Asset Management / Distributed Control

How to Get the Best Performance from Your Control System

Make sure your control system achieves the potential possible with today's instrumentation.

By Greg McMillan and Stan Weiner

Practical Installation Best Practices

Stan: We continue our conversation started last month with Tim Green, operations manager, field services at Maverick Technologies, on how to make sure a control system will meet the performance capability possible with today's instrumentation.

Greg: What guidance can you offer based on your field experience and that of your key field people?

Tim: There are many considerations, but here are the foremost installation best practices. The list is a collaborative effort that involved the Maverick construction managers and commissioning managers:

  1. Install steam and liquid transmitters below the process tap, and air and gas transmitters above the process tap. In liquid-filled applications, often the static fill will need to be zeroed out to null the effect of the liquid head pressure.
  2. For magnetic flowmeters and other meters requiring ground rings, ensure that the ground ring material is appropriate for the process liquid by referencing the instrument data sheet. A standard 316SS ground ring may not last two weeks if Hastelloy C was required.
  3. Fill differential pressure (DP) flow transmitters from the bottom to avoid the entrapment of air in the impulse lines. Conversely, drain impulse lines intended to be dry right before commissioning.
  4. Verify orifice plate materials and dimensions, and that they are installed based on the flow direction. "The square edge of the orifice plate must be facing upstream. For low Reynolds numbers the upstream edge may be either rounded or conical. This reduces the effect of Reynolds Number on the orifice coefficient to less than 2% whereas the effect could be as large as 30% for a square edge. The coefficient of a rounded of a conical edge is also less susceptible to change from wear. Thus, for streams with abrasive solids or high viscosities, a rounded or conical edge can be advantageous."
  5. Sensing elements of flow transmitters normally require a minimum straight run of pipe prior to and after the sensor location. Follow the manufacturer's recommendation. Note that there is no buffer in this number, and it may not take into account all of the piping details. The straight run must be more than the minimum. The straight run will need to be increased if there are nonplanar elbows, process equipment or valves upstream. Insertion flow meters will require greater analysis to get the sensor at the point in the pipe cross-section that is at the average pipe velocity.

    See Also: How to Avoid Problems in Electrical and Instrument Installations

  6. Install radar transmitters with the device plumb. Make sure the horn or probe is correct for the service. For coaxial probes, make sure the standoffs are installed correctly to prevent false readings. If standoffs are not used, ensure the probe is not bent, to avoid reading contact points with the housing of the bridal or internal structure of the tank. When using transmitters with horn-type radar, avoid mounting too close to the sidewall, internal structures, inflow or sparge lines. Make sure the installation does not interfere with tank venting. Be sure to run an echo graph with the tank empty, all of the dip tubes installed and the agitator running as a baseline reading that includes the effect of internals and geometry for future reference. This is especially important for detecting problems from coating of the probe and vortexing of the liquid.
  7. Always design wireless systems after a thorough site survey. The number and locations of transmitters for signal hopping must be more than adequate to make sure transmission is not blocked when a transmitter is removed from service. Locations of transmitters may need to be adjusted. Sales may not realize how critical this site survey is. Pay attention to future physical changes to the site and possible parking of tank trucks and service vehicles that may interfere with the existing wireless system.
  8. Temperature transmitters are designed for multiple sensors, so make sure the correct sensor is selected and connected to the appropriate terminals. If there is a mixture of 2-wire, 3-wire,and 4-wire resistance temperature detectors (RTDs), the risk of incorrect terminations greatly increases due to the lack of standardization. Ensure that the probe length matches the thermowell depth, the fit is tight and the tip is bottomed out in the well. Also use high-temp pipe dope to avoid thread damage.
  9. Make sure the flowmeter element is installed in the correct orientation because most flowmeters are sensitive to flow direction.
  10. Install analytical probes within the limits of the factory cable lengths between the transmitter and sensor. Attempts to extend the wiring to meet operations or maintenance requests will result in erratic readings. The factory cable must be used and junction boxes not introduced. Substituting cables and adding junctions causes shielding, termination, color coding and resistance problems.
  11. Avoid installing pH and ORP probes before process fluids are flowing. Protect the probe from physical damage and keep the wet cap on the probe during storage. This wet cap can be used to protect the electrodes before start-up, but the caps must be removed before commissioning. If the electrode tip dries out or is knocked against anything or hit by debris, the glass surface can be permanently damaged.
  12. Check motors for proper starter size. Starter sizes follow the denomination of money: dime, quarter, half dollar and dollar. NEMA 1 through 4 horsepower ratings are 10, 25, 50 and 100. When bump-testing rotation, decouple the pump from the motor to avoid loosening the impeller. Verify all thermal overload protection matches the motor nameplate. The horsepower should not be assumed.

Greg: Thermowells and electrodes need to see a representative process temperature and composition with a fast response time and negligible noise. Achieving this objective translates to specific sensor location requirements. The tip must be near the centerline of a pipeline or beyond the baffle and away from a sparger or dip tube in a vessel. The tip must see a single phase (e.g., liquid or gas) and maximum uniformity (e.g., sufficient axial and radial mixing to achieve blend and minimize noise). In addition, it must introduce a minimum amount of dead time (e.g., transportation delay from equipment to sensor must be less than the time constant of a good, clean sensor) and also sustain an adequate velocity to prevent a slow sensor time constant from fouling. Finally, it must prevent exposure of electrodes to high temperatures or high acid, base or alcohol concentrations that cause premature aging, chemical attack or dehydration of glass electrodes.

Often, there is a compromise needed in the location of the sensor where a larger transportation delay is accepted to achieve a single phase, sufficient uniformity and adequate velocity. The increase in total dead time rarely exceeds a few seconds and may actually reduce the total dead time in the system by reducing the sensor time constant or signal filter time needed. For example, a pH electrode located in a recirculation line will introduce less dead time than an electrode in a vessel due to an inherently faster time constant and by avoiding coatings. A thermowell or electrode located 25 pipe diameters downstream of a heat exchanger or static mixer will provide a more repeatable measurement with less need for a signal filter. This distance is increased for desuperheaters and static mixers with gaseous reagents (e.g., flashing ammonia) to avoid droplets and bubbles, respectively. For more details on thermowell and pH electrode installation requirements, see the books Advanced Temperature Measurement and Control and Advanced pH Measurement and Control.

Stan: We conclude with a Top 10 E&I Start-Up "Believe It or Don't" List

Top Ten E&I Start-Up "Believe It or Don't" List

(10) Inadequately sized actuators were found and replaced before start-up
(9) All of the control valves had better than 0.25% resolution
(8) Minimum straight run for flowmeters was based on piping system detailed drawings
(7) Thermowells measured the average pipe temperature
(6) Electrodes measured the average pipe composition
(5) Insertion flowmeters measured the average pipeline velocity
(4) pH electrodes were not installed until process fluid was near normal velocity
(3) Sensors location was optimum in terms of minimizing dead time and maximizing single-phase consistency and measurement repeatability
(2) All of the control loops had initial tuning settings identified automatically during high-fidelity simulation testing and operator training
(1) Innovation and advanced process control was used everywhere.

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