Process Control and Instrumentation Flashcards

The term 'turndown ratio' for a flow meter describes:

Answer: The ratio of the maximum to minimum measurable flow with acceptable accuracy

Turndown ratio defines the usable measurement range of a flow meter; a 10:1 turndown means it reads accurately from 100% down to 10% of full scale.

In a heat exchanger control scheme, the coolant flow is increased when the process outlet temperature rises above setpoint. This is an example of:

Answer: Feedback control

Feedback control measures the process output (temperature), compares it to the setpoint, and adjusts the manipulated variable (coolant flow) to correct the deviation.

What is the purpose of an interlock in a process plant?

Answer: To automatically prevent or stop an operation when an unsafe condition is detected

Interlocks enforce safe operating limits by automatically triggering actions (closing valves, tripping pumps) when process parameters reach dangerous levels.

A pressure transmitter has a range of 0-100 psig and a 4-20 mA output. What current signal corresponds to 50 psig?

Answer: 12 mA

50 psig is 50% of span; 50% of the 16 mA span (4 to 20 mA) is 8 mA, added to the zero of 4 mA gives 12 mA.

Control valve hysteresis is best described as:

Answer: The difference in valve position between increasing and decreasing signal strokes

Hysteresis causes a valve to reach different positions for the same signal depending on whether it is traveling upward or downward, degrading control precision.

When calibrating a pressure transmitter, the technician applies a known input pressure and adjusts the output to match the expected 4-20 mA value. The two adjustments made are:

Answer: Zero and span

Zero calibration sets the output at the low end of range (0% input = 4 mA) and span calibration sets the output at full range (100% input = 20 mA).