Filling a tank to 100% capacity

[kbow364]

I'm trying to convince my boss that filling a plastic tank full of diesel exhaust fluid (DEF) to the very top with no air space is a bad idea due to thermal expansion.
The tank holds about 450 gallons, is not vented, but has a small forwarding pump attached to the top. My fear is that as the fluid heats up it could cause pressure buildup and damage the tank or the pump.
Am I wrong for thinking it's a smart idea to leave a 5-10% air bubble in the tank?

 

[FluidPowerUser]

You are correct. The expansion of the fluid will always be more than the expansion of the containment vessel.

DEF fluid is 67% water and 33% urea. The water will expand more that the urea, but both will expand as the temperatures rises.

The air will expand too, so if you want your plastic tank to last, you need more expansion volume or it will split if you don't vent it.

 

[LittleInch]

You're not wrong, no.

How much air space you need though is a function of the volume of the liquid, the amount of thermal expansion of the liquid and most importantly the pressure rating of the plastic tank.

With literally no vapour gap, your tank could increase in pressure by 2 to 3 bar every degree C rise if the tanks was very stiff. Now in practice this doesn't happen with plastic tanks because the tank walls start to bulge, but the pressure does climb until something gives way in the plastic tank, either a seam or a corner or the filling neck.

It is a very smart idea to leave at least 5% air gap, but does depend ion how much temperature rise you are anticipating in the liquid.
Apparently domestic "oil" or kerosene tanks they recommend only filling to 85% of capacity. Sounds a bit big to me, but gives you a good idea.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[kbow364]

Thank you for the responses, I feel like I'm going crazy trying to explain basic engineering fundamentals to my superiors and coworkers. I don't think the volume will expand much since there is an electric heater that keeps the DEF at a fairly constant temperature, but still only a 5 deg change could cause damage if the tank was completely full.

 

[Gator]

"Ullage is the volume which is left empty in a tank so that there is space for the liquid in the tank to expand."

 

[KernOily]

I'm probably misunderstanding, but I'm not clear how this tank is operating without being air vented. Unless it is rated for full vacuum or unless it is filled at the same rate it's emptied, a vacuum will be created in the tank relative to atmosphere. Plastic tanks are not rated for vacuum as far as I know so there is a risk present of tank collapse.

 

[3DDave]

Having a tank filled full up while maintaining a constant temperature is one thing.

Filling a tank up and then maintaining a constant temperature is another.

If you get a DEF delivery and it's below freezing on the delivery truck and that's what is used to fill the tank, then the heater is certainly going to cause problems if there is no place for expansion to go.

What happens then depends on the shape and strength of the tank material. Sadly, if it stretches permanently, then the next time will stretch it a bit more.

This sort of thing is handled in cars all the time. The cooling system is maintained 100% full, but there is a pressure relief to an overflow tank that allows coolant to be drawn back when the bulk of the coolant is allowed to cool.

 

[Compositepro]

I have not heard that this issue is a really problem. I think most commercial tanks are designed so it is not an issue. The fill opening may be lower than the tank top, or there is an allowance for bulging, like in milk jugs that have a hemispherical divot in the side that can pop out. If this were a real problem, which I agree it could be if not designed for, then tanks would be bursting all the time.

 

[TugboatEng]

How do you fill it and draw it down if it's not vented?

 

[pierreick]

Hi,
"The tank holds about 450 gallons, is not vented". End of the story, the tank is going to collapse.
Probably, no overflow, no dike ....

Pierre

 

[georgeverghese]

So how do you prevent tank from imploding with partial vacuum pressure during outflow ? No details here, there must be some way air gets into this tank when this pump on tank roof runs.
How is this forwarding pump operated - manually by operator ? This must be some kind of submersible pump, else priming the pump suction will be a problem. Your concern here is that operators may forget or neglect to start this pump when level rises close to tank top?

 

[TugboatEng]

An IBC tote of DEF is 300 gallons. The operating range of the tank should be 100-400 gallons. If you're pressing that means there will be spillage. Is this mess acceptable?

I am familiar with this problem. Your tank was sized for the best case scenario. The tugboat industry is notorious for doing this because bonuses are paid at completion of construction and not after verification of design.

 

[mk3223]

Is it true that the tank is without venting?
IMO, if the tank with a top filling nozzle, it‘s normally covered with a cap with the vent hole for “breathing”.

 

[TugboatEng]

The tank is vented. OP doesn't know where or how. The situation is impossible as the tank would have collapsed on the first drawdown.