Will you help me with a fuel oil system RFI?

[ballsonwalls]

I have a main 3,000 gallon fuel tank on grade. They are pumping to a day tank in a penthouse about 70 feet above grade which are supplying boilers. The boilers require about 40 CFH. Also, this tank is supplying the day tank of an emergency generator on grade, about 20 feet away.
The generator requires 40 CFH as well. I have a 1" FOS and 1" FOR back to the main tank.

The RFI states that the return piping should be larger than the supply piping. Why? I have basically no idea what I'm doing as this is all new to me. My boss said it had to do with the velocity of the return (or overflow would be a better term) and the temperature of the oil.

I'd be asking him but he is not in today.

 

[MintJulep]

Assuming that both tanks are vented to atmosphere:

The Delta P on the supply is NPSH + Pump Head - 70 ft.

Delta P on the return is 70 ft.

For same size pipe, higher delta P -> more flow.

 

[lilliput1]

If your return line does not have enough capacity to allow gravity return (because tanks are vented) back to the main tank, the overflow will spill out the vent pipe.
The return and supply line must be sized for the capacity of the fuel oil pump the pump going to the daytank should be at least 5 gpm but the capacity of the pump selected would probably be more. Better add a flow measuring/balancing valve at the pump discharge to limit the flow at say 6 gpm. With 70' static head available the 1" return should be adequate if flow is limited per above.

The return for the generator however would probably be undersized. Remember to check the pump gpm & see if it should be limited.
Just for your information, in case you are not aware,note that the boiler & generator integral pumps are actually pumping about 2 to 4 times more than what is being burned. This has no bearing on your case because you are pumping to daytanks.
Note also that you should require that the main tank be filled with diesel fuel, not No. 2 fuel oil. Diesel fuel has antiknock properties and is needed by the generator engine. If diesel is not used, the engine generator manufacturer will void their warranty. Boilers can burn diesel or No. 2 fuel oil. Some installations have separate tank for diesel for generators and a separate tank for fuel oil for boilers.

 

[atlas06]

Contractors love to get the engineer chase a tail all the time. They tend to say we suspect that this and that violates code, etc..
It is the contrcator that has to justify his claim for the oversized pipe. Your 1" pipe is already oversized anyway. I've done at least 100 fuel oil gen set projects using 1" FOS/FOR and I never had a problem, ever.

If I were you, I'd just say: "Where did you get such an idea of a fuel oil pipe requiring oversizing on the return and it is up to you to prove that my 1" pipe is undersized."

Or ask him this: Hey mister, you get paid for building or for opinions?

I actually used that line above and I shut the heck out of the contractor in front of 15 people when he tried to get a fire damper change order. Instead of me chasing a tail, it was him spinning his tail.

Be firm, and tell him to go to hell. Trust me, it is the only language they understand AND respect.

 

[RossABQ]

"Be firm, and tell him to go to hell. Trust me, it is the only language they understand AND respect."

LOL, you better be prepared next time he finds a legitimate problem with your design! He'll bend you over big time!!