Water Flow vs Pressure

Flow is defined as the amount of water that travels through a pipe in a specific time period (usually seconds). This means that a higher water flow will have more of it travelling through the pipe than a lower flow. In order to measure an increase in flow, you would need to see more water in the pipe over seconds, minutes or hours. Pressure is defined as the force applied to water by gravity within an enclosed vessel (pipe) or system of vessels and pipes. For example, an increase in pressure would mean that the water is being pushed harder by gravity, and thus travels faster. Pressure is usually measured in pascals (1 Pa = 1 N/m2). In everyday tools like gauges, the pressure units are shown in psi.

How are flow and pressure related?

Flow and pressure are directly related to each other because they are both related to the movement of water. Any increase in the amount (flow) will cause an equal increase in water pressure. An increase in water pressure exerts more pressure on the plumbing system. Poor water pressure will affect the flow rate and the ultimate water supply.

This is because, as more water is flowing through a pipe, there are more molecules of water colliding with the sides of the pipe causing turbulent flow. This means that there are more collisions between molecules and atoms from which friction occurs during this process. As a result of these many collisions, pressure is generated causing an equal increase in flow rate.

Flow creates pressure and pressure creates flow. They co-exist and one cannot work without the other.

The pipe size used in the plumbing system will determine the amount of water passing through and the water pressure at the tap. The width and length of the pipe are crucial. Over a long distance, there will be a pressure drop.

To supply water to a house, additional factors determine how much water reaches the taps.

A water pump is another factor. A pump is very good at turning low water pressure into high water pressure. It pushes water into the pipes. The pump capacity in terms of pumping capability affects the volume of water.

Another factor is the valve control, which acts as a pressure regulator that helps control water flow. When wide open, more water passes through and vice versa. By adjusting the valves you control the pressure and water flow running through the pipes.

Additionally if many taps are open at the same time, a decrease in overall water pressure will be occur.

Does flow change with pressure?

The flow will not change with pressure if the pipe is perfectly straight. When a pipe is horizontal and there is no increase in elevation and friction, the flow will stay the same.

This means that water will continuously move at the same speed through the pipe as long as there are no changes or increases to either of these factors.

However, interestingly enough, this theory does not apply when you look at vertical pipes because gravity affects water more so than any other force in nature. This means that even tiny changes in elevation result in relatively large increases in the fluid pressure (and thus flow).

This happens because the higher up your elevation goes, the greater amount of gravitational force you feel pushing down on you from above which results in an exponentially increasing amount of pressure as you go up. This is the case when pushing water up into a water tower.

This is why, if your elevation increases even slightly, it becomes exponentially harder to push water through a vertical pipe because there is greater pressure pushing down on the water molecules when they travel upwards.

How do you calculate water flow from pressure?

Because pressure is the force applied on water, you need to express pressure in an equation relative to units of time.

Pressure = Force/Area (N/m2 x s1)
Force = Pressure x Area (N/m2 x m2)
Flow = Pressure ÷ Cross-sectional area (Liters per second, L/s)

The first step is to look at the unit of measurement for flow, which is litres per second. This means that your answer will be given in units of litres as well as seconds. The easiest way to find flow is to convert all measurements into Litres. From here we can write our formula: Flow = Pressure ÷ Cross-sectional area (Litres per second, L/s).

There are useful online water conservation guides and flow rate calculator tools that customers and your plumber can refer to, to determine the dimensions and pipes to get the best pressure and water flow reaching the taps.

Water pressure just like normal pressure is measured in pascals. It describes the force applied on the water by gravity and can be expressed in two separate ways:

Force = Pressure x Area (N/m2 x m2)
Force = Pressure ÷ Cross-sectional area (Pa x m2)

To solve for a specific equation you would need to calculate all variables that make up the formula. For example, if we wanted to find flow using pressure measurements of 20 Pa and an area measurement of 1 cm2, we could apply the formula like so:

Flow = 20 Pa ÷ 1 cm2
Flow = 20 N/m x s1 ÷ 1 m2
Flow = 2 L/s

Once you have the value for flow, convert it back to litres per second by multiplying Flow (in Litres per second) x 1000. This will give you your answer in units of Litres: 2 L/s = 2000 L/min.