ROC-M Exam key terms

A spring-loaded device that opens automatically at a set pressure to discharge fluid and protect a boiler, vessel, or piping system from overpressure, then closes again as pressure falls.

The inlet pressure at which a relief device is adjusted to begin opening under operating conditions. It is stamped on the device and must not exceed the vessel's maximum allowable working pressure.

The highest internal pressure permitted in a boiler or pressure vessel at a stated temperature, based on design thickness, material strength, and code rules. It establishes the ceiling for relief device settings.

The pressure increase above the maximum allowable working pressure that occurs while a relief device is discharging at its rated capacity. Codes limit this rise, commonly to ten percent for boilers.

A strength and leak test in which a vessel or piping system is filled with water and pressurized above its design pressure, then inspected for distortion, leakage, and permanent deformation before being returned to service.

A safety control on a boiler that senses water level and shuts off the burner when the level drops below a safe limit, preventing overheating and catastrophic failure of the pressure vessel.

The air supplied to a burner to mix with fuel and support complete burning. Insufficient supply causes carbon monoxide, soot, and flame instability, and it is verified during startup and annual servicing.

A device that automatically discharges condensate and non-condensable gases from steam lines while retaining live steam, maintaining heat transfer efficiency and preventing water hammer in the distribution system.

A pressure surge caused by the sudden stopping or reversal of fluid flow, producing a loud banging noise and potentially damaging pipes, fittings, and supports. It is prevented by proper trapping, venting, and slow valve operation.

A valve that permits flow in one direction only and closes automatically when flow reverses, protecting pumps, compressors, and piping from backflow, reverse rotation, and hydraulic shock.

A valve with a sliding wedge or disc that moves perpendicular to the flow path, giving low resistance when fully open. It is intended for isolation service, not throttling, because partial opening erodes the seat.

A valve with a plug and seat arranged so flow changes direction, producing higher resistance but fine throttling control. It is commonly used for regulating flow and for frequent open-close duty.

A modulating valve positioned by an actuator in response to a controller signal to regulate flow, pressure, temperature, or level. Correct sizing and fail-safe action are essential to loop stability.

The formation and collapse of vapor bubbles in a pump or valve where local pressure falls below vapor pressure. Collapse produces noise, vibration, and metal erosion, reducing capacity and damaging impellers.

The absolute pressure margin above vapor pressure available at a pump suction, expressed in head. Insufficient margin causes cavitation, so the available value must exceed the required value given by the manufacturer.

A rotodynamic machine that adds energy to liquid by spinning it outward through an impeller, converting velocity into pressure. Its flow varies with system resistance and it must never run deadheaded.

Relationships showing how pump or fan flow, head, and power change with speed or impeller diameter. Flow varies with speed, head with speed squared, and power with speed cubed, guiding speed control savings.

A pair of lapped faces, one rotating and one stationary, held together by spring pressure and fluid film to prevent leakage along a rotating shaft. It requires clean flush fluid and correct installation.

A predictive maintenance technique that measures machine vibration and interprets frequency spectra to detect imbalance, misalignment, bearing defects, and looseness before functional failure occurs.

Periodic or continuous measurement of machine parameters such as vibration, temperature, oil debris, and pressure to track health and schedule maintenance based on actual need rather than elapsed time.

Scheduled servicing, lubrication, adjustment, and replacement performed at fixed intervals to reduce unexpected failures. It is time-based, unlike predictive work driven by measured condition.

Inspection methods such as radiography, ultrasonic testing, magnetic particle, dye penetrant, and visual inspection that reveal defects in welds and materials without damaging the component under test.

A written document that specifies the materials, joint design, preheat, filler, current, and technique qualified by testing for a particular weld. Welders must follow it to ensure code compliance.

A safety procedure that isolates hazardous energy sources, applies locks and tags, and verifies zero energy before maintenance, preventing unexpected startup or release of stored energy such as steam or pressure.

Entry into a space with limited access and poor ventilation that is not designed for occupancy, requiring atmospheric testing, ventilation, an attendant, and a written permit before personnel may enter.

A formal written authorization describing the task, hazards, controls, and required isolations, signed before high-risk work such as hot work, confined space entry, or line breaking begins.

A control system that proves main flame or pilot establishment and supervises it continuously, shutting off fuel within a safe time if flame is lost, preventing furnace explosions from unburned fuel.

A controller that combines proportional, integral, and derivative actions to reduce error between a measured variable and setpoint. Tuning balances speed of response against stability and overshoot.

A device, powered electrically, pneumatically, or hydraulically, that moves a valve or damper to a commanded position. Its fail-safe mode determines equipment behavior on loss of power or signal.

An electronic device that varies the speed of an alternating-current motor by adjusting supply frequency and voltage, enabling flow control by speed change instead of throttling and reducing energy use.

A protective device that disconnects a motor when sustained current exceeds its rating, preventing winding overheating from overload, single phasing, or mechanical binding while allowing normal starting current.

A refrigeration machine that removes heat from a liquid, usually water, for air conditioning or process cooling. It comprises a compressor, evaporator, condenser, and expansion device in a vapor-compression cycle.

The temperature of refrigerant vapor above its saturation temperature at a given pressure. Measured at the evaporator outlet, it confirms the expansion valve setting and protects the compressor from liquid slugging.

The amount by which liquid refrigerant is cooled below its saturation temperature at a given pressure. Measured at the condenser outlet, it indicates charge level and condenser performance.

A metering device that regulates refrigerant flow into an evaporator by sensing suction superheat at the outlet, maintaining superheat within a range and matching flow to load.

A fluid used in a refrigeration cycle that absorbs heat by evaporating at low pressure and releases it by condensing at high pressure. Selection considers safety group, ozone depletion, and global warming potential.

A machine that raises gas pressure by reducing volume. Types include reciprocating, rotary screw, and centrifugal; surge, overheating, and lubricant condition are key operating concerns.

An unstable operating condition in a centrifugal compressor or fan where flow reverses momentarily, causing violent pressure pulsations, noise, and possible mechanical damage. It is avoided by staying above minimum flow.

An assembly containing a fan, coils, filters, dampers, and controls that conditions and moves air through ducts to served spaces. Balancing and filter maintenance strongly affect its delivered capacity.