Monday, September 3, 2018

Designing Generator Fuel Systems (2)

Per "Designing Generator Fuel Systems (1)", we have discussed (1) Runtime Criteria; and (2) Fuel Storage. We are going to consider:-

Fuel Oil Pumping

Gensets are equipped with gear-driven pumps that pressurize fuel in the common rail of the engine. The integral pump draws fuel from the external tank. Excess fuel not injected into the cylinders is returned back to the tank. The pump has limited capability for priming and overcoming friction losses in the fuel distribution system (piping, fittings, and filters).

Usually, two types of electric-driven fuel oil pumps are used external to the genset - gear pumps and centrifugal submersible pumps:

Gear pumps: Mounted on a separate skid and typically used for low-flow, high-pressure applications, these pumps can be internal or external gear type and suitable when pressure requirements exceed 40 psi.

Submersible pumps: Used for high-flow, low-pressure applications. It require adequate clearance above the fuel tank for accessibility and maintenance, even though the majority of the pump assembly is within the tank.

Static lift and friction losses should be reviewed in detail during fuel system design. The design flow rate of the pumping system should be two to four times the peak demand so that pumps operate intermittently to fill the auxiliary tanks instead of operating continuously.

Cooling the Fuel

Excess fuel in the common rail that isn't injected into the cylinders is sent back to the tank. The return fuel is at an elevated temperature because it absorbs heat from the injectors and water jacket. When it mixes with cooler fuel in the tank, the supply fuel temperature gradually starts to rise.

For every 12 degree Celsius rise in fuel temperature above 37 degree Celsius, the engine horsepower reduces by approximately 1%. High fuel temperature also reduces its ability to lubricate the engine fuel system components. If the temperature of fuel being supplied to the engine exceeds a certain limit (typically 60 - 65 degree Celsius), the genset shuts down because of the safety cutoff. This is especially problematic when the tank volume is relatively small (e.g., auxiliary tanks) and the return fuel temperature is not reduced.

This basic fuel oil system flow schematic reveals the main fuel storage and auxiliary tanks.

Gensets with unit-mounted radiators typically are equipped with fuel coolers. They take advantage of the engine-driven radiator fan to reject fuel heat. Gensets with remote radiators typically require an external fuel cooler to reject fuel heat. Another option is to provide a return pump at the auxiliary tank and exchange fuel with the main tank (return hot fuel and replace it with cold fuel) if fuel temperature exceeds a certain setpoint. The return pumps also can be enabled manually to empty the auxiliary tank for maintenance, or via level sensor to prevent overflow conditions.

Fuel Transfer Pipes

When designing underground site piping, a nonmetallic material, such as reinforced thermosetting resin pipe is preferred due to its inherent corrosion protection. Underground piping is almost universally double-wall, and is comprised of a carrier pipe and a containment pipe. The interstitial space between the pipes is monitored with a leak detection system.

Fuel transfer pipes located above ground in accessible areas typically are single-wall carbon steel. Note that local jurisdictions and insurance carriers may require double-wall piping for aboveground applications as well.

Fuel Oil Maintenance

Fuel oil is made up of organic compounds and will gradually degrade over time due to biological growth, water accumulation, and particulate formation. This degradation, if uncontrolled, could result in clogged filters, or could negatively impact the combustion process in the generator engine.

Degradation is not a concern for applications where fuel is used on a consistent basis and a fresh stock of fuel is introduced regularly—for example, gensets used for combined heat and power applications. For standby generator applications, fuel usage is minimal due to limited runtimes as a result of periodic testing. For such applications, a fuel maintenance or polishing system can be provided for treating fuel oil periodically (usually on weekly or biweekly basis).

Continue Reading: Designing Generator Fuel Systems (3)

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