Lesson 6 - Principles of Fluid Mechanics 3

The second major source of pressure drop in a pipe or duct system is due to the many other items which are used to construct the pipeline.  If you look around any brewery, packaging plant or even the plumbing in your own office or home and you will find endless elbows, valves, connectors, and other fittings which are a required part of the piping system function. These, all in their own way, contribute a loss of pressure, which at times can be more than the frictional pressure drop of the piping used. We will now look at these fitting pressure losses in more detail.

Fittings

If you think of a pipe or duct system, its job is to take fluid from point A to point B with the required control level of flow rate at the right time – without spilling any. It would be unusual and unsuitable for a single-straight continuous length of pipe or hose where the fluid is poured into one end and flows out of the other, to be used.

What is far more common is for the pipeline to be connected to a tank, with a control valve. This is followed by the pipeline taking a series of turns or bends horizontally and/or vertically to follow a path along a building perimeter, or round other vessels and equipment. The pipeline may change in pipe diameter if other pipelines connect to it. There may be filters, instrumentation ports, sample points, and a multitude of other fittings before the pipe enters the next tank. Examples of the types of fittings which may be involved include:

• Lengths of pipe or duct

• Bends

• Changes of pipe/duct sizes

• Tanks/vessels entry and exit ports

• Valves or dampers

• Filters

• Sensor housings

These fittings should significantly improve the function of the pipeline but come at a price – increased pressure loss. This loss has to be overcome by increasing the supply pressure somehow – most commonly done through the use of a pump or fan. As we will see in a later lesson, these fitting pressure losses (together with pipe frictional losses) directly affect the size and economics of the pump/fan required to produce flow through the pipe/duct system. Although careful and considered pipework design and selection of well-engineered fittings can reduce these fitting (and frictional) losses, they can never be eliminated. They are the price we pay to utilise a pipe or duct system.

Let us look in more detail at how we represent pressure losses due to fittings.

Fitting pressure loss representation

The flow of fluid through pipe and ductwork fittings generally results in the generation of turbulence and eddies downstream of the fitting. This is due to:

• Abrupt change of flow direction – think of pipe bends as an example. A 90° elbow is quite an abrupt change of direction which the fluid is forced through.

• Abrupt flow over steps, or changes of cross-section – even with good design and manufacturing practices, fittings can create seemingly small obstacles for the fluid to flow over, which create a drop in pressure.

• Flow around obstructions – obstructions require the flow to move around them, inevitably creating turbulence and a drop in pressure. Examples of these are thermometer wells, valve elements, and flow static mixers.

We have been talking about pressure being lost, either due to friction or in this section, due to fitting losses but where do you think the pressure goes to?

Give yourself 5-10 minutes to think about this, and then click ‘Next’ to continue with this lesson.