NiCuZn / Glossary

Process engineering glossary.

Plain definitions of the terms used in early process and basic engineering, with links to guides and calculators.

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Battery limits: The boundary of a process unit or project scope. Streams that cross it, such as feeds, products and utilities, have their conditions defined at the battery limit in the design basis. How to write a design basis →
Block flow diagram (BFD): The simplest process diagram: blocks for major process steps joined by the main streams, often with an overall material balance. Used at the concept and feasibility stage. BFD vs PFD vs P&ID →

Choked flow: The condition where increasing the pressure drop across a valve or restriction no longer increases the flow. In liquids it is linked to cavitation and flashing. Control valve Cv calculator →
Control valve Cv: The flow coefficient of a valve: the flow of water in US gallons per minute at 60 °F that passes with a 1 psi pressure drop. Kv is the metric equivalent in m³/h at 1 bar; Cv ≈ 1.156 Kv. Control valve Cv calculator →

Design basis: The agreed set of requirements and assumptions for a plant: capacity, feeds, products, battery limit conditions, utilities, site data and applicable codes. Every later engineering step depends on it. Design basis checklist →
DEXPI: An industry initiative and data standard for exchanging piping and instrumentation diagram information between engineering software tools.
Duty: The rate of heat transferred by a piece of equipment, such as an exchanger, condenser or reboiler, usually in kW or MW. Heat exchanger area calculator →

Friction factor: A dimensionless factor relating pipe pressure drop to velocity, length and diameter in the Darcy–Weisbach equation. It depends on Reynolds number and relative roughness, often via the Colebrook–White equation. Line sizing calculator →

HAZOP: Hazard and operability study: a structured team review of a design, usually node by node on the P&IDs, using guide words to identify deviations, causes, consequences and safeguards.
Heat and material balance (H&MB): The table of flow, composition, temperature, pressure and properties for every numbered stream in a process, together with equipment duties. Equipment is sized from it. Heat and material balance guide →
Hold-up time: The time a vessel's working liquid volume would last at the design outflow if the inflow stopped. Used to size drums for level control and surge. Drum sizing calculator →

Line list: A register of every process line with its number, size, piping class, fluid, design and operating conditions, insulation and the equipment or lines it connects.
LMTD: Log mean temperature difference: the effective temperature driving force in a heat exchanger, calculated from the temperature differences at each end. Corrected by a factor F for multi-pass arrangements. Heat exchanger area calculator →

Material balance: The application of conservation of mass to a process: at steady state, what goes in comes out, with component balances adjusted for reactions. Heat and material balance guide →

NPSH: Net positive suction head. NPSH available is the suction head above the liquid's vapour pressure; it must exceed the NPSH required by the pump, with a margin, to avoid cavitation. Pump power calculator →

On-stream factor: The fraction of the year a plant is expected to operate, used to convert annual production into an hourly design rate.
Overall heat transfer coefficient (U): The combined heat transfer coefficient of an exchanger, including film and wall resistances and fouling, relating duty to area and temperature difference: Q = U A ΔT. Heat exchanger area calculator →

Piping and instrumentation diagram (P&ID): The detailed diagram of a process showing all piping with line numbers and classes, valves, instruments and control loops, safety devices, vents and drains. Used for detailed design, HAZOP, construction and operation. BFD vs PFD vs P&ID →
Piping class: A specification defining the pipe material, pressure rating, schedule or wall thickness, fittings and valves allowed for a given service.
Process flow diagram (PFD): The diagram of a process showing major equipment, main process lines, numbered streams and key operating conditions, usually with a heat and material balance. The central document of basic engineering. BFD vs PFD vs P&ID →
Property package: The set of thermodynamic models and data a simulator or calculation uses to predict phase behaviour and properties. Its suitability depends on the components and conditions of the system.
Purge: A small stream removed from a recycle loop to stop inerts or impurities accumulating.

Reboiler: The heat exchanger at the bottom of a distillation column that vaporises part of the bottoms liquid to provide the vapour flow up the column.
Reflux: Condensed overhead liquid returned to the top of a distillation column. The reflux ratio strongly affects separation and energy use.
Reynolds number: The dimensionless ratio of inertial to viscous forces in a flow, Re = ρvD/μ. In pipes, flow is laminar below about 2000 and turbulent above about 4000. Line sizing calculator →

Schedule (pipe): A designation of pipe wall thickness for a given nominal size, such as Schedule 40, defined in standards like ASME B36.10. Line sizing calculator →
Souders–Brown equation: A relation for the maximum vapour velocity that still lets liquid droplets settle out in a separator: v = K √((ρL − ρV) / ρV). Drum sizing calculator →

Tear stream: A stream in a recycle loop whose values are assumed, so the loop can be calculated in sequence and iterated until the assumed and calculated values agree.
Turndown: The lowest throughput a plant or piece of equipment can operate at, usually expressed as a fraction of design capacity.

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