YQA Generator Day Tanks

07.01 Should day tanks be single wall or double wall?

Day tanks should always include a containment structure for at least 125% of the tank capacity, and in some locations by regulation 150%. Day tanks are built to a UL 142 standard for Steel Tanks for Flammable and Combustible liquid storage. This standard covers both single wall and double wall tanks.

Common constructions for day tanks are:

(a) single wall UL 142 steel tank within an open-top steel containment basin. This system has the benefit of the open top construction actining also to serve the containment for all of the pipe, valves, and fittings installed on the top of the tank. The drawbacks are problems of excluding rainwater in exterior locations, and the inability to pressure test the containment.

(b) double wall UL 142 steel tanks with the secondary top sealed to the primary tank. This system has the benefit of pressure testing capability for the secondary containment, and exterior installation capability that excludes rainwater. The drawback is that this type of construction may need a means of containment for the pipe, valves, and fittings mounted on the tank.

07.02 How big should a day tank be?

Day tank sizes are restricted by regulation to a maximum capacity that may be allowed within a room of a given occupancy. Within that restriction, there is not a standard for day tank sizing.

The recommended approach is to take the maximum generator consumption per hour, which can range up to 200 gallons per hour for larger generators, and consider the day tank size for 1, 2, 4, and 8 hours.

Here is an example: If the building code requires 4 hours of run time on a life safety generator that consumes 50 gallons per hour, then you would have 200 gallon of consumption. Since the tank is not always full, NFPA requires a 1.33 factor adjustment (to compensate based on a 75% full tank) which would be 266 gallons. Then round up to a 300 gallon day tank and check to see if the volume is within the regulatory restrictions for the room.

Another example: If the day tank has a well designed re-fill system from a bulk tank, the codes are restrictive, and space is at a premium in the generator room, then a 100 gallon capacity day tank may be appropriate, even though this is only 30 minutes of run time on a large generator with 200 GPM consumption. If the facility has well trained operating personnel, and the fuel system is monitored by the BMS, then 30 minutes of time to react to a problem with the day tank, may be an appropriate measure.

07.03 Where should the day tank be located?

The day tank should be located as close to the generator engine as possible, and preferably on the side of the engine that includes the fuel supply and return connections.

In some cases, the day tanks are located along one wall of the generator room for convenience, which in some cases could be up to 100 feet away. The generator dealer should be contacted to confirm that a generator fuel pump will operate with a remotely located day tank.

Local restrictions on the volume of fuel in a room, may require that day tanks be located remotely in fire rated rooms, separated from the generator room.

07.04 How is the tank refilled?

Day tanks are re-filled by either (a) on-board fuel transfer pumps, or (b) remote pump systems with inlet control valves at the day tanks..

On-board fuel transfer pumps are simple configurations for single generator – single bulk tank operations. They may be configured as either single or duplex pump systems. Limitations include: (a) limited suction lift on pumps means the bulk tank must be in close proximity, (b) multiple day tanks typically need dedicated suction lines, rather than sharing a common fuel supply line, (c) potential for loss of prime in suction lines.

Remote pump systems can be either positive displacement pumps or submersible turbine pumps, and are usually configured as duplex systems. Advantages include: (a) wide range of flow capacities, (b) ability to serve multiple day tanks.

07.05 How do you safeguard against overfills?

The primary safeguard against overfills is a properly sized gravity oerflow pipe from the day tank and returning to the bulk storage tank. A secondary safeguard is a high level switch in the day tank that disables and closes the inlet control valve.

The primary safeguard of gravity overflow cannot be used where the bulk tank is at a higher elevation than the day tank, or where pipe routing considerations do not allow for gravity flow back to the bulk tank. In these circumstances, a return flow / overflow pump is needed along with additional protection.

The return flow pump is configured as either (a) a separate overflow receiving tank with pump out, or (b) a return flow pump mounted on each day tank. In either case the return flow pump should be sized for 125-150% capacity of the fuel supply to the day tanks.

Additional protection can be provided by a high level stop valve in the day tank inlet piping. This is typically a normally open solenoid valve that closes when a high level sensor is activated.

07.06 What are common day tank problems?

The most common day tank problems are in re-filling: either high level or low level problems.

High level problems can be caused by: (a) leaking inlet solenoid valves, (b) inlet solenoid valve failed open, (c) inlet solenoid bypass valve is open, (d) failure of the fill stop level sensor, (e) loose wiring to the controller for fill stop level sensor.

Low level problems can be caused by: (a) failed solenoid valve, (b) closed manual valve in the day tank inlet piping, (c) failure of the fill start level sensor, (d) loose wiring to the controller for the fill start level sensor, (e) loss of signal from the controller to the pump start, (f) problem with duplex pump electrical motor starter, (g) closed valve in fuel supply piping, (h) loss of prime in pump suction piping, (i) low fuel level in bulk storage tank.

07.07 How is a day tank different for mission critical?

Mission Critical day tanks provide redundant functional elements for all of the potential failure mechanisms noted above. This type of design includes: (a) dual inlet solenoid or actuated valves, (b) high level sensor redundant to fill stop sensor, (c) low level sensor redundant to fill start sensor, (d) BMS monitoring of controller, (e) manual bypass of functional elements, (f) duplex pumping systems, (g) dual suction pipes from bulk tank to duplex suction pumps, (h) dual bulk storage tanks with auto-switch on low level.

07.08 What are common day tank accessories?

Common day tank accessories include the following: (a) level control panel, (b) inlet solenoid or actuated valves, (c) inlet strainers for solenoid valves, (d) inlet manual valves, (e) engine fuel supply connection with manual valve, (f) engine fuel return connection, (h) level sensor for high, fill stop, fill start, low, and critical low, (i) tank leak sensor, (j) direct read level gauge, (k) inspection port for manual gauging, (l) emergency vents, (m) normal breathing vent, (n) on-board fuel return / overflow pump.

07.09 Is a pump needed to transfer from the day tank to the generator?

The generator engine has an on-board fuel transfer pump that draws fuel from the day tank, and delivers the fuel to the engine at the proper flow rate and pressure. No supplementary pump is needed.

07.10 When is high temperature a problem and how is it handled?

High temperatures for generator fuel occur because the engine returns 65-95% of the withdrawn fuel back to the day tank. This return fuel flow is at a temperature that is well over 100 degrees F, without cooling measures, and this can cause the temperature in the tank to rise to dangerous levels. The engines often require relatively cool fuel, usually less than 110 degrees F, as a means of cooling certain engine components. And heated fuel can become a safety concern if it is heated above its flash point temperature.

High temperature is not a common problem because most generators include a fuel cooling radiator to treat the return fuel before it is received by the day tank. The fuel cooling radiator is mounted as a slave to the generator engine coolant radiator, and utilizes the same engine fan.

The fuel cooling radiator on the engine may not be practical where the engine utilizes a remote radiator for its coolant system. In this case several methods of fuel cooling may be used:

  • a dedicated fuel oil radiator and fan are installed at the day tank for return flow fuel from the engine.
  • fuel return from the engine is directed to a gravity return flow pipe to a bulk storage tank. Since the engine consumption is only 1 / 3 of the engine fuel pump flow, the fule supply to the day tank should be increased by about 3 times to accommodate the higher flow.
  • A temperature sensor in the day tank activates a return flow pump to pump fuel back to the bulk storage tank, allowing the day tank to be re-filled with fuel at a lower temperature.
07.11 How much fuel does a generator consume?

As a quick estimate we use 0.085 gallons per hour per generator KW at full load. So a 1000 KW generator consumes about 85 gallons per hour at full load, and a 2000 KW generator consumes about 170 gallons per hour at full load.

Generator specifications should be checked for exact calculations.

07.12 How do I size the inlet piping for the day tank?

For on-board day tank pump systems, the inlet piping should be at least the size of the pump inlet, and most likely will be increased to allow for calculated line losses on the suction side.

Remote pump systems should be sized for 2 considerations: (a) minimize line losses at full flow conditions to a maximum of about 10 PSI, and (b) size the piping sufficiently to make sure that the last generator in a series, is not starved for fuel.

07.13 Do I need an anti-siphon valve on the engine suction line at the day tank?

Anti-siphon valves may be required in the fuel supply piping from the day tank to the generator. The anti-siphon valve is considered a safety device if the fuel hose connections at the generator were to fail.

The preferred method of providing the anti-siphon protection is a DC powered normally closed solenoid valve that is activated directly by the generator controls. Spring loaded anti-siphon valves used for aboveground tanks are typically not used in this application because of the increased suction burden that they place on the engine fuel pump.

07.14 When is a return flow pump needed and how is it sized?

A return flow pump may be required for several reasons: (a) day tank overflow protection as discussed at Item 7.05 above, (b) day tank high temperature as discussed at Item 7.10 above, and (c) as a means of lowering the fuel level in the day tank for initial and periodic testing of the system. Sizing of the pump is usually governed by the overflow protection function.

07.15 Can the inlet flow to the day tank be precisely controlled?

There are 2 reasons to more precisely control day tank inlet flows: (a) to make sure that downstream day tanks are not starved for fuel flow, and (b) to make sure that the overflow protection mechanism, either gravity overflow or pumped overflow, is greater than the maximum inlet flow.

In general, a properly sized inlet control valve will allow for these functions without additional measures. A 1 / 2” solenoid valve at 25 PSI differential pressure will flow about 18 GPM. This allows for (a) a maximum normal engine consumption of about 3 GPM, (b) adequate capacity where the engine return flow goes directly back to the bulk tank requiring a maximum flow of about 9 GPM, and (c) adequate capacity for a 30 GPM rated return flow pump.

More precise flow control can be achieved by using a flow regulating valve, such as the one manufactured by Kates, that has been proven in fuel service. The device does have relatively small orifices which must be protected by a properly sized inlet strainer.

07.16 Do the day tank vents need to terminate outside?

NFPA requirements are that fuel tank vents must terminate outside of buildings. Emergency vents are often mounted directly on the day tank and these are typically acceptable since they are normally closed vents.

Some local regulations may require that emergency vents terminate outside the building as well. If the day tank is a UL 142 double wall tank with a closed secondary containment, then the tank will have a secondary emergency vent. In some cases both the primary and secondary emergency vents, as well as the normal breathing vent, must be terminated outside the building. The size and number of these vent pipe can be a routing challenge within the building, so it is an issue worth addressing with the local fire authorities early in the project.

07.17 Can a common vent pipe be used for several day tanks?

NFPA requirements are that vent pipes be independent for each tank, except in special circumstances. The special circumstances are usually vapor recovery systems for gasoline underground tanks.

In some cases, the local regulators have allowed for the combining of day tank vents into a common vent header to the outside of the building.

Another basis for combining vents is to calculate the normal and emergency venting requirements of the day tank, and provide a single properly sized vent pipe to the exterior that is normally open, and fulfills the requirements of both functions.

07.18 Can a day tank be shared by more than one generator?

Day tanks can be used for more than one generator, when they are properly sized. Each generator should have an independent fuel supply line from the day tank, and preferably an independent fuel return to the day tank.

The reason that each generator usually has its own day tank is redundancy. A failure of a single day tank would disable all of its associated generators.

07.19 What day tanks levels are specified for high and low levels?

Typical day tank level switch activations are:

  • 90% High Level
  • 85% Fill Stop Level
  • 75% Fill Start Level
  • 50% Low Level
  • 15% Critical Low Level

These can vary slightly based on the depth of the tank, because there is a minimum spacing between floats

07.20 How are day tank levels integrated with the generator?

Commonly generators use inputs from the day tank controller for high level, low level, critical low level, and leak. The critical low level input is often used as an engine shutdown condition.

In some instances these day tank signals are input to the generator control equipment, such as ASCO, RussElectric, GE Zenith, or others, rather than to the individual generator controls.

07.21 Do I need filters for the day tank?

Generators commonly have fuel filters at the inlet connection to the generator. For this reason, filters at the day tank inlets are not commonly used.

07.22 What are day tank size restrictions in building codes?

Local building code requirements will vary as to the limitations on day tank sizes, within varying types of room construction and occupancy ratings.

A common restriction is 240 gallons total per room, and 60 gallons per individual tank.