Piping Glossary

Corrosion progressing faster than the assumed rate used in design. Often caused by process changes, contamination, or incorrect material selection. Trigger for engineering review and potential re-rating.

Device that operates a valve using an external power source: pneumatic, electric, or hydraulic. Specified when manual operation is impractical due to torque, frequency, or remote operation requirements.

American Iron and Steel Institute. Provides standard designations for stainless and alloy steels. AISI 304, 316, 410 are common piping material grades.

Maximum stress permitted in a pipe wall under design conditions, per the applicable code. For B31.3, defined in Appendix A for each material and temperature. Basis of wall thickness calculation.

Pipe support that restrains all six degrees of freedom: three translational and three rotational. Transfers all thermal expansion forces to structure. Structural must confirm capacity before specifying.

Multi-port averaging pitot tube for flow measurement. Better velocity profile averaging than single-point pitot. Used on large-bore lines where orifice plate pressure drop is unacceptable.

American Petroleum Institute. Issues standards for equipment and piping in petroleum and petrochemical industries: API 610 (pumps), API 617 (compressors), API 570 (inspection), API 650 (tanks).

Piping Inspection Code: In-service inspection, rating, repair, and alteration of metallic piping systems. Essential knowledge for brownfield work. Defines inspection intervals, corrosion monitoring, and fitness-for-service.

Centrifugal pumps for petroleum, petrochemical and natural gas industries. Defines allowable nozzle loads (Table 4), materials, testing, and documentation. Governs pump specification on most process projects.

Axial and centrifugal compressors for petroleum, chemical and gas industry services. Higher nozzle load sensitivity than pumps. Dynamic analysis required on large machines.

Welded tanks for oil storage. Governs atmospheric storage tank design, materials, and fabrication. Foundation requirements, settlement monitoring, and inspection intervals specified.

Designation of plant areas by the likelihood and consequence of a flammable atmosphere being present. Zone 0/1/2 (IEC) or Division 1/2 (NEC). Governs selection of electrical equipment. Affects routing of instrument cables.

Drawing updated after construction to reflect the actual installed condition. In brownfield work, treat as-builts as a starting hypothesis — always verify physically. Discrepancies are the norm, not the exception.

American Society of Mechanical Engineers. Issues the most widely referenced piping codes: B31.1 (power piping), B31.3 (process piping), B31.4 (pipeline transport), B31.8 (gas transmission), and the ASME Boiler and Pressure Vessel Code (Section VIII for vessels).

Standard for pipe flanges and flanged fittings NPS ½ through NPS 24. Defines pressure-temperature ratings by material group and flange class (150 through 2500). The reference for every flanged joint in process piping.

Factory-made wrought butt-welding fittings. Defines face-to-face, centre-to-face, and end-to-end dimensions for elbows, tees, reducers, and caps. The dimension source for all BW fitting isos.

Forged fittings — socket-welding and threaded. Covers Classes 2000, 3000, and 6000. Limits maximum NPS to 4 for socket-weld fittings.

Process Piping code. The primary design, fabrication, examination, and testing code for process plant piping. Defines wall thickness, material selection, flexibility analysis, NDE requirements, and fluid service categories.

Welded and seamless wrought steel pipe. Defines OD and wall thickness (schedules) for carbon and alloy steel pipe. OD is fixed per NPS; schedule determines wall thickness.

Stainless steel pipe. Schedules 5S, 10S, 40S, 80S. OD identical to B36.10M for same NPS. Most process stainless is 10S or 40S.

Boundary separating process units, or separating plant from external areas. Different design basis, codes, and operating responsibility on each side. All B/L crossings require special documentation and typically isolation valves.

Reverse flow in a piping system. Prevented by check valves. Backflow at pump discharge can cause reverse rotation, mechanical seal damage, and equipment damage.

Tubular vessel housing a pig launcher or receiver. Includes quick-opening closure, kicker line, drain, vent, and bypass connections.

Isolation valve at the unit battery limit. Typically a full-bore ball valve with chain and padlock. Allows unit isolation for maintenance or emergency without affecting adjacent units.

A pipe change of direction formed by cold bending or hot bending the pipe itself, without a separate fitting. Minimum bend radius typically 5D. Used to reduce weld count on large-bore lines.

Complete list of all piping components required for a spool or line: pipe lengths, fittings, flanges, gaskets, bolts, instruments, supports. Extracted from model or manually from isometric. Basis for procurement.

Connection of a smaller pipe (branch) into a larger pipe (header/run). Connection type depends on branch/header ratio: tee for >50%, weldolet/sockolet for <50%. Reinforcement may be required per B31.3 Para 304.3.

Flanged joint inserted in a piping run specifically to allow the line to be separated for maintenance — equipment removal, valve replacement, or inspection access. Always specify in pipe rack take-offs to allow maintenance disassembly.

A project involving modification, revamp, or extension of existing operating plant. Higher risk than greenfield due to undocumented modifications, as-built inaccuracies, hazardous area constraints, and shutdown window limitations. Requires site verification before design.

Pressure relief device where a precision-engineered pin buckles at a specific set pressure. More accurate set point than spring-loaded PSV. Good for clean, non-fouling services. Non-reclosing — pin must be replaced after actuation.

The removal of a heat exchanger tube bundle from its shell for cleaning or replacement. Requires a clear space at the fixed head end equal to the full bundle length plus minimum 1000mm. Never route piping in the bundle pull zone.

A welded joint joining two pipe ends or fittings end-to-end after bevelling. Standard joint for NPS 2 and above in most process piping. Requires full penetration weld. NDE requirements per piping class.

Additional wall thickness above the pressure design minimum, added to allow for wall thinning over the design life. Typically 1.5mm to 6mm. Specified in piping class. Affects wall thickness selection and inspection intervals.

Industry-standard pipe stress analysis software (Hexagon). Accepts piping geometry, material properties, and loads; calculates stresses and nozzle loads. Your routing decisions directly determine whether Caesar II passes or fails.

The most common piping material in process plants. ASTM A106 Grade B (seamless pipe), A234 WPB (fittings), A105 (flanges and forgings). Suitable for most services up to approximately 425°C. Subject to H2S and amine cracking in specific services.

Formation and collapse of vapour bubbles in a liquid pump, caused by local pressure dropping below vapour pressure. Causes noise, vibration, erosion of impeller and casing, and eventual pump failure. Primary cause: insufficient NPSH available.

Specific location on a piping system where wall thickness is measured periodically. Position, method (UT), and frequency defined by inspection engineer per API 570. Locations selected at areas of highest expected corrosion rate.

Deliberate under-cutting of pipe length during fabrication to pre-stress the system, reducing operating loads on nozzles or supports. Requires specific assembly instructions and stress analysis verification. Controversial — many projects prohibit it.

A reducer where the centreline of inlet and outlet are coaxial. Used for size changes on horizontal lines. NOT used on pump suction — eccentric reducer required to prevent gas pockets.

Automatically throttles flow based on a process signal (flow, pressure, temperature, level). Consists of valve body, trim, and actuator. Requires upstream and downstream straight runs. Always include isolation valves and bypass for maintenance.

A metal sample installed in a pipe or vessel and removed periodically to measure corrosion rate by weight loss. Low-cost, direct measurement of actual corrosion in service conditions.

Time-dependent deformation of metal under sustained stress at elevated temperature. Significant above approximately 370°C for carbon steel, 600°C for austenitic stainless. Determines maximum allowable stress at high temperature.

Corrosion occurring on the external surface of insulated pipe, driven by water ingress and oxygen. Often invisible until severe. Major failure cause in ageing plants. Carbon steel lines cycling between wet and dry conditions most susceptible. Risk mitigated by proper cladding and sealant.

Connection made to a live pressurised pipe without shutdown, using specialised equipment. Requires engineering approval, hot work permit, and experienced contractor. Design the branch to be mechanically sound for the live operation.

A section of pipe that is normally stagnant — blocked at one end with no through-flow. Creates risk of corrosion, microbiological growth, and pressure rating uncertainty. Minimise dead leg length. Flag all dead legs in design review.

Maximum pressure at which a piping system is designed to operate. Used for wall thickness calculation and flange class selection. Typically set 10% above maximum operating pressure or the PSV set pressure, whichever is lower.

Maximum and minimum temperature used for material selection, wall thickness calculation, and flange rating selection. Maximum determines allowable stress; minimum determines impact test requirements.

Metric designation for pipe size used in European and international projects. DN 100 = NPS 4. DN does not equal actual OD — it is a nominal reference. Conversion: DN = NPS × 25 (approximately).

Two block valves with a bleed valve between them. Provides positive isolation confirmation: both blocks closed, bleed opened — if bleed shows flow, upstream block is leaking. Required for instrument isolation in many piping classes.

Connection at a low point in a piping system for removing liquid during shutdown, maintenance, or commissioning. Every low point needs a drain. Size minimum DN 25 (1 inch). Connect to closed drain unless service permits open drain.

A short vertical drop at a low point in a steam or condensate line, fitted with a steam trap or drain valve. Collects condensate and prevents water hammer. Missing drip legs are a leading cause of catastrophic steam line failures.

Cast iron with spheroidal graphite morphology. Higher strength and toughness than grey cast iron. Used for water and low-pressure services. Not suitable for hydrocarbon service at elevated temperature or pressure.

A reducer with offset centrelines — one side flat, one side tapered. Flat On Top (FOT) on horizontal pump suction prevents gas pockets. Flat On Bottom (FOB) on vertical drop maintains pipe centreline alignment.

Valve closed rapidly to isolate a section in an emergency. Must be reachable within seconds from a safe position. Access route must be clear in normal operation and when wearing full PPE including breathing apparatus.

90° elbow with centreline radius of 1.5 × pipe nominal diameter. Standard for all process piping. Lower pressure drop and stress intensification factor than SR elbow. Default choice unless space is specifically identified as critical.

90° elbow with centreline radius of 1.0 × pipe nominal diameter. Higher stress intensification factor than LR. Avoid unless space is critically constrained and stress analysis confirms acceptability.

A project document listing all equipment items with tag numbers, descriptions, sizes, design conditions, and vendor information. The designer uses it to confirm equipment dimensions and nozzle schedules before routing.

Internal pipe coating applied to inhibit corrosion or contamination. Used in water service, food-grade, and some chemical services. Limits available bore and must be considered in velocity calculations.

System that automatically shuts down a process unit or equipment on detection of a dangerous condition. ESD valves are actuated valves that fail-close (or fail-open) on ESD signal. Design routing to ensure all ESD valves are accessible.

A U-shaped section of pipe inserted in a long straight run to absorb thermal expansion. The loop acts as a flexible element. Size and geometry determined by stress engineer. Coordinate early — loops need significant space.

The dimension between the two sealing faces of a valve or fitting. Per ASME B16.10 for valves. Critical for isometric dimensioning and spool fabrication. Control valve F-F varies by manufacturer — always request vendor data sheet.

Testing performed on equipment or valves at the manufacturer's facility before shipment. Opportunity to identify defects before equipment reaches site. Witness tests defined in procurement specification.

Early project phase focused on defining scope, selecting process design, and establishing the basis for detailed engineering. Piping input: plot plan, equipment list, preliminary P&IDs, and sizing studies. Quality of FEL determines project outcome.

A weld made on-site during construction, as opposed to in the fabrication shop. Higher cost, slower, and more difficult to quality-control than shop welds. Design routing to minimise field welds — maximise shop fabrication.

Assessment of whether a degraded or damaged piping component can continue to operate safely. Performed per API 579/ASME FFS-1. Considers actual measured condition versus original design.

A projecting rim with bolt holes, used to connect pipes, valves, and equipment. Types: weld neck, slip-on, blind, socket weld, threaded, lap joint. Class rating (150 to 2500) determines pressure-temperature capability.

Pressure rating classification for flanges per ASME B16.5: 150, 300, 600, 900, 1500, 2500. Higher class = higher allowable pressure at the same temperature. Class selection depends on design pressure, temperature, and material group.

Calculation of thermal expansion stresses and nozzle loads in a piping system. Required by B31.3 for all but the simplest systems. Performed using Caesar II or equivalent. Result determines support types, anchor locations, and loop sizes.

Vibration caused by fluid flow — turbulence, vortex shedding, or acoustic resonance. Common in high-velocity gas lines, compressor discharge, and lines with control valves. Requires dynamic analysis for critical applications.

Combustion exhaust from fired heaters, boilers, or incinerators. High temperature, potentially corrosive. Routing of flue gas piping must account for high thermal expansion and corrosion allowance.

The unsupported length of pipe between two supports. Maximum span governed by allowable sag, stress, and natural frequency. Exceeding maximum span causes excessive deflection, vibration, and potential fatigue failure.

Designation on an isometric drawing indicating a weld to be made in the field, not in the fabrication shop. Shown as a circle around the weld symbol on the iso. Minimise FW count — each one adds time, cost, and quality risk.

In European/process plant practice: the detailed piping plan drawing showing routing, dimensions, and elevations at a specific level. Multiple GAs cover a unit at different elevations. Source document from which isos are extracted.

Electrochemical corrosion occurring when two dissimilar metals are in electrical contact in the presence of an electrolyte. The more anodic metal corrodes preferentially. Prevent with isolation flanges, dielectric unions, or compatible material selection.

A sealing element compressed between two flange faces. Single-use — never reuse. Type depends on service: spiral-wound (general process), ring-type joint RTJ (high pressure/temperature), full-face (cast iron flanges, low pressure).

Full-bore, bidirectional isolation valve. Wedge disc moves perpendicular to flow. Not for throttling — disc will vibrate and damage seat. Standard isolation valve for most process services.

Throttling or isolation valve with a disc moving perpendicular to a seat. Higher pressure drop than gate valve. Used for throttling, flow control, and applications requiring frequent operation. Higher Cv loss than ball valve.

Drain system relying on gravity flow to a collection point. Minimum slope typically 1:100. Pipe sizing must ensure self-cleaning velocity. Never route gravity drains upward — a trap of liquid defeats the purpose.

Alphanumeric coordinate system on plant drawings (A1, B3, C4...). Used to locate equipment and piping in communications, TQs, and punch lists. Always use grid references when describing locations.

Pipe support that restrains lateral movement while allowing free axial movement (thermal expansion along the pipe axis). Used in coordination with anchors and stops to direct thermal expansion along a planned path.

Highly toxic and corrosive gas present in many hydrocarbon streams. Above threshold concentrations, requires NACE MR0175/ISO 15156 material selection — hardness limits on all pressure-containing components. Standard carbon steel is often not acceptable.

Systematic technique for identifying hazards in a process design by examining deviations from design intent. Each node reviewed for what happens if flow, temperature, or pressure is too high, too low, reversed, or absent. P&ID must be HAZOP-reviewed before IFC.

Electrical or steam trace applied to a pipe to maintain minimum temperature — freeze protection or viscosity control. Shown on P&ID with HT designation inside insulation symbol. Electrical trace requires area classification check.

Cracking in carbon steel caused by atomic hydrogen absorption from wet H2S environments. Occurs at inclusions in the steel microstructure. Prevented by specifying HIC-resistant steel (NACE MR0103, low sulphur content).

Connection at the highest point in a piping system for releasing trapped gas or air during filling or draining. Every high point needs a vent. Missing vents cause air locks, incomplete draining, and hydrotest failure.

A design issue that cannot be resolved at the time of drawing issue. Area enclosed in a cloud on the drawing, marked HOLD with a description. Never issue For Construction with open holds on safety-critical items.

Making a branch connection on a pressurised, in-service line without shutdown. Requires a full-encirclement saddle, valve, and specialist hot-tap machine. Engineering approval mandatory. Design branch to be mechanically suitable for hot-tap execution.

High-flow water or gas flushing of new piping prior to commissioning to remove construction debris. Velocity typically 1.5× operating velocity. Requires temporary strainer, disposal connection, and flush acceptance criteria.

Pressure test using water at 1.5× design pressure (B31.3) to verify integrity of welds and fittings. Pipe and supports must be designed for water-filled weight. Minimum test temperature must be at least 17°C above nil-ductility transition temperature.

The formal drawing issue status authorising field construction. All engineering is complete, all holds closed, all approvals obtained. Changes after IFC require formal engineering change procedure.

Drawing issue status for client or discipline review. Not authorised for construction. Comments received and incorporated before IFC.

Charpy V-notch impact test measuring fracture toughness at specified low temperature. Required by B31.3 for materials used below their listed minimum temperature. Ensures ductile behaviour during upset conditions.

B31.3 classification defining NDE requirements: Normal (visual + spot RT/UT), Category D (visual only for low-hazard services), Category M (more stringent for lethal service). Higher inspection class = more welds examined.

All measurement and control devices in a process system: transmitters, indicators, controllers, switches, valves. As a piping designer, you model the connections but the instrument engineer specifies the devices and their locations.

Thermal insulation applied to pipe for heat conservation, personnel protection, or freeze protection. Types: mineral wool, calcium silicate, cellular glass (cold service), elastomeric foam. Add insulation thickness to OD when checking clearances.

A 3D-view drawing of a single piping line showing all dimensions, fittings, welds, supports, and bill of materials. The primary fabrication document for pipe spools. Extracted from the 3D model or drawn manually.

A pipe within a pipe, with the annular space carrying a heat transfer medium (steam, hot oil, coolant). Used for highly viscous materials, temperature-sensitive products, or reactive chemicals. Complex to design and maintain.

A factor (E) reducing allowable stress to account for the quality of a longitudinal weld seam in pipe. E = 1.0 for seamless pipe, 0.85 for ERW pipe (depending on NDE performed). Affects wall thickness calculation.

A horizontal or vertical vessel upstream of a compressor or flare system that separates entrained liquid from a gas stream. Critical for compressor protection and flare system integrity. Inlet nozzle often tangential to promote centrifugal separation.

3D survey technique using laser pulses to capture a point cloud of existing plant. The primary tool for brownfield modelling. A point cloud is ground truth — it captures undocumented modifications that drawings miss. Cost is a fraction of one field reroute.

Regulatory programme requiring periodic inspection and repair of fugitive emission sources — valves, flanges, connectors. Drives selection of low-emission valve packing and flange connection types in new design.

P&ID sheet defining all symbols, abbreviations, and line types used in the drawing set. Read on day one of every project. Never assume a symbol means the same as your previous project.

Project database listing every piping line: tag, P&ID reference, design pressure, design temperature, fluid, material, insulation, and pipe spec. The numerical companion to the P&ID. Cross-reference before routing any line.

Pipe with an internal lining of different material — rubber, PTFE, glass, cement, or epoxy. Used when the base pipe material cannot resist the process fluid. Full-bore fittings required to avoid lining damage at connections.

Natural gas cooled to -162°C, liquefying it for storage and transport. Requires cryogenic materials (austenitic stainless or 9% nickel steel), impact testing, and special design for thermal shock.

Drain connection at the lowest accessible point in a piping run. Drain valve size minimum DN 25 (1"). Must allow complete drainage of the line for maintenance or hydrotest water removal.

A pressure-tight access opening in a vessel or column, typically NPS 18 to 24, for personnel entry. Requires minimum 1000mm clear swing space for the door. Never route piping that blocks manway access.

A line on a drawing showing where it is cut and continued on another drawing. Always quotes the reference drawing number. Essential navigation tool when a unit spans multiple drawing sheets.

The maximum pressure at which a piping system or pipeline may be operated on a sustained basis. Determined by the weakest component in the system. For pipelines, calculated per B31.8 or B31.4 based on actual wall thickness and location class.

Maximum gauge pressure at which a pressure vessel or piping component is rated at a specific temperature. Calculated from actual thickness minus mill tolerance, corrosion allowance, and mechanical allowances.

A device preventing fluid leakage along a rotating shaft in a pump or compressor. API 682 defines seal arrangements (Plan 11, 21, 53...). Seal flush plans shown on P&ID. Piping connections must be accessible for maintenance.

Manufacturing dimensional tolerance on pipe wall thickness, typically ±12.5% of nominal. Wall thickness calculation must use minimum wall (nominal minus mill tolerance) when checking pressure containment.

Formal process for documenting, reviewing, and approving any design change after the design basis is set. Bypassing MOC is how small changes become expensive disasters. Every change — however minor — requires written confirmation and MOC if it affects safety, environment, or operability.

Pipe hangers and supports — materials, design, and manufacture. Defines MSS support types 1 through 59 referenced in stress reports and piping specifications. Know types 24 (shoe), 38 (guide), 51 (constant spring), 52 (variable spring), 54 (anchor).

Quantified list of all piping materials: pipe lengths, fittings, flanges, gaskets, bolts. Extracted from model or manually counted from drawings. Accuracy of MTO directly impacts project cost and schedule.

National Association of Corrosion Engineers. NACE MR0175/ISO 15156 governs material selection for sour (H2S) service. Defines hardness limits for all pressure-containing components in wet H2S environments.

Examination of welds and materials without destruction. Methods: RT (radiographic), UT (ultrasonic), MT (magnetic particle), PT (liquid penetrant), VT (visual). Type and percentage required by piping class and B31.3 inspection class.

Mechanical and electrical standards for steam turbines. NEMA SM23 Table 1 defines minimum allowable nozzle forces and moments for steam turbine inlet and exhaust connections. Most vendors provide more specific allowables — always request.

Temperature below which a steel transitions from ductile to brittle fracture behaviour. The minimum design metal temperature (MDMT) must be above the NDTT to prevent brittle fracture. Determined by Charpy impact testing.

B31.3 service classification for most process piping: not lethal, not high-pressure, and within standard material temperature limits. Defines required NDE percentage (random examination).

Imperial designation for pipe size, used in North America and internationally. NPS does not equal actual OD for small bore. NPS 2 has OD 60.3mm, not 50mm. For NPS 14 and above, OD in inches equals NPS number.

Measure of the energy available at pump suction to prevent cavitation. NPSH available (NPSHa) must exceed NPSH required (NPSHr) by the vendor-specified margin. Suction piping routing directly affects NPSHa.

The connecting point between a vessel, exchanger, pump, or other equipment and the piping system. Has allowable load limits (forces and moments) that stress analysis must satisfy. Nozzle orientation from vendor drawing.

Forces and moments imposed by the piping system on equipment nozzles from dead weight, thermal expansion, and dynamic loads. Must not exceed vendor-specified allowables. Excessive loads cause casing distortion, seal failure, or misalignment.

A self-reinforced branch fitting. Weldolet for butt-weld branches, Sockolet for socket-weld, Threadolet for threaded. Use when branch size is less than 50% of header size. Inherently reinforced — typically no additional reinforcing pad required.

The normal, expected pressure during routine operation. Design pressure is set above operating pressure to accommodate pressure surges and upset conditions, typically by 10% or the PSV set pressure.

A flat plate with a precision-bored hole installed in a pipe between orifice flanges. Creates a measurable pressure drop proportional to flow rate. The simplest, most reliable flow measurement element for clean liquids and gases.

The primary engineering document defining all process equipment, piping, instruments, and controls. Produced by process engineering. Piping designer models exactly what the P&ID shows. Check revision every morning before opening the model.

A ring of plate welded around a branch connection opening to restore the pressure-containing cross-sectional area removed by the opening. Required when the header wall is insufficient to self-reinforce the branch per B31.3 Para 304.3.

The piping connected directly to equipment nozzles. Routing of pipeaway is critical — it governs nozzle loads, maintenance access, and support locations. Always route pipeaway before the rest of the line.

A structural steel frame carrying multiple pipes from one area of a plant to another. The main highway of a process unit. Pipes allocated to levels by service type: process lines on main level, utilities above, cable trays on separate level.

Project-specific document defining all allowable materials, ratings, and components for a given service and pressure class. Every pipe, fitting, flange, valve, gasket, and bolt must comply with the applicable piping class.

Overall plant layout drawing showing equipment positions, buildings, roads, pipe racks, battery limits, and safe distances. Scale 1:200 to 1:500. First document to read on any new project.

Field verification of alloy material composition using X-ray fluorescence (XRF) or optical emission spectrometry. Required for all alloy components in critical services to confirm correct material was installed.

The raw output of a laser scan: millions of measured points representing the surfaces of existing plant. Processed into a 3D model or used directly in design software as a reference. Ground truth for brownfield design.

A spring-loaded valve that opens proportionally to overpressure. Used for liquid service. Closes when pressure returns to below set point. Must be mounted vertically. Outlet piping designed for reaction forces.

A spring-loaded valve that opens rapidly at set pressure to protect against overpressure. Used for gas and vapour service. Must be mounted vertically upright. Outlet to closed relief header or atmosphere per area classification.

Fluoropolymer with excellent chemical resistance, used for valve seats, gaskets, lining, and instrument tubing. Temperature limit approximately 230°C. Not suitable for fluorine, molten alkali metals.

Displacement of air (inert gas purge) or process fluid (steam purge) from a piping system before introducing process. Required before commissioning hydrocarbon lines. Design must provide purge connections at high and low points.

Non-destructive examination using X-rays or gamma rays to detect internal weld defects. The standard NDE method for butt welds in process piping. Percentage required varies by piping class and B31.3 inspection class.

A fitting that changes pipe size. Concentric = same centreline. Eccentric = offset centreline. See Eccentric Reducer for pump suction rule.

See Pad. A ring of plate around a branch opening to restore removed pressure-containing area.

The collection of PSVs, PRVs, rupture discs, and the associated collection header and flare or vent system. Design of relief headers requires careful hydraulic analysis — back-pressure on PSV must not prevent it from opening at set pressure.

A significant modification to an existing process unit to change capacity, product quality, or operating conditions. Involves tying new equipment into existing piping. High-risk activity requiring careful as-built verification and shutdown planning.

A pipe that changes elevation — typically from underground to above-grade, or from one pipe rack level to another. Requires careful support design at grade and at the rack to manage vertical thermal movement.

A flange facing type using a metallic ring gasket seated in a groove machined into the flange face. Used for high-pressure, high-temperature service where RF spiral-wound gaskets are not suitable. Requires precise alignment.

A non-reclosing pressure relief device that bursts at a precise set pressure. Single use — replaced after actuation. Often used in series upstream of a PSV to protect the PSV from corrosion or fouling.

A control system dedicated to returning a process to a safe state on detection of a dangerous condition. Independent of the basic process control system (BPCS). SIS components shown with diamond bubble on P&ID.

A connection for withdrawing a representative fluid sample for analysis. Needle valve preferred. Sample cooler often required for hot service. Closed-loop sample system preferred to minimise exposure.

A designation for pipe wall thickness. SCH 40 and SCH 80 most common. Higher number = thicker wall = higher pressure rating = smaller bore. Always confirm schedule from piping class before routing.

Cracking caused by the combined effect of tensile stress, a corrosive environment, and a susceptible material. Austenitic stainless in chloride environments (SCC), carbon steel in amine service (amine cracking), CS in caustic (caustic cracking).

Pipe manufactured without a longitudinal weld seam, formed by piercing a billet. Higher integrity than welded pipe. Joint efficiency E = 1.0. Standard for high-pressure, high-temperature, and critical services.

Classification of safety function reliability: SIL 1 (lowest) to SIL 4 (highest). Determines redundancy, testing frequency, and component quality. Defined by risk analysis. Piping isolation valves may have SIL requirements.

A flange that slides over the pipe end and is fillet-welded front and back. Lower structural integrity than weld neck flange. Not permitted in severe cyclic service or lethal service per B31.3.

A dynamic restraint that is rigid under fast loads (seismic, water hammer, PSV reaction) but free under slow thermal loads. Allows thermal expansion while restraining dynamic loads. Requires dynamic load input from analysis.

A joint where the pipe inserts into a socket in a fitting or flange, and a fillet weld is applied at the outside. Used for NPS 2 and below in most applications. Not permitted in high-purity, crevice-corrosion-sensitive, or radiographically examined services.

Piping systems handling fluids containing H2S above threshold concentration (per NACE MR0175). Requires hardness-controlled materials throughout. Special documentation requirements — Material Test Reports (MTRs) retained for life of plant.

A two-position blind fabricated as two discs connected by a bar. One disc solid (blind), one disc with a hole (open/spade). Flipped to change from flow to isolated. Position clearly indicated on P&ID.

A prefabricated section of piping assembled in a fabrication shop, transported to site, and installed. Good spool design minimises field welds, maximises shop quality, and accounts for site access and lifting constraints.

Iron-chromium alloys with minimum 10.5% chromium. Types in piping: 304/304L (general), 316/316L (better chloride resistance), 321/347 (stabilised for high-temperature), duplex 2205 (high strength, good corrosion resistance).

A pipe support that restrains axial movement while allowing free lateral movement. Used to direct thermal expansion and prevent pipe from moving axially beyond the designed range. Complement to a guide.

See SCR. Cracking driven by the combination of stress and specific corrosive environment.

A device for removing solid particles from a fluid stream. Types: Y-strainer (permanent, small bore), basket strainer (large bore, top-access), duplex strainer (online maintenance). Temporary conical strainer always installed at commissioning.

Tubular Exchanger Manufacturers Association. Defines heat exchanger types by shell, front head, and rear head configuration (e.g. BEM, AEL, NEN). Specifies design, materials, and fabrication tolerances. Required reference for HX specification.

The increase in pipe length and diameter when temperature rises. Carbon steel expands approximately 12mm per 100°C per 10m of pipe. Must be absorbed by flexibility loops, expansion joints, or guided routing to prevent excessive stresses and nozzle loads.

A pressure-tight well installed in a pipe or vessel allowing a temperature sensor to be inserted and removed without depressurising. Must be sized for the pipe bore. Insertion length must reach at least one-third of pipe diameter into the flow.

A mechanical joint using tapered or parallel threads. Used for small bore connections (NPS 2 and below) in non-critical services. Avoid in vibrating services, hydrogen service, and where regular disassembly is required.

The connection of new piping to existing in-service piping. Requires careful planning: isolation procedure, drain, purge sequence, weld procedure, inspection hold points, and construction sequence within the available shutdown window.

The reference elevation for structural steel surfaces. Pipe support base plates sit on TOS. Used for setting pipe elevation above structure. Always confirm TOS datum with structural engineer at project start.

A formal question raised to the client, process engineer, or engineering lead requesting clarification of a technical issue. Raise a TQ for every P&ID/line list discrepancy. Document the question and the response. Never assume — always confirm in writing.

A cylindrical stub welded to a pipe to carry vertical loads at a support point, especially on large-bore high-temperature lines. Avoids point loading on the pipe wall. Same material and welding procedure as parent pipe.

Non-destructive examination using sound waves. Detects internal flaws in welds (phased array UT) and measures wall thickness. Preferred over RT in many applications as it detects planar defects better. No radiation hazard.

A three-piece threaded fitting allowing pipe disassembly without unthreading the pipe itself. Used in small bore instrument and utility connections for easy maintenance access. Not suitable for high-pressure or high-temperature service.

A point in the plant providing access to utilities: water, steam, nitrogen, instrument air, plant air. Used for purging, cleaning, and emergency cooling. Coverage requirements typically specified in project layout philosophy.

A mechanism for operating a valve using power other than manual force. Types: pneumatic diaphragm (most common for control), pneumatic cylinder (higher force), electric motor (MOV, for remote isolation), hydraulic (large bore high-force).

The pressure-containing housing of a valve. Material must match the piping class. End connections (RF flanged, BW, SW, threaded) and face-to-face dimension per applicable standard.

An opening at the highest point in a piping system to release trapped gas or air. Required at every high point. Vent valve required for pressurised systems. Connect to closed system for hazardous fluids.

Dynamic loading on piping from machinery, fluid pulsation, or vortex shedding. Major cause of small-bore connection failures. Special small-bore bracing required near compressors, reciprocating pumps, and high-velocity gas lines.

A hydraulic pressure surge caused by sudden velocity change — rapid valve closure, pump trip, or steam meeting trapped condensate. Can generate forces many times operating pressure. Prevented by slow-closing valves, proper drainage, and surge analysis.

A flange with a tapered hub butt-welded to the pipe. Highest integrity flange type. Required for severe cyclic service, high-temperature/pressure, and lethal service. More expensive than slip-on but superior structural performance.

A drawing or table identifying all weld joints in a system with unique weld numbers, tracking NDE results and inspection records. Required for high-pressure and alloy piping. Maintained through fabrication and construction.

A self-reinforced butt-weld branch fitting. Installed by cutting a hole in the header pipe and welding the weldolet in place. Used for branch sizes less than 50% of header. Inherently reinforced.

See Radiography (RT). Common field term for radiographic testing of welds.

See Area Classification. The designation of hazardous zones around equipment handling flammable materials. Determines electrical equipment specification and cable routing requirements.