Methods for Protection

When selecting the appropriate method for protection of the pipe system several factors have to be considered, e.g. number of pumps in operation, on-off regulation/continuous duty, normal stop/power failure, buckling risk/fatigue problems, risk for clogging etc. Common for all methods of protection is that they must be based on full understanding of how they will affect the system and be dimensioned for each specific case.

Air chamber
An air chamber is a reservoir containing liquid and compressed air connected to the pipeline. When the pressure drops after a pump stop the air chamber discharges liquid into the pipe system. The flow in the pipe is stopped slowly and low pressures are avoided. An air chamber that is too closely connected to pumps and check valve can give rise to valve slamming.

Variable frequency drives (VFD)/Soft Stopper
A variable frequency drive can be used to protect the system from dangerous sub-pressures during normal pump stops by slowly decreasing the rotational speed of the pump(s). In some cases a soft stopper (less costly than a VFD) can be used in the same manner as a VFD but this often requires a more thorough investigation of the system since a soft stopper do not offer the same possibility of continuous speed regulation as the VFD

Common for VFDs/Soft stoppers is that they can not be used to protect the system in the case of power failure

Slow closing valves
The flow is slowly restricted with a valve before the pump is shut off. The flow in the pipe is slowly retarded and low pressures are avoided.

This method is not suitable for protection of the pipe system in case of a power failure since the valve closing procedure must have been completed before the pump is shut off.

Air inlet/release valves
Air inlet/release valves are mounted along the pipelines at locations where sub-pressure occurs, letting air in when the pressure in the pipe falls below a pre-set value. The air that is let in the system must be released again through an air-release valve in order not to create pockets of air at the locations of the pipe system with the highest elevations.

If the air is released too quickly, the liquid column forcing the air out can gain high speed. When all the air is discharged and the liquid column is abruptly stopped, high pressure can arise.

Relief valves
Relief valves are mounted where high pressure occurs. They open and discharge liquid when the pressure is high.

Fly wheel
Installing a fly wheel offers a possibility to increase the rotating system’s moment of inertia and prolongs the time the pump will rotate after shut-off and thus decrease the pressure drop.
This method is only feasible for dry installed pumps but increases the power needed to start the pumps.