Water Hammer

velocity

By closing the valve slowly, the _______________ of the water is reduced before the column is stopped. Since the momentum of the water is decreased gradually, damaging water hammer will not be produced. Sometimes, check valves can produce hydraulic shock. Swing check valves are often used to prevent liquid being drawn into spaces that are subject to intermittent vacuums. They are also applied to prevent back-flow from elevated systems when adequate pressure to raise the liquid cannot be guaranteed.

exchangers

Heat ________________ , tracer lines, steam mains, condensate return lines, and in some cases, pump discharge lines, may contain a bi-phase mix.

shockwave

Hydraulic shock occurs when a valve is closed too abruptly. When a water valve is open, a solid column of water moves from its source at the main to the valve outlet. This could be 100 pounds of water flowing at 10 feet per second, or about 7 miles per hour. Closing the valve suddenly is like trying to instantly stop a 100-pound hammer. A ________________ of about 600 psi slams into the valve and rebounds in all directions, expanding the piping and reflecting back and forth along the length of the system until its momentum is dissipated.

bi

In __________-phase systems, steam bubbles may become trapped in pools of condensate. Since the condensate temperature is usually below saturation, the steam will immediately collapse. Steam occupies hundreds of times the volume of an equal amount of water. When the steam collapses, water is accelerated into the resulting vacuum from all directions. When the void is filled, the water impacts at the center, sending shockwaves in all directions.

low

Malfunctioning steam traps may also contribute to thermal shock followed by water hammer. A steam trap that has failed open injects live steam directly into the condensate return line. If this steam is mixed with return line condensate of sufficiently ___________ temperature, it will immediately collapse, and thermal shock will follow.

drip

One likely place for thermal shock to occur is in steam utility corridors. In these areas, the _____ traps from high-pressure steam mains often discharge directly into the pumped condensate return lines. The temperature of the condensate in these lines usually ranges from 140 to 180 degrees Fahrenheit. The condensate being discharged from the steam trap is at nearly steam temperature when it passes through the trap orifice. When the trap discharge enters the low-pressure condensate line, a great deal of it flashes back into steam. The flash steam immediately collapses again when it encounters the relatively cool pump discharge water.

positive

Prevent the thermal shock that is generated by such a design by substituting a constant-purge device, such as a differential condensate controller, for the steam trap. Condensate controllers maintain a ________________ differential pressure across the coil at all times. All the tubes will be fed from the supply end only, preventing the entrapment of steam and the resulting thermal shock.

acceleration

The ________________ of the reversing column of liquid may be quite high. If the swing length of the check valve is sufficient, the column will build enough inertia to cause hydraulic shock in the time it takes the valve to slam shut. Substitute silent or non-slam check valves for swing checks to prevent water hammer in these situations. Silent check valves are center-guided to provide a much shorter stroke than swing checks. These valves also use a spring to help enclosing. The result is that silent check valves are closed by the loss of upstream pressure rather than the reversal of flow, preventing hydraulic shock.

coils

Thermal shock can also occur easily in steam ____________ since the steam is directed toward the center tube first, it can reach the return header before the top and bottom tubes are filled. Consequently, steam feeds the more remote tubes from both ends. With steam flowing into both ends of a tube, waves of condensate flow toward each other. These waves have the potential of trapping pockets of steam between them. If this happens, thermal shock will result when the pocket of steam collapses and water hammer will probably occur.

direction

Thermal shock often causes damaging water hammer. The localized sudden reduction in pressure near the wall chips away piping and tube interiors. Oxide layers that otherwise would resist further corrosion are removed, resulting in accelerated deterioration of piping and equipment. To minimize such a disturbance, the drip trap should discharge in the _____________ of condensate flow by means of a special fitting. This method of controlling thermal shock, called "sparging," reduces the concentration of collapsing steam bubbles, and keeps the action from occurring by the pipe wall.

thermal

Three distinct conditions have been identified, which provide the force that initiates water hammer. These conditions, hydraulic shock, ________________ shock, and differential shock, are common to many industrial fluid applications.

supply

To prevent the occurrence of water hammer due to thermal shock, you must reduce the concentration of collapsing steam bubbles in the condensate. If flashing condensate must be discharged into a cool condensate line, it should be discharged in the direction of condensate flow and away from the pipe wall. Precautions must be taken that heat exchanger tubes are always filled from the __________ end only.

hydraulic

To prevent water hammer due to _______________ shock, avoid suddenly stopping water columns. Ensure slow closure of valves, and install spring-loaded, center-guided, non-slam, or silent check valves that close before flow reversal when appropriate. Use water hammer arresters if necessary, but be sure they are sized and placed correctly, and are well-maintained.

arrester

Water hammer arresters, if correctly sized, placed, and maintained, will reduce water hammer. When the forward motion of the water column is stopped by the valve, part of the reversing column is forced into the water hammer __________________. The water chamber of the arrester expands at a rate controlled by the pressure chamber, gradually slowing the column, and preventing hydraulic shock.

incompressible

Water hammer begins when some force accelerates a column of water along an enclosed path. The _____________ nature of water gives it the power of a steel sledge as it slams into elbows, tees, and valves.

condensate

Water hammer can occur in any water supply line, hot or cold, and its effects can be even more pronounced in bi-phase systems. Bi-phase systems contain both _______________ and live or flash steam in the same space.

Differential

___________ shock, like thermal shock, occurs in bi-phase systems. Differential shock can occur whenever steam and condensate flow in the same line, but at different velocities, such as in high-pressure condensate return lines. In bi-phase systems, the velocity of the steam is often ten times the velocity of the liquid. If this gas flow causes condensate waves to rise and fill the pipe, a seal is formed with the pressure of the steam behind it. Since the steam cannot flow through the condensate seal, pressure drops on the downstream side. The condensate seal now becomes a piston that is accelerated downstream by virtue of this pressure differential. As it is driven downstream, the piston picks up more liquid that is added to the existing mass of the slug, and velocity increases