Glossary
Short definitions of common terms in process piping. For a list of abbreviations, see Common piping abbreviations.
A
Anchor A support that restrains a pipe in all directions, dividing a system into independent sections for stress analysis.
Acoustic-induced vibration (AIV) High-frequency vibration of piping walls caused by noise from high-flow pressure-reducing devices such as relief valves; a fatigue risk at small-bore connections and branches.
B
Battery limit The boundary of a process unit or package, where responsibility or code coverage changes.
Blowdown Rapid depressurisation of equipment to flare, which can cause very low temperatures in the piping.
Branch connection Any connection of a smaller line to a header: tee, weldolet, sockolet or reinforced stub-in.
C
Category D A B31.3 fluid service for nonflammable, nontoxic fluids at low pressure and moderate temperature, with relaxed examination and testing.
Category M A B31.3 fluid service for fluids where a single small exposure can cause serious irreversible harm.
Corrosion allowance Extra wall thickness added for expected metal loss over the design life.
Corrosion under insulation (CUI) External corrosion of pipe under wet insulation.
Critical line list The list of lines that need formal stress analysis.
D
Deadleg A section of piping with no flow, where water and solids collect and corrosion can be severe.
Design pressure / temperature The most severe coincident pressure and temperature a line is designed for.
Duplex stainless steel Stainless steel with a mixed ferrite-austenite structure, combining strength and chloride resistance.
E
Eccentric reducer A reducer with one side flat, used on pump suctions (flat side up) and on racks to keep a constant bottom of pipe.
Expansion loop A U-shaped offset in a line that absorbs thermal growth by bending.
F
Field weld A weld made on site rather than in the fabrication shop.
Fluid service The B31.3 classification of a line that sets examination and testing requirements.
G
GRE Glass-reinforced epoxy pipe, used for seawater, firewater and produced water.
Guide A support that stops sideways movement but allows axial movement.
H
Hazardous area An area where a flammable atmosphere may be present, classified into zones.
Hydrotest A pressure test with water, normally at 1.5 times design pressure in B31.3.
I
Isometric A not-to-scale 3D drawing of a single line, used for fabrication and inspection.
L
Line class See piping class.
Line stop A support that stops axial movement.
M
Mill tolerance The allowed under-thickness of pipe from the mill, usually 12.5% for seamless pipe.
MDMT Minimum design metal temperature.
N
NPS Nominal pipe size; a name, not a measurement, for NPS 12 and below.
NPSH Net positive suction head, the pressure margin above vapour pressure at a pump suction.
Nozzle load The forces and moments that piping puts on an equipment connection.
P
P&ID Piping and instrumentation diagram.
Piping class A pre-approved set of materials and components for a given rating, material and service.
PMI Positive material identification, confirming alloy composition by XRF or OES.
PREN Pitting resistance equivalent number, %Cr + 3.3 × %Mo + 16 × %N.
PWHT Post-weld heat treatment.
R
Rest A support that carries vertical load and allows sliding.
RTJ Ring type joint, a flange facing with a groove for a metal ring gasket.
S
Schedule A designation of pipe wall thickness for a given NPS.
SMYS Specified minimum yield strength.
Sour service Service with enough H2S to cause sulfide stress cracking.
Spec break The point where one piping class changes to another.
Spool A prefabricated piece of piping sized for transport and installation.
Spring support A support that carries weight while allowing vertical movement.
T
Tie-in A connection of new piping to an existing system.
Trunnion A pipe stub welded to an elbow or pipe to support it.
U
Utility station A group of hose connections for air, water, steam and nitrogen in a process area.
V
Vent and drain High-point vents and low-point drains used for testing, filling and emptying a line.
W
Weldolet A forged branch fitting welded onto a header.
WPS Welding procedure specification.