Cause of water hammering and how to reduce it

Q- What do you understand by the term water hammering and how to prevent it?

Ans - Water hammer occurs when water, accelerated by steam pressure or a low-pressure void, is suddenly stopped by Impact on a valve ar fitting, such as bend or tee, or on a pipe surface. Water velocities can be much higher than the normal steam velocity in the pipe, especially when the water hammer Is occurring at startup. When these velocitles are destroyed by impact, the kinetic energy in the water is converted into pressure energy, and a pressure shock is applied to the obstruction.

In mild cases, there is noise and perhaps movement of the pipe. More severe cases lead to fracture of the pipe or fittings with almost explosive effect and consequent escape of live steam at the fracture. Fracturing of pipes or steam system components can propel fragments that can cause injury or loss of life.

Thère are two types of water hammer: steam-flow-driven water hammer and condensate-induced water hammer.

A steam-flow-driven water bammer is an impact event, where a slug of rapidly
moving water strikes a stationary object. The exchange of momentum createş a pressure of perhaps a few hundred psi in the impact area.

A condensate-induced water hammer is the more powerful of the two types. It is a rapid
condensation event that occurs when a steam pocket, being totally surrounded by cooler condensate collapses into a liquid state. Depending on the pressures and temperatures involved,the reduction in volume may be by a factor of several hundred to well over a thousand, and the resulting in low-pressure void allows the pressurized surrounding condensate to rush in, resulting in a tremendous collision. This in turn generates a severe over-pressurization that can easily exceed
1,000 psi.

Common places to look for both types ot water hammer are steam mains, steam tracing lines and air heating colls.


Causes of water hammer

Condensate bulldup is cammon to both types of water hammer. It can be caused by 1)boiler carry-over depositing large amounts of boller water Into the steam main, 2)overwhelming the steam trap
or 3) malfunctioning steam traps or-check valves. Even reduced trap capacity caused by low steam main pressure conditions can result In backflow from condensate mains.
AT Element necessary to steam-driven water hammer Is steam flow, usually from some sort of nearby steam load producing the force that drives the slug.

For condensate-Induced water hammer, necessary elements are:
1.steamcondensation, during which a trapped pocket condenses in pooled condensate
2.a pressure drop (perhaps combined with steam process control valves opening), which can lead to a vacuum belng created

Reducing the risks

Operators can reduce their risk of water hammer by preventing or resolving steam system design Issues.

1. Drainage: Avold water hammer completely by taking steps to ensure that water (condensate) is drained away before it accumulates in sufficlent quantity to be picked up by the steam. Provide
proper drainage; do not simply deal with It by installing components with high pressure ratings or capacities. Components with generous "safety factors" do not necessarily ensure safe and effective
steam maln dralnage.

2.Steam quality: Improve steam quality, keeping steam as dry as possible at all times. Install steam-conditioning stations upstream of meters and any other critical steam system components.

3.Steam velocitles: Do not allow steam velocities to became excessive as a result of system modifications. The higher the velocity, the higher the force of impact during a steam flow-driven event.

4. Boiler and steam supply: In larger systems, consider Installing an automatic valve in the steam supply line, arranged so that the valve stays dosed until a reasonable pressure is attained In the
boiler. The valve can then be set to gradually open, allowing flow, temperature and pressure in the distribution system to reach equilibrium slowly. Install a backpressure control valve on the steam main to prevent the pressure within the boller itself from being drawn down due to some sort of upset condition.

5.Steam traps: Make sure the steam traps used are of correct type and capacity. Type can depend on the startup methods used. If operational procedures change, different types of steam traps may be needed. If in doubt, call in a steam system expert. Check steam traps regularly and maintain them properly, Never fall below a minimum pressure differential across a steam trap. Always pipe steam main isolation valves with a steam trap to allow dralnage of condensate that may form while the valve is closed. Design target unit piping to include bypass systems that allow gradual heating and pressurization on startup.

6.Open the steam valve very slowly to any cold tanks

7. Piping- Correct any occurrence of pipe sagging and lagging ,wet or damaged insulation that could cause condensate accumulation and exceed steam traps capacities