Glossary

Plain-language definitions of API, ISO, and industry terms used across wellhead, subsea, and valve engineering — now covering detailed wellhead sub-components, valve types, choke configurations, BOP elements, and completion/artificial-lift systems, each referenced to the primary API standard that governs its design, fabrication, testing, or operation.

All API standards referenced are subject to revision — verify the current edition at api.org before application. This glossary is a reference aid only and does not substitute for the full requirements of the cited standard.

A · B · C · D · E · F · G · H · I · J · K · L · M · N · O · P · R · S · T · U · V · W

API Standards Quick Reference

Standard	Scope / Subject Matter
API 5CT / 5B	OCTG – Casing, Tubing; Threading & Gauging
API 5DP	Drill Pipe Specification
API 5L	Line Pipe Specification (PSL1 & PSL2)
API 6A	Wellhead and Christmas Tree Equipment (primary wellhead standard)
API 6D	Pipeline and Piping Valves (ball, gate, check, plug)
API 7-1	Rotary Drill Stem Elements (bits, kellys, subs, drill collars)
API 7K	Drilling Equipment (mud pumps, rotary tables, top drives)
API 8C	Drilling and Production Hoisting Equipment
API 11B	Sucker Rods
API 11D1	Packers, Bridge Plugs, and Tubing Plugs
API 11E	Beam Pumping Units
API 11S / 11S1–S8	Electric Submersible Pump Systems
API 11V	Gas Lift Valves and Equipment
API 11W	Hydraulic Jet Pumps
API 14A	Subsurface Safety Valves (SCSSV) – Equipment
API 14B	Subsurface Safety Valves – Installation, Maintenance, Testing
API 16A	Drill-Through Equipment (BOPs, rams, annulars)
API 16C	Choke and Kill Equipment
API 16D	BOP Control Systems (pods, HPU, panels)
API 16Q	Marine Drilling Riser Systems
API 17D	Subsea Christmas Trees and Wellheads
API 17F	Subsea Production Control Systems / Umbilicals
API 17G / 17H	Completion / Workover Risers; ROV Intervention
API 510	Pressure Vessel Inspection Code (in-service)
API 520 / 526 / 527	Pressure Relief Devices – Sizing, Design, Seat Tightness
API 594 / 599	Check Valves / Plug Valves – Design and Testing
API 598	Valve Inspection and Testing
API 609	Butterfly Valves
API 610	Centrifugal Pumps (ISO 13709)
API 617 / 618	Centrifugal / Reciprocating Compressors
API 650 / 653	Welded Steel Tanks – Design & Inspection
API 660	Shell-and-Tube Heat Exchangers
API MPMS Ch. 5 & 14	Petroleum Measurement Standards (Flow Metering)
API Spec 4F	Drilling and Well-Servicing Structures (Derricks)
API RP 2SK / 2I	Offshore Mooring Systems – Design & Inspection
API RP 9A / 9B	Wire Rope Specification and Application
API RP 12J / 12K / 12L	Oilfield Separators, Heaters, and Tank Field Assembly
API RP 14C / 14E	Surface Safety Analysis; Flow Line Design
API RP 53	Well Control Equipment (BOP installation and testing)
API RP 67	Wireline Well Service Operations
API RP 579	Fitness-for-Service Evaluation
API RP 1102 / 1110	Pipeline Crossings; Pressure Testing Steel Pipelines

Equipment Glossary (A–W)

A

Term	API Standard	Definition
Accumulator (Hydraulic BOP)	API 16A / API 53	A nitrogen pre-charged pressure vessel storing hydraulic fluid at operating pressure to provide emergency power for BOP closure. API 53 requires sufficient accumulator capacity to close all BOP functions and still retain a minimum system pressure of 200 psi above the pre-charge pressure.
Adjustable Choke Valve	API 6A	A variable-orifice choke in which the bean size can be changed while the well is flowing, using a rotating or sliding needle/cage mechanism. Available in hydraulic and manual configurations; rated per API 6A pressure classes up to 20,000 psi working pressure.
Annular BOP (Cylindrical Type)	API 16A	A type of annular BOP using a cylindrical piston and wedge system to compress the packing element radially inward. Common in Hydril GK series. API 16A governs material grades, pressure ratings (2,000–10,000 psi), and bore sizes.
Annular BOP (Spherical Type)	API 16A / API 53	A blowout preventer using a spherical housing to confine and compress the packing element. Manufactured by Cameron (Type D) and others; spherical design provides uniform rubber compression around the drill string. API 16A specifies hydrostatic and function test requirements.
Annular BOP Packing Element	API 16A	The elastomeric donut-shaped sealing element inside an annular BOP. Manufactured from nitrile, neoprene, or natural rubber with embedded steel segments that force the rubber inward to seal. Replaces as a primary wear component and is tested per API 16A acceptance criteria.
Anti-rotation Pin (Wellhead)	API 6A	A pin or key fitted into the casing hanger profile to prevent rotation of the hanger within the wellhead bowl during setting, torquing, or running operations. Required per API 6A hanger design specifications.
API Flange (Ring Joint / RTJ)	API 6A	A pressure-containing flange connection using a soft metal ring (oval or octagonal cross-section) seated in precision-machined grooves on mating flanges. API 6A defines ring numbers (R, RX, BX), material, hardness, and dimensional tolerance for wellhead service.

B

Term	API Standard	Definition
Ball Valve (Floating Ball)	API 6D / API 6A	A quarter-turn valve in which the ball is not rigidly supported at the trunnion but is free to move slightly downstream under line pressure, loading the downstream seat. Standard in lower-pressure pipeline applications per API 6D.
Ball Valve (Trunnion-Mounted)	API 6D / API 6A	A ball valve in which the ball is fixed at top and bottom trunnions; line pressure loads the upstream seat against the ball. Preferred for high-pressure and large-bore pipeline service. API 6D specifies seat-load testing and shell test requirements.
Bean (Fixed Choke)	API 6A	A precisely-machined interchangeable orifice disc or cylinder that restricts flow to a fixed rate. Installed in a choke body; bean sizes are specified in 64ths of an inch. Used for production rate control and back-pressure management.
Blind Flange	API 6A / ASME B16.5	A solid disc used to seal the end of a piping or wellhead connection. API 6A blind flanges are rated to the same pressure class as the matching wellhead component and are hydrostatic-tested before installation.
Blind Ram (BOP)	API 16A / API 53	A BOP ram assembly with solid rubber-faced rams that close across a wellbore with no pipe in the hole, sealing the wellbore completely. Must be distinguished from pipe rams and shear-blind rams in BOP stack configurations per API 53.
Bonnet Assembly (Gate Valve)	API 6A	The pressure-containing closure at the top of a gate valve body through which the stem passes. Includes the stem packing gland, packing rings, stem seal energiser, and bonnet bolts. API 6A requires stem seals to hold full rated working pressure.
BOP Control Unit (Driller’s Panel)	API 53	The hydraulic control system console at the driller’s position that operates BOP functions remotely. Houses pressure gauges, actuating valves, accumulator manifold controls, and position indicators for each BOP component. API 53 specifies response time and redundancy requirements.
BOP Stack (Marine Riser)	API 16A / API 16Q	The complete assembly of annular and ram BOPs, connectors, and kill/choke lines deployed below a floating drilling rig, connected to the wellhead via a low-pressure marine riser. API 16Q covers riser interface requirements; API 16A covers each BOP component.
Breech Block Connector	API 16A / API 17D	A quick-release connector using a rotating outer ring with interrupted threads (breech-lock) to engage mating profiles. Used to connect BOP components and subsea wellhead equipment. API 17D specifies connector qualification testing for subsea service.
Bridge Plug	API 11D1	A downhole plugging device set inside casing to provide a pressure barrier for zone isolation, well abandonment, or zonal testing. Can be permanent (mill-out) or retrievable. API 11D1 specifies temperature/pressure qualification testing and marking requirements.
Butterfly Valve	API 609	A quarter-turn valve using a disc rotating about a central shaft to control or shut off flow. Lightweight, compact, and economical for large-bore, lower-pressure applications. API 609 specifies design, face-to-face dimensions, and testing for butterfly valves in the petroleum industry.

C

Term	API Standard	Definition
Casing Hanger (Mandrel Type)	API 6A	A device that suspends a casing string in the wellhead and provides an annular seal between the casing and the hanger bowl. Mandrel hangers use a tapered shoulder landing in the bowl. API 6A specifies load ratings, seal qualification, and dimensional tolerance.
Casing Hanger (Slip Type)	API 6A	A wellhead hanger using serrated slips that grip the casing OD to support casing weight. The slip assembly locks into the bowl when set. Used where casing is cut and the top is accessible. API 6A governs load capacity and qualification testing.
Casing Hanger Seal Assembly	API 6A	The metal-to-metal or elastomeric seal installed between the casing hanger and the wellhead housing bore to contain annulus pressure. Dual-barrier seal designs use inner primary and outer secondary seals. API 6A mandates pressure qualification to rated working pressure.
Casing Head (Wellhead)	API 6A	The first permanent wellhead component, welded or threaded to the surface (conductor) casing string. Provides the sealing bowl and hanger profile for the next casing string. Includes casing head outlets (side valves) for annulus access. Rated per API 6A pressure-temperature material classes.
Casing Head Outlet Valve (Annulus Valve)	API 6A	A gate or ball valve installed on the casing head side outlets (flanged or threaded) providing access to the A-annulus (casing-tubing annulus) or B-annulus (surface-intermediate casing annulus) for pressure monitoring, bleeding, or fluid injection.
Casing Spool	API 6A	An intermediate wellhead component installed between the casing head and the tubing head that provides the hanger bowl for an intermediate or production casing string, along with side outlets for annulus access. Available in single-studded and double-studded end configurations per API 6A.
Check Valve (Non-Return Valve)	API 6D / API 594	A valve that allows flow in only one direction and closes automatically when flow reverses. Types include swing, lift, ball, and dual-plate. API 6D covers pipeline check valves; API 594 covers wafer-type check valves, specifying design, face-to-face dimensions, and testing.
Choke Body (Manifold)	API 6A / API 16C	The pressure-containing body housing the choke insert (bean or cage). Attached to the choke manifold downstream of the BOP stack kill/choke lines. Must withstand full well shut-in pressure; API 6A governs wellhead-rated bodies; API 16C covers choke and kill manifolds.
Choke Cage (Positive or Adjustable)	API 6A	A precision-machined cylindrical cage with flow ports positioned over a sleeve or plug; the flow area varies with sleeve position. Cage chokes offer better sand and erosion resistance than needle-type chokes. Trim material selection (Tungsten Carbide, Stellite) is critical for abrasive service.
Choke Manifold	API 16C	An assembly of high-pressure valves, choke bodies, gauges, and piping used to control wellbore pressure during well-killing, pressure testing, and production testing. API 16C specifies design, test, and marking requirements for choke and kill systems.
Choke Needle Valve	API 6A	A fine-metering valve using a tapered needle and seat to restrict flow to very small rates during well testing or pressure bleed-down. Needle valves are rated to full API 6A working pressure class of the assembly.
Christmas Tree (Subsea)	API 17D	A subsea wellhead valve assembly installed on the seafloor to control production flow from subsea wells. Dual-bore or mono-bore design; includes production and annulus master valves, crossover valve, and production choke. API 17D specifies design, test pressures, and qualification requirements.
Christmas Tree (Surface)	API 6A	The assembly of valves, spools, fittings, and chokes at the top of the wellhead controlling production flow from one or more zones. Comprises master valves, wing valves, swab valve, and production choke. API 6A governs pressure class, material class, temperature class, and qualification testing.
Christmas Tree Adapter Spool	API 6A	A spacer spool installed between the tubing head and Christmas tree to accommodate different flange sizes, add a downhole safety valve control line access, or provide additional instrument connections. Rated to the same API 6A class as mating components.
Clamp Hub Connector	API 16A / API 17D	A pressure-containing pipe connector using a segmented clamp and ring gasket rather than bolted flanges. Provides quick make-up and break-out for BOP stack components, wellhead connectors, and subsea tree interfaces. API 17D specifies subsea clamp connector qualification.
Control Line (Hydraulic)	API 14A / API 53	Small-diameter hydraulic tubing run from surface to subsurface safety valve (SCSSV) or other downhole control devices to supply operating fluid pressure. API 14A specifies control line OD, wall thickness, and material requirements for SCSSV systems.
Crossover Sub / Adapter	API 6A / API 5B	A short threaded or flanged fitting used to connect components of different sizes, pressure ratings, or thread types. Common in wellhead and Christmas tree assemblies to mate API 6A flanges to API 5B pipe threads.

D

Term	API Standard	Definition
Derrick (Standard / Portable)	API Spec 4F	The structural steel tower erected over a wellbore to support hoisting equipment and the drill string. API Spec 4F provides wind load, hook load, and rotary load ratings plus fabrication requirements. Rated by maximum hook load capacity (e.g., 500,000–2,000,000 lbf).
Diaphragm Valve	API 598 (testing ref)	A valve using a flexible diaphragm to isolate the fluid from the valve body internals, ideal for corrosive or abrasive slurry service. Common in water injection and chemical injection systems on production platforms.
Drill Collar	API Spec 7-1	A thick-walled, heavyweight tubular section of the drill string providing bit weight (WOB) and stiffness in the bottomhole assembly (BHA). API Spec 7-1 specifies OD, ID, connection types, material grade, and magnetic permeability for non-magnetic drill collars.
Drill Pipe (Premium / Standard)	API Spec 5DP / API RP 7G	Hollow steel pipe with flush or upset ends and tool joint connections used to transmit torque and drilling fluid. API Spec 5DP defines grades (E-75, X-95, G-105, S-135), dimensional tolerances, and inspection classes; API RP 7G covers field inspection and retirement criteria.
Drillstring Float Valve	API Spec 7-1	A check valve installed in the drill string (above the bit) to prevent reverse flow of wellbore fluids up the drill pipe when the pumps are off. Available in flapper and ball types; sized to API Spec 7-1 drill collar and bit sub connections.
Dual Completion Wellhead	API 6A	A wellhead configuration permitting simultaneous production from two independent zones through separate tubing strings run side-by-side. Requires a dual tubing hanger and dual-outlet Christmas tree. API 6A governs hanger design and seal qualification for dual completions.

E

Term	API Standard	Definition
Elastomeric Seat (Valve)	API 6D / API 6A	A resilient rubber or PTFE seat in a ball or gate valve providing soft-seated, zero-leakage closure. API 6D Annex B specifies seat leakage testing. Soft seats are used in clean-service pipelines; metal seats are preferred for high-temperature or erosive service.
Electric Submersible Pump (Motor)	API RP 11S4	The downhole induction motor that drives the ESP centrifugal pump stages. Runs at 3,500–3,600 RPM on fixed-frequency power or variable speed with a Variable Speed Drive (VSD). API RP 11S4 covers motor performance testing and winding requirements.
Electric Submersible Pump (Seal Section / Protector)	API RP 11S6	The section between the ESP motor and pump that prevents wellbore fluid ingress to the motor and equalises internal pressure with wellbore pressure. Contains labyrinth chambers and mechanical face seals. API RP 11S6 covers seal performance testing.
Emergency Disconnect System (EDS)	API 16Q / API 17H	An automated system on floating drilling rigs that disconnects the lower marine riser package (LMRP) from the BOP stack in an emergency (e.g., drive-off, loss of position). API 16Q specifies riser disconnect connector requirements; API 17H addresses ROV intervention interfaces.
Emergency Shut-in Valve (ESIV)	API RP 14C	A fail-safe-closed valve installed in the production flow stream that closes automatically upon activation of the Emergency Shutdown System (ESD). Part of the surface safety system; API RP 14C analysis determines required valve placement locations.
Erosion-Resistant Choke Trim	API 6A	Hardened choke internals (beans, cages, seats) manufactured from Tungsten Carbide, Stellite 6, or hardened stainless steel to withstand high-velocity sand-laden flow. API 6A does not mandate trim material but governs the choke body pressure qualification.
Expansion Joint (Wellhead / Riser)	API 16Q / API 2RD	A telescoping pipe joint in a marine drilling or production riser that accommodates vessel heave while maintaining a pressure-containing conduit. Consists of an inner and outer barrel with elastomeric or metal seals. Rated per API 16Q for drilling risers.

F

Term	API Standard	Definition
Fail-Safe Actuator (Spring-Return)	API 6D / API 6A	A valve actuator incorporating a compressed spring that drives the valve to a pre-defined safe position (open or closed) upon loss of instrument air, hydraulic supply, or electrical power. API 6A requires SCSSV actuators to be fail-safe closed.
Fixed Choke (Positive Choke)	API 6A	A choke valve in which the flow-controlling orifice (bean) has a fixed size and cannot be adjusted while the well is flowing. Used in low-risk, stable-flow production wells. Bean replacement requires well shut-in and choke disassembly.
Flapper Valve (SCSSV)	API 14A	The closure element in a subsurface safety valve consisting of a hinged flapper disc retained by a flow tube. When hydraulic control pressure is removed, the flapper spring-closes against the seat. API 14A specifies flapper seat leakage and closure force testing.
Flex Joint (Riser)	API 2RD / API 16Q	An elastomeric ball-and-socket or metal articulated joint installed at the top and bottom of a marine riser to allow angular deflection (±10°–15°) and accommodate vessel motion without bending stress concentration in the riser string.
Flow Cross / Crossover (Christmas Tree)	API 6A	The main body spool in a Christmas tree that distributes wellbore flow to production and test wing outlets. Typically a large-bore forged steel block with integral valve bores machined for the master valves and wing valve seats.
Flow Line Swivel	API 6A / API RP 14E	A rotating connection between the wellhead/Christmas tree and the surface flow line, permitting limited angular movement to prevent flow line bending stress from being imposed on the wellhead. Rated to full working pressure of the connected equipment.
Flow Tube (SCSSV)	API 14A	The cylindrical sliding component inside a tubing-retrievable subsurface safety valve (TRSCSSV) that holds the flapper open when hydraulic pressure is applied. When pressure is lost, the flow tube retracts and the flapper spring-closes.
Forged Body (Wellhead / Valve)	API 6A	A wellhead or valve pressure-containing body manufactured from a solid steel forging rather than a casting or fabricated assembly. API 6A Section 5 requires that pressure-containing parts be forged for rated working pressures above 3,000 psi.
Full-Bore Gate Valve	API 6A / API 6D	A gate valve in which the port through the gate is the same diameter as the pipeline or tubing bore, allowing pigging and wireline tools to pass through without restriction. API 6D specifies full-opening bore dimensions; API 6A governs wellhead gate valves.

G

Term	API Standard	Definition
Gate Valve (Expanding Gate)	API 6A	A gate valve using a two-piece segmented gate that expands outward against both upstream and downstream seats simultaneously when in the closed position, providing a double-block seal. Preferred for wellhead master valves in high-pressure service.
Gate Valve (Slab Gate)	API 6A / API 6D	A linear-motion valve using a flat rectangular gate with a circular port that slides between two parallel seats to open or shut flow. The slab gate provides a metal-to-metal seal and is the most common wellhead valve configuration per API 6A.
Globe Valve	API 6D (reference) / API 623	A linear-motion valve using a plug or disc that seats against an orifice in the flow path; the flow path makes two 90° turns through the body. Provides precise flow control but has high pressure drop. API 623 covers steel globe valves for refinery and petrochemical service.
Grease-Injected Valve Stem Seal	API 6A	A wellhead valve design in which sealant grease can be injected around the stem packing under pressure to restore a leaking stem seal while the valve is in service and the well remains on production. API 6A permits this as an emergency measure only.

H

Term	API Standard	Definition
Hanger (Tubing)	API 6A	A device that suspends the tubing string from the tubing head spool and provides an annular seal between the tubing and the tubing head bore. Mandrel-type hangers use a tapered profile; slip-type hangers grip the tubing OD. API 6A specifies load ratings and seal qualification testing.
Hanger Packoff (Tubing)	API 6A	The seal element installed above the tubing hanger to isolate the A-annulus (tubing-casing annulus) from the Christmas tree bore. Available in metal-to-metal and elastomeric types. API 6A requires packoff qualification to rated working pressure and temperature.
High-Integrity Pressure Protection System (HIPPS)	API RP 14C / IEC 61511 (referenced)	A safety instrumented system (SIS) that prevents overpressure in a downstream system by rapidly closing high-integrity shut-in valves upon detection of abnormal pressure. Designed to SIL (Safety Integrity Level) 2 or 3 per IEC 61511 with API RP 14C application guidance.
Hydraulic Connector (Subsea Wellhead)	API 17D	A remotely operated pressure-containing connector used to land and lock a subsea BOP stack or tree onto a subsea wellhead housing. Incorporates a metal-to-metal seal and a hydraulic locking mechanism operable by ROV or dedicated hydraulic circuit. Qualified per API 17D.
Hydraulic Power Unit (HPU)	API 53 / API 16D	The surface accumulator and pump skid that supplies hydraulic fluid to BOP control lines and sub-sea control systems. API 16D covers the design and testing of BOP control systems (pods and HPU); API 53 specifies minimum accumulator sizing and test pressures.

I

Term	API Standard	Definition
Injection Valve (Chemical)	API 6A / API 14A	A small-bore high-pressure valve used to inject corrosion inhibitors, scale inhibitors, or methanol into the wellhead, flow line, or downhole control line. Includes a check valve to prevent backflow and a metering needle valve for flow rate control.
Integral Flange (Wellhead)	API 6A	A flange forged as a single piece with the wellhead body, eliminating a potential leak path compared to a welded-on flange. API 6A requires that integral flanges meet the same dimensional and pressure qualification requirements as companion flanges.

J

Term	API Standard	Definition
Jet Pump (Surface Hydraulic)	API RP 11W	A hydraulic artificial lift device using high-pressure power fluid directed through a nozzle to entrain reservoir fluid via jet suction through a throat and diffuser, lifting the mixture to surface. API RP 11W covers performance rating, sizing, and installation.
Journal Bearing (Valve Stem)	API 6A / API 6D	A sleeve-type bearing used to support and guide the stem of a gate or ball valve, reducing friction and wear. API 6D specifies stem bearing material requirements; in high-pressure wellhead valves, stem bearings must withstand rated working pressure and mechanical stem loads.

K

Term	API Standard	Definition
Kelly Cock (Upper / Lower)	API Spec 7-1 / API 53	A full-opening ball valve installed above (upper) and below (lower) the kelly in the drill string to shut off flow through the drill pipe. API 53 requires kelly cocks as part of the well control equipment stack. Must operate under full rated pressure.
Kill Line	API 16C / API 53	A high-pressure line from the BOP stack choke/kill outlet to the kill manifold, used to pump weighted kill fluid into the well. API 16C specifies pipe, fittings, valves, and testing for choke and kill systems. Must be rated to full well shut-in pressure.
Kill Manifold	API 16C	The high-pressure piping assembly used to route kill fluid from the mud pump high-pressure line to the kill line and into the well during well control operations. Typically includes isolation valves, a pressure gauge, and connections to the standpipe manifold.

L

Term	API Standard	Definition
Landing Nipple (X / XN / R / No-Go)	API 11D1 / API 14A	A short section of tubing with an internal machined profile for seating wireline-set plugs, standing valves, or safety valves. X-nipples accept selective-set tools; No-Go nipples include a reduced ID to stop selective tools from passing below the intended nipple.
Lock Screw / Lockdown Screw (Wellhead)	API 6A	A threaded grub screw or bolt installed through the wellhead housing body that bears against the casing hanger OD to prevent upward movement (pressure-energised lift) of the hanger during high annulus pressure conditions. Specified in API 6A wellhead design.
Locking Mandrel (Wireline)	API 14A	A downhole locking device that anchors a wireline-retrievable plug or safety valve in a landing nipple profile. Includes collet fingers or dogs that expand into the nipple groove; used to install and retrieve downhole flow control equipment.
Lower Marine Riser Package (LMRP)	API 16Q	The lower portion of a marine riser that connects the drilling BOP stack to the marine riser string. Typically includes an annular BOP, the riser connector, flex joint, and LMRP connector. API 16Q specifies design and test requirements for LMRP components.

M

Term	API Standard	Definition
Master Valve (Lower / Upper)	API 6A	The primary isolation valve in a Christmas tree, installed in the vertical bore directly above the tubing head. Lower master valves are the first valves in the wellbore flow path; upper master valves provide a secondary barrier. Both are full-bore gate or ball valves per API 6A.
Metal-to-Metal Seal (MxM)	API 6A / API 17D	A pressure seal between two precision-machined metallic surfaces, eliminating reliance on elastomers. Used in wellhead casing hanger packoffs, BX ring joint flanges, and subsea connector seals. API 6A requires metal seals to qualify at 1.5× rated working pressure.
Mud Pulse Telemetry (MWD Tool)	API RP 67 (referenced)	A measurement-while-drilling (MWD) sensor package in the BHA that encodes downhole data (direction, inclination, gamma ray, pressures) into pressure pulses in the drilling fluid column, decoded at surface. Not strictly API-governed; API RP 67 covers MWD surface equipment.
Mule Shoe / Guide (Tubing)	API Spec 5CT / API 14A	A bevelled or notched shoe on the bottom of a tubing or casing string that guides the string into the wellbore and prevents hang-up on ledges or casing collars. API 5CT specifies casing shoe dimensions; API 14A references shoe design for SCSSV installation strings.
Multiport Valve (Flow Router)	API 6A	A valve with more than two ports used to direct flow between multiple flow paths, such as routing a well to a test separator or production header. Also known as a diverter valve or production selector valve. API 6A governs pressure-containing bodies.

N

Term	API Standard	Definition
Needle Valve (High Pressure Choke)	API 6A	A choke configuration using a tapered needle advanced into a carbide seat to throttle flow from very high shut-in pressures during well testing or initial flow-back. Tungsten carbide trim is standard for erosion resistance in gas or sandy well streams.
Needle Valve (Instrument)	API 6A / API RP 14C	A precision flow-control valve with a tapered needle and machined seat used for instrument isolation, bleed-down, and chemical injection. Rated to full API 6A working pressure in wellhead service; must be lockable in the closed position for safety critical isolation.
Non-Rising Stem Valve	API 6A	A gate valve design in which the stem threads engage the gate (not the bonnet) and does not move axially during operation; only the gate moves. The compact stem design is preferred where vertical clearance is limited on wellhead assemblies.

O

Term	API Standard	Definition
Offset Wellhead	API 6A	A wellhead configuration in which the casing strings have different nominal ODs requiring adapter spools with offset bore centres. Common in older wells that were re-drilled or deepened using a smaller drilling assembly.
Oilfield Christmas Tree Lubricator	API 6A	A pressure-containing tube installed above the swab valve on the Christmas tree to allow wireline tools and lubricator valves to be pressured up against wellbore pressure before entering the well. API 6A rates the lubricator to match the tree working pressure class.
O-Ring Seal (Wellhead)	API 6A	An elastomeric toroidal ring seal used in wellhead equipment for face and radial sealing in lower-pressure or secondary seal applications. API 6A mandates qualification testing for elastomeric seals; material selection (NBR, HNBR, FKM) depends on service fluid and temperature.
Outline Flange (Test Flange)	API 6A	A temporary blind flange with a central port used to pressure-test wellhead components during assembly and pre-production testing. Rated to the same API 6A pressure class as the assembled wellhead string.

P

Term	API Standard	Definition
Packer (Production / Permanent)	API 11D1	A downhole device set in the casing to support tubing weight and isolate the tubing-casing annulus from production zone pressure. Permanent packers require milling to remove; retrievable packers can be unset with wireline or workover tools. API 11D1 specifies qualification testing.
Packer (Retrievable Hydraulic)	API 11D1	A production packer set by applying hydraulic pressure through the tubing string; can be unset by picking up weight and rotating the tubing to release the setting mechanism. API 11D1 temperature/pressure performance classes govern qualification testing for this type.
Pipe Ram (BOP)	API 16A / API 53	A BOP closing element with a rubber-faced ram body containing a semi-circular opening sized to seal around a specific pipe OD. Available in variable-bore (VBR) versions that seal on a range of pipe sizes. API 16A specifies ram material, seal geometry, and pressure qualification.
Pipe Ram (Variable Bore – VBR)	API 16A / API 53	A ram BOP element incorporating a flexible elastomeric packing that self-adjusts to seal around different drill pipe, tubing, or casing sizes within a defined range. API 53 requires VBRs to be tested at each pipe size within the manufacturer’s rated range.
Plug (Bridge / Wireline Set)	API 11D1	A downhole pressure barrier device set on wireline inside a landing nipple or in open casing. Used for zone isolation, pressure testing above the plug, or well abandonment. API 11D1 specifies performance qualification and marking requirements.
Polished Bore Receptacle (PBR)	API 11D1 / API 14A	A section of tubing or casing with a honed and polished internal bore used to accept a seal assembly or stinger, providing a metal-to-metal or elastomeric seal for completion tie-back strings and subsea completions.
Pressure Gauge (Wellhead)	API RP 14C / ASME B40.100	A mechanical or electronic instrument installed on wellhead spool outlets to display casing/tubing annulus or wellhead bore pressure. API RP 14C defines required gauge locations; ASME B40.100 specifies gauge accuracy class and case material for the hydrocarbon environment.
Production Choke (Wellhead)	API 6A	The choke assembly installed on the production wing outlet of a Christmas tree to control the well’s producing rate, wellhead flowing pressure, and prevent reservoir pressure depletion. API 6A governs the choke body pressure containment; bean or cage sizing determines flow characteristics.
Production Wing Valve	API 6A	The valve on the horizontal side outlet of a Christmas tree flow cross that controls the main production flow path to the flow line. Normally a full-bore gate or ball valve, motor-operated or manual, rated to the same API 6A working pressure as the tree.

R

Term	API Standard	Definition
Ram BOP (Double / Triple Stack)	API 16A / API 53	A BOP configuration housing two or three independent sets of rams in a single body (e.g., pipe ram, variable bore ram, blind/shear ram), reducing riser stack height. API 53 specifies BOP stack configurations for different well types and pressure environments.
Relief Valve (Pilot-Operated)	API 520 / API 526	A pressure relief valve in which a small pilot valve senses system pressure and controls the opening and closing of the main relief valve disc. Provides better set-pressure accuracy and seat tightness than spring-loaded valves in high-pressure wellhead systems.
Ring Gasket (API BX / RX / R Type)	API 6A	A precision-machined soft iron or stainless steel ring seated in grooves of mating API ring-type joint flanges. R-type (oval/octagonal) is standard service; RX is pressure-energised; BX is the self-energised design used in 5,000–20,000 psi wellhead flanges per API 6A.
Rising Stem Valve (RSV)	API 6A / API 6D	A gate valve design in which the stem rises out of the bonnet as the gate opens, providing a visual open/closed indicator. API 6A wellhead master valves are typically rising-stem to allow visual confirmation of valve position from the wellhead deck.
ROV (Remotely Operated Vehicle) Intervention Panel	API 17D	A panel on a subsea Christmas tree or BOP stack with actuator interfaces accessible to an ROV for manual override of hydraulic valve functions when normal control is lost. API 17D specifies ROV torque tool bucket and override valve requirements.

S

Term	API Standard	Definition
Safety Relief Valve (Spring-Loaded)	API 526 / API 520	A self-actuating pressure relief device using a spring-loaded disc to open at a set pressure and close when system pressure drops below the set point. API 526 provides flange-rated valve specifications; API 520 covers sizing calculations for vapor and two-phase relief applications.
Seal Bore Extension (Wellhead)	API 6A	A component added below the tubing hanger or casing hanger to extend the polished seal bore depth, accommodating a deeper packoff installation. Increases seal bearing length, reducing unit stress in higher-pressure wellhead applications.
Shear Ram (Blind Shear Ram – BSR)	API 16A / API 53	A BOP closing element capable of shearing the drill pipe and sealing the wellbore simultaneously. The most critical well control component; must shear through all drill string elements including tool joints in emergency scenarios. API 16A specifies shear testing requirements.
Side Valve (Wellhead)	API 6A	Gate or ball valves installed on the side outlets of casing heads and tubing heads to provide access to the casing-tubing or inter-casing annuli. Used for pressure monitoring, annular fluid injection, and gas lift. Rated per API 6A to the housing working pressure class.
Slips (Casing / Tubing)	API 7K / API Spec 8C	Wedge-shaped, serrated gripping elements used in a rotary table or wellhead bowl to support the weight of casing or tubing while connections are made up. Slip bodies are made of ductile iron or steel; inserts are replaceable. API Spec 8C governs load ratings.
Spacer Spool (Wellhead)	API 6A	A tubular component with flanged ends used between wellhead components to provide additional height for accommodation of BOP equipment, Christmas tree components, or downhole safety valve control line access ports. Rated per API 6A to the assembly pressure class.
Stem (Valve)	API 6A / API 6D	The mechanical linkage transmitting actuating force from the handwheel or actuator to the gate or ball closing element. API 6A requires stems to be blow-out proof (cannot be ejected by wellbore pressure) and the stem-to-bonnet seal must hold rated working pressure.
Stinger / Tie-Back Stinger	API 11D1 / API 17D	A downward-extending mandrel from a completion string or subsea tree that stabs into a polished bore receptacle (PBR) to establish a pressure-containing connection without a threaded joint, allowing angular misalignment tolerance.
Straddle Packer	API 11D1	Two packers connected by a perforated or slotted production tube, isolating a single zone between them for selective stimulation, testing, or injection. Used in multi-zone wells where adjacent zones must be independently accessed.
Studs and Nuts (Wellhead Flange)	API 6A / ASTM A193/A194	The bolting used to join API 6A flanged connections. API 6A specifies stud diameter, length, and required torque for each flange size and pressure class; ASTM A193 Grade B7 studs and ASTM A194 Grade 2H heavy hex nuts are the standard material specification.
Swab Valve (Christmas Tree)	API 6A	The topmost valve on the Christmas tree vertical bore, used primarily to provide isolation at the top of the tree during well intervention operations (wireline, coiled tubing). Normally open during production; closed when attaching a wireline lubricator.
Swivel Joint (Flow Line)	API 6A / API RP 14E	A rotating pipe connector that allows angular and rotational movement between the wellhead outlet and the surface flow line, absorbing thermal expansion and preventing torsional load transfer. Contains high-pressure ball-type or roller bearings and primary and secondary O-ring seals.

T

Term	API Standard	Definition
Test Wing Valve (Christmas Tree)	API 6A	A valve on the secondary (test) side outlet of the Christmas tree flow cross, used to route well fluids to a test separator for production testing while the production wing valve isolates the main flow line. API 6A governs bore size, pressure class, and seat testing.
Torque Anchor (Tubing)	API 11D1 / API RP 5C1	A downhole device that prevents tubing string rotation caused by pump torque reaction in rod-pump wells. Latches into casing perforations or grips the casing ID. Prevents tubing unscrewing and allows higher-torque pump installations.
Tubing Hanger (Mandrel)	API 6A	A mandrel-type wellhead hanger that suspends the production tubing string from the tubing head spool using a tapered shoulder profile. Incorporates control-line and injection-line feed-throughs, a lockdown mechanism, and primary + secondary metal-to-metal seals per API 6A.
Tubing Hanger (Slip)	API 6A	A slip-grip tubing hanger used where the tubing is cut at the tubing head level; slips grip the tubing OD and lock in the bowl. Used in high-angle wells or where the tubing hanger string weight exceeds mandrel-type ratings.
Tubing Head Spool	API 6A	The topmost wellhead pressure housing installed above the casing spool, providing the bowl and landing profile for the tubing hanger, the A-annulus side outlets (valves), and the lower flange that mates to the Christmas tree lower flange. Rated per API 6A pressure and material classes.
Tubing Plug (Through-Bore)	API 11D1	A pressure-containing plug set inside a landing nipple or in the tubing itself to temporarily isolate the well during workover or intervention operations. Through-bore plugs allow passage of smaller tools through the plug ID. API 11D1 covers qualification and performance requirements.
Tubing Retrievable Safety Valve (TRSCSSV)	API 14A	A subsurface safety valve that is part of the production tubing string and requires a workover to retrieve and replace. Provides a large flow bore for maximum production efficiency. API 14A specifies design, qualification, and performance testing requirements.

U

Term	API Standard	Definition
Umbilical Termination Assembly (UTA)	API 17F / API 17D	The end fitting of a subsea umbilical that mates with a subsea tree or manifold flying lead connection panel. Contains individual tube stab connections for hydraulic supply/return lines and electrical connectors. API 17F specifies UTA design and testing.
Unitized Wellhead System	API 6A	A wellhead design in which casing heads, spools, and tubing head are pre-assembled and tested as a single unit before installation, reducing rig time. API 6A governs unitised wellhead qualification as a complete system rather than individual components.

V

Term	API Standard	Definition
Valve Actuator (Electric – MOV)	API 6D	A motor-operated valve (MOV) actuator using an electric motor with a gearbox to drive a valve gate or ball. Available with local and remote control, position feedback, and torque limiting. API 6D specifies functional test requirements for motor actuators on pipeline valves.
Valve Actuator (Hydraulic)	API 6D / API 6A	A double-acting hydraulic cylinder coupled to a gate or ball valve stem/trunnion to open and close the valve remotely using hydraulic fluid. API 6D specifies actuator interface dimensions and override requirements; API 6A requires fail-safe capability for wellhead SCSSVs.
Valve Actuator (Pneumatic / Gas)	API 6D	An actuator using instrument gas or compressed air to operate a valve, typically through a rack-and-pinion or scotch-yoke mechanism for quarter-turn valves. API 6D specifies actuator mounting flange dimensions per ISO 5211 and breakaway torque requirements.
Valve Body (Forged / Cast)	API 6A / API 6D	The main pressure-containing shell of a valve. API 6A mandates forged bodies for wellhead valves above 3,000 psi rated working pressure. API 6D permits cast valve bodies for pipeline valves with appropriate NDE requirements per ASTM A216/A351 or equivalent.
Valve Seat (Metal / Soft)	API 6A / API 6D	The seating surface against which the gate or ball closes to achieve shut-off. Metal seats (Stellite, Inconel) tolerate high temperature and erosive service; soft seats (PTFE, PEEK, nylon) give zero-leakage in clean-fluid service. API 6D Annex B specifies seat leakage test classes (A–D).
Vent Valve (Wellhead)	API 6A / API RP 14C	A valve on the wellhead or Christmas tree used to bleed pressure from the well bore or casing annulus in a controlled manner. Must be rated to full wellhead working pressure and include a downstream restriction to prevent uncontrolled release.

W

Term	API Standard	Definition
Wellhead Housing (High-Pressure / Low-Pressure)	API 6A	The primary structural and pressure-containing tubular body of the wellhead system, welded to the conductor or surface casing. Provides the bowl profiles for casing hangers and the lower flange for the casing head. High-pressure housings (10,000–20,000 psi WP) use H4 or similar connectors per API 6A.
Wing Valve (Production / Test)	API 6A	Gate or ball valves on the horizontal outlets of the Christmas tree flow cross controlling the production and test flow paths. Production wing valves are normally open during production; test wing valves are operated intermittently. Both rated per API 6A to tree working pressure class.
Wire Rope (Drilling Line)	API Spec 9A / API RP 9B	Multi-strand steel wire rope running from the drawworks drum over the crown block sheaves to the travelling block for hoisting the drill string. API Spec 9A covers construction, grade, and breaking strength; API RP 9B provides ton-mile calculation methods for retirement scheduling.
Wireline (Slickline / Braided)	API RP 67	A continuous single-strand (slickline) or multi-strand braided steel wire used to run downhole wireline tools (plugs, gauges, perforating guns) into the well. API RP 67 covers equipment and operations requirements for wireline well service units.
Wye Spool (Christmas Tree)	API 6A	A Y-shaped fitting in a Christmas tree or flow line assembly that splits flow into two outlets or combines two streams into one. API 6A governs the pressure-containing body and flange rating of wye spools used in wellhead assemblies.