YQA 27 Fuel System Special Issues
27.1 When is ant-siphon protection needed and how is it done?
Anti-siphon valves are used at aboveground tanks where fluid levels would be above the point of use with the potential for inadvertent siphoning of fuel out of the tank. The valve is installed in the fuel supply piping from the tank at the top of tank elevation. Anti-siphon valves are also required for submersible pumps in aboveground tanks, since fuel can flow through the pump under siphon conditions. There are 2 common valves used for anti-siphon protection: (a) spring loaded valves, or (b) normally closed electric solenoid valves or actuated valves. The electric actuated valves are sometimes preferred where there is a concern over the capability of a suction pump in reliably overcoming the spring force resistance of a spring loaded valve. The spring loaded valve is a spring loaded angle check valve with a spring force sized to resist the static fuel head. They are selected as 0-5, 5-10, 10-15, 15-20 feet of head resistance and are typically adjustable within that range. The fuel transfer pump must overcome the spring force to allow the pump to open and flow. Electric actuated valves are normally closed valves that open and allow flow when energized. Solenoid valves for submersible pumps are sometimes energized with the same electrical circuit as the pump to simplify control.
27.2 What is fluid hammer and how is it avoided?
Fluid Hammer is a pressure wave within a piping system that can cause excessive vibration of the piping and other system components. Fluid hammer occurs when the fluid in a piping system is rapidly pressurized, most commonly from a pump starting or a valve closing, particularly when refilling generator day tanks. The easiest way to avoid water hammer in fluid systems is through the control programming that (a) allows a pump to start only after a valve is day tank inlet valve is opened, and (b) allowing a day tank inlet valve (or the last open valve in a multi-day tank system) to close only after the pump has stopped. An alternative is to slow the start and stop of pumps using VFD drives, and to use actuated valves rather than fast closing solenoids at tank inlets. Other alternatives are (a) the use of hydraulic actuated valves to slow the pressure builup in piping systems, (b) the use of accumulators or bladder tanks to absorb and dissipate pressure waves.
27.3 What is important about fuel meters?
Accurate fuel metering depends on (a) removal of entrained air in suction pump systems, (b) minimizing turbulence at meter inlet and outlet connections using minimum lengths of straight pip, (c) measurement of fluid temperature and compensation (other than for mass flow meters). Meters used for custody transfer are commonly regulated by State and local agencies for accuracy, including periodic independent testing of accuracy. Meters are used in generator fuel systems for a number of purposes:
- Measurement of fuel delivered to various generators when multiple tenants draw fuel from a common tank
- Filtration systems that operate based on the volume filtered in a particular cycle
- Measurement of generator or boiler fuel consumption for air quality regulation recordkeeping
27.4 How is fuel consumption measured?
Fuel consumption measurement for diesel engines ins complicated by the fact that the engine consumes only about 1 / 3 of the fuel flow. There are 2 common methods of determining consumption: (a) measure the fuel flow into a day tank or reservoir tank and compensate for start volume, end volume, and temperature, (b) use 2 meters one on the engine fuel supply plus one on the engine fuel return, compensate for temperature and net the difference. The second method of measuring inflows and outflows can be subject to inaccuracy because a very small measurement error is multiplied by many times with repeated flow. Highly accurate mass flow meters are preferred for this method.
27.5 What are the causes of tank overfills and the appropriate protections?
Overfills are the most common cause of release from aboveground tanks. The key idea in prevention is redundancy. What several methods should be used to prevent overfills:
- a method – such as an electronic tank gauge- of determining the fuel volume in the tank, and calculation of the available free space to the 90% fluid level,
- a secondary method for levels / volumes such as a direct reading level gauge,
- a third method for level / volume such as a manual gauge stick,
- procedures for filling tanks only when facility personnel are present for observation and monitoring and lockable fill equipment to control access,
- a high level audible and visual alarm to warn the fill operation personnel when the tank reaches 85% capacity,
- an overfill prevention valve to close and stop flow when the tank level reaches 90%.
27.6 What are the causes of piping overpressure and the appropriate protections?
The most common cause of over-pressure in piping is caused by thermal expansion, an increase in temperature of a pipe section that is blocked between 2 closed valves. The situation occurs because relatively cool fuel may be drawn from a tank into a higher temperature piping system, or an exterior piping system may increase in temperature as the ambient temperature increases. Thermal expansion of fluid will cause a pressure increase that exceeds the pressure rating of pipe and valves in the fuel system. It must be relieved through safety relief valves installed in the piping system. Relief valves should be installed in any piping section that could be blocked at both ends by closed valves. The safety relief valves should discharge to a return flow pipe or dedicated discharge pipe that is open to the fuel supply tank. Some fuel system valves are available with internal pressure relief devices, however in general external relief devices are used.
27.7 What are the causes of suction pump problems?
Suction pump problems are caused by the following conditions:
- there is a leak in the suction piping allowing air to flow into the pipe,
- the vertical lift of the suction pump exceeds the maximum lift of the suction pump
- the pump inlet piping is under-sized causing a high line loss
- Valves or other devices, such as clogged strainers, cause high line losses
- The pump itself has poor suction characteristics