Alternatives is proposing to install a new cross flow turbine at their Ring Shop Dam in Whitingsville, MA. The Town of Northbridge is concerned about the possibility of “outgassing” occurring. Outgassing is a phenomenon that occurs when cavitation takes place in a hydraulic turbine. The two are intrinsically related.

Cavitation is defined as the formation of voids within a body of moving liquid when the local pressure is lower than the vapor pressure and the particles of liquid fail to adhere to the boundaries of the passageway. The failure of the particles to adhere to boundaries occurs when there is insufficient internal pressure within the liquid to overcome the inertia of the moving particles and force them to take sufficiently curved paths along the boundary. The voids thus formed fill with vapour of the liquid and result in vapour bubbles. The effect of cavitation is to cause pitting of the boundary surfaces. This pitting is the actual destruction of the metal surface caused by the waterhammer associated with the violent collapse of the vapour bubbles created by cavitation.

If the cavitation is such that the vapour pressure also drops below the partial pressure of the gases composing the air, dissolved in the water, the air will break down. Both oxygen and nitrogen gases will be released into the water column. As such, an early predictor of changes in the levels of both dissolved oxygen (DO) and dissolved nitrogen (DN) is the incipient state of cavitation.

Since cavitation and pitting cause destructive deterioration of both pumps and hydraulic turbines, hydraulic laboratories have extensively studied why cavitation takes place. As a result of these extensive investigations, a cavitation predictor know as the plant sigma, Thoma number or cavitation coefficient has been developed. This parameter can be expressed mathematically as:

Sigma= (ha-hv-hs)/h

where: sigma= cavitation coefficient

hs= difference in elevation between the minimum tailwater elevation and

the cavitation reference point at the outflow from the runner, ft.

ha= atmospheric pressure head, ft

hv= vapour pressure head at the temperature of the water issuing from the

turbine, ft.

h= net effective head, ft.

The formation of cavitation results in noise, vibration and loss of performance. Hydraulic laboratories have studied numerous small scale models to relate the inception of cavitation to degradation of turbine parameters such as efficiency and power. The U.S. Department of the Interior has published a useful experience curve that is used to determine a safe turbine setting. Conversely, for a given turbine selection, it can be used to predict if cavitation is likely to occur. This curve relates the acceptable plant sigma to the specific speed, ns. Specific speed is a homologous number which predicts the rpm a turbine design will achieve if it were to operate at one foot of head.

The proposed hydraulic turbine is a cross flow design. Cross flow turbines are ideally a two stage impulse machine. The design incorporates a nozzle that focuses a rectangular jet of water into a two stage runner which resembles a squirrel cage blower. The water enters the runner from the side, flows through the blades into the center of the machine and exits the other side through the blades on the far side of the runner. Impulse machines extract energy from the momentum change created by the change of direction of the flowing water. An ideal impulse machine does not utilize a draft tube. Due to the nature of impulse machines, they are usually used on sites in excess of 1000 feet of head. Cross flow turbines, because of their low cost and simplicity, are being used on progressively lower heads. At lower heads, it becomes necessary to extract all the energy from the falling water. As an example, at Alternatives, the total head is 12 feet. The proposed turbine runner will be installed horizontally and is three feet in diameter. It is necessary to have the turbine at least a foot above tailwater in order to be able to work on it. This makes the centerline of the machine 2 ½ feet above tailwater and the nozzle exit at four feet above tailwater. Without a draft tube, the machine would operate on only 8 feet of the 12 feet of head on the site. In order to recover this energy, a draft tube is installed. The draft tube creates a vacuum in the water column equal to the height of water from the runner to the tailwater. This vacuum creates a higher differential across the nozzle. The higher differential pressure results in a higher velocity jet which allows for more energy recovery in the two stages.

A problem with utilizing a draft tube on an impulse machine is the friction losses that are created when the column of water in the draft tube rises to the elevation of the downside of the runner. At this point, the runner is contacting the draft water. This condition is known as ”wading “. Wading creates both a drag on the runner and a power loss. In order to avoid this condition, the draft chest of a cross flow turbine is provided with an adjustable valve known as a vacuum breaker. This valve is adjusted so that air is allowed into the water column. The level of vacuum can be adjusted so that the water column is just beneath the level of the runner without touching it ie: a cavity of air exists between the runner and the draft water. This has the additional effect of increasing the total aeration of the water column re-entering the river.

As long as the pressure in the turbine does not create cavitation, the additional air will neither increase DN nor DO levels. Given the limited data on the proposed Ring Shop Dam Project, it is possible to predict if the proposed site will create cavitation and outgassing.

Site Data:

A discussion with the manufacturer of the hydraulic turbine, Mr. Harry Terbush of Windsor Machine, provided the following information. The proposed turbine is a cross flow with a 36 inch diameter runner. The runner will be 50 inches wide. It should be 18 to 24 inches above tailwater to prevent flooding of the vee-belt drive. The runner will turn at 87 rpm. It will produce 68 horsepower and use 66 cubic feet per second. Its specific speed is 32 rpm.

The elevation of the crest of the flashboards is 284.5 msl and the minimum tailwater set by the backwater from the highway bridge is 272.5 msl. The maximum net effective head on the site is 12 feet. Assuming the worst case scenario, the suction head would be defined at the exit of the first stage which is approximately 2 feet above tailwater plus 2 ½ foot diameter runner or 4 ½ feet above tailwater. This places the cavitation point at approximately ½ way in the water column.

At 284.5 msl the atmospheric pressure is approximately 33.7 feet of water and at 80 degrees Fahrenheit the water vapour pressure is 0.17 feet of water.

Calculations:

Data:

Determine the plant sigma:

Sigma= (ha-hv-hs)/h

where: sigma= cavitation coefficient

hs= difference in elevation between the minimum tailwater elevation and

the cavitation reference point at the outflow from the runner, ft.

= 4.5 feet

ha= atmospheric pressure head, ft

=33.7 feet

hv= vapour pressure head at the temperature of the water issuing from the

turbine, ft.

= 0.17 feet

h= net effective head, ft.

= 12 feet

Sigma= ( 33.7 ft-0.17 ft- 4.5 feet )/ 12 feet

= 2.42

Determine the critical sigma:

From the Department of the Interior:

From the experience curve, for a specific speed of 30 the critical cavitation coefficient is 0.06.

Conclusion:

Since the calculated plant sigma of 2.42 is larger than the critical sigma of 0.06 it is unlikely that cavitation will occur. Since the graph only goes to 1.0 and the calculated plant sigma of 2.42 is literally “off the graph” it is highly unlikely that cavitation will occur at this plant. In the absence of cavitation, it is impossible for outgassing to occur.

The use of a cross flow turbine at the Alternatives site will not degrade water quality. The introduction of additional aeration from the vacuum breaker and the mixing effects of the cross flow runner will increase water quality standards downstream of the project.

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Last modified: 10/06/08