Fluid flow

Fluid flow is one of the first subjects that undergraduate chemical engineers are taught. I guess there are few, if any, chemical production plants in the world which don’t use pipes with fluids flowing in them.

In many ways, fluid flow is an unglamourous subject. Graduate chemical engineers tend to think of “Unit Operations” (our name for the processes of heat transfer, mass transfer (such as distillation, liquid extraction, etc), chemical reactors, etc) as being the vital parts of a chemical plant. But fluid flow is vital to them all, and a solid understanding of fluid flow is essential for all chemical engineers.

As a rule of thumb, process piping costs around 25% of the total cost of a new chemical production plant. Hence getting process piping right is important!

When I started to learn chemical engineering, we used slide rules and log tables to do our calculations. I was one of the first students in my department at Leeds University to get a (very simple) electronic calculator! Hand calculations are vital to do in our learning years to understand the calculation procedures and underpinning science. However, nowadays, we do all our routine calculations by computer programs.

The commonest situation is that of fluids flowing down pipes to get from place A to place B. There are many computer programs to calculate such fluid flow problems. When in ICI I used Process Engineering Library (PEL) ( see http://www.pelsoftware.com/) which has been massively developed since then and is still sold by ABB, who bought most of ICI Engineering when ICI was “restructuring”. It seems to be a good suite of programs now. In recent years I preferred to use Pipe Flow Expert https://www.pipeflow.com/ which is an inexpensive, easy to use, comprehensive program for such problems. It has a lot of built-in information about pipe fittings (“Crane” data) to make such calculations speedy and repeatable. As with all such programs, information about special or unusual items has to be input by the user. However it is a very quick and accurate program to use for straightforward fluid flow problems.

For less common problems, like gravity flow, two-phase flow, slurries, unusual rheology such as thixotropic fluids, and transient flow, more specialised programs or even hand calculation are needed.

For example, when designing tanks draining by gravity, there comes a point as the liquid level falls such that air can be entrained into the outlet pipe as a vortex is formed in the exit branch. The presence of air or gas in the outlet liquid makes a profound effect on the nature of the flow, and the flow can become unstable. I once witnessed a 36 inch overflow from a cooling tower header about 50m high, which was jumping up and down by several inches due to this. What was happening was that

  • Overflowing water was entraining air, which caused the flowrate to fall.
  • This made the water in the header increase in depth, and the air bubbles were able to escape.
  • So then the flowrate increased, when again air was entrained, slowing the flow down.
  • And so on, the cycle was repeating, and risking breaking the pipe off as the mass of water in the pipe was significantly changing every few seconds, bending all the supports!
  • Many tonnes of pipe falling off would have caused a lot of damage, so the plant production rate and hence the cooling water flowrate had to be decreased to stop this happening until a larger overflow pipe could be fitted.

So, fluid flow is actually a very interesting area of chemical engineering. Transient flow was the specialised area in which I got interested. There’s a separate page on this site for that subject – see https://cedcs.com/transient-fluid-flow/