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Tech Topics: All about Water Hammer by Bob Pelikan

We’ve all experienced water hammer at home: The shower valve is abruptly closed, or the sprinkler system valve closes at the end of the cycle, or the washing machine fill cycle ends and bam, all the pipes in the house rattle. What’s happening? Any time you have a high-flow velocity, particularly in a long pipe run, and there is a sudden change in velocity, water hammer can occur.

The resulting pressure wave is transmitted throughout the system, subjecting every component in the system to pressures approaching 10 times the normal system pressure. The pressure returns to normal only when it is dissipated by friction loss, pipe expansion or when something gives – a valve or section of pipe. Let’s take a closer look at the causes of water hammer and consider some remedies.

For water hammer to occur, four conditions must be present – sufficient flow velocity, an abrupt change in velocity, a sufficiently long pipe run, and a rigid piping system. Regarding the first condition – flow velocity – it generally is agreed that water flow velocities below 5 feet per second (fps) preclude the possibility of water hammer. The abrupt change in velocity can be either rapid acceleration (pump start/valve opening) or rapid deceleration (pump stop/valve closure). To quantify these changes in velocity, we use the term, “critical time factor.”

This first formula below is used to calculate the critical time factor, the velocity change timeframe below which water hammer occurs, based on the length of the pipe run. The second formula offers a means for calculating the maximum pressure surge resulting from water hammer in a rigid piping system.

The formula used to calculate the critical time factor, which is the minimum time duration for a valve to open or close or for a pump to start or stop without causing water hammer, is as follows:

Critical Time Factor in seconds: Tc = 2L/S
Where: L = length of pipe in feet
S = Speed of sound in water (4,860 fps)

For example, if we have a 400-foot pipe run from the well to the house the critical time factor would be:

Tc = 2 X 400 / 4,860
Tc = 0.16 seconds.

This means that if the pump takes less than 0.16 seconds to start, or if a valve in the house is closed in less than 0.16 seconds, your pressure tank better be in good working order. The pressure tank acts like a shock absorber, and normally will absorb the shock wave. However, if it is waterlogged, you may have a water hammer every time the pump starts or a valve is closed.

What, then, would be the worst-case peak pressure caused from water hammer? The Instrument Society of America (ISA) suggests the following formula for a rigid piping system based on the assumption that the shock wave will travel at the speed of sound, 4,860 fps for water. Of course, no piping system is absolutely rigid, so the actual peak pressure would be a little less than these numbers.

Formula: Ps = Po + (64V X SG)
Where: Ps = Maximum pressure surge in PSIG
Po = Normal system operating pressure in PSIG
V = Velocity of fluid in FPS
SG = Specific gravity of fluid (1 for water)

Let’s assume a 30/50 pressure switch, 20 gpm in 1-inch schedule 80 pipe. The velocity of 20 gpm in 1-inch schedule 80 pipe would be about 7.5 fps. This information is in the pressure loss charts available from most pipe and pump manufacturers.

Therefore, the maximum pressure surge would be:

Ps = 50 + (64 X 7.5 X 1) = 537.5 psig

Now that we know how to calculate the severity of the water hammer problem, how can we protect our system? Start with a good system design. Size the piping such that the velocity does not exceed 5 fps. In the above example, if 11⁄4-inch pipe had been used instead of 1-inch, the velocity at 20 gpm would have been on the order of 4 fps vs. 7.5 fps, and water hammer would not have been an issue.

Also, make sure the pressure tank is checked regularly, and at the first sign of air loss, fix the problem or replace the tank. Make sure there is not a check valve between the house and the pressure tank. You want any shock waves generated in the house to expand into the pressure tank, and a check valve would prevent that from happening. The check valve needs to be between the pressure tank and the pump.

Consider using a flow control valve to reduce the peak flow velocity to below 5 fps. Consider installing a stand pipe or small surge tank near the source of the water hammer. Consider replacing quarter-turn ball valves with gate valves, which open more slowly, or train the ball valve users to open and close them slowly.

In an existing residential system, it often is possible to fix a water hammer situation simply by shutting off the pressure to the house and opening the lowest drain valve in the system to drain out some of the water in the pipes. When houses are plumbed, it is standard practice to extend the piping inside the walls a foot or so above each valve so that a pocket of air is trapped above the valve to act as a water hammer shock absorber. Over time, the air pocket will be absorbed into the water, and water hammer begins to occur. Draining some of the water from the piping system refreshes these air pockets, and water hammer is eliminated. Try it.

In larger systems, pilot-operated flow and pressure control valves can control the rate at which the velocity changes, be it on the pump end or in the distribution system. Electronic soft-starts and variable-speed drives are another good solution to water hammer caused by pumps because they can be programmed to slowly ramp up and ramp down the pump speed.

Whether it be a small residential water system or large commercial piping system, water hammer can be a major problem. With an understanding of the factors contributing to the problem and access to the necessary tools, it is possible to tame this insidious gremlin and provide your customer with a quiet, long-lasting pumped water system.



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