Bypass noise: How low can you go?

The electric power industry is only now beginning to learn what it needs to know about the design, operation, and maintenance of air-cooled condens­ers (ACCs)—or so it appears. This is a bold statement given the first direct air-cooled plant installed in the US, the 18-MW coal-fired Neil Simpson Unit 1, began commercial operation 40 years ago. The Black Hills Power facility in Wyoming is still in service.

ACCs have become popular here only recently, primarily because of water-resource issues. Today, many new plants in the West, virtually all combined cycles, are ACC-equipped. Even some new gas-turbine-based generating facilities in the East have ACCs. One example: The nominal 500-MW 2 × 1 Astoria Energy plant in Queens (access www.combinedcy­clejournal.com/archives.html, click 4Q/2006, click “Astoria Energy” on cover).

NV Energy, Las Vegas, may have the largest installed base of ACCs (megawatt basis) in the nation, which is why the utility launched the ACC Users Group late last fall. It wanted to share experiences industry-wide on such things as the effects of wind speed and direction on performance, fan and gearbox failures, flow-accel­erated corrosion, particulate trans­port, and other issues. You can access the presentations at http://acc-users­group.org.

However, you won’t find much, if any, information in those presenta­tions on how to reduce noise radiated from the large steam ducts that char­acterize ACCs. Most powerplants are in remote locations and in-plant noise levels that meet OSHA limits are unlikely to affect area residents. But because combined cycles are eco-friendly compared to most other types of large-generation alterna­tives, they often are built close to or within population centers sensitive to noise, which can be an issue in some locations.

A case in point is Hawaii Elec­tric Light Co’s Keahole Generating Station on the Big Island of Hawaii. Its ACC-equipped 56-MW combined cycle, which began commercial opera­tion in 2009, is located near a resi­dential area (Fig 1). Most readers can appreciate the challenges the util­ity faced in permitting a plant on an island paradise.

The process began over a decade ago. Land-use and environmen­tal permitting delays, and related administrative proceedings and liti­gation, prevented commercial opera­tion of the combined cycle’s two dis­tillate-fueled LM2500 gas turbines (designated CT-4 and CT-5) until mid 2004. Conversion to combined cycle was planned and work initiated on the Rankine cycle portion of the facility about two years after the GTs were installed, according to Hawai­ian Electric Light’s Production Man­ager Norman Verbanic.

The “steam” plant consists of two once-through steam generators from Innovative Steam Technologies (IST), Cambridge, Ontario, Canada; an 18-MW Siemens Energy SST-300 steam turbine/generator; a GEA Air Cooling Systems condenser; and sys­tem auxiliaries. Sounds simple, and it would have been if not for the permit requirement to limit the noise level at the plant fence to 55 dBA during the on-peak period 7 a.m. to 10 p.m. and 45 dBA off-peak—about 20 dB lower than required of any other generating facility known to the editors.

Mitigation of noise associated with bypassing of main steam into the turbine exhaust duct during startup, full-load trip, etc, was the challenge. The gas turbines are in “soundproof” enclosures equipped with inlet and exhaust silencers so they were not an issue. Neither were the steam turbine/generator and cycle auxiliaries, located in a new building designed by Black & Veatch International, Cary, NC, which provides the necessary attenuation.

Recall that in powerplants with ACCs, turbine exhaust steam is car­ried to the condenser through a large, uninsulated cylindrical duct (Fig 2). Noise sources that discharge into a thin-wall duct have much less atten­uation than in a water-cooled con­denser.

At Keahole, the condenser is 40ft from the steam turbine building and the 104-in.-diam duct is elevated approximately 24 ft above ground level (Fig 3). Thus it’s easy to understand how high noise lev­els at the duct surface can generate unacceptable noise levels at the plant boundary—only 100 ft away in this case—and in neighboring communities. In addition, noise transmitted into the exhaust duct radiates into the ACC producing additional noise sources.

Duct-radiated noise is particularly challenging in combined-cycle plants, which have 100% turbine bypass systems. Exhaust flow, together with cooling water evaporated in the desuperheater, is about one and a half times the turbine steam flow; plus, its enthalpy is higher than that of the exhaust steam.

When such a large mass flow is discharged to the duct through a so-called “dump device” that is smaller than the turbine exhaust nozzle, noise is concentrated in a very small area. Steve Freitas, director of fossil power R&D for CCI—Control Components Inc, Rancho Santa Margarita, Calif, told the editors that single-stage con­trol valves and dump elements can generate noise levels in excess of 130 dBA at a distance of 3 ft from the duct surface and more than 75 dBA 1000 yards away.

Specifications for most plants, he continued, establish near-field sound pressure levels of 90 dBA for insulated pipes to ensure a safe working environment as specified by OSHA. However, in plants with ACCs, the far-field requirements usually dictate the near-field lev­els. For example, to achieve 60 dBA 400 ft from the duct may require engineers to achieve 85 dBA at 3 ft.

Keep in mind the exhaust duct is not insulated because of the high cost involved and the desire to reduce the steam’s energy via conduction. This means the attenu­ation capability of a bypass system for an ACC-equipped combined cycle must be significantly better than that for a conventional steam plant with a water-cooled condenser. Whatever noise and vibration come from the bypass valve are transmitted directly into the thin-wall duct.

Freitas said bypass-system noise comes from two primary sources: the steam-bypass control valve (Fig 4) and the final dump element that directs exhaust steam into the ACC duct (Fig 5). The sound power and peak frequency of each source must be controlled to reduce overall system noise, he added.

The dominant noise source in large power stations is the final dump element. The most common designs feature a drilled plate, fish-mouth device, or dump tube (Fig 5D). Such arrangements can generate noise levels in excess of 130 dBA 3 ft from the duct surface. The concen­trated sound power creates vibration conducive to crack­ing of the duct walls and dump-element mounting ring. Erosion by wet steam is another concern.

The noise produced by the dump element at the turbine exhaust duct surface can be reduced significantly by using a combination of small orifice sizes and multi-stage pressure reduction. The former shift the peak frequencies of jets discharg­ing from the dump element, the lat­ter reduces discharge velocity. In some cases, such as Keahole, both approaches are necessary to reduce noise to an acceptable level. But for this Hawaiian plant, even this did not meet requirements.

Freitas recalled taking a call from the Black & Veatch engineer, Jason Rowell, who asked if CCI could supply a bypass system that would limit noise 3 ft from the duct surface to the 65 dBA necessary to meet the fence-line requirement of 45 dBA. Most specifications call for about 65 dBA at the plant boundary.

The valve designer remembered thinking how difficult it can be to achieve 85 dBA 3 ft from the duct sur­face to make 65 dBA at the fence. CCI’s top product, which uses the company’s DRAG® resistors as the final dump element, normally achieves 85 to 90 dBA at 3 ft. Fig 5 shows the disks (A) stacked to build the resistor (B) cre­ate the multiple flow paths and stages to reduce fluid pressure and velocity with minimal noise.

A point to remember: Noise gen­eration is proportional to the fourth or fifth power of velocity, so limit­ing velocity is critical to achieving near- and/or far-field sound-power objectives.

Black & Veatch asked CCI for a solution. The valve manufactur­er began by calculating just how much noise attenuation could be achieved with its traditional offering, the design of which was constrained by a limit on the physical size of the dump resistor.

The Black & Veatch engineers determined that the resistor could not occupy more than 15% of the cross-sectional area of the duct because it would adversely impact turbine back­pressure. CCI engineers—including Graham Clark, Shelley Notarnicola, and Mike Barrett—estimated that a resistor of maximum allowable size would produce a noise level of 72 dBA 3 ft from the duct surface and 55 dBA at the fence.

Back to the drawing board. Freitas and his colleagues believed a Drag-resistor dump element in series with a vent silencer might work. CCI’s Fluid Kinetics Div, which man­ufactures vent silencers, joined the team. The collaborative effort result­ed in an integrated noise control system that met the plant’s needs. It consisted of the following elements:

  • Drag-type bypass valve (refer back to Fig 4) with 20 stages of pressure reduction, outlet diffus­er, and a 20-in.-diam outlet.
  • A 32-in.-diam Drag-type dump device with a 20-in.-diam inlet and 12 stages of pressure reduc­tion.
  • An 8-ft-diam vent silencer.

While this solution met the func­tional requirements, it was not opti­mal for the overall plant design and Black & Veatch’s engineers want­ed one silencer to accommodate the bypass systems from both HRSGs; plus, they wanted an expansion joint between the bypass valves and vent silencer. These requirements returned CCI engineers to the drawing board.

The unique solution (Fig 6), shows dump elements mounted side-by-side at the silencer inlet, which required increases in silencer diam­eter (from 8 to 12 ft), length (to 25 ft), and weight (to 20 tons). The expansion joint presented a special challenge, Freitas said, because it had to support the weight of a 1500-lb dump element.

Black & Veatch located the entire bypass system—including valves, resistors, silencer, etc—on a dedicat­ed platform inside the turbine build­ing. This had two benefits: (1) It pro­vided a straight run of pipe between the desuperheater and the dump device, thereby allowing sufficient time for mixing and evaporation of spray water before the mixture entered the resistor. (2) It restricted any noise from the bypass system upstream of the resistor and silencer to the turbine building.

When the editors spoke with Ver­banic at the end of 2009, the plant had been in operation for about six months and was consistently meeting the 55 dBA / 45 dBA requirement at the plant fence. Typically, one gas tur­bine/HRSG train starts daily, so the bypass system gets plenty of exercise.

The takeaway from this experi­ence: If neighbors are complaining about bypass-system noise from your facility, retrofit of this latest solution during the plant’s next outage might warrant consideration. ccj