PE Control Systems flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

4-20 mA live zero

In a 4-20 mA loop, 4 mA represents 0 % of span and 20 mA represents 100 %; a 0 mA reading indicates a broken wire or lost loop power.

Orifice flow equation

For a differential-pressure flow element, volumetric flow is proportional to the square root of differential pressure, so the transmitter or control system applies square-root extraction to linearize the signal.

Pt100 RTD

A platinum resistance thermometer with 100 ohms at 0 degC and a temperature coefficient near 0.00385 ohm/ohm/degC, chosen where accuracy and long-term stability exceed thermocouple capability.

Cold junction compensation

Thermocouples measure only the difference between hot and reference junction voltages, so the reference junction temperature must be measured and compensated to obtain an accurate absolute temperature.

Coriolis mass flowmeter

A Coriolis meter measures mass flow directly from the phase shift of vibrating tubes and also reports fluid density, typically with 0.1 to 0.2 percent of rate accuracy.

Magnetic flowmeter limitation

Magnetic flowmeters require a conductive liquid, generally above 5 microsiemens per centimeter, and a full pipe; they cannot measure hydrocarbons, gases, or other non-conductive fluids.

Gauge versus absolute pressure

Gauge pressure is referenced to local atmospheric pressure while absolute pressure is referenced to perfect vacuum; psia equals psig plus atmospheric pressure, about 14.7 psi at sea level.

Ideal PID equation

Controller output equals Kc times the sum of error, the integral of error divided by Ti, and derivative time Td times the derivative of error.

Ziegler-Nichols ultimate gain tuning

From the ultimate gain Ku and ultimate period Pu at which the loop cycles continuously, PID settings are Kc = 0.6 Ku, Ti = Pu/2, and Td = Pu/8.

Damping ratio and overshoot

For a second-order loop, fractional overshoot equals exp(-pi zeta / sqrt(1 - zeta squared)); a damping ratio of 0.5 gives roughly 16 percent overshoot.

IMC lambda tuning

For a first-order-plus-dead-time process, internal model control sets controller gain Kc = tau / (K (lambda + theta)) and integral time Ti = tau, where lambda is the chosen closed-loop time constant.

Gain margin

Gain margin is the amount in decibels by which loop gain may increase at the 180 degree phase crossover before instability; a minimum of about 6 dB is commonly targeted.

Cascade secondary loop speed

In cascade control the inner secondary loop should be at least three to five times faster than the outer primary loop so the primary controller sees nearly instantaneous correction.

Integral windup

When controller output saturates while error persists, the integral term keeps accumulating; anti-windup logic stops or limits integration to prevent large overshoot when the loop recovers.

Valve flow coefficient Cv

Cv is the number of gallons per minute of 60 degF water that flows through a wide-open valve with a pressure drop of one psi across it.

Equal percentage valve characteristic

With equal-percentage trim, each equal increment of stem travel changes flow by a constant percentage of current flow, producing a nearly linear installed characteristic when valve pressure drop falls with increasing flow.

Air-to-open actuator

An air-to-open actuator with spring return closes the valve when air supply is lost, so it is specified wherever the process safe state requires the valve to be shut.

Cavitation in control valves

Cavitation occurs when vena contracta pressure falls below vapor pressure and bubbles then collapse downstream, causing noise, vibration and trim damage; flashing occurs when downstream pressure stays below vapor pressure.

Valve positioner

A positioner compares the controller signal with actual stem position and adjusts actuator air accordingly, overcoming packing friction and unbalanced forces to reduce hysteresis and improve accuracy.

Valve rangeability

Rangeability is the ratio of maximum to minimum controllable flow through a valve, typically about 30:1 to 50:1 for globe valves before installed performance degrades.

Three-element drum level control

Boiler drum level control that trims feedwater using drum level feedback, steam flow feedforward, and a feedwater flow inner loop, compensating for shrink and swell during load changes.

Compressor anti-surge control

Anti-surge control opens a recycle valve when compressor flow approaches the surge line, maintaining flow above the minimum needed to prevent damaging flow reversal.

pH control nonlinearity

The pH titration curve is extremely steep near neutrality, so a fixed PID gain becomes unstable; gain scheduling or adaptive control based on the measured slope is used instead.

Fuel-air cross-limiting

In fired heater control, cross-limiting increases air flow before fuel on load increases and cuts fuel before air on load decreases, keeping the mixture below flammable limits during upsets.

Column tray temperature control

Distillation composition is often inferred from a tray temperature and controlled by manipulating reflux flow or reboiler steam, because online analyzers are slow and expensive to maintain.

Selective auctioneering control

A selector passes the highest or lowest of several controller outputs, for example overriding on compressor suction pressure or protecting a pump near the end of its curve.

ISA instrument tag letters

On a P&ID the first letter gives the measured variable, such as F flow, T temperature, P pressure or L level, and later letters give the function, such as transmitter, controller or valve.

SIL 2 PFD range

Safety integrity level 2 corresponds to an average probability of failure on demand between 10^-3 and 10^-2 for a low-demand safety instrumented function.

1oo1 PFDavg formula

For a single one-out-of-one channel, average probability of failure on demand is approximately lambda_DU times TI divided by 2, where lambda_DU is undetected dangerous failure rate and TI the proof-test interval.

Class I Division 1

A hazardous location classification for areas where flammable gases or vapors are present during normal operation, requiring explosion-proof or intrinsically safe equipment and wiring methods.

Intrinsic safety barrier

An intrinsic safety barrier uses zener diodes and current-limiting resistors to limit energy entering a hazardous area so no spark or hot surface can ignite the atmosphere, even under fault conditions.

SIS independence from BPCS

A safety instrumented system must be functionally separate from the basic process control system, with its own sensors, logic solver and final elements, to satisfy IEC 61511 requirements.