Pressure reducing orifice plates

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Discussion Overview

The discussion revolves around solving a problem involving two pressure reducing orifice plates in series, specifically in the context of a ship's firemain system using seawater as the medium. Participants explore the relationships between supply pressure, flowrate, and pressure drops across the orifice plates, while considering the implications of varying supply pressures.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Clint seeks to determine flowrate and pressure after the orifice plates, given known orifice diameters and supply pressure, but lacking overall flowrate and pressure drops.
  • Some participants suggest that the oil cooler requires a minimum flow, indicating the need for design considerations that include safety margins.
  • Clint expresses interest in estimating resultant flowrates when reducing supply pressure from 9 bar to lower values (6, 4, and 2 bar) while keeping orifice dimensions constant.
  • One participant mentions that flow may be choked, implying limited dependence on pressure in certain conditions.
  • Additional resources for orifice plate calculations are provided, suggesting external tools for further exploration.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the methods for calculating flowrate and pressure drops, with various viewpoints on the impact of supply pressure and flow conditions being presented.

Contextual Notes

There are unresolved aspects regarding the iterative nature of the calculations, dependencies on Reynolds number, and the specifics of choked flow conditions. The discussion lacks a clear resolution on how to approach the problem effectively.

clint84
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Hi,

I would like to know how would i be able to solve a problem involving two (2) pressure reducing orifice plates in series with seawater as medium.

I know the orifice diameters, the supply pressure, but i don't know the overall flowrate, and pressure after the orifice plates.

The Ship's firemain which is the source of pressure = 9 bar, has a branch which is connected to an
Oil cooler downstream. Downstream from the oil cooler the pipe discharge into the sea (3m below waterline). The total length of branch pipe is 10m and the supply point is 5 m above the discharge point, with the discharge point being 3 m below waterline.

If I know the pressure but don't know the pressure drops or flowrate, is it possible to solve for the flowrate and pressure after the multiple orifice plates by hand?

I'm guessing this would be an iterative equation because the pressure drop over the orifice plates will be dependent on velocity - which i don't know, and K, which is dependent on Reynold's number and also related to velocity. Is there a method to solving this problem?

The only standards i find are on flowrate measuring orifice plates which are not the same as pressure reducing orifices.

Any help would be much appreciated

Cheers

Clint
 
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The oil cooler will require a minimum flow and so you must design for that, with a safety margin.
You know your input pressure, outlet head, pipe sizes and the pressure drop across the oil cooler at that design flow.
The two orifice plates will have to each drop half the remaining pressure.
 
I already know the minimum flow into the oil cooler, but what if i wanted to calculate/estimate the resultant flowrate from reducing supply pressure from 9 bar down to 6, 4, and 2, keeping orifice plate dimensions constant?

The orifice plates in series at the moment are designed for a specific supply pressure and design flowrate into oil cooler. I want to know how the flowrate and pressure drop over the orifices will change if i deviate from the design supply pressure, i.e. supply pressure is reduced significantly
 

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