I would think that the liquid entering the truck will push the gas out and provide the energy to produce the flowing vapor pressure drop at flowrate in the vapor pipe to the main header with only inches water gage pressure drop. Maybe there is some other reason for a blower but I don't know of one myself.

The vapor lines from a bunch of tank and truck unload stations and one truck loading station tie into a header leading to the thermal oxidizer.

Is the problem the existing header size? Can the header be looped so it can flow either way?

More detail, better answers.

Good Luck,
Latexman

The truck tank may be an ISO container with a very low design pressure, and this may most likely not be high enough to flow these vapors out to the TO, given the max developed backpressure during operation. Backpressure buildup will include components such as the waste gas burner, any inline detonation arrestors and line frictional drop. If the vapor space in the truck isotainer contains O2, then the common header may also need an air sweep to keep the flammable vapor composition below LEL.

Quote (abot8765)

I don't think I understand why a blower would be needed in this line.

1/ Provide negative pressure in truck&piping to assure that toxic vapors will not escape.
2/ Provide design dP across burner to assure proper mixing and therefore degree of VOC destruction.
3/ Prevent inadvertent backflow in case of hydraulic issues or operator error.
4/ Suck vapors from truck tank before filling start to avoid product swing/contamination.
5/ Hasten vapors evacuation during truck tank pressure/vacuum swing inerting.
6/ Provide ventilation of piping low points and pockets to avoid stagnant conditions and undesired polymerisation, condensation etc.

Vapors are hard to be controlled and those not always want to go to a point it was intended. There are many hidden&sudden events that have a potential to force a system work another way it was designed/intended/considered with no a clear evidence to operator. A pressure changer (e.g. blower) will help to control flows.

Abot.

The key issue as my fellow posters have stated is what is the MAX pressure allowed in the tank truck?

Often this can be very low pressure - think low inches of water column. Hence there is very little pressure to flow the gas to the burner which will have a pressure requirement of its own.

The vent headers are often large to allow this flow.

So find the lowest tanker pressure at the facility and that will give you an idea of how much or little pressure is available.

Blowers though have a risk of implosion of trucks so need to be designed very carefully to avoid this.

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

Thank you everyone for the great responses to my questions. It really helps provide some insight.

We will be getting a better picture of the existing vent header system in terms of layout, flows, pipe sizes, etc., and will be getting fan curve(s) for the thermal oxidizer.

For the truck loading line of interest, we should also be getting more information on layout, in-line losses, and truck max pressure. There is a vacuum relief valve set at five inches of water column, to be installed in the vapor vent line.

Thanks again for all the reasons why a blower can be useful, I am hopeful that digging deeper into the gas pressure/flow calculations of the system will help too.

Quote (LittleInch)

Often this can be very low pressure - think low inches of water column.

Truck tank is able to absorb multiple daily suspension shocks when driving loaded on highway but is unable to withstand tiny overpressure (inches of water) when staying still during loading. Am I correct? All truck tanks I have ever seen before seemed very rigid and able to withstand a small explosion nearby. You have another experience? Is the situation different where you are from? Can you explain please?

shvet,

Physical strength is different to internal pressure. Often the issue is not the tank itself, but all the connections. But even a few psi when there are thousands of square inches available, rapidly add up to a lot of force.

Tankers like LPG are rated for pressures up to 25 bar, but most tankers are I suspect "atmospheric" pressure, a bit like large storage tanks.

Bit difficult to find a definitive source, but 0.2 to 0.4 bar or 4 psi for "normal" liquids seems to crop up a lot. So maybe a bit more than "inches of water", but start low and go up when it comes to tanks.

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