Process Flow and Instrumentation Flashcards

The main reason to install instruments with a 4-20 mA signal rather than 0-20 mA is that 4-20 mA allows:

Answer: Detection of a broken wire or dead instrument (live zero)

The 4 mA 'live zero' means a reading of 0 mA indicates a fault such as a broken wire, not a valid zero value.

A control loop is 'hunting' or oscillating continuously around setpoint. This is most often caused by:

Answer: Too much gain or too little damping in the tuning

Excessive gain or insufficient damping causes the loop to overshoot repeatedly and oscillate.

Which device is used to physically isolate a section of pipe for maintenance while providing positive visual confirmation it is blocked?

Answer: A blind flange or spectacle blind

A blind flange or spectacle blind provides a positive, visible mechanical block for isolation during maintenance.

An RTD (resistance temperature detector) measures temperature by sensing the change in:

Answer: Electrical resistance of a metal element

An RTD relies on the predictable increase in electrical resistance of a metal (often platinum) as temperature rises.

On a P&ID, a dashed line forming a circle around a group of instruments usually indicates they are:

Answer: Part of a shared display or distributed control system function

A bubble with an enclosing form or shared symbol denotes shared display/control such as a DCS function.

A check valve is installed in a pump discharge line primarily to:

Answer: Prevent reverse flow (backflow) when the pump stops

A check valve permits flow in one direction only and prevents backflow that could damage the pump.

The term 'span' of a transmitter refers to:

Answer: The difference between the upper and lower range values

Span is the algebraic difference between the upper and lower range limits of the measurement.