What is water hammer?
Any change of the water flow velocity in a pipeline causes pressure fluctuation (called "Water hammer" or "surge").
As the velocity change is larger and faster- so are the pressure changes. Fast stoppage of high velocity flow may cause dangerous high / low pressure oscillating waves, exceeding the safe operating limits of the pipeline, accessories, pump casings etc.
Sub- atmospheric pressure, common phenomena in the pressure oscillation, may damage the pipeline by cavitation and collapsing of the pipe due to the external pressure.
Common causes of rapid change in flow velocity:
- Start-up/ Shut- down of pumps.
- Fast closure / opening of valves.
- Filling of drained pipe at excessive flow rate.
How can we prevent water hammer?
- By slowing- down the flow variations in the network.
- By relieving returning flow, after pump's shutdown, out of the pipe.
- By preventing fast filling of empty networks.
- By admitting air or water into the pipe, when sub- atmospheric pressure is expected.
DOROT supplies comprehensive solutions for surge-prevention in water systems, based on computer analysis and long-time expertise of their engineers.
The equipment is extremely simple for installation, adjustment and maintenance, proved in numerous large and small systems across the world.
Some of DOROT product descriptions:
Hydraulic Quick- Relief valve (code "QR")
The Dorot "QR" valve Prevents pressure surges caused by pump start, valve closure etc.
The main valve is a direct sealing diaphragm valve activated by line pressure and controlled by an adjustable pilot with large water passages.
The valve is mounted on a "TEE" junction in the network and adjusted to open instantly whenever the network exceeds its pressure rating.
The valve relieves the excess pressure instantly, protecting the network form dangerous pressure surges.
When the network pressure drops to the safe level, the pilot allows the valve to close slowly. The speed of closing is adjustable (unlike spring - loaded safety valves) so that no secondary surge, caused by sudden closure of the valve, will occur.
The "QR" may be added to other automatic control functions (Float valves, for example) whose fast closure may create surges.
Pump control valve (code "BC")
Dorot "BC" valves are designed to prevent pressure surges caused by electric pumps start- up and shut-off procedures.
The main valve is a direct sealing diaphragm valve, activated by the mainline pressure and controlled by the pump control panel.
The valve opens slowly after pump startup, increasing the network flow gradually. When fully open, the energy loss is negligible.
When the "Stop" command is given to the pump, the valve closes slowly so it reduces the network pressure gradually. Only when the valve is fully closed, the pump motor is stopped automatically.
In the event of a power failure, the main valve closes immediately and prevents main line drainage.
Various control functions (pressure and flow restrictions) may be added to this model.
Two- stages opening valve (code "TO")
Dorot "TO" valve is a direct sealing diaphragm valve, activated by the mainline pressure and controlled by a hydraulic pilot valve.
On pump startup or main valve opening the network pressure is extremely low and the flow rate is very high. When all of the air is expelled from the network the flow rate drops suddenly, and a very high water hammer may be generated in the network.
In order to avoid this risk, the "TO" valve allows for adjustment of the filling pace of the network by limiting the initial flow rate. The main valve will open fully only when normal operating conditions have been established in the network.
Deep-well control valve (code "DW")
The Dorot "DW" valve is designed to prevent pressure surges caused by start-up and shutdown of deep well vertical turbine pumps or submersible pumps.
The main valve is a direct-sealing diaphragm valve activated by the pipeline pressure and controlled the pump control panel.
The valve is mounted on a junction on the pump discharge side, upstream of the check valve.
The valve is in "open" command before the pump starts, relieving the initial flow out of the network when the pump is started.
The valve then closes at an adjustable pace, increasing network pressure and flow velocity gradually, until all pump discharge is directed to the pipe system.
Upon shut-off command the valve opens slowly, reducing pressure and velocity of the main at adjustable pace while the pump is still running.
The motor stops automatically when the valve is fully open.
Surge- anticipating valve (code "RE")
The Dorot "RE" valve protects pumping system from the water hammer in the event of instant shut down of the pump (as caused by power failure).
The main valve is a direct sealing diaphragm valve, activated by two "Quick Response" pilots or by a battery- supplied electric command ("RE/EL" version).
The valve is assembled as a Relief Valve on a tee junction downstream of the check valve, near the pump. It opens instantly when the pump shuts-down, remaining open to relieve the returning high-pressure wave.
The valve closes slowly when the network pressure increases to the normal static pressure of the system.
The valve also opens instantly when the network pressure exceeds the preset safe level and closes gradually when the network pressure returns to normal ("QR" function).
In effect the valve is a "Safety Fuse" for any pressurized system.
"Stages" valve closure (code "SP")
Closure of a valve in the downstream end of a long pipeline generates pressure surge, which may exceed the safe operating pressure of the pipe.
As the pipe is longer, or normal flow velocity higher, the closure time of the valve must be extended considerably to avoid dangerous pressure waves oscillating along the pipe.
The standard closure restriction devices, such as nozzles and needle valves, may be insufficient when very long closure time is needed.
The "SP" feature is added to the standard control circuit of float valves, remotely -controlled electric valves and other automatic valves. It senses the upstream pressure while the valve is closing.
When the pressure exceeds the static pressure of the valve location- The SP device stops instantly the closure process.
The closure resumes when pressure drops to normal.
The above- mentioned process generates "stages closure", where the valve adjusts, automatically, its closure pace to the hydraulic characteristics of the network.