Gate valve: how it works, types and pressure ratings

What a gate valve is

A gate valve is a shut-off valve with a gate, usually a wedge, that slides across the bore. Turning the handwheel winds the stem and lifts the wedge out of the way; turning it back drives the wedge down between two seats in the body. When it is open, the water runs straight through, which is why Crane describes its gate valves as the choice where minimum pressure drop matters.

The same design is its limit. A gate valve is built to stand fully open or fully closed. In between, the wedge starts to flutter in the flow, causing turbulence and wear on the wedge, body and sealing faces, as the German valve dealer ACI explains. For regulating flow, a globe valve or another control valve is the right tool.

Wedge, stem and seal: gate valve types

Gate valve types are told apart by three things:

• Wedge. A solid (stiff) wedge is one piece; a flexible wedge has two plates joined by a bridge, which ACI says is most common today because it seals well and is light. Wedge valves seal from both sides, so flow can run either way.
• Stem. On a rising stem valve the stem climbs as the valve opens, so you can see its position from outside; Valve.tr notes non-rising stems suit tight spaces. With a non-rising stem, the threaded part sits inside the body, in the water.
• Seal. Metal-seated valves seal metal on metal. Soft-sealing, or resilient-seated, valves have a wedge fully covered in NBR or EPDM rubber; ACI says they are used mainly in water and wastewater and seal more tightly than a rigid metal wedge.

Brass and bronze gate valves: pressure and temperature

Small gate valves for house and building supply lines are usually threaded brass or bronze. Two British data sheets show what the rating on the body means:

Brass and bronze gate valves: pressure and temperature

	Crane D156	Hattersley 33X
Body	brass (CW617N)	bronze
Rating	PN16, BS EN 12288 Series B	PN20, BS EN 12288 Series B
Pressure at −10 to 100°C	16 bar	20 bar
At high temperature	7 bar at 170°C	9 bar at 180°C
Stem	non-rising	non-rising
Wedge	not stated	solid
Ends	taper thread, ISO 7-1 (formerly BS 21)	taper thread, ISO 7-1; NPT to order
Sizes	¼” to 2” to BS EN 12288	¼” to 4”

Sources: the Crane D156 data sheet and the Hattersley 33X data sheet. The pressure a valve may carry falls as the water gets hotter, so the PN figure is the cold rating. Hattersley’s bronze valve also carries the Kitemark and is WRAS-approved for drinking water; Crane states that the D156 is not for Group 1 gases. Sizes are sold in inches, as people search for them: ¾, 1½ and 2½ inch gate valves all fall within the Hattersley ¼ to 4 inch range, where a ¾ inch valve weighs 0.55 kg and a 2 inch valve 2.55 kg.

The ends of these valves are taper-threaded, so the joint is sealed on the thread with PTFE thread tape or a jointing compound.

Gate valve or ball valve

Many buyers weigh a ball valve against a gate valve for the same job. A ball valve turns a drilled ball through 90 degrees, so a quarter turn of the lever opens or shuts it, and the Wikipedia article on ball valves adds that the handle lies along the pipe when it is open. A gate valve is wound open and shut with a handwheel and shows its position only if it has a rising stem.

Neither is a flow regulator. Crane calls gate valves impractical for throttling, and the Wikipedia article notes that ball valves lack fine control in throttling. Where a line needs fast, visible shut-off, the quarter-turn ball valve is quicker to use; where the valve sits open for years and is closed only for repairs, a full-bore gate valve gives the lowest pressure loss.

Sluice valves on larger pipes

A sluice valve is the same kind of valve on a bigger scale: Cair Euromatic, an Indian valve maker, writes that sluice and gate valves only start or stop the flow, and that sluice valves are generally the size of the main they sit on, up to 300 mm. For water and wastewater, ACI points to the soft-sealing type described above, and Cair Euromatic adds that the term sluice valve is also used for valves in slurry service.

What to check when you buy

• Size and thread. Inch size matching the water pipe, and the thread type: taper ISO 7-1 (BSPT) or NPT.
• Pressure rating. PN16 or PN20 on the body, and the rating at your water temperature.
• Body. Brass or bronze for small threaded valves; for drinking water, an approval such as WRAS.
• Stem. Rising if you need to see whether the valve is open; non-rising where space is short.
• Seal. Resilient-seated for water and wastewater mains where tight shut-off matters.
• Use. A gate valve for full-bore isolation; a ball valve for quick shut-off; neither for regulating flow.