In part 1 of this article we looked at the use of check valves to reduce waterhammer. Today we’ll examine at how surge relief valves, air valves and surge tanks can be used to prevent waterhammer damage.
Surge relief valves
Surge relief valves open to allow the high pressure spikes caused by waterhammer to vented, thus protecting the pump, check valve and pipeline. They can either be triggered by low pressure or by high pressure.
- Surge anticipating valves are triggered by low pressure. When a pump trips it creates a low pressure wave. This triggers a self actuating control valve. By the time the return high pressure wave arrives this valve is open and the pressure is released.
- Fast acting relief valves are triggered by the high pressure wave. They open fast enough to vent the pressure before it does any damage. They may require gas actuation to react fast enough.
Air release valves
Air dammages pipelines. It gets into pipleines in many ways:
- Via from open sources and pump stations.
- Through the release of dissolved air due to pressure and temperature changes.
- Incomplete discharge of air during filling of empty pipes.
- Controlled air penetration to prevent negative pressures.
We need to remove air from pipelines because:
- It reduces the capacity leading to higher energy consumption.
- It leads to more corrosion of internal pipe surfaces.
- It causes innacurate flow metering and damage to such metering equipment.
- Sudden changes in velocity will be magnified by air due to its compressibility. This compounds waterhammer problems.
Air can be removed from pipelines by hydraulic and mechanical means.
As long as the liquid is flowing fast enough air will move. You can trap and bleed it at sensible points along the pipeline. The minimum flow velocity necessary to move entrapped air along the pipeline can be calculated using Wisner’s formula*. Check this between air valves on the flattest sections of the pipe.
Mechanical removal usually happens through manually or automatically operated valves located at points where the air would naturally accumulate. Using equal-tees or other trapping mechanisms ensures that the air doesn’t just flow past.
Air valves need to be positioned carefully. We won’t dive into those calculations here but please yell if you need some help. I’ve got a computer program for sizing and positioning air valves.
Surge tanks allow water to be injected into a pipeline at points where low pressure vapour cavities are formed when pumps trip, valves close rapidly or just at low points on the hydraulic gradeline.
The most common formats are:
- Closed and pressurised with air. This requires an installed compressor and instrumentation with high maintenance to ensure continued correct operation.
- Open tanks at low pressure points. These are normally concrete or steel structures with associated problems of maintenance and remote location.
- Bladder tanks. These are by far the most suitable solution as they have a rubber bladder interface between the water and air and do not suffer from high maintenance costs.
A more complete version of this article including formulae and calculation examples is available here.