How to Succeed – Part 4

We continue this series with some helpful hints for control system design and implementation. This week we look at  measurement location and selection. Since a control system deals with change, the prevalent theme is how to improve the detection and correction for change. In the coming weeks we will take a look at control valves and how to get the most out of your PID controller.

A control system’s job is to correct for undesired changes in the process rejecting disturbances and to make desired changes in the process achieving new setpoints. You cannot control something you cannot measure. Furthermore the control system must not be fooled by extraneous changes. Here we look at sensor location and selection for best control system performance.

(1) SENSOR LOCATION OBJECTIVES (in approximate order of importance):

a. Maximize the detection of changes in the process from disturbances and setpoint changes. For composition, pH, and temperature choose the location that shows the largest change in both directions for a positive and negative change in the ratio of the manipulated flow to the feed flow realizing there are cross sectional and longitudinal temperature and concentration profiles in pipes and equipment. Areas behind baffles or near the surface or bottom of an agitated vessel or at the outlet of inline equipment may not be as well mixed. Temperature and pH sensor and analyzer sample tip should be near the center of pipe and extend well past equipment walls. Packed and fluidized bed equipment may have uneven composition and temperature distribution from channeling of flow. A series of temperature sensors across a fluidized bed at several longitudinal distances is often necessary with averaging and signal selection to get a representative measurement and prevent hot spots. The insertion length of the thermowell should be more than 5 times the diameter of the thermowell to minimize thermal conduction errors from heat conduction along the thermowell wall between the tip and process connection. Calculations should be run with program supplied by manufacturer on the allowable maximum length in terms of preventing vibration failure from wake frequencies. Resistance temperature detectors (RTDs) are more prone to vibration failure than thermocouples (TCs). Programs today may only be looking at thermowell failure. The tip of a pH electrode must be pointed down at a 30 to 60 degree angle to prevent the internal bubble in the glass electrode from lodging in the tip or at the internal electrode.

b. Minimize noise over the whole operating range reducing extraneous changes. The real definition of measurement rangeability must take into consideration the increase of noise at extremes of the range. Noise at low flow is the principle limitation to the rangeability of a differential head meter. Sufficient straight runs upstream and downstream have a critical effect. Liquid purging can cause transients from changes in the process pressure and purge flow. A location with good mixing and a single phase will minimize fluctuations in temperature and concentration and the disruption of bubbles or solids in liquids and liquid droplets in gasses hitting temperature or pH sensors or getting into sample lines for analyzers or into impulse lines for pressure and level measurements. Pressure probes in high velocity gas streams and furnaces must be designed to minimize momentum and vacuum effects. Sensors and sample probes tips should be not be on pump suctions and should be downstream of strainers and at least 25 pipe diameters downstream of the outlet of a static mixer or heat exchanger. The distance between a desuperheater and temperature sensor should provide at least 0.2 seconds of residence time, which doesn’t sound like much until you release this corresponds to 20 feet for a steam velocity of 100 feet per second (see Straight Talk and Secret Installation Effects). Solids are more problematic in that they may not dissolve in a reasonable residence time. The use of calcium hydroxide (lime) or magnesium hydroxide as a reagent may seem inexpensive until you consider the cost of poor control and solids going downstream. A dehydrated pH electrode or a pure water sample can cause a noisy measurement due to high glass and water resistance, respectively. A non aqueous solvent can cause dehydration and excessive sample resistance. The spikes from ground potentials and electromagnetic interference (EMI) can be eliminated by wireless transmitters. RTDs are less susceptible to EMI than TCs due to a higher level signal.

c. Minimize sensor deadtime and lag by reducing transportation delays and increasing velocities. The transportation delay in a pipe or sample line is the volume divided by the flow rate or the distance divided by the velocity. The lag time of temperature and pH sensors decreases with velocity by an increase in the heat transfer and mass transfer coefficient. Fouling also decreases with velocity. A thin film can dramatically increase the lag time of a sensor especially a pH glass electrode. A liquid velocity of 5 to 7 fps has been shown to greatly reduce fouling of probes. Velocities less than 1 fps significantly increase the lag time of sensors. The velocity in even highly agitated vessels is less than 1 fps unless the probe or thermowell is near the impeller tip. The air gap between the sensor and the interior tip of the thermowell must be minimized. The tip of the sensor must touch the bottom of the thermowell and annular clearance between the TC or RTD element and thermowell wall should be less than 0.02 inches allowing for temperature expansion. An aged or dehydrated or extremely cold pH electrode or a highly acidic sample can cause a large lag time in the pH response.

(2) SENSOR SELECTION OBJECTIVES (in approximate order of importance):

a. Maximize threshold sensitivity, resolution, and repeatability over the whole operating range reducing undetected and extraneous changes . The sensitivity of RTDs is more than an order of magnitude better than TCs. The sensitivity of Coriolis meters are more than an order of magnitude greater than vortex meters. Differential head meters may have good repeatability but suffer from noise plus uncertainty from pipe inside diameter and roughness and orifice edge wear.

b. Minimize drift eliminating loss of process knowledge, running at the wrong operating point, and the need for recalibration. Drift results in an offset of the measured value from the true value. An offset can be automatically corrected by upper level loop in cascade or model predictive control. Thus loops with a cascade or remote cascade setpoint are less affected by drift. However, knowledge of the process is degraded. For example, while the offset in a flow measurement is corrected by a setpoint change in a cascade loop, the error messes up material balances (process flows), energy balances (utility flows), and online process metrics for process analysis. Flow ratio control must be corrected by a composition loop for flow measurement drift. For custody flow meters, an offset is unacceptable. Smart transmitters and advances in sensor design have in many cases reduced drift and the effect of extraneous process and ambient conditions on installed accuracy by an order of magnitude. Drift in analytical, temperature, or pH is particularly troublesome because these are upper level loops often closely related to product quality. Operations may have adjusted setpoints to compensate for offsets in upper level loops but such adjustments are ad hoc and undone by the replacement of a sensor or transmitter. When there is an operational problem, the first question is often what maintenance was done. The drift of TCs is unpredictable and can be one to two orders of magnitude larger than the drift of RTDs. The drift of new pH electrode designs from sterilization and high temperature exposure has been greatly reduced. Solid state pH reference electrodes tend to drift for hours to days after installation due slow equilibration of the reference and high reference junction potential.

c. Minimize maintenance by eliminating drift by the use of the aforementioned advances in smart transmitters and sensors and by eliminating impulse (sensing) lines, sample lines, wires, and terminations. In-line flowmeters, close coupled differential pressure and pressure transmitters, in-situ probes, retractable insertion pH electrodes, and wireless transmitters greatly reduce the time spent analyzing real or perceived problems. Once an instrument requires maintenance, the device is a suspect whenever there is a problem. Platforms should provide access to sensors and transmitters. Analyzer shelters should be used for sophisticated at-line analyzers. For maximum onstream time and reliability use middle signal selection of 3 measurements that is capable of inherently riding out a single failure of any type and eliminating unnecessary maintenance by recognition of relative performance. The use of middle signal selection is particularly important for pH.

d. Minimize nonlinearity that cannot be corrected by a smart transmitter. RTDs can be consistently linearized by the use of Callendar-Van Dusen equation eliminating the error when sensors are changed. The interchangeability error for TCs is much greater than RTDs due to greater nonlinearity and unpredictability.