In an earlier article, I covered a few items that should be defined when setting out to specify a diesel fuel storage tank for an emergency generator.
If you have decided that an above-ground tank is the way to go for your project, here is a quick checklist to guide your design. If you need support in any of these areas, please reach out for a consultation.
- UL-142 or UL-2085?
Your specifications should require compliance to the UL-142 standards for fuel tank construction. Where a “Protected” tank is required, UL-2085 compliance should be specified.
- Cylindrical vs. Rectangular vs. Vertical
Cylindrical tank designs are most common. They are less costly than their rectangular counterparts, and are less susceptible to the accumulation of contaminants (because there is no flat bottom). Rectangular designs can offer space savings, and allow better fitment inside rooms with a limited height. Vertical designs have the lowest footprint but, in larger sizes, their height can be problematic for fuel transfer systems that cannot lift fuel from top-mounted tank connections.
- Paint Systems
Most manufacturers use a standard paint system that consists of a single-step application (a self-priming finish coat is sprayed in one step). An optional multi-step system provides separate application of a primer base, an intermediate epoxy primer and a final urethane coat. The multi-step system offers the best protection against coastal environments, and is recommended for installations exposed to airborne chemicals (i.e: waste-water treatment plants).
- Tank Access Ladders, Stairs and Platforms
For safe access to the top of the tank (for maintenance purposes), your specifications should require a ladder (or stairs), and a safe platform to stand. Vertical ladders should be provided with a safety cage and gate to restrict access. Stairs should be provided with handrails. Once atop the tank, a platform or catwalk with handrails should be provided. In all cases, climbing devices should comply with OSHA requirements.
- Internal Access
Internal access to a fuel tank is normally accomplished through a removable manhole cover. These manholes are provided by the tank manufacturer with, or without, an internal ladder.
- Tank-mounted Accessories
Tank-mounted accessories may include overfill prevention valves, emergency vent caps, level monitors, inspection ports, and electronic sensors for remote monitoring devices.
✅ Piping Connections
- Tank Openings
Per NFPA-30, all piping connections to the tank shall be made above the maximum liquid level. Consider the overall fuel system and look to specify all openings required to accommodate piping connections, sensors, vents, etc. You may specify spare connections that can always be capped-off, if found to be unnecessary.
- Tank Venting Requirements
At a minimum, tank venting requirements should follow NFPA guidelines. Local AHJs may have additional requirements for the routing or termination of fuel vent piping. Reputable tank manufacturers will know what size venting is required for a given tank capacity.
- Fuel Fill Connections
Fuel fill connections may be provided directly on the tank (common for smaller tanks), or by use of a “fuel port” at ground level. When tank filling is performed from a location some distance away from the tank, an automatic fuel fill station offers a convenient system to allow safe, ground-level filling operations, while also monitoring fuel level and leak detection sensors.
- Fuel Supply / Fuel Return Connections
An internal drop tube should be furnished for supply of fuel to the emergency generator system. This drop tube is generally set 6″-8″ from the bottom of the tank, and should be equipped with an extractable foot valve. A fuel return connection should be provided with a drop tube and diffuser.
- Fuel Filtration System Connections
Connections for an automated fuel filtration system should include a suction pipe that reaches to the bottom of the tank. This will allow pickup of settled water and contaminants. The fuel return pipe (which brings clean fuel back from the filtration system) should be located at the opposite end of the tank. Locating the supply and return connections at opposite ends will promote fuel circulation inside the tank.
- Tank Monitors and Gauging Systems
Depending on the fuel system complexity, your tank monitor and gauging systems may consist of simple mechanical indicators for fuel level and leak detection, or may involve several electronic sensors that communicate with remote monitoring devices, or SCADA systems.
✅ General Design Considerations
Tank clearances should follow requirements listed by NFPA, and your local AHJ.
- Spill Containment
Spill containment is normally required at any connection(s) used for filling of the tank. For filling operations conducted on the top of the tank, a “spill bucket” is the most common method used to contain spills. For fill operations at ground level, a fuel fill port is recommended. Automatic fuel fill stations must be specified where the operator does not have a clear line-of-sight to the tank level monitor.
- Secondary Containment
Most tanks today are furnished as “secondary-containment tanks”. These tanks avoid the need for external diking. Secondary-containment tanks feature 360º double-wall construction, with an interstitial space to contain (and monitor) any leaks from the primary tank.
- Fuel Quality / Fuel Maintenance
Emergency generators are often designed with several days worth of stored fuel. Much of this fuel is unused and stored for months, if not years. Maintaining the quality of fuel is a requirement of NFPA. This topic is discussed in more detail here.
- Anti-siphon Protection
A rupture in a section of exposed fuel piping connected to a tank may allow fuel to be siphoned out of the tank. Anti-siphon devices address this risk by closing off a line that is not active. These devices permit flow through the fuel line when sufficient suction is created by a fuel transfer pump. Don’t forget that anti-siphon protection is also required on the fill line between a remote fuel fill station and a fuel tank.
- Integral vs Remote Fuel Transfer Pump Systems
Fuel tanks may be equipped with submersible-type pump systems, or external fuel transfer pump systems. Each type has benefits and drawbacks that should be considered on a case-by-case basis.
- Tank Selection in Multi-tank Systems
Controlling the direction of fuel flow is important where multiple bulk tanks are designed to serve multiple day tanks. This topic is covered in detail here.
These points should provide a quick checklist to help you select an above-ground fuel storage tank. If you are getting started on a design project, and could use a little (or a lot!) of one-on-one design support, my Design Advisory Services may be a good next step!