Propane tank sizing for standby generators
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The rise of natural gas and propane as relatively cheap, plentiful fuel sources has created a virtuous cycle when it comes to commercial standby generators. With building operators seeking fuel cost savings and the ability to avoid the extra maintenance required with diesel fuel storage, generator manufacturers have responded with new, cost-competitive gas and propane options.
“Investments by generator set manufacturers to expand and invest in their product lineups has resulted in some positive momentum for gaseous generators in this space,” says Mark Taylor, a technical marketing advisor at generator manufacturer Cummins. In fact, he says, prices for gaseous generators in capacities up to 200 kW are in line with diesel options.
For architects and engineers specifying generators in that size range, propane generators are more likely to be part of the consideration set than in the past. They’re a viable option for projects without access to natural gas or in jurisdictions where natural gas solely provided by the utility isn’t reliable enough for life-safety loads.
But to maximize the resilience benefits of propane generators, it’s helpful for engineers to brush up on propane storage tank sizing to make sure the generators function as expected. Luckily, Taylor says, sizing and selecting a propane fuel system can be very approachable for consulting or design engineers once they start down the path.
Sizing for vaporization rate
For both natural gas and propane, one major concern is to ensure the fuel source can provide adequate fuel pressure needed by the generator set at full load. When it comes to propane storage tank sizing, engineers should be careful to size the tank not only to last for the duration of an outage but also to provide an adequate vapor withdrawal rate.
Even if the tank is sized for overall capacity to meet the building’s NFPA 110 Class — the minimum number of hours the emergency system should run without refueling — a larger tank may be needed to meet the generator set’s fuel consumption at full nameplate rating, Taylor says. The local ambient temperatures can also impact this calculation and should be considered.
Sizing propane storage for vaporization rate is a factor that should be considered for any project using propane, notes Jim Bunsey, director of Superior Energy Systems, which engineers and installs propane systems. He recalls a correctional facility he recently worked with that doubled the building’s square footage and decided to simply double its backup propane storage, from two 1,000-gallon tanks to four.
While that amount of storage would have been enough to supply the facility for a day, the boilers and other equipment in the addition required a vaporization rate higher than those four tanks could produce.
Propane storage solutions
Adding more or larger propane storage tanks isn’t the only option. In some cases, moving a tank underground can keep it warm enough in cold climates to improve the vaporization rate. In others, engineers can turn to direct-fire vaporizers.
“It works like a water heater, except we’re actually heating the propane to boil it off, to create vapor pressure so they can run their system,” Bunsey says. That’s the solution the correctional facility employed, allowing the facility to maintain the storage in place. For standby generators, vaporizers can often utilize heat from the generator’s engine coolant.
A vaporizer can also help solve tricky engineering challenges, such as property restrictions that limit the size and placement of propane tanks. “We have found in the commercial space, real estate can be a crucial decision driver,” Taylor says. “Anything to decrease the footprint of the system is always a key marketing selling point. And so a vaporizer and liquid withdrawal can get you a smaller tank.”
Backup power for building resilience
Engineers are experts at predicting and planning for the known risks and challenges to a design, and enhancing a building’s resilience is all about navigating potential risks.
“In the world of emergency backup power, your power system is as strong as your weakest link,” Taylor says. “In certain scenarios, the resilience of the fuel supply could be that weak link.” Buildings with diesel-fueled generators should ask how long it can take to get a fuel truck on site if a prolonged outage occurs.
Propane fits right in as a means of mitigating against the shortcomings of a diesel-only or a natural-gas-only fuel strategy, Taylor says. “Diesel fuel requires continued maintenance; it does have a shelf life,” he says. “A gaseous generator set that is fueled by utility natural gas and on-site propane with automatic changeover offers the best of both worlds. Propane basically lasts the life of the tank, so adding it to the fuel readiness of utility natural gas, without the maintenance of diesel, can be a very compelling option for many end users.”
And, of course, when utility natural gas is out of the picture, using propane on its own to fuel a generator set also maintains the benefits of a low-maintenance fuel, Taylor adds.
Your propane supplier or a specialist such as Superior Energy Systems can help you design your propane storage system to ensure your backup power meets your resilience expectations. And generator manufacturers such as Cummins can help make fuel selection decisions, in addition to offering online resources and technical content. Finally, check out our Ultimate Guide to Standby Power Generation for a collection of valuable resources to help create more resilient buildings.