Liquid Flow Calculation out of a pressurized tank

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Discussion Overview

The discussion revolves around calculating the discharge times and flow rates of liquids from a pressurized tank, specifically in the context of a tanker transporting various liquid chemicals. Participants explore the effects of overpressure and gravity on fluid discharge, as well as the application of Bernoulli's equation to determine exit velocities.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant inquires whether to add the force of overpressure to the force due to gravity when calculating discharge times from a pressurized tank.
  • Some participants affirmatively respond to the initial inquiry, indicating that the overpressure should be considered in calculations.
  • Another participant seeks clarification on applying Bernoulli's equation, specifically regarding unit cancellation in the calculation of exit velocity and expresses confusion about the term (P_tank - P_atm)/density.
  • A later reply suggests that the participant needs to multiply by g_c to resolve the unit discrepancy in their calculations.

Areas of Agreement / Disagreement

There is some agreement on the necessity of considering overpressure in calculations, but the application of Bernoulli's equation and unit consistency remains contested and unresolved.

Contextual Notes

Participants express uncertainty regarding the correct application of fluid dynamics principles, particularly in the context of varying pressure conditions and unit conversions.

Who May Find This Useful

Engineers and students involved in fluid dynamics, chemical transport, or related fields may find this discussion relevant for understanding the complexities of fluid discharge calculations under pressure.

Evan Jones
I am an engineer who needs to help my chemical transport girl friend with a little basic physics. It has been about 35 years since basic physics for me so I am rusty. She wants to understand how to calculate the following:

She drives a large tanker (cylindrical) which carries a variety of liquid chemicals and wants to understand how to calculate the discharge times. When they discharge the chemicals at the destination, they pump (and maintain) an over pressure of air in the tank during the discharge process. The fluid drains through a hose into an open (non-pressurized) tank. I know the fluid properties of the various chemicals, I just need to see if anyone can assist with the calculation. Am I just adding the force of the over pressure to the force due to gravity? If this were an open tank, I know how to make that calculation from the tank, through the hoses, and into the destination tank. I just don't understand how to account for the over pressure.
 
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The answer to your question is Yes.
 
Chestermiller said:
The answer to your question is Yes.
Can I ask you @Chestermiller to please expand on your answer. I am working on a similar problem whereby I need to determine the flow rate out of a tank that is under constant pressure from a hydraulic ram plate. I have looked at Bernoulli's equation but, I cannot make the units cancel out when calculating the exit velocity. Specifically, the term (P_tank - P_atm)/ density. The units for Pressure are in pounds-force per ft^2 while the units for density are in pounds-mass per ft^3. What am I missing for the equation v = (2*(P_tank - P_atm) / d)^1/2. Any assistance is much appreciated.
 
Last edited by a moderator:
fcroma said:
Can I ask you @Chestermiller to please expand on your answer. I am working on a similar problem whereby I need to determine the flow rate out of a tank that is under constant pressure from a hydraulic ram plate. I have looked at Bernoulli's equation but, I cannot make the units cancel out when calculating the exit velocity. Specifically, the term (P_tank - P_atm)/ density. The units for Pressure are in pounds-force per ft^2 while the units for density are in pounds-mass per ft^3. What am I missing for the equation v = (2*(P_tank - P_atm) / d)^1/2. Any assistance is much appreciated.
You need to multiply by g_c.
 

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