Plumbing Connection Winter 2016

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PLUMBING CONNECTION

WINTER 2016

PIPE SYSTEM LOSSES

n the last edition I discussed the fundamentals of

pressure and related it back to pipe system design using

some everyday examples. You may remember I used

the term “maximum theoretical pressure” in some of my

examples in the last article because there are a number of

other factors that will cause pressure losses in pipework

systems such as pipe friction and form losses.

Pipe system pressure losses are very important and they

can significantly affect expected pipework flowrates and

velocities, as well as system performance.

In this issue I will provide a simple explanation of some of

the fundamental causes of pressure losses in fluid systems.

There are generally two types of pipe flow: pressurised

pipe flow (i.e. water supply systems - Fig. 1a) and open

channel pipe flow (e.g. stormwater drainage pipes - Fig. 1b).

The difference between pressurised pipe flow and

open-channel flow is in the fundamental mechanism that

drives the flow. For open-channel flow, gravity is the only

driving force acting on the fluid, i.e. water flows downhill.

However, for the flow in pressurised (full-pipe) systems,

while gravity may also play a role, the main driving force

is likely to be a pressure gradient along the pipe (i.e. the

fluid can flow uphill in a pressurised full-pipe system). For

example, in Figure 1a, when the pressure at point (1) is

greater than the pressure at point (2), a positive pressure

gradient exists which drives the fluid along the pipe from

point (1) to point (2). Importantly, if the pipe is not full

(Fig. 1b), it is not possible to maintain this pressure

difference (P1= P2) so the fluid does not flow (unless it is

affected by gravity).

Before we get into pipe pressure losses, I think it is a good

time to explain the concepts of hydraulic grade line (HGL)

and energy grade line (EGL). The HGL is a line that can be

drawn along a pipe to indicate the elevation to which the

fluid would rise if open to atmospheric pressure. This is

shown in by the piezometer tubes in Figure 2. It is also the

same height that the fountain of water would theoretically

raise if you punched a hole in the top of the pipe.

The HGL is an indicator of static pressure and is not

affected by the velocity of the fluid. The HGL is also often

called the Pressure Head and the units are usually measured

in metres height of fluid (m). In an open-channel, or a

partially full pipe, the HGL follows the water surface. The HGL

is the most important indicator for designers of stormwater

drainage systems because it indicates the water surface

levels (and potential overflows) in any of the system

components open to the atmosphere (i.e. in gutters or

drainage pits). It also identifies locations of any potentially

excessive system pressures.

The energy grade line (EGL) is a line that can be drawn

along a pipe indicating the total energy at any point in the

pipe. The EGL values generally include the Potential Head,

DR TERRY LUCKE

EXPLAINS PIPE SYSTEM LOSSES, THE REASONS WHY THEY OCCUR AND HOW TO AVOID THEM.

FIGURE 1 – DIFFERENT PIPE FLOWS

HYDRAULIC CLASSROOM

DR TERRY LUCKE

(A)

PRESSURISED PIPE FLOW

(2)

(B)

OPEN-CHANNEL PIPE FLOW

≠ P

(1)

= P

(2)

(1)