Water Hammer Flashcards
What is water hammer?
A phenomenon in a piping system caused by an abrupt change in flow at some section. The disturbance, which might be caused by changing a valve opening, travels through the system at the speed of a sound wave in the fluid.
What is water hammer in terms of kinetic energy?
If a valve is suddenly closed, a dynamic pressure, in addition to the normal static pressure, is created within the pipe. This dynamic pressure is commonly called water hammer. It is caused by the sudden transformation of kinetic energy to pressure energy
What are 5 causes of water hammer?
Valve-induced water hammer
Void-induced water hammer
Flashing-induced water hammer
Steam-propelled water slug
Condensate-induced water hammer
What happens in Valve-Induced Water Hammer?
-Valve is closed causing the water column velocity to stop - faster closing valves cause greater water hammer effects
-Causes pressure waves to propagate through the water upstream of valve
-Valve closure can also create a void downstream of the valve – when the void collapses a pressure wave propagates downstream of the valve
What happens in Void-Induced Water Hammer?
-Line is refilled after a void has formed – air from suction break, air introduced after drain down or maintenance
-Water fills void until it impacts other water boundary
-Causes pressure waves to propagate in both directions
What happens in Flashing-Induced Water Hammer?
-Pressure drop where the pressure is less than the liquid vapor pressure – reduction in flow area, pump cavitation
-Water turns to steam resulting in two phases
-Propelled slugs of liquid or collapsing steam pocket impacts other water boundary
-Causes pressure waves to propagate in both directions
What happens in Stream-Propelled water slug?
- Water slug (such as collected condensate) is propelled by steam (or other gas) into a system boundary such as an elbow
- Can damage elbow or piping support from impulse
- Avoided by proper condensate (or other liquid) draining and proper system startup
What happens in Condensate-Induced Water Hammer?
-Rapid condensation of steam by subcooled water
- Steam flowing over subcooled water, ripples or waves seal to top of pipe, condensation of steam causes collapse of steam pocket and impact of water boundaries
How is energy absorption used to prevent water hammer?
- Surge tanks (generally provide a gas volume above the fluid or within the fluid separated by a bladder, diaphragm, piston) - Energy is absorbed by compression of the gas.
- Flow out of a fast acting relief valve -(Energy is absorbed by ejecting the fluid through an orifice with the kinetic energy now outside the system.)
- Slow valve closure
- Damage to system by deforming walls of piping - (Shouldn’t be a primary way of dealing with kinetic energy but ductile materials make a fluid system more forgiving)
Application to Steam Systems
- Follow procedures to drain condensate, and startup system
- Maintain critical components such as condensate drains
- Proper selection of components, such as requiring ductile steel instead of cast iron
- Listen for evidences of water hammer and deal quickly with problems (water hammer isn’t acceptable)
Application to Water Systems
- Maintenance of surge suppressors
- ullage gas level for non-bladder suppressors
- precharge pressure, with water drained, for bladder suppressors
- Follow procedures to hard fill prior to Firex checkout or similar procedures
- Bleed and slowly fill system after maintenance
- Proper design, selection, and configuration of control valves to control opening and closure
Application to Test Systems
-Bleed gas from liquid systems prior to operation
-Proper design of systems used with dynamic valves including evaluating dynamic pressures
-Review of test data to ensure system is operating within desired limits
-Proper selection and scaling of pressure transducers
-Ensure that data rates are adequate to capture dynamic pressures
