Succinct Field Instrumentation Guidance for the Automation Generalist Tips

The amount of information that the generalist needs to know is staggering. The knowledge needed is buried in a hundred thousand pages of publications and presentations that are oriented toward the specialist. Here I provide the essentials for the best field instrumentation system. The emphasis in this guidance is on performance rather than cost.

I think we short change ourselves in terms of implications of supposed upfront cost savings in terms of long term cost of maintenance and process variability and reliability especially when it comes to measurements and valves. The biggest mistakes in my career have been the result of attempts to save on hardware cost as exemplified by the case history noted in the last blog where valve positioners were omitted or replaced by boosters on fast loops.  

I don’t have time to reference all my articles, blogs, books, and columns that provide more details. I suggest you peruse the subjects in my ISA Interchange Insights, Control Talk Blogs, Control Magazine articles and Control Talk columns, InTech articles and ISA books, Momentum Press books, Emerson Deminars, and MYNAH Seminars.

(1) Use radar sensors for maximum level measurement accuracy for inventory and material balance control (e.g., reactor residence time and distillate level control).

(2) Use smart transmitters with separate compartments for wiring and electronics for maximum reliability and maintainability.

(3) Use integral mounted transmitters or close coupled transmitters to eliminate sensor wires, filled capillary systems, and impulse lines if the sensor connection is safely accessible, surface temperature is less than 50 degrees C, and connection vibration is negligible. Avoid like the plague the direct wiring of temperature sensors to a DCS or PLC input card.

(4) If impulse lines must be used, minimize length and ensure the fill is a single phase of constant density.

(5) If capillary systems must be used and there is adequate pressure range and protection from excessive vacuums, make the seals and capillary lengths the same, minimize the length of the filled line for best response time, maximize the size of the seal for best sensitivity and ensure the lines are secured (do not move in wind) and are protected from the sun and rain.

(6) Use tip sensitive and vibration durable 4-wire Resistance Temperature Detectors (RTDs) for maximum sensitivity and accuracy and minimum drift for process temperatures less than 500 degrees Centigrade.

(7) Use ungrounded sheathed premium thermocouples (TCs) of appropriate type for process temperatures greater than 500 degrees Centigrade.

(8) Use spring loaded compression fitting to ensure RTD and TC tip touches thermowell bottom with minimal annular clearance to minimize response time from air acting as insulator.

(9) Use tapered thermowell to reduce thermal conduction error and vibration failure.

(10) Specify and purchase the TC or RTD assembled in thermowell with integral mounted transmitter via a pipe union as an assembly from the supplier’s factory with a transmitter calibration to match sensor nonlinearity.

(11) Install the thermowell in an elbow pointed into the flow at the pipe center line to minimize response time and error from cross sectional variation in temperature profile.

(12) Use shrouded spherical high temperature pH glass and a replaceable reference junction for best accuracy, reliability, and maintainability.

(13) Install pH electrode pointing 45 degrees down with tip in the centerline of a pipe whose velocity is 5 to 10 fps to minimize coatings and response time.

(14) Use sliding stem valve for smaller lines and non-abrasive and non-plugging applications from a manufacturer whose heritage is control valves and not piping valves or on-off valves.

(15) Use v-ball or contoured butterfly valves for larger lines and applications best served by a rotary valve from a manufacturer whose heritage is control valves and not piping valves or on-off valves.

(16) Use a diaphragm actuator instead of a piston actuator unless the valve size or pressure drop cannot be handled by even the higher air pressure diaphragm actuators available today. 

(17) Always use a tuned digital positioner. Add a booster to the positioner output(s) with an integral bypass valve slightly opened if the valve needs to be faster.

(18) For tight shutoff use an isolation valve from a piping or an on-off valve manufacturer that is coordinated to automatically work in conjunction with the control valve for throttling.

(19) Use equal percentage trim if the valve pressure drop is less than 50% of the total system pressure drop to provide better rangeability and linearity. Note that surge control valves, pH reagent valves, and gas pressure vent valves tend to have a valve to system pressure drop ratio approaching one and benefit greatly from a linear valve trim.

(20) Design the valve drop at maximum flow to be greater than 20% of the total system drop to prevent excessive loss of rangeability.

(21) Use a pulse width modulated variable frequency drive (VFD) with high resolution input cards, torque to speed control system in the drive, minimum deadband and speed rate limiting, noise resistant cables, ventilated and if necessary fan cooled motors with higher service factor if energy savings and a more precise and faster response is justifiable provided the static head is less than 10% of the total system pressure drop.

(22) Use Coriolis meters for smaller line sizes to maximize accuracy and rangeability and to minimize drift and maintenance.

(23) Use magmeters for liquid flow in moderately large line sizes.

(24) Use vortex meters in moderately large line sizes where magmeters cannot be used with straight run requirements similar to orifice meters

(25) Use flow nozzles in very large line sizes to minimize permanent pressure loss and change in meter coefficient with wear and piping conditions with dual smart transmitters to maximize rangeability. Make sure straight run requirement is based on actual piping installation.

(26) For maximum reliability and on-stream time, use three sensors with transmitters and middle signal selection. This setup is almost essential for any pH measurement used for process control.