diaphragm plate connected to a steel rod called a stem populate because of low cost and high mechanical advantage g. Single acting i. Single acting spring opposed actuator like diaphragm, type the piston is opposed by a spring ii. Air cushion type piston actuator has pressure trapped under the piston and air compresses as the piston is pushed down as the signal is reduced the tapped compressed air pushes piston back up iii. Double acting type pressure is routed to both sides of the piston driving the stem to required position by balancing the pressure on either side of the piston need to be controlled with a positioner e. Valve positioners 24. Function is to make the valve position match the controller output signal 25. Locates moving parts of valve in accordance with a predetermined relationship with the instrument signal received from the loop controller 26. Can be used to modify the relationship between the input and output instrument air signals h. Positioning the valve i. Reverse the action j. Mimic a valve trim type k. Provide split-range control xviii. Valve positioner operation 27. Valve movement may be predicted by observing pressure gauges l. Instrument input pressure gauge indicates signal pressure form controller or I/P transducer and is important while troubleshooting the loop for problems, gauge can be used to determine output signal form the controller m. Output pressure gauge indicates output pressure applied to the actuator that may or may not be equal to the instrument signal pressure also reads pressure required for valve position n. Air supply pressure gauge indicates instrument supply pressure IV. Output Signals f. Output of controller may need to be revered g. Accomplished by configuring the current to pneumatic transducer for the valve position were to respond in reverse to the signal h. Designed with a particular fail safe position and is forced into fail safe position if instrument air is lost so if the output of the controller is reversed does not matter since the control fails into the position demanding by the opposing spring V. Valves for control Configurations i. Globe control valves xix. Most common type of valve xx. Flow can change direction j. Three-way control valves xxi. Special type of globe body valve with three connecting ports instead of two xxii. Designed to mix two flowing input seams together with one outlet or one flowing stream to two output ports xxiii. Can be used for diverting or blending k. Butterfly control valves xxiv. Lower manufacturng costs and higher flow capacities xxv. Actuator used with these use rotary motor which produces less wear on the packing then aa sliding stem valve does xxvi. Eupped with spring and diaphragm actuators or piston types of actuators xxvii. Flow characteristics between linear a quick opening xxviii. Can be used for liquids and gases l. Ball or segmented ball control valves xxix. Rotary valve that contains a spherical plug with a hole at the center xxx. The attached top of the ball is attached to a stem that rotates it to open or closed xxxi. May be used as a tight shutoff or modulating valve xxxii. Used for fluid slurry to minimize settling and straining of materials VI. Instrument air regulators xxxiii. Self contained self-actuating controlling device used to regulate variables xxxiv. Generally, receive energy to move from the process stream itself xxxv. Air regulator can either reduce the supplied pressure upstream or relieve pressure downstream xxxvi. Important parts 28. Inlet: supplies pressure from a source 29. Outlet: regulates pressure 30. Diaphragm: senses the pressure on the outlet side of the regulator 31. Pilot valve assembly: working part of the regulator; entire mechanism works to either supply pressure or relieve pressure from outlet side xxxvii. Instrument air regulator procedure follows the sequence: 32. Handwheel is turned 33. The spring compresses 34. The diaphragm is pushed down 35. The pen and plug move from their seat 36. Air rushes in, increasing downstream pressure 37. Output gauge responds accordingly 38. Equilibrium is established xxxviii. The regulator is self-contained controlling device it ahas elements integrally located inside m. Back pressure regulators xxxix. Used to regulate and or contorl the pressure of a process fluid upstream of the location n. Pressure reducing regulators xl. Used to control the pressure of a process fluid downstream of the location Checking for Understanding 1. In a globe valve, as the plug moves away from the set, the flow through he valve **a increases.** 2. A control valve configured with a spring and diaphragm, actuator is said to be required air to close, if there is an air or power loss, the valve will b**. be forced completely open by the spring.** 3. During a power failure, you want a valve that tis allows the addition of material to a tank to stop the flow. Therefore, this valve should be a: **a fail closed valve** 4. A(n) \_**d. actuato**r\_\_ is a device that reacts to an instrument signal by creating a linear or rotational motions. 5. If a rising stem control valve and its spring and diaphragm actuator are both direct acting, then the control valve will \_**a. Fail open**\_\_ if the air is lost to it. 6. Which of the following can a valve positioner do? A **position the valve stem in reference to the instrument signal** 7. What are the three pressure gauges on pneumatic valve positioner? **A. instrument, output and supply** 8. The output signal form a controller can be revered by **c. valve positioner** 9. In the processing industry, the most common control valve body style is the \_**c. globe**\_\_ 10. A butterfly valve is opened with a **b. rotary motion** 11. A three way valve can be used to **a. divert a flowing stream into two separate pipes, b. mix two separate flowing streams together into one pipe** **and d. replace two valves** 12. What is the recommended supply pressure for may air supply control valves **b20** 13. 14. Properly identify each valve a. I b. III c. II