Return pumps: Not just for returning fuel
Are there any fuel oil handling systems that don’t need return pumps? Are there any advantages to not having return pumps? While it is true that I have worked on system designs and executions that have not included return pumps, my general answer is “no,” there are not any advantages. And there are many different functional reasons why, especially within Mission Critical and First Response handling systems. The purpose of this white paper is to list several different uses for return pumps under various circumstances, and to demonstrate how every single installation can benefit from them.
We all know that return pumps are great for keeping things under control, even if humans seem determined to find a way to spill fuel—running supply pumps in “hand,” or opening valves that should remain closed, and preventing a day tank overflow. This is the most basic function of a return pump, and by far the most popular reason for their inclusion in both older and more modern designs.
FUEL RETURN: Sending fuel from a day tank, to a main tank
Fuel return is a return pump’s most basic function. During typical, normal operation, the return pump will be a lonely piece of equipment, but will also be one of the important safety features within the entire fuel oil handling system (FOHS), potentially saving very significant environmental and financial damages.
On many basic systems, the return pump will only activate when a “high” level float switch is activated, or similar signal, from a Controller. An improvement is to have the return pump interlocked with a “high” level switch, which will start the return pump even in the event the Controller is offline or disabled.
Be sure to size the return pump appropriately to the application. Calculate the highest possible rate of fuel entering the day tank. On multiple day tank systems, this may mean the highest flow rate possible from all supply pumps running simultaneously, while filling that one particular day tank, and no others. The return pump should be sized at a significantly higher rate than that potential high-flow rate. How much higher will depend on pump type, available flow rates, etc., but should be 150-200% of the highest flow rate possible from the supply pumps.
INSPECTIONS AND REPAIRS: Emptying a day tank for repair or replacement
What if you find fuel in your day tank’s secondary containment? After looking for the usual leaking suspects, such as a threaded connection, weld, valve, etc., you may come to the conclusion that your primary tank is leaking. Whether you intend on inspecting the tank, repairing or replacing it, you will need to drain it first. But, if you have only a top/side-mounted overflow, how do you get the fuel out?
A return pump makes this process much faster and less messy, sending the good, clean fuel exactly to where you want it—back into the main tank. You are also eliminating potential problems with big, messy barrels, which will contaminate the fuel and cause you to dispose of hazardous materials, increasing the hassle—not to mention the expense—even more.
FUEL FILTRATION: Cleaning ALL of the fuel, not just the fuel in the main tank
One of the most common reasons for emergency generators to fail to run is bad fuel. The best way to prevent bad fuel is to filter it. ALL of it. This means polishing the fuel that is trapped in supply lines, return lines, and day tanks. Generators get their fuel directly from their day tanks, but most fuel is only polished in the main tank!
Return pumps are great for circulating fuel. Modern control systems can be programmed to run polishing sequences, including activating return pumps (and supply pumps) that will circulate the fuel through the day tanks and allow it to be cleaned at the main tank(s). Running a day tank “turnover” sequence, in combination with a filtration/polishing sequence, ensures much cleaner fuel throughout your entire fuel system. The best generators on Earth won’t run on bad fuel!*
*ALL Mission Critical facilities should be on a strict fuel filtration and polishing regimen (Please look out for other white papers about Filtration from Preferred Utilities). In fact, any facility that requires a backup generator of any type has a responsibility to ensure, to the best of their ability, that the generator runs in an emergency. That’s what they’re there for!
THERMAL MANAGEMENT: Decreasing fuel supply temperatures for generators
Generators use diesel fuel oil not only for internal combustion, but also for cooling the engine’s injectors. Less than half of the fuel that the generator draws from the day tank is actually used for combustion; the remainder of the fuel is returned to the day tank by the engine, and at a higher temperature. As continuous running creates a continuous fuel temperature increase, this can adversely affect the performance of the generator, up to, and including, generator shutdown.
There are two main contributors to overheating a day tank’s fuel:
- Ambient temperature. Perhaps the day tank is outside, or on a rooftop, in a warm climate, or all of the above. If the fuel in the day tank is already at 95 degrees F, for example, it’s going to rise fairly rapidly when the generator engine starts. Unfortunately, in places like California, many power outages occur during the hottest days of the year, due to excessive demand on the grid.
- Day tank size versus generator engine size. A large generator engine paired with a small day tank will increase the day tank fuel temperature quickly, regardless of any other environmental conditions. Local, state, or national code may inhibit the installation of larger day tanks.
Return pumps can assist in decreasing fuel temperatures for generators under both scenarios. By simply circulating the hot fuel out of the day tanks, and replacing it with cooler fuel from the main tank(s), the generators will continue to run, and run more efficiently.
This fuel circulation can be automated. The day tanks can be equipped with Resistance Temperature Detector (RTD) probes, which will monitor the day tank temperature. When the day tank temperature reaches a pre-determined threshold, the RTD will signal the master control system (“Controller”), which will start a day tank cooling sequence. We sometimes refer to a day tank cooling sequence as, “level bouncing.” A level bouncing sequence would look something like this hypothetical example:
- RTD reports temperature threshold met on Day Tank 1 to Controller.
- Controller activates Day Tank 1 Return Pump. Day Tank 1 begins to pump out.
- Day Tank 1 reaches “Supply-Pump-On” lower level, which creates a Call For Fuel.
- Controller turns Return Pump off.
- Controller activates Supply Pump, and opens Day Tank 1’s inlet valve.
- Supply Pump fills Day Tank 1.
- Day Tank 1 reaches “Supply-Pump-Off” high level.
- Controller deactivates Supply Pump.
- RTD monitors temperature, and…
- RTD reports temperature threshold met on Day Tank 1 to Controller, and sequence repeats… or…
- RTD reports temperature acceptable. No action occurs.
Return pumps are useful for far more than just pushing fuel back to a main tank. They are an integral part of any system and do not only save us from a messy cleanup and a lot of explaining, but also enable us to truly and completely clean a system, cool a day tank, or just do a more thorough inspection.
For more information, or if you have any questions, please contact:
District Sales Manager, West
PREFERRED UTILITIES MFG. CORP.