Ball valves and butterfly valves both open and close with a quarter turn. On a seawater system (intake, cooling, fire main, ballast), they still do different jobs. The choice depends on pipe size, function (isolation or throttling), mounting, materials and, on board a ship, the applicable rules.
Two designs, one quarter turn
The ball valve
A bored ball turns between two seats. The US Department of Energy (DOE) mechanical science handbook credits it with quick operation, compact size and tight sealing at low torque. In the full-port version, the bore of the ball equals the inside diameter of the pipe, so water flows through unobstructed.
Its weak point is throttling. When partly open, the exposed part of the seat wears quickly under the jet (DOE).
The butterfly valve
A disc, roughly the inside diameter of the pipe, turns on a shaft in the middle of the flow. The DOE highlights savings in weight, space and cost, especially in large sizes, and suitability for large flows at relatively low pressure. It can be held part-open to throttle. In return, the disc stays in the flow even when fully open.
Selection criteria on a seawater system
Pipe size
In the RPM catalogue, ball valves run from DN6 to DN300 and butterfly valves from DN25 to DN1000. Below DN25, the threaded ball valve is therefore the only one of the two: it is the valve screwed onto a small through-hull. In large sizes, a flanged ball valve becomes heavy and bulky, and the butterfly valve takes over (DOE).
Isolation or throttling
To isolate a strainer, a pump or an intake, either valve will do. To throttle, prefer a butterfly valve, operated by a notched lever, a handwheel gearbox or an actuator. Gearboxes and actuators fit onto a mounting flange standardised by ISO 5211 (F05, F07, F10…), shown on the product pages.
Mounting: wafer, lug or flanged
- Wafer: the valve is clamped between two flanges. It is the cheapest arrangement, but it needs a flange on each side: a wafer valve alone cannot close off the end of a line (A-T Controls).
- Lug: each flange is bolted to threaded lugs on the valve body. Downstream pipework can then be removed while the upstream side stays under pressure, within the limit given by the manufacturer.
- Flanged (double-flanged): the stiffest option, and the heaviest.
Ball valves come in threaded, flanged and wafer versions.
Pressure
The valve's nominal pressure (PN) must cover that of the system. In the catalogue, butterfly valves range from PN6 to PN16 (and ANSI 150); some ball valves go to PN40 and beyond.
Materials and seals
Seawater calls for careful material selection: see our guide Bronze, brass, 316 stainless steel or copper-nickel. The Copper Development Association recommends copper-alloy valves in copper-alloy pipework and advises against unprotected ferrous valves in such systems. On the catalogue's butterfly valves, the disc is mostly aluminium bronze; on ECON models, the body is ductile iron (data sheet 5820). Ball valves are available in bronze, stainless steel and DZR brass, among others.
The seal matters as much as the metal. According to the Parker O-Ring Handbook, EPDM handles hot water and steam but not mineral oils or fuels, whereas NBR (nitrile) resists mineral oils and diesel. On a line that may carry hydrocarbons, such as bilge pumping, NBR is the logical choice.
On board a ship: what the rules say
On ships covered by the International Convention on Load Lines, valves and shell fittings for scuppers, inlets and discharges must be of steel, bronze or other approved ductile material; valves of ordinary cast iron are not acceptable (regulation 22). Since 1 January 2020, SOLAS regulation II-1/12 (resolution MSC.421(98)) allows cargo ships to fit a butterfly valve on pipes piercing the collision bulkhead, provided it is suitably supported by a seat or flanges and can be operated from above the freeboard deck.
The ship's classification society then approves the choice: see our guide to 3.1 and 3.2 certificates and type approval and the valves whose product page shows a type approval.
At a glance
| Criterion | Ball valve | Butterfly valve |
|---|---|---|
| Sizes in catalogue | DN6 to DN300 | DN25 to DN1000 |
| Function | Isolation (on/off) | Isolation and throttling |
| Valve open | Unobstructed (full-port version) | Disc in the flow |
| Large sizes | Heavy and bulky | Light and compact |
| Mounting | Threaded, flanged, wafer | Wafer, lug, flanged |
Frequently asked questions
Can a ball valve be used to throttle flow?
It is not recommended: partly open, it wears its seat (DOE). Use a butterfly valve or a control valve instead.
Can a wafer butterfly valve close off the end of a line?
No: it needs a flange on each side. To be able to remove downstream pipework, choose a lug valve and check the pressure rating for that case on its data sheet.
Are butterfly valves accepted on board ships?
Yes, within the rules. Since 2020, SOLAS accepts them on cargo ships for pipes piercing the collision bulkhead, under conditions. In every case, the ship's classification society has the final say.
Which valve goes on a through-hull fitting?
The seawater intake configurator pairs the through-hull with a female-female ball valve of the same size, in a compatible material. See also our guide Choosing a seawater intake and through-hull fitting.
Sources
- U.S. Department of Energy, DOE Fundamentals Handbook — Mechanical Science, vol. 2 (DOE-HDBK-1018/2-93), module 4 "Valves", 1993: energy.gov.
- A-T Controls, technical document DI00036 on dead-end service with butterfly valves: a-tcontrols.com.
- Copper Development Association, Seawater System Design: Guidelines (archived copy, 14/12/2024): web.archive.org.
- Parker Hannifin, Parker O-Ring Handbook (ORD 5700), § 2.2 "Basic O-Ring Elastomers": discover.parker.com.
- IMO, International Convention on Load Lines (1966, 1988 Protocol), regulation 22, as reproduced by the Philippine Maritime Industry Authority: marina.gov.ph.
- IMO, SOLAS regulation II-1/12 as amended by resolution MSC.421(98), Bahamas bulletin published by IRClass: irclass.org.
- ISO 5211:2026, Industrial valves — Part-turn actuator attachments: iso.org.