Water hammer

[lgb]

Could anyone please tell me how water hammer effect happens and how can I predict/calculate the same for my system? I think we broke a differential pressure gauge in the lab due to water hammering. I need to replace it and I want to replace the gauge with a rugged one. Suggestions please. Thank You.

[anihita]

Water hammer (or, more generally, fluid hammer) is a pressure surge or wave caused when a fluid (usually a liquid but sometimes also a gas) in motion is forced to stop or change direction suddenly (momentum change). Water hammer commonly occurs when a valve closes suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe. It's also called hydraulic shock.

This pressure wave can cause major problems, from noise and vibration to pipe collapse. It is possible to reduce the effects of the water hammer pulses with accumulators, expansion tanks and other features.

Rough calculations can be made either using the Joukowsky equation, or more accurate ones using the method of characteristics.

[anihita]

Mitigating Measures:

Water hammer has caused accidents and fatalities, but usually damage is limited to breakage of pipes or appendages. An engineer should always assess the risk of a pipeline burst. Pipelines transporting hazardous liquids or gases warrant special care in design, construction, and operation.

The following characteristics may reduce or eliminate water hammer:

Reduce the pressure of the water supply to the building by fitting a regulator.
Lower fluid velocities. To keep water hammer low, pipe-sizing charts for some applications recommend flow velocity at or below 1.5 m/s (4.9 ft/s)
Fit slowly-closing valves. Toilet fill valves are available in a quiet fill type that closes quietly.
High pipeline pressure rating (expensive).
Good pipeline control (start-up and shut-down procedures).
Water towers (used in many drinking water systems) help maintain steady flow rates and trap large pressure fluctuations.
Air vessels work in much the same way as water towers, but are pressurized. They typically have an air cushion above the fluid level in the vessel, which may be regulated or separated by a bladder. Sizes of air vessels may be up to hundreds of cubic meters on large pipelines. They come in many shapes, sizes and configurations. Such vessels often are called accumulators or expansion tanks.
A hydropneumatic device similar in principle to a shock absorber called a 'Water Hammer Arrestor' can be installed between the water pipe and the machine, to absorb the shock and stop the banging.
Air valves often remediate low pressures at high points in the pipeline. Though effective, sometimes large numbers of air valves need be installed. These valves also allow air into the system, which is often unwanted.
Shorter branch pipe lengths.
Shorter lengths of straight pipe, i.e. add elbows, expansion loops. Water hammer is related to the speed of sound in the fluid, and elbows reduce the influences of pressure waves.
Arranging the larger piping in loops that supply shorter smaller run-out pipe branches. With looped piping, lower velocity flows from both sides of a loop can serve a branch.
Flywheel on pump.
Pumping station bypass.
Hydroelectric power plants must be carefully designed and maintained because the water hammer can cause water pipes to fail catastrophically.

[anihita]

More information can be found in google and following link:

[link Point to another website Only the registered members can access]

[carl_e]

Or you just buy and install (two for differential) dampener/pressure limiting valves that protect the instrument and be done with it.
They are designed for impulse tubed instrumentation.

h*t*t*p*://www.midwestinstrument.com/limiting/limiting_main.asp

[sgm2]

It is caused when a moving fluid is stopped rapidly (perhaps a valve closed very rapidly). A moving fluid has momentum and if a valve closes very quickly, the momentum of the fluid is transferred to the pipe. The pipe or components connected to the pipe can react as if being hit by a hammer and may recoil causing the shock wave to bounce back and forth a few times. You can minimize water hammer by reducing sudden changes in fluid velocity or by providing some sort of cushion where flow can be redirected and the fluid's kinetic energy to disperse more slowly. Plumbers will often leave a section of pipe filled with air (although over time the air will dissolve into the water). An air bladder would work much better and allow the energy to be dissipated over a longer period of time instead of rocking the pipe and fittings.

The idea is to increase the period of the wave being generated. If there is no path for the fluid to follow once a valve closes, the fluid will compress and recoil - which you will interpret to be water hammer.

Modeling it can be done with basic fluid flow mechanics. The underlying mechanism is simple. That said, solving the problem mathematically will not help you solve the problem in the field. Solving the problem in the field is designing a damped path for the fluid to go when the valve closes. Or close the valve very slowly if you can.

[zinokabyl]

In our plant AGR acid gas removal we have an exchanger preheater in stalled just upstream of the absorber and this preheater is used to maintain the feed gas temperature below it's dew point (maintain its gaseous phase) and to maintain a temperature of 40 degree which is suitable for an ideal absorption of acid gas H2S and CO2 by using tertiary amine named UCARSOL, this exchanger is tube and shell type where vapor is used in the shell side for the given purpose the vapor pressure and temperature are 15 bar and 250 degree C we observed always hammering occurs in this exchanger, and the outlet of vapor is routed to a condensate pot which is a vessel to collect the water, for more info upstream bleeder of PRV of this shell side water is coming /sometimes water sometimes vapor intermittently, and the PRV always installed at the top side (top platform), which means clearly that the shell side of the exchanger is full with water....as per what described below what's the solution, is it related to the design of the exchanger or anything else, me I suggested to add an other drain line from the shell side like an elbow which will be connected to main drain going to the condensate pot.. in order to accelerate the accumulated condensate water inside the shell side ?? just for sharing and discuss...your reply is highly appreciated