Chapter 2 Control Loop Hardware and Troubleshooting
Overall Course Objectives • Develop the skills necessary to function as an industrial process control engineer. • Skills • Tuning loops • Control loop design • Control loop troubleshooting • Command of the terminology • Fundamental understanding • Process dynamics • Feedback control
Control Relevant Aspects of Control Loop Hardware • Necessary for control loop troubleshooting: • To determine if each subsystem (control computer, actuator system, and sensor system) is functioning properly • To understand the proper design and operation of all the components that make-up each of the subsystems of a control loop
Controllers/Control Computers • Pneumatic controllers • Electronic analog controllers • Supervisory control computers • Distributed Control Systems (DCS) • Fieldbus technology
Pneumatic Controllers - Phase I • Introduced in the 1920’s • Installed in the field next to the valve • Use bellows, baffles, and nozzles with an air supply to implement PID action. • Provided automatic control and replaced manual control for many loops
Pneumatic Controllers - Phase II • Transmitter type pneumatic controllers began to replace field mounted controllers in the late 1930’s. • Controller located in control room with pneumatic transmission from sensors to control room and back to the valve. • Allowed operators to address a number of controllers from a centralized control room.
Electronic Analog Controllers • Became available in the late 1950’s. • Replaced the pneumatic tubing with wires. • Used resistors, capacitors, and transistors based amplifiers to implement PID action. • Out sold pneumatic controllers by 1970. • Allowed for advanced PID control: ratio, feedforward, etc.
Computer Control System • Based upon a mainframe digital computer. • Offered the ability to use data storage and retrieval, alarm functions, and process optimization. • First installed on a refinery in 1959. • Had reliability limitations.
Distributed Control System- DCS • Introduced in the late 1970’s. • Based upon redundant microprocessors for performing control functions for a part of the plant. SUPERIOR RELIABILITY • Less expensive per loop for large plants. • Less expensive to expand. • Facilitates the use of advanced control.
DCS and Troubleshooting • The data storage and trending capability of a DCS greatly facilitate troubleshooting control problems. That is, the sources of process upsets can many times be tracked down through the process by trending a group of process measurements until the source of the process upset is located.
Control Relevant Aspects of a DCS • The most important control aspect of a DCS is the cycle time for controller calls. The shortest cycles times are typically around 0.2 seconds while most loops can be executed every 0.5 to 1.0 seconds. These cycle times affect flow control loops and other fast control loops.
PLCs • PLCs can withstand has industrial enviroments. • PLCs are used for discrete and continuous control. • Discrete control is used for startup and shutdown and batch sequencing operations. • Ladder logic is used to program PLCs.
PLCs vs. DCSs • Advantage of PLCs: • Better to withstand harsh operating enviroments, faster cycle time are possible, easier to maintain due to modular nature and lower cost for small and medium sized applications. • Advantage of DCSs: • Lower cost per loop for applications involving a large number of control loops.
Fieldbus Technology • Based upon smart valves, smart sensors and controllers installed in the field. • Uses data highway to replace wires from sensor to DCS and to the control valves. • Less expensive installations and better reliability. • Can mix different sources (vendors) of sensors, transmitters, and control valves. • Now commercially available and should begin to replace DCSs.
Actuator System • Control Valve • Valve body • Valve actuator • I/P converter • Instrument air system
Types of Globe Valves • Quick Opening- used for safety by-pass applications where quick opening is desired • Equal Percentage- used for about 90% of control valve applications since it results in the most linear installed characteristics • Linear- used when a relatively constant pressure drop is maintained across the valve
Effect of Linearity in the Installed Valve Characteristics • Highly nonlinear installed characteristics can lead to unstable flow control or a sluggish performance for the flow controller.
Analysis of These Examples • Note the linear installed valve characteristics over a wide range of stem positions. • If the ratio of pressure drop across the control valve for the lowest flow rate to the value for the highest flow rate is greater than 5, an equal percentage control valve is recommended.
Control Valve Design Procedure • Evaluate Cv at the maximum and minimum flow rate using the flow equation for a valve (Eq 2.3.3). • Determine which valves can effectively provide the max and min flow rate remembering that, in general, the valve position should be greater than about 15% open for the minimum flow rate and less than 85% open for the maximum flow rate. • Choose the smallest valve that meets the above criterion for the minimum capital investment or choose the largest valve to allow for future throughput expansion.
Additional Information Required to Size a Control Valve • CV versus % open for different valve sizes. • Available pressure drop across the valve versus flow rate for each valve. Note that the effect of flow on the upstream and downstream pressure must be known.
Valve Sizing Example • Size a control valve for max 150 GPM of water and min of 50 GPM.
Determine CV at Max and Min FV • Use the valve flow equation (Equation 2.3.3) to calculate Cv • For DP, use pressure drop versus flow rate (e.g., Table on page 82)
Analysis of Results • 2-inch valve appears to be best overall choice: least expensive capital and it can provide up to a 50% increase in throughput. • 3-inch and 4-inch valve will work, but not recommended because they will cost more to purchase. The 2-inch valve will provide more than enough extra capacity (i.e., something else will limit capacity for it)
Valve Deadband • It is the maximum change in instrument air pressure to a valve that does not cause a change in the flow rate through the valve. • Deadband determines the degree of precision that a control valve or flow controller can provide. • Deadband is primarily affected by the friction between the valve stem and the packing.
Valve Actuator Selection • Choose an air-to-open for applications for which it is desired to have the valve fail closed. • Choose an air-to-close for applications for which it is desired to have the valve fail open.
Optional Equipment • Valve positioner- a controller that adjusts the instrument air in order to maintain the stem position at the specified position. Greatly reduces the deadband of the valve. Positioners are almost always used on valves serviced by a DCS. • Booster relay- provides high capacity air flow to the actuator of a valve. Can significantly increase the speed of large valves.
Adjustable Speed Pumps • Used extensively in the bio-processing industries (better to maintain sterile conditions and relatively low flow rates). • Fast and precise. • Do require an instrument air system (i.e., 4-20 mA signal goes directly to pump). • Much higher capital costs than control valves for large flow rate applications.