Normally, water hammer analysis is handled by process engineer. However, piping designer should be familiar with this subject and in this blog I will briefly discuss water hammer and explore events that cause it.
Air in the piping system
Water hammer can be created by air presence in piping system and I have discussed that topic in my blog Air in the piping system (link).
Vacuum in the air system
Water hammer may occur when upstream valve in pipe closes, water downstream of the valve attempts to continue to flowing creating the vacuum that may cause the pipe to collapse. This problem can be particularly acute if the pipe is on the downhill slope. Please see the figure below. To prevent this air and vacuum valves are installed downstream preventing vacuum occurring.
Similar situation occur when a piping being drained from water. Vacuum cause negative pressure that might collapse the pipe. Please see the figure below.
Closing upstream valve in process piping can also cause water hammer. Please see the figure below.
Water hammer can occur in steam piping and possible causes are: condensate build up, equipment stall and thermal shock-introducing hot steam into cold environment. I have already discussed thermal sock in my blog Steam piping. Water hammer in steam piping has been very well presented on TAV site.
Change of steady state
Piping system is subject to water hammer whenever there is a change in steady-state condition such as pump start up or shutdown. The video below shows pressure rise during pump start up.
For smaller pumps, valve should be opened slowly during pump start up to avoid surge pressure. Some large pumps might require control valve installed to slowly introduce water into the piping system and discharge pipe must be filled with water during pump start up. Changes from steady-state condition also happens when valve is instantaneously closed, causing water hammer. Pressure surge is calculated by using Joukowsky equitation. Below is the snapshot of water surge calculation for 12″ steel pipe, water-filled, having 7 ft/sec velocity.
For the condition above, pressure surge is 123.35 psi. Instantaneous valve closure is defined to occur if the valve is closed faster than the wave travel time which is defined as 2L/c. Below is the snapshot of valve’s closing time calculation for 25 feet pipe length having wave speed from the example above.
It is evident that pressure surge can be control by valve closure time. Closing time can be set at calculated value for automatic valves. For valve operated by hand closure time must be assumed. For example valve operated by hand requiring only 90 degree turn to close, such as ball or butterfly valve, closure time of well under a second, probably 1/4 sec. For a closure time of 1/4 sec, pipe that has length of 125 meters is long enough to experience Joukowski head. In vulnerable situation, like this, good solution is installing hand valve operated with gearbox so that it is not possible for an operator to close them suddenly. Installing hand valve operated with gear box is preferable option, rather than relying on ‘good maintenance practice’ (closing valve slowly). Pumps can run down due to power interruption in any length of time from less than a second to tens of seconds, according to their inertia and internal friction. The rate of slow-down decreases as the speed decreases. However a typical pump will run down to under 50% of its initial speed in 2 to 3 seconds, causing a similar decrease in flow rate. The effect of this can be estimated as if it were a closing valve, which was discussed above.
Vacuum in the elevated place
Vacuum in the piping system can be formed in elevated places such as pipe bridge. In the case of pump trip, vacuum can be formed at the elevated places where barometric leg is above tank. After pump restart liquid columns will collide resulting in high pressure surge. Note that column separation might occur even if elevated section is less than barometric leg. Please see figure below.
Possible solution to remedy this problem is installing closing valve downstream that will be automatically closed when power interruption is occur.
When a flow is restricted by a restrictor orifice that is close to the end of a pipe, a problem with pressure surge can occur on startup. On startup, when the pipe is empty, the liquid runs at high rate until it reaches the restrictor, when its flow suddenly falls. This causes a surge event with the velocity being the change in velocity caused by the passage of liquid through the restrictor. Please see the figure below.
Bladder Surge tank is device that can alleviate pressure surge and it is commonly used in piping system that experience Joukowski head. Please see video bellow
Piping system should be design in such way that Jaukowski pressure will not occur. Of course, there will be pressure increase during valve closing or pump start up. This increased pressure shall be taken into consideration when we conduct pipe stress analysis. Bellow is the snapshot of water surge force calculation for pressure increase of 123.35 psi in 12″ pipe.
Calculated force (13950.6 lb) will be included in stress analysis, imposing force on the elbow in the 3D piping model (Caesar II or Autopipe). Please see the figure below.
Pipes can withstand pressures well in excess of their design pressure for short periods of time. Note that Process piping code B31.3 (302.2.4) allow to exceed pressure of allowable stress for pressure design not more than 33% for no more than 10 hours or 20% for not more than 5 hours. It is important that pipe supports also can withstand water hammer force. Generally, pipes longer than 250 meters or pipes with fluid velocity greater than 10 ft/sec are considered as potentially at risk from pressure surge. In the video bellow, combination of long pipe run and high velocity (4 m/sec) are causes for generating water hammer force.
Video bellow shows magnitude force during water hammer occurrence.
In some cases, dynamic analysis of piping system might be required, but that is out of scope.