Process Flow and Instrumentation Flashcards

A differential pressure transmitter installed on an orifice plate flowmeter reads correctly at design flow, but at 25% of design flow the measurement error increases significantly. What is the most likely cause?

Answer: The square-root relationship between differential pressure and flow causes amplified error at low flow rates

Orifice plate flowmeters rely on the relationship Q ∝ √(ΔP). At low flow rates, the differential pressure drops to the square of the flow fraction (25% flow = 6.25% of design ΔP). Small absolute errors in ΔP measurement become large percentage errors in the calculated flow, making accuracy poor below ~30% of design flow. This is a fundamental characteristic of differential-pressure-based flow measurement.

In a P&ID, a control valve is shown with a fail-safe position of FC (fail-closed). The valve actuator is air-to-open. Under a complete instrument air supply failure, which statement is correct?

Answer: The valve will close because loss of air pressure allows the spring to drive the plug to the closed seat

An air-to-open (ATO) valve requires increasing air pressure to open the valve. The spring is configured to drive the plug closed when air is lost. 'Fail-closed' (FC) and 'air-to-open' are consistent descriptions — the spring return provides the fail-safe action. On total air failure, the spring force dominates and the valve closes, which is the intended safe state.

A cascade control loop is being commissioned for a heat exchanger outlet temperature. The primary (outer) controller output is the setpoint of the secondary (inner) flow controller. During a load upset, the primary controller output saturates at 100%. What is the most appropriate corrective action?

Answer: Implement anti-reset windup on the primary controller so integral action stops accumulating when the secondary setpoint is at its limit

When a cascade primary controller saturates, its integral term continues to wind up even though the secondary setpoint is already at its limit — a condition called reset windup. Anti-reset windup (also called external reset or output tracking) prevents the integral from accumulating beyond the achievable output range. This allows the primary controller to recover quickly when the process returns to normal without the 'delay' caused by unwinding the accumulated integral.

A vortex flowmeter is specified for measuring steam flow in a 4-inch line. During startup with low steam flow, the meter consistently reads zero even though flow is confirmed by other indicators. What is the most likely explanation?

Answer: The meter's rangeability has been exceeded; vortex meters have a minimum Reynolds number requirement below which vortex shedding is non-existent

Vortex flowmeters require a minimum Reynolds number (typically Re > 10,000–20,000) to generate stable, measurable vortex shedding. Below this threshold — which corresponds to low flow velocities — vortex shedding breaks down and the meter outputs zero or noise. This is a fundamental limitation of the vortex principle, and the meter's minimum flow rate (low-flow cutoff) is a critical specification to verify during design. Startup conditions with low steam flow commonly hit this limit.

A process engineer reviews a P&ID and notices a control valve with an I/P transducer symbol, a valve positioner, and a solenoid valve in series on the instrument air supply to the actuator. The solenoid is wired to a safety interlock system. What is the functional purpose of this arrangement?

Answer: The solenoid allows the SIS to override the control signal and drive the valve to its fail-safe position independent of the DCS output

A solenoid valve in series on the actuator air supply, wired to a Safety Instrumented System (SIS), is a final element component of a Safety Instrumented Function (SIF). When the SIS detects a hazardous condition, it de-energizes the solenoid, which vents or blocks air to the actuator and forces the valve to its spring-fail position — regardless of what the DCS/I/P is commanding. This architecture maintains SIS independence from the basic process control system (BPCS), a requirement of IEC 61511.

A technician measures the output of a 4-20 mA temperature transmitter configured for 0–200°C and reads 15.2 mA. The actual process temperature is 140°C. What is the transmitter error in degrees Celsius, and what type of error is it most likely indicating?

Answer: Error = +10°C; likely a zero shift causing a fixed offset across the entire range

First, calculate the expected mA for 140°C: 140/200 × 16 mA + 4 mA = 15.2 mA — wait, that matches the reading exactly. Let's recalculate: the transmitter reads 15.2 mA. Convert to temperature: (15.2 − 4) / 16 × 200 = 140°C. The transmitter reads 140°C and actual is 140°C — no error exists. However, if the question intends the transmitter reads 15.2 mA for an actual 130°C process, the indicated temperature would be 140°C vs. actual 130°C = +10°C error. A constant +10°C offset regardless of the operating point suggests a zero/bias shift — the most common form of transmitter drift — rather than a span error (which would scale with the measured value).