Parts Selection for Fluid Flow Systems

Posted by Kevin McWilliams on Mar 28, 2017 2:04:00 PM
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Many engineering projects, whether they are consumer products, industrial equipment, or even scientific gear, use flowing fluids inside the device.  Selecting the correct approach to containing and controlling the fluid can be crucial to a successful design.  There are several things to consider when deciding on an approach to designing fluid flow paths (IE plumbing):

  • What is the nature of the fluid - is it corrosive, toxic, or otherwise hard to handle?
  • What mechanical constraints must be considered - does plumbing need to be easily disassembled or be flexible, how precisely must parts align, what is the operating environment?
  • What is the sensitivity to cost in the plumbing assembly?

 In most cases, the selection of the fluid for a system is driven by other engineering considerations.  It must possess certain qualities, such as thermal or lubricating properties that are required to make the system function correctly.  The selection of materials and connection types for the plumbing then flows from this requirement.  The first order of business, once a fluid has been specified, is to understand what materials it is compatible with, and more importantly any materials the fluid is NOT compatible with.  

For instance, if we wish to move acetone through flexible plumbing, we should select EPDM or butyl tubing, as the acetone will aggressively degrade most other rubber formulations and problems will occur in short order.  On the other hand, if we are moving gasoline, EPDM or butyl are terrible choices.  Some liquids, such as liquid oxygen should only be contained in cleaned stainless steel plumbing or explosions and fire can result.  The selection of valves and other parts is also driven primarily by fluid compatibility of seal materials and temperature/pressure ratings.

The second primary consideration for plumbing design should be mechanical.  What sort of environment is the system operating in?  What fluid pressures must be contained?  In nearly every case, flexible rubber plumbing and plastic components will be significantly less costly than metallic parts, but may not be suitable for the application.  It is entirely possible to build flexible plumbing using entirely stainless steel rated for high pressures, but it is also quite costly to do so.  

A couple of additional mechanical concerns should be considered.  Firstly, how frequently must the plumbing be disassembled or disconnected?  Is the use of quick-disconnect fittings warranted, though they are much more expensive than standard plumbing connections?  Another concern may be the ease of assembly for plumbing.  For instance, threaded fittings are cheap, easily available in just about any material, and can be rated to extremely high pressures.  

However, it is nearly impossible to design precise threaded plumbing assemblies, because there is variation in how far together any two plumbing parts must be threaded to obtain a liquid-tight seal.  It is also typically rather difficult to obtain reliable clocking of features (like a pipe sticking out of a tee fitting) with threaded connections for the same reasons.  Finally, it is frequently difficult to reliably seal threaded parts, as pipe tape or dope must be used.  Use of flexible tubing with barbed fittings is one way around these problems, and is suitable for many common liquids at low pressures.  

In the case of more difficult liquids, high temperatures, or harsh environments, metallic plumbing must be used.  Compression fittings and drawn tubing are frequently the best solution in these cases.  Compression style fittings work by creating an interference fit between precise metallic surfaces to make a reliable seal around metallic tubing.  Tubing has the advantage that arbitrary shapes (within limits) can be bent from a single piece of tube and the fitting will seal at any clocking.  Plumbing of this type is nearly ubiquitous in laboratory and aerospace systems, where cleanliness and reliability are of paramount concern.  Of course, this is also typically the most expensive choice when it comes to plumbing design.  

 

Topics: Design, Fluid, Materials