Piping Terms Glossary

Piping design and engineering uses a large number of abbreviations, acronyms and technical terms. This glossary is a practical A–Z reference to the terminology used in piping design, piping engineering, process plants and project delivery. Where an abbreviation has more than one meaning, each meaning is listed under the same term.

Conventions vary between companies and projects; approved project specifications, standards and procedures always take precedence.

A

A: Absolute — see ABS.; Air — used to define the commodity in a line number.; Analog signal — normally indicates the existence and type of signal from an instrument to the control system.; Inlet nozzle — see Note 1.; Pipe anchors — a fixed anchor restricting pipe movement in horizontal and vertical directions.
ABS: Absolute — a unit-of-measure qualifier, as in absolute pressure.
AC: Air Conditioner — equipment designation.; Air to Close — indicates a control valve that is spring-to-open and requires air to close.
ADAPT.: Adaptive control mode.
AE: Architect-Engineer — a firm or project type covering architectural and engineering design, usually without procurement or construction (listed as ‘EA’ in the legacy glossary).
AFC: Approved For Construction — document status indicating that the document has been approved for construction.
AFD: Approved For Design — document status normally used for an early design document such as a P&ID.
A/G: Above Ground — commonly used on a P&ID to indicate a portion of a line that is above ground.
AG: Above Ground — alternative form; see A/G.
AGA: American Gas Association.
AISC: American Institute of Steel Construction.
AISI: American Iron and Steel Institute.
Allowable Stress: Maximum stress the governing code allows for a material at temperature.
ANSI: American National Standards Institute — coordinates and accredits US standards. The B16 and B31 piping standards are written by ASME, so ‘ANSI Class 150’ flanges are now correctly called ASME Class 150.
AO: Air to Open — indicates a control valve that is spring-to-close and requires air to open.
AP: Plant Air — used to define the commodity in a line number.
API: American Petroleum Institute.
AS: Air Supply — used to define the commodity in a line number.; Sprinkler system.
ASA: American Standards Association — historical. Predecessor of ANSI (renamed in the 1960s); still seen on old drawings and specs, e.g. ‘ASA 150’ flanges.
As-Built: Drawings updated to show what was actually installed, usually from site redline mark-ups.
ASME: American Society of Mechanical Engineers.
ASME B16.5: Standard for pipe flanges and flanged fittings NPS ½ to 24.
ASME B16.9: Standard for factory-made wrought butt-welding fittings.
ASME B31.1: Power piping code used for power plants and boiler external piping.
ASME B31.3: Process piping code used for refineries, chemical, pharmaceutical and similar plants.
ASME B36.10M: Dimensional standard for welded and seamless wrought steel pipe.
ASME B36.19M: Dimensional standard for stainless steel pipe (the ‘S’ schedules).
ASME BPVC Section IX: Rules for qualifying welding and brazing procedures and personnel.
ASTM: ASTM International (formerly the American Society for Testing and Materials) — develops material, product and test standards such as A106, A312 and A105.
ATM: Atmosphere.
AVG.: Average.
AWS: American Welding Society.
AWWA: American Water Works Association.

B

B: Outlet nozzle — see Note 1.
BA: Base Anchor — a rigid support connecting piping to a paving or floor surface.
Ball Valve: Quarter-turn valve with a bored ball, giving quick, tight shut-off.
BBE: Beveled Both Ends — an end-preparation qualifier for fittings such as reducers and swedges.
BBL: Barrel — commonly used for plant or tank capacity. A standard oil barrel contains 42 US gallons.
BBP: Bottom of Base Plate — the bottom ring plate of a vertical vessel support or bottom plate of a saddle support.
BC: Bolt Circle — may refer to a flange, vertical-vessel support or manhole.
BD: Blowdown — used to define the commodity in a line number.
BF: Blind / Blank Flange.
BFW: Boiler Feed Water — cleaned, treated and preheated water supplied to a boiler.
BG: Base Guides — supports under low piping designed to permit movement in a specific direction.
BL: Battery Limit — an invisible boundary around a defined portion of a process plant.
BLE: Beveled Large End — an end-preparation qualifier for reducers and swedges.
BLK: Black.
BLVD: Boulevard.
BOF: Bottom of Face of Flange — a reference point used with an elevation.
BOM: Bill of Material — may refer to the material list on an individual drawing or a consolidated material summary.
BOP: Bottom of Pipe — the bottom of a horizontally oriented pipe.
Brownfield: Work on or within an existing operating plant.
BS: British Standard — published by BSI. Many piping standards are now European or international standards adopted in the UK as BS EN or BS EN ISO (e.g. BS EN 1092-1).
BSE: Beveled Small End — an end-preparation qualifier for reducers and swedges.
BTM: Bottom.
BTU: British thermal unit — unit of measure.
Butterfly Valve: Quarter-turn valve with a rotating disc; compact and common on large-bore water and utility lines.
BVLD: Beveled — normally referring to the end preparation of pipe and fittings.
BW: Butt Weld or Butt Welded — a welded joint in which the pipe or component ends are prepared to the applicable weld profile.

C

C: Celsius (°C) — temperature unit; ‘centigrade’ is the older name.; Clean Drain — system qualifier used on P&IDs and other drawings.; Condensate — used to define the commodity in a line number.; Condensate Nozzle — see Note 1.; Cradles — curved metal supports fitted beneath insulated pipe at supports to prevent crushing of insulation.; Patchboard or matrix board connection.
CA: Corrosion Allowance — extra wall thickness to allow for metal loss over the design life.
CAESAR II: Hexagon’s widely used pipe stress analysis software.
C-C: Centre to Centre — dimension measured between the centrelines of two items.
CC: Corrosion Coupon — a piece of metal exposed to process conditions to monitor corrosion.
CDO: Certified Dimension Outline — vendor drawing whose dimensions and connections the vendor has certified after buyer comments; used for final piping design.
CENT: Centigrade — older name for Celsius; see C.
CFM: Cubic Feet per Minute — volumetric flow unit, common for air and gas.
Check Valve: Valve that lets flow pass in one direction only, e.g. swing, lift, dual-plate or piston types.
CHU: Centigrade Heat Unit — obsolete heat unit (heat to raise 1 lb of water by 1 °C).
CI: Cast Iron — brittle iron used for some valves, pumps and drainage; largely replaced by ductile iron.
CL: Centreline — the main reference line through a pipe, nozzle or piece of equipment, used to locate and dimension it (also shown with the ℄ symbol).
Clash Detection: Checking the 3D model for physical interferences between pipes, steel, equipment and other disciplines.
cm: Centimetre.
CO: Chain Operated — valve fitted with a chain wheel so it can be operated from below when out of reach.; Clean Out — access point on underground drainage for rodding and cleaning.
Commissioning: Bringing systems into operation by introducing process fluids and proving performance.
Concentric Reducer: Reducer with both ends on a common centreline, typically used in vertical lines.
Constant Spring Hanger: Support giving nearly constant load over its travel; used where vertical movement is large.
COP: Critical Operating Parameters — key process limits (pressure, temperature, flow) a system must be kept within.
Cr: Chromium — alloying element giving corrosion and oxidation resistance; basis of stainless and chrome-moly steels.
Critical Line: Line meeting criteria (size, temperature, equipment connection) that require formal stress analysis.
CrMo: Chrome-Moly — low-alloy steel with chromium and molybdenum for high-temperature service, e.g. P11, P22, P91.
CS: Carbon Steel — the most common piping material for non-corrosive service, e.g. ASTM A106 Gr. B.; Cold Spring — deliberately fabricating a line short (often by about half its expected thermal growth) so that operating loads and stresses are reduced; see PS.
CSC: Car Seal Closed — valve designation indicating that the valve is to be secured against inadvertent opening.
CSO: Car Seal Open — valve designation indicating that the valve is to be secured in the open position.
CTR: Center.
CU: Cubic.
CV: Control Valve — an actuated valve used to control process conditions; the body may be globe, ball, butterfly, pinch or another type.
CWP: Construction Work Package — a defined segment of project construction work together with its drawings, material lists, specifications and related documentation.
CWR: Cooling Water Return — used to define the commodity in a line number.
CYR: Cycle Water Return — used to define the commodity in a line number.
CYS: Cycle Water Supply — used to define the commodity in a line number.

D

D: Derivative control mode — commonly used with instrumentation.; Digital signal — commonly used with instrumentation.; Drain — used to define the commodity in a line number.; Drain or draw-off connection — see Note 1.
DBB: Double Block and Bleed — two isolation valves with a bleed between them, giving a verifiable isolation.
DBM: Design Basis Memorandum.
DCS: Distributed Control System.
Dead Leg: Section of piping with no flow in normal operation, prone to corrosion, freezing or contamination.
DEG: Degree — unit of measure.
Design Pressure: Pressure a line or component is designed for, set above the maximum expected operating pressure.
Design Temperature: Temperature a line or component is designed for, covering the most severe expected condition.
Detailed Design: Project phase producing construction-ready drawings and documents.
DIA: Diameter.
DIFF: Difference — used mathematically to indicate subtraction.
DIM: Dimension.
DIN: Deutsches Institut für Normung — German standards body. Many DIN piping standards (e.g. the DIN 2630-series flanges) are withdrawn and replaced by European standards such as EN 1092-1, published in Germany as DIN EN.
DIR: Direct-acting.
Displacement Stress Range: Cyclic stress caused by thermal expansion and imposed movements, checked against an allowable range.
DO: Drawing Office.
dP: Delta P — pressure differential.
DRG: Drawing — the usual abbreviation in UK and Irish practice. DWG is more common in North American usage (see DWG).
DS: Dummy Support — a short length of pipe attached to an elbow or other location to provide support where the normal support arrangement is not practical.
Duplex Stainless Steel: Austenitic-ferritic stainless steel with high strength and chloride resistance; super duplex is a higher-alloy grade.
DW: Drywall — gypsum board partition; appears on building and architectural drawings.
DWG: Drawing — the common North American abbreviation; see also DRG.

E

E: East — compass direction, used with plant coordinates.; Voltage signal — instrumentation signal type.
E3D: AVEVA Everything3D — AVEVA’s current 3D plant design system and successor to PDMS.
Eccentric Reducer: Reducer with offset centrelines giving a flat side (see FOT and FOB) to avoid pockets, e.g. on pump suctions.
ECN: Engineering Change Notice — formal document recording, assessing and approving a design change, including its cost and schedule impact.; Engineering Charge Number — project-specific cost code for booking engineering hours.
EDS: Engineering Design Specification — project document setting out design requirements.
E-E: End to End — dimension measured between the ends of an item.
EFW: Electric-Fusion-Welded — pipe made from formed plate with a fusion-welded seam, typically large-diameter pipe.
EL: Elevation — vertical height of a point relative to the plant datum, e.g. EL +105.500.; Elbow — not preferred; see ELL.
ELB: Elbolet — not preferred; see EOL.
ELL: Elbow — fitting that changes the direction of a line, normally 90° or 45°, in long- or short-radius form.
EN 1092-1: European standard for steel flanges with PN designations; replaced many DIN flange standards.
EN 13480: European standard for metallic industrial piping.
EOL: Elbolet — branch outlet fitting shaped to sit on an elbow, giving a reinforced branch, often used for thermowells or vents.
EOP: End of Pipe — dimensioning reference at the open end of a pipe, such as an atmospheric vent.
EPC: Engineering, Procurement & Construction — a project or contracting model covering engineering, procurement and construction.
EPC&C: Engineering, Procurement, Construction and Commissioning.
EPCM: Engineering, Procurement & Construction Management — engineering and procurement with construction executed by subcontractors under management of the EPCM contractor.
ERW: Electric-Resistance-Welded — a method of manufacturing welded pipe.
ES: Electric Supply.
ESD: Emergency Shutdown — system and actions that shut down a plant or section in an emergency; see also ESS.
ESS: Emergency Shutdown System.
ESV: Emergency Shutdown Valve — a valve designated for emergency isolation.
ET: Electrically Traced — indicates heat tracing using electrical resistance cable.
ETA: Estimated Time of Arrival.
Expansion Loop: U-shaped arrangement of pipe that absorbs thermal expansion by bending.

F

(F): Furnished — item supplied with, or as part of, another item such as a vendor package.
F: Fahrenheit (°F) — temperature unit.; Feed nozzle — see Note 1.
FAB: Fabrication — the process of making pipe spools, or a fabricated spool itself.
FAHR: Fahrenheit — see F.
FBW: Furnace-Butt-Welded — older method of making small-bore welded pipe by forge-welding heated strip; e.g. ASTM A53 Type F.
FC: Fail Closed — on loss of power or instrument air, the valve defaults to the closed position.
FCN: Field Change Number — a system for indexing and controlling changes originating in the field.
F&D: Faced and Drilled — flange face machined and bolt holes drilled.
FD: Flex-Disc Valve — a gate-valve design using a flexible disc.
FD&SF: Faced, Drilled and Spot-Faced.
FE: Flanged End.
FEED: Front End Engineering & Design — the early engineering phase establishing the basis and requirements for a project.
FF: Face to Face — dimension between the end faces of a valve or component.; Flange Face — dimensioning reference at the face of a flange.; Flat Face — flange facing that is flat across its full face, used with full-face gaskets, e.g. against cast-iron equipment.; Full Face — gasket covering the full flange face, including the bolt circle.
FFW: Field Fit Weld (also Final Fit Weld) — field weld with extra pipe length (typically 100–150 mm) left on one side for trimming at site; made after other field welds on the line.
FG: Fuel Gas — commodity code in a line number.
FI: Fail Indeterminate — valve with no defined position on loss of power or air (compare FC, FO and FL).
FIV: Fire Isolation Valve — see ESV.
FL: Fail Locked.
Flange Class: Pressure–temperature rating designation of a flange, e.g. Class 150, 300 or 600 to ASME B16.5, or PN ratings to EN 1092-1.
FLG: Flange.
FLGD: Flanged End — alternative abbreviation; see FE.
Flushing: Cleaning lines with water (or blowing with air or steam) to remove debris before start-up.
FMU: Fitting Make Up — two or more standard fittings or flanges welded together without pipe between them.
FO: Fail Open — on loss of power or instrument air, the valve defaults to the open position.
FOB: Flat on Bottom — commonly used with eccentric reducers and other non-concentric components.; Free On Board — Incoterms rule for sea and inland waterway shipments: the seller’s risk ends once the goods are loaded on board the vessel at the named port of shipment.
FOT: Flat on Top — commonly used with eccentric reducers and other non-concentric components.
(F&P): Furnished and Piped — supplied by the vendor with the associated piping already installed.
FP: Full Port — valve whose bore matches the pipe bore; important for pigging and low pressure drop.
FPT: Female Pipe Thread.
FRP: Fibre-Reinforced Plastic — glass-fibre reinforced thermoset piping; see GRP.
FS: Far Side — drawing/dimensioning qualifier, opposite of NS (Near Side).; Field Support — secondary support (such as under a control-valve manifold) often located or fitted in the field.; Forged Steel — steel shaped by forging, used for small-bore fittings, flanges and valve bodies.
FT: Flow Transmitter.
FTG: Fitting — general term for elbows, tees, reducers, olets and similar components.
FV: Full Vacuum — design condition where a line or vessel must withstand complete vacuum.
FW: Field Weld — weld made at site rather than in the fabrication shop; components either side are made to exact dimensions.; Fire Water — commodity code in a line number.

G

g: Gram — SI unit of mass.
G: Gas — used to define the commodity in a line number.; Glycol — used to define the commodity in a line number.; Level Gauge or Gauge Glass.; Pipe Guides.
GA: General Arrangement — drawing showing equipment or piping positions in plan and section; a piping GA (piping plan) shows pipe routing within an area.
GAL: Gallon — unit of measure.
GALV: Galvanized — corrosion-resistant metallic coating.
Gasket: Sealing element compressed between flange faces to prevent leakage.
Gate Valve: Isolation valve with a wedge or slab moving across the flow; meant to be fully open or fully closed.
Globe Valve: Valve with a disc that moves onto a seat, suitable for throttling.
GO: Gear Operated — used with a valve description.
GPH: Gallon per Hour.
GPM: Gallon per Minute.
GR: Grade — commonly used for paved or unpaved plant surfaces.
Greenfield: New facility on an undeveloped site with no existing plant.
GRP: Glass-Reinforced Plastic — glass-fibre reinforced polyester or vinyl ester piping (GRE uses epoxy), used for seawater and corrosive service.
GS: Gas Supply — used to define the commodity in a line number.
GT: Glycol Traced — heat tracing designation.
GV: Gauge Valve — an isolation valve used with instrumentation.

H

H: Hand hole — see Note 1.; Horizontal.; Hot condensate drain — see Note 1.; Hour — see HR (the SI symbol is h).; Hydraulic signal — used to define the commodity in a line number.
HAZOP: Hazard and Operability study — structured, line-by-line P&ID review to identify hazards and operability problems.
HDPE: High-Density Polyethylene — thermoplastic pipe joined by fusion welding, common for buried water and firewater.
HDR: Header — a main supply pipe for a utility or a main branch/header in a piping configuration.
HEX: Hexagonal.
Hg: Mercury — used to define the commodity in a line number.
HH: Hand hole — see Note 1.
HOA: Hand / Off / Auto — commonly used for pump controls.
HP: High Point.; High Pressure.; Horse Power.
HPC: High Pressure Condensate — used to define the commodity in a line number.
HPP: High Point of Finished Pavement — see HP.
HPS: High Pressure Steam — used to define the commodity in a line number.
H. PT.: High Point — see HP.
HPT: Hose-Pipe Thread.
HR: Hanger Rods — a device providing support from above.; Hour — unit of time (SI symbol h).
HS: Hose Station — see US.; Hydraulic Supply — used to define the commodity in a line number.
HTR: Heater — equipment designation.
HVAC: Heating, Ventilation and Air Conditioning — may refer to the discipline, equipment or work.
Hydrostatic Test: Pressure test using water at a code-specified multiple of design pressure (hydrotest).

I

I: Current (electrical) signal.; Inorganic waste — used to define the commodity in a line number.; Interlock.
IA: Instrument Air — used to define the commodity in a line number.
IAS: Instrument Air Supply — used to define the commodity in a line number.
ID: Inside Diameter (also written Internal Diameter) — the bore of a pipe or component.
IE: Invert Elevation — see Inv. El.
IFA: Issued For Approval.
IFB: Issued For Bid.
IFC: Issued For Construction.
IFD: Issued For Design.
IFH: Issued For HAZOP.
IFI: Issued For Information / Issued For Implementation.
IFR: Issued For Review.
IMP: Imperial — units such as feet and inches.
INS: Insulation — non-metallic covering applied to piping or equipment for heat or cold conservation.
INST: Instrumentation.
Instrument Tag: ISA-style code where the first letter is the measured variable and the following letters the function, e.g. FIC-101.
Inv. El.: Invert Elevation — the bottom point inside a drain pipe, used to establish and maintain drainage slope.
IPS: Iron Pipe Size.
IRI: Industrial Risk Insurers — historical. A US property insurer whose fire-protection requirements appear on older specifications; it no longer operates under this name.
IS: Inside Screw — valve-stem arrangement.
ISA: International Society of Automation — publishes instrument symbol and tagging standards (ISA-5.1).
ISBL: Inside Battery Limits — the defined process-unit area within the battery limits.
ISO: Isometric Drawing — a pictorial drawing of a piping line with everything needed to buy, fabricate, test and install it.; International Organization for Standardization — publishes international standards, many adopted as EN ISO / BS EN ISO.
Isogen: Automatic isometric drawing generator used by most 3D plant design systems.
IS&Y: Inside Screw and Yoke — valve design.
ITP: Inspection and Test Plan — document listing inspections, tests and hold/witness points.

J

J: Pump-out nozzle — see Note 1.

K

k: Kilo — SI prefix meaning × 1000.
K: Carbonate — commodity code in a line number.
kg: Kilogram — SI unit of mass.

L

L: Level instrument nozzle — see Note 1.; Liquid — process qualifier, e.g. HLL = High Liquid Level.
LA: Level Alarm — instrument function.
Laser Scanning: 3D survey of existing plant producing a point cloud used as a background for brownfield design.
lb: Pound — unit of mass/weight.
LC: Lock Closed — valve designation indicating the valve is to remain closed unless authorised otherwise.
LG: Level Gauge.
LI: Level Indicator.
LIC: Level Indicating Controller.
Line List: Register of every line with size, service, class, design/operating/test conditions and insulation (also called a line designation table).
Line Number: Unique identifier for a line, typically combining size, service code, sequence number, pipe class and insulation code.
Line Stop: Support that prevents movement along the pipe axis while allowing other movement.
LL: Liquid Level — used with prefixes such as H, L, HH and LL, e.g. HLL (High Liquid Level), LLLL (Low-Low Liquid Level).
LLI: Long Lead Item — equipment or material with a long delivery time that must be ordered early.
LO: Lock Open — valve designation indicating the valve is to remain open unless authorised otherwise.
LOL: Low Oxygen Level.
LP: Low Point — see L.PT.; Low Pressure.
LPC: Low Pressure Condensate — used to define the commodity in a line number.
LPS: Low Pressure Steam — used to define the commodity in a line number.
L.PT.: Low Point — critical low elevation, e.g. on paving or a drained line (opposite of HP, High Point).
LR: Long Radius — for an elbow, normally 1.5 times nominal diameter.
LS: Level Switch.
LSTK: Lump Sum Turnkey — project/contract model including engineering, procurement, construction and commissioning.
LT: Level Transmitter.; Light-wall — used with pipe.
LTCS: Low-Temperature Carbon Steel — impact-tested carbon steel for low design temperatures, e.g. ASTM A333 Gr. 6.

M

m: Metre — SI unit of length.
M: Manhole — see Note 1.; Mega — SI prefix meaning × 1 000 000.; Motor actuator / Motorised — valve with an electric motor operator.
MACH: Machined.
Maintenance Clearance: Free space kept around equipment, valves and instruments for operation, removal and maintenance access.
MATL: Material.
MAWP: Maximum Allowable Working Pressure — highest pressure a vessel, component or system may operate at for a given temperature.
MAX: Maximum.; Maximum control mode — instrumentation.
M/C: Machine.
MC: Mechanical Completion — stage where a system is installed and tested as designed, ready for commissioning.
MCC: Motor Control Centre — electrical assembly or room housing motor starters and feeders.
MFR: Manufacturer.
MI: Malleable Iron — heat-treated cast iron used for small screwed fittings.
MIN: Minimum control mode.; Minute — unit of time.
mm: Millimetre.
Mo: Molybdenum — an element found in stainless and alloy steels.
MOC: Management of Change — formal process for reviewing and approving changes to design or operations.
Model Review: Formal staged review of the 3D model (e.g. 30/60/90%) by the client and disciplines.
MOS: Maintenance Override Switch.
MP: Medium Pressure.
MPC: Medium Pressure Condensate — used to define the commodity in a line number.
MPS: Medium Pressure Steam — used to define the commodity in a line number.
MPT: Male Pipe Thread.
MR: Material Requisition — technical document defining items to be bought, sent to vendors for quotation.
MS: Mild Steel.
MSF: Mill Scale Free — material that has been or must be cleaned of mill scale.
MSS: Manufacturers Standardization Society of the Valve and Fittings Industry — publishes the MSS SP standards for valves, fittings, flanges and pipe supports (e.g. SP-58).
MT: Magnetic Particle Testing — detects surface and near-surface defects in ferromagnetic materials; see NDE.
MTC: Mill Test Certificate — manufacturer’s certificate of chemical and mechanical properties, often to EN 10204 type 3.1.
MTO: Material Take-Off — the process, and the resulting list, of quantifying piping materials from drawings or the 3D model for procurement.
MW: Manway — see Note 1.

N

N: Nitrogen — commodity code in a line number.; North — compass direction used with plant coordinates; may be true, plant, platform or magnetic north.; Reboiler connection — see Note 1.
Navisworks: Autodesk model review and clash detection tool.
NC: Normally Closed — normal position of a valve as shown on the P&ID.
NDE: Non-Destructive Examination — inspection methods that don’t damage the part, mainly VT, PT, MT, RT and UT (also called NDT).
NEMA: National Electrical Manufacturers Association.
NFPA: National Fire Protection Association.
NG: Natural Gas — used to define the commodity in a line number.
Ni: Nickel.
NIC: Not In Contact.
NIPP: Swaged Nipple.
NLL: Normal Liquid Level.
NNF: Normally No Flow.
NO: Normally Open.
Nozzle Load: Forces and moments applied by piping on equipment nozzles, checked against vendor or code allowables.
NPS: Nominal Pipe Size.
NPSC: Straight pipe thread for pipe couplings.
NPSF: Dryseal straight pipe thread.
NPSH: Net Positive Suction Head.; Straight pipe thread for hose couplings and nipples.
NPSI: Dryseal internal straight pipe thread.
NPSL: Straight pipe thread for locknuts.
NPSM: Straight pipe thread for mechanical joints.
NPT: National Pipe Thread / tapered pipe thread.
NPTF: Dryseal taper pipe thread.
NRS: Non-Rising Stem — valve stem arrangement where the stem turns without moving up, so the handwheel position doesn’t show whether the valve is open or closed; common on buried and space-limited gate valves.
NRV: Non-Return Valve — see Check Valve.
NS: Nitrogen Supply — used to define the commodity in a line number.; Near Side — drawing/dimensioning qualifier, the opposite of FS (Far Side).
NV: Needle Valve.

O

O: Electromagnetic or sonic signal — instrumentation signal type.; Oil — commodity code in a line number.; Organic Waste Drain — commodity code in a line number.
OD: Outside Diameter — external diameter of pipe; for NPS 14 and above it equals the nominal size in inches.
Offsite: Facilities outside the process units; see OSBL.
OG: Oxygen — commodity code in a line number.
Olet: Generic name for integrally reinforced branch outlet fittings welded onto a run pipe (weldolet, sockolet, threadolet, elbolet, etc.).
Onsite: Process unit areas; see ISBL.
OP: Operating Valve — valve used in normal operation, as opposed to isolation-only.
OPT: Optimizing control mode — instrumentation.
Orifice Plate: Plate with a bore, installed between orifice flanges, that measures flow by differential pressure.
OS: Outside Screw — valve stem with its thread outside the body.
OSBL: Outside Battery Limits — offsite facilities such as tank farms, utilities and interconnecting pipe racks; counterpart of ISBL.
OSHA: Occupational Safety and Health Administration — US federal workplace safety regulator.
OS&Y: Outside Screw and Yoke — valve stem arrangement with the thread outside the body, so the stem position shows whether it is open or closed.
OVHD: Overhead — used to describe equipment, nozzles or process streams located overhead.
OWD: Oily Water Drain — used to define the commodity in a line number.
OX: Oxide.

P

P: Process line — project-specific designation.; Pneumatic signal.; Pressure connection — see Note 1.; Proportional control mode.; Purge or flushing device.
PA: Plant Air Supply — used to define the commodity in a line number.
PBE: Plain Both Ends.
PC: Pressure Controller.; Pumped Condensate — used to define the commodity in a line number.
PCV: Pressure Control Valve.
PDMS: AVEVA PDMS — legacy 3D plant design system, still common on brownfield projects.
PE: Plain End.
PED: Pressure Equipment Directive (2014/68/EU) — EU law covering the design and conformity of pressure equipment, including piping.
PFD: Process Flow Diagram — a preliminary process schematic showing the principal process equipment and streams.
PFI: Pipe Fabrication Institute.
PG: Pilot Gas — used to define the commodity in a line number.
PI: Pressure Indicator.
P&ID: Piping and Instrumentation Diagram — the principal process document defining process equipment, piping, valves, instruments and control requirements.
Pipe Class: A set of piping materials and ratings for a defined service range, identified by a class code in the line number; see PMS.
Pipe Rack: Elevated steel or concrete structure carrying many pipelines (and often cable trays) between areas of a plant.
Pipe Stress Analysis: Engineering check that a piping system’s stresses, loads and displacements are within code limits.
PIV: Post Indicator Valve — commonly used for fire-water isolation.
Plant 3D: AutoCAD Plant 3D — Autodesk plant design tool with P&ID and isometric modules.
PLC: Programmable Logic Controller.
Plot Plan: Scaled site drawing fixing the locations of units, equipment, roads, pipe racks and buildings; the starting point for piping layout.
PMI: Positive Material Identification — on-site test (XRF or OES) confirming the alloy composition of installed items.
PMS: Piping Material Specification — defines the allowed components, materials, ratings and branch tables for each pipe class.
Pneumatic Test: Pressure test using air or nitrogen where water can’t be used; needs extra safety measures because of stored energy.
PO: Pump Out — commodity code in a line number.; Purchase Order — commercial order placed with a supplier.
POE: Plain One End — end-preparation qualifier for nipples and swages.
POS: Point of Support — the location where a pipe is supported.
PQR: Procedure Qualification Record — record of the test weld and results that qualifies a WPS.
PR: Pressure Regulator — self-acting valve holding a set downstream pressure.
Pre-FEED: The concept/feasibility stage before FEED, assessing options, scope and an order-of-magnitude cost to support a funding decision; see FEED.
PRV: Pressure Reducing Valve — a functional description, not a specific valve type or manufacturer.; Pressure Relief Valve — a functional description for a valve that relieves pressure when a defined set pressure is exceeded.
PS: Pipe Support — a general term covering primary and secondary pipe-support devices.; Pre-Spring — setting a deliberate dimensional offset at installation to offset thermal movement; similar to cold spring (see CS).; Process Sewer — used to define the commodity or service in a line number.
PSI: Pound per Square Inch.
PSIA: Pound per Square Inch Absolute.
PSIG: Pound per Square Inch Gauge.
PSV: Pressure Safety Valve — see PRV.
PT: Pressure Tap — may identify a valved and plugged future connection.; Penetrant Testing — liquid dye penetrant inspection for surface-breaking defects; see NDE.; Pressure Transmitter.
PU: Pick-Ups — a type of secondary pipe support using a larger pipe or steel member and U-bolts.
Punch List: List of outstanding items; category A items must be closed before start-up, category B may follow.
Pup: Pup piece — a short length of pipe used between fittings, flanges or other components to achieve a required dimension.
PW: Potable Water — used to define the commodity in a line number.
PWHT: Post Weld Heat Treatment — controlled heating after welding to reduce residual stresses and achieve required metallurgical properties.

R

R: Automatic-reset control mode.; Radius.; Reflux nozzle — see Note 1.; Reset.; Reset of fail-locked device.; Resistance signal.
RED: Reducer.; Reducing.
REF: Reference.
REQ: Requisition — procurement term or document.; Required.
REV: Reverse-acting.
RF: Raised Face — a flange facing arrangement.
RFI: Request for Information.
RFP: Request for Proposal — an invitation to bidders to submit a technical and commercial proposal for goods or services.
RFQ: Request for Quote.
RJ: Ring Joint — see RTJ.
R/L: Random Length — commonly used for purchased pipe; exact lengths depend on the applicable specification.
RPM: Revolutions per Minute.
RS: Rising Stem — valve-stem arrangement.
RT: Reducing Tee.; Radiographic Testing — X-ray or gamma-ray weld inspection; see NDE.
RTD: Resistance Temperature Detector.
RTJ: Ring Type Joint — flange facing/gasket arrangement.
RW: Raw Water — used to define the commodity in a line number.

S

S: Pipe Shoes — supports used under hot insulated piping to raise the pipe above the primary support level and protect insulation.; Solenoid actuator.; South.; Steam or sample connection — see Note 1.; Storm Water Drain — used to define the commodity in a line number.
SAE: Society of Automotive Engineers.
SC: Sample Connection.; Steam Condensate — used to define the commodity in a line number.
SCH: Schedule — pipe wall thickness designation, e.g. SCH 40, SCH 80, SCH 10S.
SCRD: Screwed — threaded joining method for small-bore pipe, fittings and valves.
SCV: Sample Connection Valve — valve at a sample point.
SCV/T: Sample Connection Valve with Thief — sample valve fitted with a sample thief/collector.
SD: Shutdown.
SDV: Shutdown Valve — automated valve that closes (or opens) fully on a safety signal.
SECT: Section — a cut view on a drawing.
SF: Spot-Faced — machined flat area on the back of a flange so nuts or bolt heads sit square.
S.G.: Specific Gravity.
Si: Silicon — alloying element in steels and cast irons.
SIF: Stress Intensification Factor — multiplier for fittings and branches that accounts for local stress concentration.
SIL: Safety Integrity Level — rating (1 to 4) of the reliability required of a safety instrumented function.
SIS: Safety Instrumented System — independent system that brings the process to a safe state when limits are exceeded.
SKT: Socket — socket-end joining method for small-bore pipe, fittings and valves.
SMLS: Seamless — pipe made without a longitudinal weld seam.
SO: Slip-On — flange type that slides over the pipe and is fillet-welded inside and out.; Steam Out — connection used to steam-clean equipment before maintenance.
SOL: Sockolet — integrally reinforced branch outlet fitting for socket-weld connections (a Bonney Forge trade name; see Olet).
Sour Service: Service containing H2S that needs materials resistant to sulphide stress cracking (NACE MR0175 / ISO 15156).
SP: Sample Point.; Set Point.; Standard Practice — MSS terminology.; Steam Pressure.
SP3D: Smart 3D (S3D, formerly SmartPlant 3D) — Hexagon’s database-driven 3D plant design system.
Spec: Specification — a defined set of requirements governing a product, material, system or activity.
Spec Break: Point on a line where the piping material class changes, shown on P&IDs and isometrics.
Spectacle Blind: Figure-8 shaped plate with a solid blank and an open ring, rotated to positively isolate or open a line.
Spiral Wound Gasket: Gasket made of wound metal strip and soft filler with a guide ring; the usual choice for process flanges.
Spool: A contiguous fabricated section of piping represented by an isometric or pipe line; it may range from a small assembly to a larger transportable fabrication.
Spool Drawing: Fabrication drawing of an individual shop spool with cut lengths and weld numbers.
SQ: Square.
SQ.RT: Square Root.
SR: Short Radius — elbow whose centreline radius equals the nominal pipe size (1D), versus 1.5D for long radius.
SS: Sanitary Sewer — used to define the commodity in a line number.; Spring Supports.; Stainless Steel.; Steam Supply — used to define the commodity in a line number.
SSO: Intermittent hot process drain — used to define the commodity in a line number.
SST: Stainless Steel — alternative abbreviation; see SS.
SSV: Soft Seat Valve.
ST: Steam Traced / Steam Tracing — heat tracing using steam tubing run alongside the pipe.; Steam Trap.; Support Trunnions.
STD: Standard.
STM: Steam — used to define the commodity in a line number.
STR: Straight.
Strainer: Device with a screen that removes solids from the flow (Y-type, basket, or temporary cone strainers used to protect pumps at start-up).
STS: Storm Sewer — used to define the commodity in a line number.
Support Standard: Company or project catalogue of standard pipe support types with tags and details.
Sustained Stress: Stress from long-term loads such as pressure and weight.
SW: Socket Welding.
SWG: Swage.; Swaged Nipple.
SWP: Standard Working Pressure.; Steam Working Pressure.

T

T: Temperature.; Temperature connection — see Note 1.; Threaded.; Trap — steam trap.
TB: Traced Body — valve with heat tracing on the body.
TBB: Traced Body and Bonnet — valve with heat tracing on body and bonnet.
TBE: Thread Both Ends — end-preparation qualifier for nipples and swages.
T&C: Threaded and Coupled — pipe supplied threaded both ends with a coupling fitted on one end of each length.
TC: Temperature Controller.; Test Connection.
TE: Threaded End.
Tee: Three-way fitting for a branch connection; equal tees have all three ends the same size (see RT for reducing tee).
TEF: Teflon — trade name for PTFE (polytetrafluoroethylene), used for seats, gaskets and linings.
TEMA: Tubular Exchanger Manufacturers Association — sets mechanical standards for shell-and-tube heat exchangers.
Test Pack: Package of isometrics, P&IDs and records defining a pressure-test boundary and its results.
T/F.F: Tangent to Face of Flange — vessel dimension from tangent line to a nozzle flange face.
TGT: Tangent.
Thermal Expansion: Growth in pipe length with temperature, which must be absorbed by flexibility, loops or expansion joints.
Thermowell: Pressure-retaining sleeve inserted into a pipe to protect a temperature sensor.
TI: Temperature Indicator — local temperature gauge or display.
Tie-in Point: Location where new piping connects to existing piping or equipment, usually numbered on a tie-in list.
TL: Tangent Line — where the straight shell of a vessel meets the curve of the head; a key dimensioning reference.
TLE: Thread Large End — end-preparation qualifier for swage nipples.
TOC: Top of Concrete — elevation of the finished top of a foundation or slab.
TOE: Threaded One End — end-preparation qualifier for nipples and swages.
TOL: Threadolet — branch outlet fitting with a threaded outlet; see Olet.
TOP: Top of Pipe — elevation at the outside top of a horizontal pipe.
TOS: Top of Support.
TPI: Threads per Inch.
TS: Twin Seal — a type of gate valve.
TSE: Threaded Small End — end-preparation qualifier for swage nipples (also written ‘Thread Small End’).
T/T: Tangent to Tangent — length of the straight shell of a vessel between tangent lines.
TT: Temperature Transmitter.
TYP.: Typical.

U

UA: Utility Air — used to define the commodity in a line number.
UDD: Utility Distribution Diagram — a plot-plan-oriented drawing showing utility distribution headers and branches.
UFD: Utility Flow Diagram — similar to a process P&ID but used to define plant utility generation/distribution systems.
U/G: Underground — a type of drawing or buried piping system.
UG: Underground — see U/G.
UNC: Unified Coarse — bolt thread designation.
UNF: Unified Fine — bolt thread designation.
UNO: Unless Noted Otherwise.
UNS: Unified Numbering System — material designation system.
UON: Unless Otherwise Noted.
US: Utility Station — a collection of utility services such as steam, plant air, water and nitrogen.
UT: Ultrasonic Testing — weld and thickness inspection using sound waves; see NDE.

V

V: Valve.; Vanadium — alloying element in some steels, e.g. P91.; Vapour outlet or vent connection — see Note 1.; Vent — commodity code in a line number.; Vertical.
Variable Spring Hanger: Spring support whose load varies slightly with vertical movement; used where thermal movement is moderate.
VB: Vortex Breaker — internal device at a vessel’s pump-suction nozzle that stops vortices forming.
Vert: Vertical.
VT: Visual Testing — visual weld inspection, the basic NDE method; see NDE.

W

W: Relief valve nozzle — see Note 1.; Water — commodity code in a line number.; West — compass direction used with plant coordinates.
W/: With.
Water Hammer: Pressure surge caused by a sudden change in flow velocity, e.g. fast valve closure or pump trip.
WC: Water Column — piping arrangement used for attachment of level or pressure instruments.
WE: Welded End.
WGT: Weight.
WLD: Weld / Welded.
WN: Welding Neck — flange type.
WOG: Water, Oil and Gas.
WOL: Weldolet — integrally reinforced branch outlet fitting for butt-weld connections (a Bonney Forge trade name; see Olet).
WP: Working Point — reference point used on drawings, particularly for sloping piping, to establish coordinates/elevations.; Working Pressure.
WPS: Welding Procedure Specification — qualified written procedure stating how a weld is to be made.
WS: Water Supply — used to define the commodity in a line number.
WT: Weight — alternative abbreviation; see WGT. Also widely used for Wall Thickness of pipe.

X

x: Multiply.
X: Unclassified actuator — used with instruments.
XH: Extra-Heavy — pipe wall designation.
XS: Extra-Strong — see XH.
XV: On-off valve — ISA tag for an automated on/off valve, often used for shutdown duty; see SDV.
XXH: Double Extra Heavy — pipe wall designation.
XXS: Double Extra Strong — see XXH.

Notes

Some single letters are used to show the purpose or function of a nozzle on a vessel (for example A = inlet, B = outlet). These conventions are not universal; always follow the project-specific piping, vessel and instrumentation conventions.

Editorial note: this glossary has been revised and expanded from the original PipingDesigners.com glossary. Project and client standards take precedence where terminology differs.

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