UNIVERSIT DEGLI STUDI DI SALERNO Bachelor Degree in
UNIVERSITÁ DEGLI STUDI DI SALERNO Bachelor Degree in Chemical Engineering Course: PROCESS INSTRUMENTATION AND CONTROL (Strumentazione e Controllo dei Processi Chimici) VALVE TECHNOLOGY OF CONTROL VALVES Rev. 2. 5 – March 13, 2019
ACTUATORS Actuators are the mechanical equipment that supply the force necessary to open or close a valve. Actuators are, essentially, the alternative to manual operation of a valve. Tasks of an actuator The actuator provides the forces able to: • • • put the closure member in the specified position exert the force necessary to maintain hermetic seal on the seat (if the full closing of the valve is requested) win all the resistant forces, e. g. : Ø friction forces exerted on the stem Ø force due to the pressure drop on the two sides of the closure member Types of actuators • • pneumatic (with a membrane) oleo-dynamic (hydraulics) electric manual 03/03/2021 Process Instrumentation and Control - Prof. M. Miccio 2
MANUAL ACTUATOR MANUAL HANDWHEEL It is set up on the pneumatic actuator of a control valve in order to allow functioning in the absence of air or in case of emergency, e. g. • crack of the diaphragm, • failure of the controller or during the start-up, • etc. UNIDIRECTIONAL HANDWHEEL It is the most common manual emergency control for the simplicity of construction and for the little maintenance. The handwheel always acts opposing to the spring and therefore moves down the closure member. It can be set up on a wide range size of valves. 03/03/2021 Process Instrumentation and Control - Prof. M. Miccio 3
GLOBE VALVE: FORCES ACTING ON THE CLOSURE MEMBER HYPOTHESES: P 1 P 2 • P 1 > P 2 • No stem bending F • Sott= area of the closure member on which the fluid pressure acts supposed equal both upstream and downstream. F 1 = P 1 Sott (upstream closure member force) F 2 = P 2 Sott (downstream closure member force) F = F 1 - F 2 = (P 1 - P 2)Sott # 0 03/03/2021 Process Instrumentation and Control - Prof. M. Miccio 4
SINGLE OR DOUBLE SEAT VALVES F F 03/03/2021 F Process Instrumentation and Control - Prof. M. Miccio 5
DOUBLE SEAT GLOBE VALVE from BELIMO glossary: Leakage on closure is high since getting the seats to touch the discs simultaneously is difficult. 03/03/2021 Process Instrumentation and Control - Prof. M. Miccio 6
1 st Example of a CONTROL VALVE with PNEUMATIC ACTUATOR Yoke Construction drawing 03/03/2021 Construction drawing Process Instrumentation and Control - Prof. M. Miccio 7
MEMBRANE PNEUMATIC ACTUATOR THE PNEUMATIC ACTUATOR IS THE MOST COMMON AND TRANSFORMS THE PNEUMATIC SIGNAL IN A UNIFORM AND LINEAR MOVEMENT OF THE CLOSURE MEMBER. IT IS MADE OF: DIAPHRAGM OR MEMBRANE IT IS MADE OF NEOPRENE WITH INSERTION OF NYLON WITH GIVES A GOOD MECHANICAL RESISTANCE AND A HIGH OPERATING TIME. DIAPHRAGM DISK IT IS MADE OF STEEL AND IS A UNIQUE ITEM WITH THE SHAFT OF THE SERVOCONTROL WITH A THREAD COUPLING. IT ALLOWS THE INVARIABILITY OF THE CALIBRATION OF SERVOCONTROL IF THE MEMBRANE IS REPLACED. YOKE MONOBLOC IN CAST IRON IT IS CONNECTED TO THE DIAPHRAGM CASING BY MEANS OF HEX CAP SCREWS AND BLOCKED TO THE BODY OF THE VALVE THROUGH A SCREWED RING NUT. TRAVEL INDICATOR IT IS MADE OF STAINLESS STEEL, CO-AXIAL AND COUPLED TO THE STEM BY MEANS OF TWO SCREW NUT. IT ALLOWS THE TRAVEL OF THE CLOSURE MEMBER BY MEANS OF A GRADUATED SCALE PLACED ON THE YOKE. 03/03/2021 Process Instrumentation and Control - Prof. M. Miccio 8
MEMBRANE PNEUMATIC ACTUATOR ENEL Sp. A Thermal power plant Sulcis – Sardegna Valve and pneumatic Actuator Picture 03/03/2021 Process Instrumentation and Control - Prof. M. Miccio 9
MEMBRANE PNEUMATIC ACTUATOR Construction drawing Animated drawing actuadordiafragma. swf (su Safari) 03/03/2021 Process Instrumentation and Control - Prof. M. Miccio 10
MEMBRANE PNEUMATIC ACTUATOR Animated drawing Bottom valve Operating scheme 03/03/2021 Process Instrumentation and Control - Prof. M. Miccio 11
SERVO-POSITIONERS They are accessory devices which measure the stem position and regulate it to a specified reference value ADVANTAGES • They improve the positioning reducing the effects of hysteresis, frictions, pressure drop of the supplied air, delay time of the response. • They limit the positioning velocity (upper limit). • They allow to some extent a linear response with a first-order dynamic response in a small range of the stem position (the dynamic response is characterized from the saturation of the velocity for high variations). 03/03/2021 Process Instrumentation and Control - Prof. M. Miccio 12
Schematic drawing CONTROL VALVE with POSITIONER 03/03/2021 Process Instrumentation and Control - Prof. M. Miccio 13
2 Schematic drawing CONTROL VALVE with POSITIONER from US Department of Energy Instrumentation and Control Module 2 03/03/2021 Process Instrumentation and Control - Prof. M. Miccio 14
direct action PNEUMATIC MEMBRANE ACTUATOR flow-to-close reverse action flow-to-open Air-to-close valves are valves which are held open by the valve spring and require air pressure (a control signal) to move them towards the closed position. see also: Air-to-open valves are normally held closed by the spring and require air § 13. 5 - Stephanopoulos, “Chemical process control: an Introduction to theory and practice”, Prentice Hall, 1984 pressure to open them - they open progressively as the air pressure increases. 03/03/2021 Process Instrumentation and Control - Prof. M. Miccio 15
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