Gate, butterfly or OS&Y? Choosing the isolation valve for a fire main
In a fire system the isolation valve is opened maybe twice in ten years — and both times matter enormously. Choosing between the three families is mostly about who has to see the valve position and how fast it must close.
Gate valve, resilient seated
The default for isolation on a riser or a zone. Full bore when open, so it adds almost no pressure loss to the pump curve. The disc travels slowly, which is what you want on a large main: a fast-closing valve of that size would hammer the line.
It is a poor choice for throttling. A gate valve is meant to be either fully open or fully closed; part-open it and the disc erodes.
Butterfly valve
Quarter turn, compact, and much cheaper than a gate valve above about DN200. Available wafer, lugged or grooved — the grooved pattern drops straight into a grooved line with no flanges to align.
The trade-off is a disc sitting in the flow path and a valve that closes fast enough to need a slow-closing gearbox on large sizes. It is the usual choice for pump room headers and for zone valves where space is tight.
OS&Y (outside screw and yoke)
The stem rises through the handwheel as the valve opens, so anyone walking past can see the position without a gauge or a switch. That visual indication is why many fire codes and insurers ask for it on the main isolation valve.
It is bulky, more expensive, and the exposed stem needs lubrication. You fit it where the authority requires visible position, not because it is better engineering.
What to specify alongside
- Listing. UL/FM or the local approval, stated per valve — not per product family.
- Supervisory switch. A tamper switch that signals the panel when the valve leaves the open position. On butterfly valves it is almost always required.
- Flange drilling. EN 1092-2 PN10/16 or ANSI B16.5 Class 150. Get this wrong and nothing bolts up.
- Operator. Handwheel, gear, or an electric actuator for a remotely controlled zone.
- Trim and seat. EPDM for water; the disc material matters where the water is aggressive.
Which one where
| Riser isolation | Gate valve, or OS&Y where visual position is required |
| Pump room header | Butterfly, grooved or lugged |
| Zone control, tight space | Butterfly with tamper switch |
| Large diameter, DN300 and above | Butterfly on cost, gate on pressure loss |
| Underground, pit | Gate valve with extension stem |
| Throttling or pressure control | Neither — use a pressure reducing valve |
Where we fit
We cast and machine resilient-seated gate valves, flanged and grooved butterfly valves, and pressure reducing valves, in ductile iron, from DN50 to DN1000, drilled to EN or ANSI on request. Send the line list and the standard you are working to.