When the flow of fluid through a system is suddenly halted at one point, through valve closure or a pump trip, the fluid in the remainder of the system cannot be stopped instantaneously as well. Water hammer is caused by a pressure or shock wave that travels through the pipes, generated by a sudden stop in the velocity of the water, or a change in the direction of flow. If the pipe is suddenly closed at the outlet, the mass of water before the closure is still moving forward with some velocity, building up a high pressure and shock waves. As fluid continues to flow into the area of stoppage (upstream of the valve or pump), the fluid compresses, causing a high pressure situation at that point. Likewise, on the other side of the restriction, the fluid moves away from the stoppage point, creating a low pressure (vacuum) situation at that location. Fluid at the next elbow or closure along the pipeline is still at the original operating pressure, resulting in an unbalanced pressure force acting on the valve seat or the elbow.
Common industrial hardware like relief valves, solenoid valves, valves in general, centrifugal pumps, positive displacement pumps, and regulators can and will cause heavy hammer effects.
Water hammer can generate a large impact strong enough to instantly damage a valve, etc., or cause smaller damage over a long period of time. Whichever the case, both of these situations can lead to serious accidents, so countermeasures must be taken.
The effects of the water hammer vary, ranging from slight changes in pressure and velocity to sufficiently high pressure or vacuum through to failure of fittings, burst pipes and pump damage. Pump stop can create hard-to-handle water hammer conditions; the most severe conditions result from a sudden power failure that causes all pumps to stop simultaneously.
Hydraulic water hammers
When there is a a sudden stop of the steam system (as happens when the rapid shut-off of a valve suddenly blocks the movement), the force engendered by the mass and the speed (kinetic energy) of the column of liquid creates a pressure blow upstream of the valve. Since liquids are incompressible, they cannot absorb this blow of pressure, which spreads in all directions. Downstream from the valve, the flowing movement does not stop immediately and induces a partial vacuum next to the valve. This vacuum then sucks the liquid upstream. The liquid bounces against the valve, thus creating another shock wave. This shock wave is replicated a number of times before fading away.
Thermal water hammers
Thermal water hammer is produced when a hot steam bubble meets cold condensate. The steam bubble condensates very rapidly, causing a vacuum, and the condensate brutally irrupts and clashes. We say that these steam bubbles implode. The steam bubbles can be dragged by the surface of hot condensate towards colder condensate. The steam bubbles can result from a decrease in the pressure of the condensate, be dragged towards colder condensate and then implode.
Ignoring friction and pipe elasticity, the equation for the pressure change in a pipe due to a "sudden" valve closure is
Delta P = - rho x c Delta p
Where,
rho = density of the fluid,
C = sonic velocity of the fluid;
Delta p = change in pressure.
Causes of water hammer
-Water hammer forming in a pipe
-Faulty operation of regulating equipment or drain valve (insufficient draining)
-Use of non-standard equipments
-Poor planning of an installation (banking-up of unwanted condensate at low points, negative slopes, etc.)
-Sudden closing or opening of valves
-Speed of the steam flow higher than speed of the condensate flow : this causes a dragging of water drops that later meets obstacles and cause hydraulic water hammers.
Factors that affect the water hammer
1) Pipe line length
2) Moment of inertia
3) Profile of pipe line
4) Pipe material and dimensions
Consequences of water hammer
Water hammer impact can have the following consequences:
The effects of the water hammer vary, ranging from slight changes in pressure and velocity to sufficiently high pressure or vacuum through to failure of fittings, burst pipes and pump damage. Pump stop can create hard-to-handle water hammer conditions; the most severe conditions result from a sudden power failure that causes all pumps to stop simultaneously.
- Broken pipe joints
- Broken valves
- Damage in heat exchanger internal tubes
- Broken pipe welding or pipes
- Ripped pipe supports/fasteners
- Collapsed elements in all designs of steam traps
- Overloaded steam gauges
- Distorted internal mechanisms
- Split steam traps
- Broken pipe joints
- Broken valves
- Damage in heat exchanger internal tubes
- Broken pipe welding or pipes
- Ripped pipe supports/fasteners
- Collapsed elements in all designs of steam traps
- Overloaded steam gauges
- Distorted internal mechanisms
- Split steam traps