A pipeline needs one or more scour valves so it can be drained – usually for maintenance or repair.
When a scour valve is opened, water is expelled from the pipe because of the difference between the pipeline pressure and atmospheric pressure. This can cause a very high flow velocity through the valve.
Consider for example a 400mm pipeline operating at 16bar. If a 100mm scour valve is fitted the velocity through the valve would be more than 30m/s.
Few valves are designed for such high velocity. In one case I know of, wafer butterfly valves were installed as scour valves on a pipeline operating at 16bar. They were destroyed after a single use.
Wedge gate valves.
Traditionally wedge gate valves were used as scour valves. They’re common and fairly robust.
But, they have a groove under the gate which traps dirt, and because scour valves are located at low points in the pipeline this is exactly where dirt tends to accumulate. The trapped dirt makes the valve difficult to open or close. If operators force the valve closed with dirt trapped it can be damaged.
Resilient seated gate valves.
Later resilient seated gate valves (RSVs) were introduced. They have a full flow port without the groove at the bottom so dirt was less likely to be trapped.
But early versions of the valve were prone to having the rubber seal torn because of the high flow velocity. Rusting inside the gate could also lead to the rubber seal being dislodged.
The design was improved to include rubberising the inside of the valve to prevent rust. And the manufacturing process was changed from glueing the seal in to molding it at high temperature and pressure.
This made these valves much more reliable. They should be the obvious choice but memory of the early failures has meant that the take up has been slow.
RSVs are currently not made to handle pressure above 25 bar so the wedge gate valve is more commonly used at higher pressures.
Fixed cone sleeve valves.
South Africa’s water authority – the Department of Water and Sanitation – has long specified the use of fixed cone sleeve valves for scour applications. They’re expensive, but are designed to handle high velocities and tolerate dirt particles often found at low points where scour valves are installed.
Air valves are required on the pipeline, at both sides of the scour valve, to prevent low pressure damage.
One problem to be aware of with an RSV gate valve is that if the valve is open-ended, rats may cause damage to the rubber. This can be resolved by putting a steel end cap on the valve.