OS&Y Valve
A fire protection gate valve whose rising stem shows if it is open.
Quick Answer
An OS&Y valve (outside screw and yoke) is a gate valve whose stem rises out of the valve as it opens, so anyone can see its position at a glance. It is used to isolate fire sprinkler and standpipe supplies, and estimators count it each by size, often with a tamper switch priced separately.
The Full Picture
OS&Y stands for outside screw and yoke. The stem threads sit outside the valve body, supported by a yoke, so when the handwheel turns, the stem rises as the gate lifts. A raised stem means the valve is open, and a stem that is down means it is closed. That visible indication is why this valve is used to control water supplies to fire protection piping: a person walking by can tell if the system has been shut off. It is a gate valve, meant to be fully open or fully closed, not to throttle flow.
In a sprinkler or standpipe system, OS&Y valves appear on the main supply to the system, on floor or zone control assemblies, at the base of a riser, and ahead of the fire department connection or backflow assembly. Which locations require them comes from the design and the adopted installation standards, such as NFPA 13 for sprinklers and NFPA 14 for standpipes, and from the local authority. Valves are commonly supervised, meaning a tamper switch reports a closed position to the fire alarm panel, if the spec requires it. The inspection and testing of valves after installation falls under NFPA 25 and the owner's maintenance program.
On the drawings, OS&Y valves show as a gate valve symbol on the fire protection plan or the riser diagram, tagged by size, and often listed in a valve schedule or described in the sprinkler specification. Sizes follow the pipe they serve, running from small zone valves up to large mains of 6 in or more. End connections are flanged, grooved or threaded depending on size and the spec, and an estimator should check which, since it changes the price and the fittings needed to connect it.
The takeoff unit is each, by size and end type, counted from the riser diagram, the plans and the valve schedule. A tamper switch, the supervision wiring, the pipe and fittings that connect the valve, and any valve identification sign are separate lines. Common pricing mistakes are to count only the valves on the plan and miss those shown only on a riser diagram or detail, to price a plain gate valve instead of the listed OS&Y type, or to leave out the switches when the spec calls for supervised valves. Closed valves reduce the water available to a sprinkler system, and OSHA expects an automatic supply that can deliver design flow for 30 minutes, which is one reason valve position is treated as a safety item.
Real Examples
Common Misconceptions
People assume: An OS&Y valve is just another gate valve.
Actually: The rising stem makes it a visible-position valve, and fire protection systems use listed valves for control. A plain gate valve without that feature is not a substitute when the spec calls for OS&Y.
People assume: OS&Y valves can be used to regulate flow.
Actually: A gate valve is intended to be fully open or fully closed. Partially closing it is not a flow-control method for sprinkler supply, and a closed valve takes water away from the system.
People assume: The valve count is the same on every sheet.
Actually: Some valves are shown only on the riser diagram, details or schedule. Check all of them against each other before settling on a quantity.
Does MeltPlan Solve This?
Partially — adjacentThe number of control valves by size is a count taken from the fire protection plans, and MeltPlan's takeoff service covers that counting work: AI pulls quantities from plans and US-based experienced estimators review and adjust them. Confirming which valves the spec requires, supervision and end connections stays with your estimating team.
Get valve counts by size taken off from fire protection plans →Frequently Asked Questions
What does OS&Y stand for?
OS&Y stands for outside screw and yoke. The stem threads are outside the valve body, so the stem rises when the valve opens and shows its position.
Where are OS&Y valves used?
They isolate water supply to fire sprinkler and standpipe systems, including main supply, riser and floor control locations. The design and the adopted standards decide where they are required.
How do you take off an OS&Y valve?
Count each valve by size and end connection from the riser diagram, the fire protection plan and the valve schedule. Tamper switches, connecting pipe and fittings and signs are counted separately.
What is the difference between an OS&Y valve and a butterfly valve?
An OS&Y valve is a gate valve with a rising stem that shows if it is open. A butterfly valve turns a disc, and the position is shown by an indicator in the operator. Both are used for sprinkler control when listed for it, and the spec says which.
Who maintains OS&Y valves after installation?
The building owner does. Inspection and testing are covered by NFPA 25, and for employers, OSHA's sprinkler rule requires them to properly maintain an automatic sprinkler system.