See Photo:   Pipe Operating Vibration Reduction Systems
 

VIBRATION CONCERNS AT GEOTHERMAL POWER PLANT

A well-head at the California Energy Company’s COSO Geothermal Project was experiencing significant vertical thermal growth, sufficient to cause the lift-off of attached 24-inch diameter piping from vertical supports. Loss of this support on lines carrying multiphase flow caused high vibration, and led to concerns for the early fatigue failure of the piping and connections.

ANCO was asked by Mission Power Engineering, the plant A&E, to obtain quantitative measurements and suggest remedial action. Accelerations and displacements on the piping system were found to be as large as 0.8 g’s and 0.9 inches, as shown in the accompanying figure. This information, the measured mode shapes, plus engineering calculations indicated that fatigue failure of the piping could occur within a few months to a few years. Hence, ANCO and Mission Power designed and installed a restraint system using a single GERB Viscodamper and constant force spring hanger. This retrofit reduced the pipe vibration to acceptable levels and extended the calculated fatigue life of the piping to the life of the plant. Subsequently, Mission Power ordered 25 additional GERB units for installation on other lines throughout the COSO field.

TRIAXIAL PIPING DAMPER

VISCODAMPERS, manufactured by the GERB company of Germany, are made up of four elements: (1) a vertically mounted container, fixed either to the structure or piping; (2) a plunger which fits into the housing, which is immersed in a viscoelastic medium and is fixed either to the piping or the structure; (3) the viscoelastic medium which partially fills the container and surrounds the plunger; and (4) a flexible, protective boot to keep foreign material from entering the viscoelastic medium.

Depending upon operating temperature and expected temperature range, viscodampers can be provided with either of two viscoelastic damping media. For limited temperature ranges, a bituminous, tar-like medium is used; if wide temperature fluctuations are anticipated, a synthetic, silicone medium is used. Viscodampers conform to the ASME piping code by reference to MSS (SP-58) Type 47, "Restraint Control Devices."

Viscodampers transmit no static load. They are used to triaxially restrain vibration in the frequency range of 1-30 Hz or more. For slow thermal expansion, there is little resistance to movement in any direction. At higher frequencies, the forces generated are proportional to velocity. The damping resistance is approximately constant in the 5-25 Hz range.

EXTENSIVE PRIOR USE

"Over 5,000 viscodampers have been successfully used for vibration control in a variety of utility and non-utility piping installations over the last fifty years throughout the world," explained Mr. Steve Keowen of ANCO. "In the United States there are currently over a thousand damper-years of experience, without failure or loss of function. In many cases, viscodampers have replaced multiple hydraulic and mechanical snubbers. ANCO has used GERB viscodampers to reduce vibration at several plants, including the 25 units at the COSO Geothermal Project, 20 units on 4-inch to 18-inch reheater lines at the Shearon Harris Plant for Carolina Power and Light, 10 units on piping at the Comanche Peak Plant, for TU Electric, and 32 units on air handler systems in Montana Power’s Coal Strip electric power plant. Viscodampers offer an excellent and reliable backfit solution to flow, water hammer, and seismic vibration problems. They can also slash restraint costs if considered at the piping design stage."

Viscodampers, in conjunction with helical springs, have also been used to provide full base isolation of turbine pedestals, full-sized buildings, and other industrial structures.