Converting PSI Pressure Loss in a Pipe to Fluid Loss Rate

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Discussion Overview

The discussion revolves around calculating the fluid loss rate in gallons per hour from a pressure loss measured in PSI during a pipe testing scenario. Participants explore the implications of pressure drops in an 8" PVC line, considering factors such as pipe length, fittings, and the behavior of water and air within the system.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • The original poster seeks a formula to convert PSI loss to gallons per hour based on a 10 PSI drop over time.
  • Some participants inquire about the length of the pipe and whether water is being replenished during the test, suggesting that air may replace leaked water.
  • One participant mentions the concept of bulk compressibility and questions its relevance if the pipe's volume remains unchanged.
  • Another participant discusses the compressibility of water, the stretching of the pipe, and the presence of air bubbles, suggesting a method to measure water usage during pressurization to determine leak rates.
  • There is a calculation presented estimating that a 10 PSI decrease corresponds to about 5 ounces of water loss based on certain assumptions.
  • Another participant calculates approximately 0.65 gallons of water expelled due to pipe contraction for a 10 PSI drop, referencing standard PVC dimensions.
  • Concerns are raised about temperature changes affecting the data, with one participant suggesting that temperature fluctuations could lead to significant variations in the perceived leak rate.
  • One participant questions the design of the test and suggests alternative methods for measuring leaks, such as draining the water and comparing volumes.
  • There is a call for clarification on who defined the testing criteria and whether they are aware of the limitations involved in the current testing approach.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the calculations and methodologies for determining fluid loss rates, with no consensus reached on the best approach or the implications of the test design.

Contextual Notes

Participants highlight several assumptions, such as the negligible change in pipe volume and the effects of temperature on measurements, which remain unresolved. The discussion also reflects uncertainty about the adequacy of the testing criteria and its implications for the design of the test.

CWPeltzer
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TL;DR
Pipe Testing
Summary: Pipe Testing

I need some help on how to calculate PSI loss to gallons per hour?

We will be testing this 8" PVC line at 150 PSI for 4 hours. If the test gauge drops 10 PSI per hour, what is the formula to determine the gallons per hour lost?

Thank you for your help.
 
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Welcome to the PF. :smile:

How long is the pipe? You are not replenishing the water in the pipe while running this test? So the leaked water is replaced by air that bubbles in via the leak?
 
berkeman said:
Welcome to the PF. :smile:

How long is the pipe? You are not replenishing the water in the pipe while running this test? So the leaked water is replaced by air that bubbles in via the leak?
500' long with roughly 60 offset fittings. no we will not be adding water through test.

Thank you for the help
 
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Are you aware of the property known as bulk compressibility? If so, how is it defined? Could you analyze this if the pipe did not get any smaller in total volume?
 
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When you pump the water into the pipe, you are compressing:
1) The water, as @Chestermiller pointed out, and
2) Stretching the pipe, mostly increasing the diameter, and
3) Compressing any air bubbles trapped inside. With 500 feet of pipe and 60 fittings, I can guarantee there will be air bubbles.

It will take about 1300 gallons of water to fill that pipe, plus more water to pressurize it. I suggest filling the pipe until it is full, but not pressurized. Then measure the amount of water to pressurize it. One way is by pumping the water in through a water meter. Record the total amount every 10 PSI or so, and plot the results to get a graph of volume vs pressure. Draw a smooth line through the appropriate data points, and you will know exactly how much water per 1 or 10 PSI pressure drop. Here is one possible meter: https://www.mcmaster.com/4119k41. They also have other water meters.

The PVC might creep a little the first or second time you run the test, so repeat the test if the results are marginal. If the apparent leak rate decreases with successive tests, blame it on creep and/or trapped air bubbles dissolving into the water.
 
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Do you have a pass/fail leak criteria for your pipe system; and, if so, who is determining this criteria i.e. is it your's internally or is it a customer's specification.
 
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Assuming negligible change in pipe volume and no dissolved air, for a water bulk modulus of 320000 psi, a 10 psi decrease corresponds to about 5 oz on 1300 gallons.
 
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Using std 8" sch 40 PVC pipe dimensions; and, PVC material physical data from the below reference, I calculate about 0.65 Gal / 10 psi will be expelled due to pipe diametrical and longitudinal contraction while reducing the test pressure from 150 to 0 psig with an initial 1300 gallon pipe filled volume at 0 psig.

https://www.usplastic.com/knowledgebase/article.aspx?contentkey=557
 
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Temperature changes will probably swamp your data. A 10 PSI decrease could be a 5 oz leak, or it could be a 0.2C temp decrease.
 
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Dullard said:
Temperature changes will probably swamp your data. A 10 PSI decrease could be a 5 oz leak, or it could be a 0.2C temp decrease.
@JBA showed that the pipe contraction swamps the liquid water expansion, and that the actual water loss for a 10 psi decrease will be more like 85 oz. And this is only for a 10 psi change.
 
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  • #11
So what y'all are saying are that this is an ill-advised (and poorly designed) test. Can you make suggestions to the OP for a better test? The pipe is kind of long, but if it were shorter, they could maybe drain the water out afterward and compare that to the initial volume to find the leaked volume?

And to @CWPeltzer -- Who defined/required this test? Do they understand these pretty severe limitations? Do they just want to know if the fittings will leak?
 

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