Parts of a Butterfly Valve – Labeled Diagram

All 12 parts on the diagram

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Body 4

Body Everything else is carried in it. The seat lines its bore and the stem passes through it from top to bottom.
Lugs A lug-style valve is bolted to each pipe flange. Each has its own set of bolts and no nuts. One side of the pipe can then be taken off without disturbing the other.
Top flange (mounting flange) Its bolt pattern follows a standard. So a lever, a gear operator or a powered actuator can be fitted to the same valve.
Wafer body It is installed between two pipe flanges.

Moving parts and operators 6

Disc A quarter turn fully opens or shuts it. Open is edge-on to the flow, shut is square across the pipe. It is the butterfly of the name.
Lever handle It is fitted on the top end of the stem. A quarter turn takes the valve from open to shut.
Stem (shaft) Turning the stem turns the disc. Some makers call it the shaft.
Bushings They isolate the stem from the body. It then turns freely and stays in line.
Gear operator It is a worm gear, which stays where it is left. So the disc can be set part open to throttle the flow.
Position indicator It shows the position of the disc: open, shut or in between.

Seals 2

Seat (liner) When the valve is shut the disc's edge presses into it. It presses all the way round, which makes the seal.
Stem seal They are O-rings on some valves and packing on others.

How this list was compiled: Every name checked on 2026-09-30, a name kept only where two independent sources use it or describe the part: Wikipedia, Butterfly valve (text only), and four valve makers' manuals read as full text on archive.org (https://archive.org/details/manualzilla-id-5808936, https://archive.org/details/manualzilla-id-5940907, https://archive.org/details/manualzilla-id-6881720, https://archive.org/details/manualzilla-id-5687458). No maker is named on the page. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; a schematic of no maker's product, nothing traced. Licence: own work

How it works

A butterfly valve opens and shuts a pipe with a disc that turns a quarter turn on a stem. The drawing shows a lug-style valve from the front, then a valve cut through along its stem.

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A disc on a stem

The disc sits in the middle of the pipe, on a stem that comes out through the top of the body. Turn the stem a quarter turn and the disc goes from edge-on to the flow, open, to square across the pipe, shut, its edge pressed into the rubber seat. Numbers 1 to 6 are on the front view. Numbers 7 to 9 are in the cut-through. Numbers 10 to 12 are on a second valve, with a wafer body and a gear operator.

Lug or wafer? A lug body has threaded ears round it; a wafer body is a plain ring.
Lever or gear? A lever turns the stem directly; a gear operator turns it through a worm gear that holds its position.

Questions and answers

What are the main parts of a butterfly valve?

Four: the body, the disc, the stem and the seat. The body sits between the pipe flanges, the disc closes the pipe, the stem turns the disc, and the seat is the lining the disc seals against. A handle or gear operator sits on top.

What is the difference between a lug and a wafer butterfly valve?

A lug-style body has threaded ears, so it is bolted to each flange separately and one side of the pipe can be disconnected without disturbing the other. A wafer-style body has no threaded ears and is held between the two flanges.

How does a butterfly valve work?

A disc in the middle of the pipe is mounted on a rod. A quarter turn of the rod turns the disc from parallel to the flow, fully open, to across it, fully shut. The disc stays in the flow even when open.

What does the seat do in a butterfly valve?

It makes the seal. On a rubber-lined valve the seat is a soft liner round the inside of the body, and the edge of the closed disc presses into it all the way round.