Can a larger pressure release port decrease pressure loss over time?

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Discussion Overview

The discussion revolves around the relationship between the size of a pressure release port and the rate of pressure loss over time in a vessel filled with seawater. Participants explore how increasing the diameter of the port affects the speed of pressure release, particularly in a scenario where the pressure must drop from 70 bar to 3 bar before a valve can be fully opened.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant inquires about the time it would take to release pressure if the port diameter is increased from 5mm to 10mm, questioning the direct relationship between the size of the hole and the rate of pressure release.
  • Another participant suggests that if the pressure drop occurs over a short length, mass flow may be proportional to the area, proposing that doubling the diameter could reduce the time to release pressure by a factor of four, assuming no time is needed for the opening process.
  • A follow-up request for references or formulas related to the assumption of mass flow rate and area is made, indicating a desire for further clarification on the topic.
  • Another participant notes that for short ports, the exit velocity is determined by the pressure difference and the shape of the port, rather than the area, suggesting that this is a common topic covered in fluid mechanics textbooks.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between port size and pressure release rate, with some suggesting a proportional relationship while others emphasize the role of pressure difference and port shape. The discussion remains unresolved regarding the exact dynamics at play.

Contextual Notes

Assumptions about the length of the port and the conditions under which mass flow is considered proportional to area are not fully established, and references to fluid mechanics principles are not provided in detail.

sionglx
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Greetings All,

I have a question regarding Pressure Loss through a Small hole vs Time.

My simplified scenario is that I have a vessel filled with Sea Water Pressured up to 70 bar. At the outlet valve, there is a pressure release port of 5mm dia which is able to relieve the pressure down to 3 bar is a matter of 3 seconds.

To describe the scenario better, only after the pressure has been released can the valve be opened fully.

Question:- How long will it take if i increase the Pressure release port to say 10mm. I would like to make the pressure release even faster. Is there a direct relation of rate of pressure release to size of hole??

Am unable to find anything regarding this topic.

There is a similar thread but cannot make anything out of the response there.

https://www.physicsforums.com/showthread.php?t=171660&highlight=pressure+loss+hole+time

MAny Thanks & Regards.
 
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It will depend on your setup. If the pressure drop in your port happens over a short length (<5mm), it should be a reasonable assumption that mass flow is proportional to the area. In that case, doubling the diameter should give 1/4 the original time. This assumes that the opening process does not need any time...
 
mfb said:
It will depend on your setup. If the pressure drop in your port happens over a short length (<5mm), it should be a reasonable assumption that mass flow is proportional to the area. In that case, doubling the diameter should give 1/4 the original time. This assumes that the opening process does not need any time...

mfb,

thank you for your quick reply. i would have to confirm that the port length is very short.
COuld you please quote some reference where you get the assumption of mass flow rate and area?
Or if there is a formula i could refer to?

BR,
 
For short ports, the exit velocity is determined by the pressure difference (and the shape of the port), but not the area. Every textbook about fluid mechanics should cover that.
 

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