56 TECHNICAL
Basic theory of fluid mechanics for plumbing( Part 2)
Last month in Part 1 of this series, Plumbing Africa dealt with water and some of its properties and characteristics. Now let us continue with pressure.
By
Chris Kyle
About the author
Chris Kyle is a qualified commercial pilot and flying instructor, accredited CETA trainer and assessor, CPD course writer and presenter to the architectural fraternity, and professional plumbing industry licensed plumber. Chris has plotted his course in the building industry from his early days as national specifications manager for Cobra Watertech, to where he is today as the general manager of Calafrica.
1. Figure 1: Pressure is force over area.
2. Figure 2: A column of water 10 metres in height is equal to one atmosphere.
3. Figure 3: Static pressure is the pressure at a point in a system when no flow is taking place.
What is pressure as it relates to plumbing? Well simply put, pressure is the force that causes water to flow in a plumbing system and thus the amount of pressure available will determine the performance that the user of a plumbing system will experience, either good or bad.
Pressure is the force that a fluid exerts perpendicularly on a body and is expressed as the amount of force exerted over an area. One pascal is the pressure exerted by a force of one newton applied perpendicularly upon an area of one square metre. Thus, the formula for pressure is force over area, or P = F / A.
The formula for pressure is: P = F / A
Where: P = Pressure F = Force( mass) A = Area( surface area)
For plumbing, we generally use the kilopascal( kPa) as the unit of pressure measurement. So, what is a kilopascal? Where is it derived from? First, let us understand atmospheric pressure. ATMOSPHERIC PRESSURE At sea level, if we were to measure the weight of an air column extending all the way up into space and pressing down on a plate having a square area of one centimetre, then that column of air would weigh 1 013 grams( if standard conditions prevailed). That means it would exert a pressure of 1 013 hectopascals( that is, 1 013 grams per square centimetre). Dividing this figure by 10 will bring it to 101.3 kilopascals, rounded off to 100 kilopascals.
Thus, one atmosphere is equal to roughly 100 kilopascals( rounded off). See Figure 1.
Pressure is also often expressed in bar, and one bar is equal to 100 kilopascals. It can be expressed in many ways; for example, pounds per square inch( psi), where 14.7psi is equal to 1 atmosphere or 100 kilopascals.
100 kilopascals = 1 bar = 14.7 pounds per square inch( psi)
How does this relate to water? As we know, water weighs much more than air, so a column of water would exert a much greater force on a surface area than air. How much more?
One cubic metre of air at sea level weighs roughly 1 184kg, whereas one cubic metre of water has a mass of 1 000kg. Without even thinking too hard, we can imagine that the height of a column of water would be far less than a column of water to exert an equivalent force.
How much? See Figure 2.
A column of water 10 metres high exerts a pressure equivalent to one atmosphere. 1013hPa 100kPa.
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September 2018 Volume 24 I Number 7 www. plumbingafrica. co. za