Sluice gates, weir gates and stoplogs: choosing a water control gate

Water and wastewater plants rely on simple gates to isolate channels, hold water levels and divide flow between process lines. The three most common types are sluice gates (also called penstocks), weir gates and stoplogs. They look alike but do different jobs. A sluice gate lets water pass underneath and is used to shut off or throttle a channel. A weir gate lets water pass over the top and is used to set a level. Stoplogs are stacked in a frame to isolate a structure, usually for maintenance.

At a glance

• Sluice gate / penstock: isolation and flow control in channels, chambers and tank openings.

• Weir gate: level control, flow splitting and decanting, with water flowing over the leaf.

• Stoplog gate: simple, dependable isolation using separate logs placed with a lifting beam.

How each gate works

Gate	How water passes	Main job	Operation
Sluice gate / penstock	Under the leaf as it is raised	Isolation and flow control	Threaded spindle and headstock, by hand wheel or electric actuator
Weir gate	Over the top of the leaf as it is lowered	Setting a level, splitting flow, decanting	Twin spindles linked through gearboxes, by hand wheel or actuator
Stoplogs	Blocked; the level is set by the number of logs	Maintenance isolation, temporary level control	Logs placed or removed with a lifting beam and hoist

Seating and unseating head

When water pushes the gate leaf onto its seat, the gate is said to work under seating head. The water pressure helps the seal. When water pushes the leaf away from its seat, it works under unseating (or off-seating) head, and the frame, wedges and seals must hold the leaf tight against that pressure. Many gates see both, for example where a channel can fill from either side. Tell us the maximum water depth on each side of the gate so we can design for the worst case.

Rising or non-rising spindle

With a rising spindle, the spindle moves up with the gate, so the gate position is easy to see from the operating floor. It needs headroom above the headstock and usually a spindle cover. With a non-rising spindle, the spindle turns in a nut fixed to the gate leaf and does not move up. It needs less headroom, but the threaded part sits in the water, so it suits clean water better than heavily loaded effluent.

Mounting and materials

Gates can be mounted on the face of a wall over an opening, or set into a channel with the frame fixed to the channel sides. A flush bottom seal leaves no step across the channel floor, so solids do not collect at the gate. Frames and leaves are made in coated carbon steel or stainless steel, with rubber seals chosen to suit the water. Seawater, aggressive effluent and frequent operation all affect the choice.

Checklist: what to send us

• Gate type, and the opening width and height

• Maximum water depth on each side of the gate

• Mounting: on a wall face or in a channel

• Height from the gate invert to the operating floor

• Manual or electric operation, and the power supply available

• Acceptable leakage, if your specification sets a limit

• Material and the type of water

• Number of gates

Frequently asked questions

Is a penstock the same as a sluice gate?

Yes. Penstock is the usual name for a sluice gate in treatment plants in the UK, India and many other countries.

Can a weir gate control a level automatically?

Yes. With an electric actuator and a level sensor, the gate can move up or down to hold a set level as the flow changes.

When should I choose stoplogs instead of a gate?

When isolation is needed only now and then, and simplicity and cost matter more than quick operation. Stoplogs are also used as a back-up behind a gate so the gate itself can be maintained.

Will a gate seal completely?

Every gate leaks a little. Rubber seals and accurate frames keep leakage low. If your specification sets a leakage limit, include it in the enquiry so the gate is designed and tested to it.

Gates are made to suit each opening. Send us your details and we will come back with a proposal. Customers in the European Union can also order through our European company, VDT Europa OÜ in Tallinn, Estonia.