## 30 January 2017

### Flow and Pressure

There must be a minor difference between flow and pressure.

Water Flow :
Water Flow is a measurement of how much water is delivered at a particular outlet over a set period of time. For example, if you place a 10 litres bucket under the tap in a sink and it takes 10 seconds to fill the bucket you can see that the flow rate is 1 litre per second.

Water Pressure :
Water Pressure is a measurement of the force exerted by the water.
We understand it by one example a cold water storage cistern in the attic may be used to supply water to a basin in a bathroom and a basin in a downstairs cloakroom. Assuming everything else is equal you will notice that the pressure at the downstairs tap is considerably more than that at the one upstairs. The increased pressure is due to the height of the cistern in relation to the tap.

Higher pressure will cause greater flow through any given pipe size, but as the flow increases, the pressure will decrease downstream due to friction loss because water velocities increase as well.

For any flow to happen, there is a requirement of pressure gradient and not the pressure. Higher the pressure gradient, keeping all other things (fluid, pipe diameter, length) constant, higher is the mass flow rate. But higher pressure does not reveal anything about the flow.

• The pressure is defined as the force acting perpendicular to the surface of an object per unit area over which the force is distributed.
• In the case of gases, this force is because of the collision of the gas particles with the surface. So, the pressure exerted by the gases in a given environmental condition is more of a statistical average than average value.
• In terms of liquids, the pressure exerted is the weight of the liquid over a surface, acting on that surface.

• The definition of flow is subjective depending on the time and length scales considered. Since we defined flow as the bulk movement of fluid particles.

If we need more water, so increase the pipe size so we don't lose more pressure to friction loss.

PSI:
Pounds per square inch, the standard measurement of pressure in the United States.

Water Velocity:
The accepted standard for water velocity in piping systems is 5 feet per second or less. As flow increases in any given pipe size, the velocity of that water also increases. As velocity and/or flow increases in any given pipe size, the PSI loss also increases. The means of decreasing pressure loss for a given flow is to increase pipe size. (diameter)

Friction Loss:
The PSI loss which results from friction against the interior walls of pipes, directional fittings, valves or any other obstruction to the irrigation water. Once again, as flow increases so do friction loss. Friction loss is synonymous with PSI loss.

GPM, GPH, GPD:
Gallons Per Minute, the standard measure of flow; Gallons Per Hour, often used for low-volume flow such as drip irrigation; Gallons Per Day, a measure of overall water use on a daily basis.