HKIE Professional Assessment - Chemical Discipline key terms

The lowest temperature at which a liquid gives off enough vapour to form an ignitable mixture with air near its surface, measured under specified test conditions. It underpins flammable-liquid classification and storage and handling precautions.

The minimum concentration of fuel vapour or gas in air, expressed as volume percent, that will support combustion. Below this concentration the mixture is too lean to burn, so LEL underpins gas-detection alarm settings and ventilation design.

The lowest temperature at which a flammable vapour or gas ignites spontaneously in air without an external ignition source. It governs the maximum permissible surface temperature of electrical equipment in hazardous areas and the design of hot surfaces.

A systematic team-based technique that applies guide words such as no, more, less, reverse and other-than to process parameters at each node, identifying deviations, causes, consequences and safeguards. It is the standard method for hazard identification in process plant design.

A defined section of a process line or vessel, bounded by isolation points, within which a HAZOP team examines deviations. Choosing nodes well keeps the study tractable while ensuring no significant inventory or equipment is overlooked.

An engineered or administrative measure that prevents a deviation from occurring or mitigates its consequences, credited in a HAZOP or risk assessment only if it is independent, testable and effective. Examples include relief valves, interlocks and operating procedures.

A semi-quantitative method that estimates the likelihood of a hazardous scenario and credits independent protection layers, each with an assigned probability of failure on demand, to judge whether risk meets a target. It often follows a HAZOP.

A discrete rating, from SIL 1 to SIL 4, expressing the required reliability of a safety instrumented function in reducing risk, defined by target probability of failure on demand and architectural constraints. It links risk assessment to instrumented protection design.

An independent instrumented system, comprising sensors, logic solver and final elements, that brings the process to a safe state when demand arises. It must be designed, verified and maintained to the SIL claimed for its safety functions.

A logic function that prevents an unsafe action or forces a safe state when defined conditions are met, such as tripping a feed pump on low flow. Interlocks may be implemented in the basic control system or in a separate safety instrumented system.

A tabular document mapping each initiating cause, such as a trip or alarm, to the resulting actions, such as valve closures and pump stops. It is the functional specification from which interlock and safety instrumented system logic is programmed and tested.

A document that fixes the feedstocks, product specifications, capacity, operating envelope, site conditions, utilities, codes and assumptions on which a process design is developed. It is the reference against which design changes and reviews are controlled.

A schematic showing the main equipment, streams, flow directions and key operating conditions of a process, without detailed piping. It is used for material and energy balances, process description and hazard study preparation.

A detailed drawing showing equipment, piping, valves, instruments, control loops and interfaces with their identification tags. It is the primary document for HAZOP studies, control design, construction and plant modification.

Deliberate spare capacity or over-design built into equipment sizing, such as a fouling allowance or a pump head margin, to cover uncertainty, deterioration and future debottlenecking. Excess margin raises capital cost and can create its own operability problems.

The process of translating performance from laboratory or pilot equipment to commercial size, accounting for changes in mixing, heat and mass transfer, residence-time distribution and wall effects. It usually requires pilot demonstration and staged design checks.

The statistical description of how long fluid elements spend in a vessel or reactor, obtained from tracer tests or models such as tanks in series. It links mixing behaviour to conversion, selectivity and product quality.

A parameter relating the molar flux of a species to its concentration driving force across a phase boundary or film. It depends on hydrodynamics, diffusivity and geometry, and is central to absorber, extractor and reactor design.

A combined coefficient, based on a chosen area, that accounts for convective film resistances on both sides of a wall plus conduction through the wall and any fouling layers. It is used with the log mean temperature difference to size exchangers.

An additional thermal resistance, expressed as a cleanliness allowance, added to an exchanger design to account for deposits that build up in service. Over-generous allowances waste area; too small an allowance causes early underperformance.

A factor correcting the ideal-solution concentration of a species to its effective thermodynamic activity, reflecting non-ideal intermolecular interactions in the liquid phase. It is essential for accurate vapour-liquid equilibrium and reaction equilibrium calculations.

An effective partial pressure that replaces the true partial pressure so that equilibrium relations keep a simple form for real gases. The ratio of fugacity to pressure is the fugacity coefficient, which measures departure from ideal-gas behaviour.

The condition in which a liquid and its vapour coexist with no net transfer of any component, so that component fugacities are equal in both phases. VLE data and models underpin distillation, absorption and flash calculations.

The ratio of the vapour-liquid distribution coefficients of two components, indicating how easily they can be separated by distillation. Values close to one imply many stages or a large reflux requirement.

The reflux rate at which the specified separation would need an infinite number of theoretical stages. It sets the lower bound for economical design, and the operating reflux is normally chosen as a multiple of it.

An ideal contacting stage in which the leaving streams reach equilibrium. Real columns need more actual trays or packing height than theoretical stages, and the ratio of the two is the stage efficiency.

The number of times a catalyst's active sites process reactant molecules per unit time, expressed per site per second. It measures intrinsic catalytic activity and allows comparison of catalysts independent of loading and dispersion.

The volumetric feed rate divided by catalyst volume, typically per hour, used to express how hard a catalytic reactor is being pushed. Liquid hourly space velocity is the analogous quantity based on liquid feed.

The loss of catalytic activity or selectivity over time through coking, poisoning, sintering or fouling. It determines regeneration cycles, catalyst replacement cost and reactor operating strategy.

A dimensionless group comparing the reaction rate to a transport rate, such as convection or diffusion. Large values indicate transport limitation, while small values indicate kinetics control, guiding reactor and experimental design.

Extra wall thickness added to equipment so that a predictable rate of metal loss over the design life does not compromise integrity. It must be justified by material selection and inspection data, not assumed.

A planning method that ranks equipment for inspection by combining the likelihood and consequence of failure, so effort concentrates on the highest-risk items. It is central to mechanical integrity programmes in process plants.

A drawing that marks the selected material and any corrosion allowance or lining for each section of a plant, showing where changes occur. It communicates materials decisions to designers, fabricators and inspectors.

The sum of a project's future cash flows discounted to today at a chosen rate, minus the initial investment. A positive NPV indicates the project adds value at that discount rate.

The rate used to convert future cash flows into present value, reflecting the time value of money and the risk of the investment. Choosing it consistently is essential when comparing project alternatives.

The time required for cumulative cash inflows to recover the initial investment. It is simple to communicate but ignores cash flows after payback and the time value of money.

Under Hong Kong safety legislation, the person or organisation responsible for ensuring that plant, work systems and premises are safe and without risks to health, so far as reasonably practicable. Duties extend to employees, contractors and the public.

A legal test requiring duty holders to weigh the risk against the cost, time and difficulty of controlling it. It does not demand the impossible, but cost alone rarely justifies tolerating serious risk.

The professional and ethical standards members must uphold, covering competence, honesty, public safety, conflicts of interest and respect for confidentiality. Breach can lead to disciplinary action by the institution.