Whether the goal is meeting business needs or conforming to code requirements, most large organizations include generators in new facility designs. But planning for back-up power is no simple task, and the sizeable investment involved makes it essential that facility executives understand the pivotal decision-making points in a generator project.
The first step is identifying what the generator will power. Every building that has a generator will have two types of loads: those for which codes require emergency power and those for business-critical systems. Each load must be documented. It is wrong to assume that a generator will cover the entire electrical system and that it will be business as usual in the event of a utility outage. This level of backup requires a significant outlay of capital; whether that investment is called for can only be determined by analyzing a facility’s needs.
Life safety code requires its branch of the emergency power system to be picked up by the generator within 10 seconds of utility failure. The critical branch in medical facilities also has the same 10-second requirement. This rather abrupt load can be difficult for the generator to handle. Sequencing loads to come onto the generator after 30 seconds of running can help smooth out generator start-up.
All related loads should also be considered in the documentation. For example, if computer systems are on the generator, it is necessary to put HVAC cooling for the computer room on the generator as well. To close the loop on items like these, all key decision makers should be involved in planning. In addition, outside consultants often are employed to determine how codes will affect a project.
NFPA standards that must be taken into consideration include NFPA 110 (generators), NFPA 70 (electrical systems), NFPA 13 (sprinklers), NFPA 30 (fuel storage), NFPA 37 (stationary engines), and NFPA 54 (fuel gas code). In addition, EPA has established four tiers of emissions requirements for generators, dividing them by horsepower. Tier 4, the most restrictive level, covers the largest generators. For some building types, the generator may be required to have a UL 2200 listing.
When the documentation is complete, an engineer can size the generator and transfer switches. This sizing will indicate the electrical and physical requirements, which in turn affect the location of the generator.
A second important step for facility executives is deciding where to locate the generator. Generators are more frequently located outside than inside, but both locations have pros and cons. A third option is placing a generator in a penthouse or on the roof.
The decision to locate a generator outdoors raises issues specific to each facility. If visibility is an issue, for instance, a generator can be placed behind a screen wall or landscaping. However, this can cause security problems in some areas.
Climate plays a role as well. If solid screen walls are used, the area around a generator must be large enough to allow for adequate cooling in hot weather; leaves must be cleared away in the fall and snow removed in winter months to prevent ice damming.
Every generator needs to be kept warm so that it can start up and take on the code-required loads in 10 seconds. In cold climates, locating a generator indoors keeps the unit at starting temperature without oversized heating elements. Despite this advantage, indoor generators require extra precautions. Because they will house some fuel and may contain the transfer switches and panels for the emergency system, generator rooms must have a two-hour fire rating and may need a sprinkler system. Indoor generators can also cause special noise and vibration problems.
Placing a generator on the roof or in a penthouse can be particularly advantageous in areas prone to flooding. A penthouse also keeps the unit at starting temperature in cold climates. However, height can amplify vibration, and the costs of meeting structural requirements should not be overlooked.
The third step — addressing noise and vibration — must be dealt with whether the generator is indoors or out. Outdoor generators can sometimes be placed far enough away from a facility that the noise is not a problem. However, smaller sites may not have this option, and even for larger facilities, the best screening area may be close to the building.
In residential areas location of the generator is critical. Moving the generator away from the facility may create a noise problem for the homes near the site. Many residential areas have noise ordinances in effect that are much more restrictive than in commercial areas. The hours of operation can play a key role. During normal business hours the noise restrictions are not as tight as in the evening. Restricting the hours of operation to normal business hours, when that is possible, can help overcome objections to zoning permits by a neighborhood association.
An outdoor generator placed close to a building has similar effects to one placed indoors. Significant effort must be committed to managing the noise generated, and the first line of noise management is the most basic: Keep sensitive occupants farthest way from the source of the noise. The space adjacent to a generator is best used for storage rooms, data centers that are not frequently occupied and loading docks.
Although the biggest source of noise from a generator is exhaust, the engine block and the air intake/relief also contribute to the problem. An improved muffler or silencer can keep exhaust noise to a minimum, but that does not eliminate the engine noise or the noise from the radiator fan.
Another thing to keep in mind is that noise does not emanate from a generator in all directions equally. Manufacturers of sound-attenuating enclosures provide data to help orient a generator for maximum noise control.
Remote radiators permit heat to be removed from a tight location or an indoor unit without having large air ducts or chases, which carry sound. Sound-attenuating enclosures for outdoor units use scoops at the air intake and outlet to direct noise up or down.
All generators produce vibration. Outdoor units do not affect the building much, but the vibration of indoor units can cause problems for medical imaging, laser surgery, delicate manufacturing processes and overall occupant satisfaction. Vibration isolation is used on most generators, but indoor units must do more. The structure around the generator must be designed with the generator’s vibration and weight in mind. For instance, structural steel in a standard building will transmit vibration from a heavy generator quite well. Therefore, the vibration isolator should be a premium-grade unit.
Along with size and location, the facility executive needs to select the fuel source — step four. The generator’s size and the area’s seismic classification influence this decision. Although cost varies from region to region, a good rule of thumb is that natural gas-fueled generators are less expensive in sizes below the range of roughly 150 to 175 kw, at which point natural gas and diesel break even.
For generators with a capacity of 350 kw or more, diesel is the most cost-effective option. These sizes require high horsepower engines to provide adequate electrical output, and natural gas-driven models are rare and very expensive because there is little demand for them in other industries.
Most clients prefer natural gas because it doesn’t require storing large quantities of fuel or signing a fuel delivery contract. But facility executives in a seismic area are limited to diesel or a dual fuel unit. A dual fuel unit allows facility executives to store a smaller volume of diesel fuel on site and contract for on-demand diesel fuel delivery only. However, duel fuel units cost more.
Step five involves addressing other switch and exhaust details in a generator project that shouldn’t be overlooked.
First, facility executives shouldn’t forget about transfer switches. The new generator will need to feed one or more transfer switches to distribute the power it produces. For indoor generators, this can be done in the same room. Outdoor units will need to feed into a transfer switch room that is separate from the utility electrical room. The transfer switch room should have a fire rating and should also contain some of the emergency panels.
Facility executives also need to consider potential problems with exhaust. The risk of exhaust gases finding their way into the HVAC air intake can be an issue for all generators, but that’s especially true for roof-mounted units. Code requires at least 25 feet of space between the generator exhaust and the HVAC air intake, but this distance is rarely enough; a building’s architecture combined with prevailing winds can lead to entraining air when the distance is many times the requirement. The bigger the building, the bigger the generator and HVAC air intake — and the bigger the potential problem. In addition to careful design, exhaust piping and adsorption filters are two ways to address the problem.
Of course, the project won’t be a success if the generator isn’t used and maintained properly, and making sure those things happen is step six. The user interface, maintenance and emergency response plan are critical. So is training: The operation of the generator annunciator may not be intuitive to the person who monitors it, but some may not even know that it is related to the generator. The location of the emergency shutoff button needs to be clearly marked, and signage needs to be clearly placed for responding firefighters. Diesel units require a plan for emergency fueling — a five-gallon gas can will not do when the generator consumes 10 gallons per hour.
In addition, relevant information should be documented in clear language and archived for future reference. This information includes user manuals, procedures, maintenance schedules and instructions, as well as information about what systems the generator is intended to cover.
A generator is a big investment in terms of both time and money. And it’s an investment that will be part of the facility for the next 20 years. Understanding the process of generator selection and carefully considering all the issues will help ensure that the organization makes an investment it’s happy to live with — and one that might save lives some day.
Mike Daugird, P.E., is an electrical engineer for Marshall Erdman & Associates, an integrated design-build firm that specializes in healthcare. Daugird has more than 10 years of engineering experience and has collaborated on more than 75 healthcare facilities.
Who is Generating Code Requirements?Requirements for generators are developed by a diverse range of groups, including:
Model codes are adopted, often with amendments, and enforced locally by the Authority Having Jurisdiction. — Mike Daugird |
