I've never trimmed a dry valve nor have I tested or drained a system myself. I'll also admit that for longer than I should have I never investigated the details of a dry valve.
Hopefully with today's post if you've been in that same boat there's enough to better understand the basic components of a dry pipe valve.
How Dry Pipe Valves Function
1. The valve is in the closed position (clapper inside the valve is horizontal) and air pressure keeps the clapper closed.
2. A sprinkler activates, releasing air pressure at the sprinkler.
3. Air pressure within the dry pipe system begins to reduce.
4. After sufficient air pressure is reduced, the water pressure below the clapper becomes greater than the pressure above the clapper, and the valve opens upward.
5. Water flows through the opening in the valve, past the clapper, and into the system.
6. After entering through the valve, the water gravitates towards the opening in the system where the air pressure is being released, and discharges through the sprinkler at the fire interface.
The clapper is the interface between the air and the water within a dry pipe valve.
It's purpose is to remain closed when air pressure is sufficient in the system, and open when the air pressure falls.
The surface area of the clapper is disproportionately larger on the air side than the water side - which is intentional. To hold the clapper closed, the force on the clapper must be greater on the air side than the water side. Since force is pressure x area, a larger surface area on the air side means the air can be kept at a much lower pressure and still keep the clapper assembly closed - often to a ratio of 4:1 or 5:1 (allowing 40 psi of air to 120 psi of water pressure, for instance).
The External Reset Knob shown in this model is used in lieu of priming water, which otherwise sits on top of the clapper and helps distribute pressure across the top interface of the clapper.
The Knob incorporated in this dry valve is depressed to "unlatch" an open clapper, allowing it to reset back into its normal, closed position. It is a convenient function where a user doesn't have to open the face of the valve in order to reset it.
Low Body Drain Valve
The low-body drain valve allows the air-side of the valve to be drained completely with the clapper closed. Since the main drain valve is below the clapper, the clapper must be opened for the main drain to be used to drain the system.
Main Drain & Intermediate Chamber
The main drain is used to drain water from the system, and is located below the clapper assembly.
For dry valves, some models permit the main drain to be used to test the waterflow pressure switch without opening the clapper.
In other models, there's an intermediate chamber between the water and air side which is normally dry. When water enters the intermediate chamber after the valve opens, the waterflow pressure switch senses water pressure and activates. In this configuration, there's usually a ball valve that can be used to test the waterflow pressure switch and fill the intermediate chamber without opening the dry valve.
The water supply pressure gauge and the system air pressure gauge are included to monitor the incoming water supply pressure and the system air pressure. These gauges are usually attached to a gauge test valve with a plug that permits removal and cleaning of the gauge orifice.
There are typically two pressure switches on the dry pipe valve assembly. The first is a pressure switch which monitors the air supply. When the air supply on the system drops to a pre-determined level, this pressure switch will typically send a supervisory signal to the fire alarm control panel, allowing an early warning that the dry valve is about to open and flood the system.
The second pressure switch is included to activate when there is water in the intermediate chamber or is flooding the system. This is typically an alarm signal that also activates fire alarm notification in a building.
Supply Shut-Off Valve
This valve is typically separate from the dry pipe valve, but allows the system to be shut off after a fire has been sufficiently suppressed. It also can be closed when a system needs to be isolated, such as for a modification, repair or remodel.
Outside of the dry valve assembly, an air maintenance device allows the incoming air to be regulated to a preset air pressure and for that pressure to be maintained in the system.
In order to supply the air, or in many cases now nitrogen, an air compressor or nitrogen generator are provided to supply pressured air or nitrogen into the system.
While this valve is only one model, many of the components between dry valves are designed similarly. For further reading and detail on dry valves, I'd recommend reading NFSA's Layout, Detail and Calculation of Fire Sprinkler Systems (2nd Edition by Kenneth E. Isman, P.E.) and any product data for a dry valve you're reviewing, installing, or specifying.
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Joe Meyer, PE, is a Fire Protection Engineer out of St. Louis, Missouri who writes & develops resources for Fire Protection Professionals. See bio here: About