Out in the field I’ve noticed a particular practice in fuel tank installations that I think should be avoided. Perhaps due to a lack of available ports on the tank, or to cut down on piping costs, installers are combining the fuel suction and return connections on the bulk fuel storage tank. While this seems convenient, I’ve got a few reasons why you would not want this.
#1 – Fuel quality
Returning fuel to a bulk storage tank is pretty rare. It usually happens only if:
- the engine fuel return connection is piped directly to the bulk tank (no day tank in place),
- a day tank has overfilled, and excess fuel is sent back to the bulk tank, or
- an operator wants to send stale or hot fuel from a day tank back to the bulk tank (to exchange for fresh fuel).
When piping a return line to a bulk tank, the best practice is to connect it at the opposite end of the tank from the suction (supply) connection. Why? This encourages the fuel to move across the tank, and helps prevents water and contaminants from settling at the bottom. If contaminants are allowed to sit, they can cause serious tank deterioration over time.
This principle becomes even more important when you have a fuel polishing system in place (here’s why you should always have one). In those cases, it’s standard practice to have the filtration pickup and return at opposite ends of the tank. Moving the fuel this way helps keep contaminants suspended so they can be captured and removed by the filtration system.
When the suction and return connections are at the same location on the tank, you risk degrading fuel quality and shortening the life of your fuel tank.
#2 – Fuel recirculation
Fuel recirculation can be a big issue, especially with relatively small tanks connected directly to generators. If your fresh fuel supply is being pulled from a spot too close to where the hot return fuel is entering the tank, the engine could end up pulling warmer fuel. Consider a longer duration outage, where the generator is operating for many hours. It is easy to see how the fresh fuel becomes quite warm. This is a critical issue because, when the fuel gets too warm, engine performance (and generator output power) drops.
By spacing the suction and return connections further apart, you allow the hot return fuel more time to mix with the cooler fuel in the tank. This will aid in dissipating some of the heat, and ultimately prevent “hot fuel temperature” problems.
#3 – Air in the fuel supply lines
Another risk with this setup is the introduction of air into the engine’s fuel lines. If the piping arrangement has the return fuel splashing into the tank, above the normal fuel level, it can create substantial turbulence and foaming. If the suction pipe is close to the returning fuel, any air bubbles will get sucked right back into the fuel supply transfer pump. Worst yet, the air can travel to the engine if the engine is connected directly to the bulk tank. Sure, there are fuel filters that can remove air before the fuel reaches the engine, but introducing air into an engine’s fuel supply system is never a good idea.
How to avoid splashing and unnecessary turbulence when return fuel enters the tank? I recommend that all fuel return connections to a tank include a drop tube that extends well into the normal fuel level. This drop tube should be terminated with a diffuser.
So, what would a better arrangement look like?
Here is an example with my recommendations:
In Summary…
Designing a fuel system isn’t just about the big components; it’s also the small details that can make or break a system, especially a mission-critical system supporting emergency generators. The location of your fuel connections might seem like a minor point, but overlooking it can lead to serious problems down the line.
On-Site Power Advisor covers several subjects related to emergency power supply systems, and fuel system design is a critical design topic. If you’re working on a project that pushes the limits of your fuel system expertise, don’t hesitate to ask for help. Partnering with On-Site Power Advisor can make all the difference in ensuring that you design and deliver the best possible fuel system for your client’s needs.