Sound Attenuation at a North Carolina Wastewater Treatment Plant
Modeling aerodynamics with accoustical analysis tones down the noise at McDowell Creek.
As Larry Hansen, principle engineer for Engineered Aeroacoustics in Minneapolis, MN, describes it, there are two approaches to sound attenuation. The ideal is to proceed step by step, each step documented, beginning with due diligence and concluding with system vesting. The advantages are obvious. Sound-attenuation requirements are considered from the get-go and are included in building design. There are no surprises, such as doors left off drawings or gaping conduit openings. The sound consultant works with a clean slate, and critical decisions are reviewed at each step, which puts everyone—engineers, contractors, suppliers, and the client—on the same page.
The opposite are projects in which the charge is to get the job done quickly and cleanly. Often as not, the sound consultant is called in after decisions on building design have been made, often with insufficient time and thought given to how standby equipment will operate or noise will be controlled. Are the drawings complete? Will the generators run efficiently in the space provided? Has someone with good intentions designed an outmoded or inefficient sound-attenuation device?
At the McDowell Creek Wastewater Treatment Plant in Huntersville, NC, Hansen used a combination of aerodynamic and acoustical computer modeling to get the job done. The McDowell Creek facility is undergoing a $78 million expansion that will double its wastewater treatment capacity to 12 million gallons a day, an upgrade that necessitated new emergency generating equipment. Although the area where the plant is located is rural, the facility is immediately adjacent to a 1,000-acre nature preserve and across the highway from a heavily used county park. A neighbor who lives opposite the plant monitors operations and has complained to plant superintendent Pete Goins about lighting and noise. Twice during routine tests of the standby generators, Goins opened his door to find himself face to face with police officers investigating the neighbor’s complaints.
Given these circumstances, Goins says he didn’t want any surprises. All the more since the two new Cummins 2,700 kW high-speed diesel generators he installed were designed not only to satisfy state requirements for standby power but will run approximately 150–200 peak-saving hours a year under an agreement with the local utility Goins estimates the generators save him over $10,000 a month off the facility’s electric bill.
Charlotte-based Southeastern Consulting Engineers Inc., which has a track record developing power generation for Charlotte-Mecklenburg Utilities, designed the power generation operation and the building where it’s housed. “We planned for three generators to handle the eventual load,” says vice president Mike Dougherty. “The peak electrical load of the plant is 1,300 kW and right now they can run the entire facility off one generator. A third generator and the necessary switchgear will be added when the load warrants. In addition, all the relaying and controls have been designed and tested so the plant can sell power back to the grid when the utility gives the OK.
“These generators are fuel efficient, but one of the things about these large units is the fans are not engine-driven as they are with smaller generators. Instead, each generator had four separate fans cooling the radiator; we run 480 volts of power off our own power grid to power them.”
The two Cummins generators are housed in a 60-foot square room separated by a firewall from a 60-foot by 24-foot room containing controls and switchgear. There is a viewing window between the two rooms as well as a door that contains a small window. Dougherty says the control room has a sound attenuating liner in the laid-in grid ceiling. The ceiling’s sound attenuation was specified by Southeastern Consulting Engineers, which also spec’d acoustical thresholds on doors for a solid sound-seal.
“In cases like this,” says Hansen, “when you are inheriting architecture and design features that are predicated upon the building’s location and how it will be used, you don’t always have the luxury of being able to have the air path as clean as you would design it if you were beginning at the beginning. In this instance, part of the design we inherited was an old approach to noise control that involved the construction of an artificial architectural structure in front of the building, essentially an air chase for both the air intake and discharge. Unfortunately it was extremely inefficient aerodynamically.
“The structure involved erecting a concrete wall 7 feet in front of the air intakes for the generators and 15 feet from the radiator discharge. The air had to turn and come through an orifice near the top of the building, drop down into the building, be drawn in by the radiator fans on the generators, and then be ejected into a similar walled chase where it would dead-head smack into a concrete wall. The design was inefficient both acoustically and aerodynamically, particularly aerodynamically.” But not, says Dougherty, aesthetically. The same architecture had been used elsewhere and Charlotte-Mecklenberg Utilities liked the clean façade the building presented to the street.
“In these types of remediation projects, where you’re left with the physical constraints of somebody else’s thinking,” says Hansen, “the goal is to try and live with the perimeters and fit the acoustics package into that.”
Another challenge was to determine local sound-attenuation criteria. Dougherty checked first with the City of Charlotte, which dictates a maximum noise level of 65 dBA after 11 p.m., and then with the police department in the city of Huntersville, where the plant is actually located. Dougherty ascertained that Huntersville does not have a sound attenuation ordinance, but police authorities agreed to accept Charlotte’s numbers, and sound attenuation was targeted for 65 dBA at 22 meters from the property line, which Dougherty says was calculated as the center line of the highway the plant is built on.
“The recreational land made this class one,” says Hansen, “which is critical for noise. The parkland immediately across the street from the generator building was sometimes used as an outdoor theater. Controlling noise couldn’t have been more critical.”
At Dougherty’s suggestion, Hansen held a one-day seminar to bring engineers at Charlotte-Mecklenburg Utilities and other customers of Southeastern Consulting Engineers up to speed on sound attenuation. “We started with the basics of acoustics,” says Hansen, “then moved on to the origins of noise as it occurs on an engine generator, breaking out the components of mechanical noise, turbo noise, vortex shedding from the radiator, and exhaust noise. Then we talked about how noise can be treated at each of these steps.”
From there, Hansen wasted no time building what he refers to as an aero-acoustic model of the new generator and control rooms. Although standard procedure on all Engineered Aeroacoustics jobs, computer modeling was critical on this project, where the goal was to get sound attention elements in place and guaranteed with a minimum of procedural tie-ups.
Hansen built the McDowell Creek facility into the computer with all three generators up and running. He considers the most important characteristic of the Engineered Aeroacoustics model its capability to track air flow while identifying obstructions, direction changes and expansions, diffusions, and contractions that disrupt its path.
“The model simulates the aerodynamic losses,” says Hansen, “and alerts us when we’re getting near the maximum-rated static-pressure loss for the particular engine fan being used. The model tells us how much noise is being propagated against the inflow of the air intake and how much is being propagated with the discharge flow of the radiator discharge. This is why we call it an aero-acoustic model—it looks at the aerodynamics and acoustics simultaneously. With the aerodynamics established, we can begin to get into the creative side of our business.
“Using the computer, we begin implementing noise remediation efforts while simultaneously establishing devices such as aero-acoustic turning vanes, to move the air more efficiently. Balancing is critical. When you look at a building for noise control, you should think of it as a containment device, like a submarine. Because your measure of success is going to be inversely proportional to the sum of the leaks, every opening has to be accounted for. Every noise source has to be accounted for. Say for instance the sound attenuation criteria were 50 dBA at a given distance, and say you’re going to bring the radiator down to that, the same with the ventilation system. The fact is when you get out to the criterion distance, the sum will be 53 dBA because all the noise sources will double. What you have to do is bring the noise sources down in concert with each other. Some will have to be down well below the stated criteria.
“The sources that will take the least pressure drop,” says Hansen, “are the ones that you allow to be the loudest. You drive down the noise levels of the sources that can take more pressure drop well below the others. All of this is predictable. For example, because noise propagation is going against airflow, the air inlet noise is always going to be lower than the radiator discharge noise, and it’s also the quiet end of the generator. The radiator discharge side has an interesting phenomenon called negative slope, which refers to the fact that noise intensity goes down with increasing frequency. So as you go up in frequency, the intensity goes down. This is the case with the mechanical noise of the generator, the block noise—because of engine combustion you get the preponderance of noise at the lower frequency.
“When you add a radiator cooling system to an engine generator,” Hanson continues, “you add a device that has a positive slope, which gives you Vortex shedding. The air passing through the core causes an increasing intensity of noise level with increasing frequency. So you get two curves crossing themselves. All that has to be accounted for in the aero-acoustic model.
“At the radiator discharge end we’ve got a minimum of compounded noise source from at least three items. We’ve got turbo noise from the turbo chargers, which is a high frequency component. We’ve got the increasingly high frequency component coming off from the vortex shedding of the radiator core and then the mechanical noise from the engine block. So you’ve got a minimum of three distinct noise signatures summing in the radiator.”
In a turnkey project Hansen might present findings from his aero-acoustic model to his client, but for the McDowell Creek project it was full speed ahead. “We’re evolving a system and in most cases, that’s what a consultant like Southeastern Consulting Engineers wants to know, the system,” Hanson explains. “They want to know how we’re going to fix the problems we’ve identified. From the model come the aero-acoustic engineering perimeters we need to design the hardware that will provide the actual sound attenuation. From there we go right into hardware design and then interface the hardware with the existing building drawings. This is critical at this particular stage. We don’t want to be at odds with the building engineers. We don’t want them cutting holes in the walls that we can’t fill or that aren’t large enough to accept our hardware.
“Basically, it goes back to the submarine analogy. You’re building a containment system, and anytime you cut an opening, you’re violating the containment system, which means all openings have to be acoustically treated. Often it’s a function of how complete the drawings are. Some of the finishing details never show up on the drawings. There may be ventilation ports, for example, that don’t show up until they turn up to be a ‘gotcha.’ Another prime example is trenching in and out of the building to handle spills from the coolant system or piping. Trenches breach walls and become noise leaks. So the more complete the drawing package we receive the more complete our sound attenuation design will be.
“Using the computer, we can model how we’re going to deal with the building design by modeling the different hardware configurations. At computer speeds this is not time-consuming. Remember, it’s not enough to provide a certain area for air to flow; you must be aware of any aerodynamic distortions that might be present. The aerodynamics of a system can be complicated—like they were in this case—by the way the air approaches an inlet. To solve the air intake problem, we added banks of silencers and an aero-acoustic turning vane that turns the air 90 degrees, minimizing aerodynamic loss. In the same way, we ran the radiator discharge into a short bank of silencers that were balanced with the turning vane and the discharge plenum to get the viscous flow of air to move in concert with the rest of the air. Instead of the dead air heading right into a wall smack in front of the radiator discharge, we actually started turning the air at the bottom so it was pushing against itself and up through the vertical plane.”
“The openings of the silencer banks Engineered Aeroacoustics built were 16 by 15 feet and have only 4 feet between them,” says Dougherty. “So you have 4 feet of concrete block and then this big opening. The air has to go up 20 feet over the plenum wall then down into the plenum room, which is about 7 feet wide, and then through the wall openings where the silencer is, through the room, across the generator, then through the radiator fans. Then it blows into another 7-foot-wide plenum room and the turning vane, which helps get rid of back pressure and turbulence and noise. The plenums also have wall treatment on the outside, and that’s how Aeroacoustics has done it all. For the job we’re working on together right now, the McAlpine Creek Wastewater Management Facility, we’ve eliminated the plenums and are installing architectural louvers on the outside and passing the air straight through.”
“If we had been in the loop earlier, we would have eliminated those plenum chambers and put in slightly larger silencers,” says Hansen, “which would have made the footprint of the McDowell Creek building smaller and saved labor and construction costs.”
But plant manager Goins is satisfied. “I never thought we could quiet it down so much,” says Goins. “Even with earplugs that attenuate the sound down 32 decibels, inside it’s still extremely loud. But outside the building there’s just a hum. I can stand there on one side and talk in a loud voice, yelling, and someone standing on the other side of the wall hears it as whisper.”
Southeastern Consulting Engineers specked GT Exhaust 3/16 stainless steel combination engine exhaust silencers-spark arrestors rated for a minimum sound attenuation of 30 dBA. “We looked at the data furnished by the engine manufacturer,” says Hansen, “and also our own computer simulations for reciprocating internal combustion engines, and came up with an analysis for the exhaust system. This is critical because you don’t want too much, but you don’t want too little either. For years, mufflers have been undersized. To meet economies, projects have tried to make mufflers as small as possible. But the result is the internal gas velocity in the muffler is extremely high, which can cause them to become noise generators, much like a siren. The result is we’ve run into instances where the muffler generated more noise in the upper frequency than it was attenuating at the lower frequencies.”
Hansen also designed hardware to attenuate noise generated by the ventilation system. “When the generators are running,” says Hansen, “there’s enough air flow that it will usually meet the cooling requirements of the building, and the generators, and the engines. When the engines are shut down, you have a lot of heat rejection into the atmosphere that has to be removed from the building.” Dougherty describes Hansen’s solution. “He took some sound-attenuated duct work and—beginning at the bottom of the roof-mounted exhaust—extended it about 12 feet toward the peak of the gabled roof. This brings hot air toward the fan so that it’s exhausted through the roof while it quiets the fans, which can cause a lot of noise outside.”
Even with the modeling, Hansen had to remediate some gotcha details. A vent area above a door was missed and had to be filled in with masonry after the sound equipment was installed. Pipe penetrations going from the engine room into the control room had to be sealed and a lightweight roll-up door attended to. “We’ve had problems with these light-weight doors in the past,” says Hansen, “because they don’t have sufficient mass. What we usually do is install an air lock as a secondary containment system. An air lock door has higher acoustic elevation than is desirable, but in this case it wasn’t unduly out of specifications. It was at a point right outside the building and fortunately there was sufficient distance to the criteria distance that it was not an issue.”
Engineered Aeroacoustics provided the custom-designed intake and discharge silencers for the plenum rooms, the aero-acoustic turning vane, and the ventilation system silencing package. The company itemized the hardware required, which it added to the master drawing so Southeastern Consulting Engineers could see how the sound attenuation aspects of the project came together. Once Hansen’s fix was approved, Engineered Aeroacoustics supplied installation drawings. Hansen did not supervise the installation as he would have had this been a turnkey operation. Nor did he test the system once it was installed. “This was a straightforward application,” says Hansen. “What they want in a situation like this is a guarantee that what we recommend will work, and that’s what we gave to them.”
Author's Bio: Journalist Penelope Grenoble is a frequent contributor to Forester Media, Inc. publications.