How much vapor in tank after pumping liquid out?

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It's been three long years since I took the PE and used my brain in an engineering manner. Now that I am allowed the freedom to solve problems, I find myself in a bit of a pickle... I seem to have forgotten much of my gray matter's ability.

So, a storage tank has the liquid inside being pumped into a rail car or tanker truck. We want to know exactly how much liquid pumps from one to the other and the flow meter is in question because there may be two-phase flow. Some of the liquid in the storage tank, while pumping, turns into vapor, so if the tank drops in level by 1000 gallons, it could actually be 990 gallons liquid transferred and 10 gallons that vaporized in storage tank (just random numbers).

Fluids isn't my cup of tea, but could anyone help me determine how to calculate exactly how much liquid has been transferred from the storage tank during any given transfer operation given the scenario above? Thanks :)

 
This all sounds like an exercise in Thermo and Fluids.

Changes in state are caused by changes in Enthalpy (i.e. addition/deletion of heat, or changes in P-V). See P-V or enthalpy diagram for your fluid.

What is this mystery liquid (properties: rho, Cp, heat of vaporization, compressible or incompressible)?

Is the tank vented (i.e. If the liquid in the storage tank is vaporized during transfer, does a negative pressure produce this)?

See formula for Extended Bernoulli, heat of vaporization (latent heat of Liquid's vaporization), and Ideal gas law (for quantity of vapor).

I can try to help, but you need more info. Helps to set up a control boundary and list the state properties at A and B.

 
What is the fluid? What is the tank pressure?

I wouldn't fuss around with Bernoulli equations and heat of vaporization.

You need to ensure you have the proper about of back pressure (by use of a control valve) on the section of line with the meter to keep the fluid in liquid state, or you will never be sure if you are getting an accurate number of meter pulses.

Or you can use a Coriolis Meter.

 
Is there a way to monitor the liquid level drirectly in the rail car or tanker that's receiving it?

 
This was for loading a tanker truck. We installed an orifice plate to keep pressure up for the flowmeter. Simple enough and I can't say we've had any fussing from operations yet on the fix. We also thought about just weighing the trucks before and after loading, but that would yield the greatest accuracy. Thanks

 
why not install scales where the trucks get loaded and get continuous weight data as they load.

 
Last edited by a moderator:
You need the liquid's molecular weight, vapor pressure (closed tank), air velocity, pool size (area of tank top opening), ambient pressure, and liquid temperature, or just estimates of these quantities, to approximate the overall evaporation rate in mass/time. You will use this for mass diffusion equation to estimate the mass loss per time. The flow rate will just follow from this approach. If you need the equation, just reply on this.

 
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