Control Systems & Instrumentation Flashcards

According to IEC 61511, what is the difference between a Safety Instrumented System (SIS) and a Basic Process Control System (BPCS)?

Answer: A SIS is designed to bring the process to a safe state upon demand, independent of the BPCS

IEC 61511 requires the SIS to be independent of the BPCS so that a BPCS failure does not compromise the safety function.

A vortex flow meter operates on which physical principle?

Answer: The frequency of vortices shed by a bluff body is proportional to fluid velocity

Vortex meters count the frequency of alternating vortices shed from a bluff body (Von Kármán effect), which is directly proportional to volumetric flow velocity.

In ISA-5.1 notation, what does the letter 'Y' as a succeeding letter in a tag identifier indicate?

Answer: A relay or computing function (e.g., computation, conversion, switching)

Per ISA-5.1, 'Y' as a succeeding identifier letter denotes a relay, computing, or converting function such as a signal selector or arithmetic device.

What is 'reset windup protection' designed to prevent in a PID controller?

Answer: Accumulation of integral action while the output is at its limit, causing overshoot on recovery

Reset windup protection (anti-windup) limits integral accumulation when the output saturates, preventing large overshoot when the output constraint is lifted.

Which of the following best describes 'feedforward control' in a process control application?

Answer: Predictive control action based on measured disturbances before they affect the process variable

Feedforward control measures known disturbances and takes corrective action before the disturbance affects the controlled variable, complementing feedback control.

What is the typical output signal range of a pneumatic controller in industrial process control?

Answer: 3-15 psi

The standard pneumatic signal range for industrial controllers and valve actuators is 3-15 psi (approximately 20-100 kPa).

During loop tuning, if increasing the proportional gain causes sustained oscillations at a constant amplitude, the loop is operating at:

Answer: The ultimate gain condition where the phase margin equals zero

Sustained constant-amplitude oscillations indicate zero phase margin (ultimate gain condition), where the open-loop phase shift equals -180° at the gain crossover frequency.