March/April 2010

Key design components of final control elements

The brass tacks

  • The deadband, resolution, speed, and turndown of final control elements determine control system performance.
  • Whether a valve or variable frequency drive has a better dynamic response depends on the application and adherence to best practices.
  • Special variable frequency drive cables and installation considerations are needed to prevent damage and interference from electrical noise.
 

A final control element is the device manipulated by a control loop to affect the process, principally by means of changing a flow. Final control elements are an essential part of nearly every process control system. Without final control elements, there is no way of controlling the process. We could not change operating points or correct for disturbances. There may be several layers of control loops, but it is usually a flow that a final control element ends up changing in a process. The most notable exceptions are heater or electrode current and mixer speed.

By far, the most common final control element is the control valve, with its attendant positioner, actuator, and other components. Variable speed peristaltic pumps are used for the exceptionally small flows of bench top and pilot plant operations. Variable speed positive displacement pumps are used for small additive and reagent flows in production. For large flows in plants and powerhouses, variable frequency drives and dampers are sometimes used instead of control valves to reduce capital and operating costs.

Axial and centrifugal blowers, fans, and pumps are used for the flow ranges normally associated with gas and liquid streams in industrial plants. A variable frequency drive (VFD), particularly in large utility flow applications, can save energy by the elimination of a control valve and its pressure drop. However, the energy savings is usually overestimated for process streams by not taking into account the service time and efficiency at low flow and the loss in turndown due to static head.

A damper can reduce the cost of the final element or fit in a non-circular duct. Dampers are commonly used in HVAC systems, boilers, furnaces, and scrubbers to manipulate air and vent gas flows. Dampers have a lower pressure drop than a control valve, but generally the performance (e.g., rangeability, resolution, sensitivity, speed, and seal) of a damper is not as good as a control valve. The leakage and limited dynamic response and materials/ruggedness of construction of dampers relegate their application to mostly utility and vent systems.

Valve design, dynamics

The shaft of the actuator and the stem of the internal closure component (plug, ball, or disk) of the control valve are normally separate. The closure component may be cast and forged with the stem or the stem may be connected during valve assembly. The actuator shaft moves the stem that moves the closure component. (While "shaft" and "stem" are more appropriate terms for the actuator and the closure component, respectively, in practice the terms "stem" and "shaft" are used interchangeably.) The amount of play (looseness or gap) in the connections between the shaft, stem, and closure component is backlash that creates deadband and determines, in part, how well the valve will respond to small changes in signal. Excessive seal friction of a closure component that is rotated (e.g., ball or disk) can result in shaft windup. The location and type of connection of the positioner feedback mechanism for valve travel determines whether the positioner is seeing the response of just the actuator or the actual response of the closure component.

Previous methods of testing valve response involved making much larger changes in the valve signal than would normally be made in closed loop control. Most valves will look OK with these large changes in requested position. In service, the change in controller output from scan to scan is generally small (e.g., < 0.2%), except during the start of an operation or process. For small changes in valve signals, the resolution limit from sticktion and deadband from backlash that prevent a good response and create a sustained oscillation (limit cycle) are observable. Current test methods established by the ISA-75.25.01-2000 (R2006) standard address the effect of step size on response.

Control valves with excessive sticktion, backlash, and shaft windup can actually increase process variability when the loop is in automatic by the creation of oscillations from the continuous hunting of integral action to find a position it cannot attain exactly.

Smart digital positioners with a good closure component measurement have the sensitivity and tuning options to mitigate the consequences of stick-slip and backlash by fast feedback control. Built-in diagnostics can pinpoint problems such as packing friction besides monitoring the dynamic response of the valve. 

Sliding stem (globe) valves have the least amount of deadband because of the direct connection between the actuator shaft and trim stem, and low trim friction. For rotary valves, connections can be problematic since there is the need to convert the linear motion of a piston or diaphragm shaft to rotary motion and the changes in the effective lever arm length. Rotary valves originally designed by piping valve manufacturers for on-off or manual operation often have a non-representative position measurement and a degree of excessive backlash and shaft windup that cannot be corrected by a positioner.

Valve best practices

For best performance, users should consider the following during the specification of control valves:

  • Actuator, valve, and positioner package from a control valve manufacturer
  • Digital positioner tuned for valve package and application
  • Diaphragm actuators where application permits (large valves and high pressure drops may require piston actuators)
  • Sliding stem (globe) valves where size and fluid permit (large flows and slurries may require rotary valves)
  • Low stem packing friction
  • Low sealing and seating friction of the closure components
  • Booster(s) on positioner output(s) for large valves on fast loops (e.g., compressor anti-surge control)
  • Online diagnostics and step response tests for small changes in signal
  • Dynamic reset limiting using digital positioner feedback

VFD cable problems

Belden Inc. has studied the radiated noise from cables between the VFD and the motor. Unshielded VFD cables can radiate 80V noise to unshielded communication cables and 10V noise to shielded instrument cables. The radiated noise from foil tape shielded VFD cables is also excessive. A foil braided shield and armored cable performs much better. Still, a spacing of at least one foot is recommended between shielded VFD and shielded instrumentation cables. The cables should never cross. As a best practice, separate trays to isolate VFD and instrumentation cables should be used to avoid mistakes during plant expansions and instrumentation system upgrades.

VFD turndown

Since the inverter waveform is not purely sinusoidal, it is important to select motors that are designed for inverter use. These "inverter duty" motors have windings with a higher temperature rating (Class F). Another option that facilitates operation at lower speeds to achieve the maximum rangeability offered by a pulse width modulation (PWM) drive is a higher service factor (e.g., 1.15).

The turndown of a VFD could drop to 4:1 for the following systems:

  • Older VFD technologies such as 6-step voltage (excessive slip at low speed)
  • Systems with a high static head (flow plummets to zero at a low speed)
  • Operation on the flat portion of the prime mover curve (cycling at low speed)
  • Hot gases (motor overheats at a low speed)

VFD controls

The turndown (rangeability) of a VFD can be increased by ensuring the pump head is large compared to the static head, by using PWM inverters, and by dealing with the heating problems at low speeds. Turndown also depends upon the control strategy in the variable frequency drive.

Which is faster: A valve or VFD?

Exceptionally fast loops can ramp off-scale in milliseconds. These loops have essentially a zero process dead time and may have a high process gain due to a narrow control range (e.g., fractional inches of water column for furnace pressure). These loops require DCS scan times of 0.05 to 0.1 seconds. Special fast scan rate digital controllers or analog controllers are needed. DCS scan time requirements of 0.2 seconds or less signify a VFD opportunity. A properly designed VFD has no measureable dead time, while control valves and dampers take anywhere from 0.2 to 2.0 seconds to start to move. For example, an incinerator pressure and polymer pressure loop that could get into trouble in less than 0.1 second required a VFD and analog controller to stay within the desired control band.

A VFD has a negligible response time delay unless a deadband or dead zone is introduced into the drive electronics to slow response to process measurement noise, or if a low resolution input card is used. A control valve or damper has a dead time proportional to the resolution limit (e.g., from stiction and windup) and dead band (from backlash and windup) divided by the rate of change of the process controller output. For large or fast changes in signal, this dead time disappears.

VFD best practices

With a VFD, a tachometer or inferential speed feedback signal should be sent to the process controller in the DCS that is sending the signal to the drive. The speed feedback should be used in a similar way to the position feedback from a digital positioner to prevent the process controller output from changing faster than the VFD can respond. The use of the dynamic reset limit option for the loops in the DCS can automatically prevent the process controller from outrunning the response of any type of final element. For best performance, users should consider the following during the specification and implementation of variable frequency drive systems:

  • High resolution input cards
  • Pump head well above static head on-off valves for isolation
  • Design B TEFC motors with class F insulation and 1.15 service factor
  • Larger motor frame size
  • XPLE (cross-linked polyethylene) jacketed foil/braided or armored shielded cables
  • Separate trays for instrumentation and VFD cables
  • Inverter chokes and isolation transformers
  • Ceramic bearing insulation
  • Pulse width modulated inverters
  • Properly set deadband and velocity limiting in the drive electronics
  • Drive control strategy to meet rangeability/speed regulation requirements
  • Dynamic reset limiting using inferential speed or tachometer feedback

Source: Essentials of Modern Measurements and Final Elements in the Process Industry: A Guide to Design, Configuration, Installation, and Maintenance by Gregory K. McMillan (ISBN: 978-1-936007-23-3) www.isa.org/finalelements