# Understanding The Relationship Between Flow And Pressure

Once a piping system is confirmed, the flow rate is directly related to the square root of the piping system’s pressure difference; the greater the difference in pressure, the greater the rate of flow. If a man-made pressure loss system such as a regulating valve is installed in the piping system, the effective pressure difference is reduced, thus reducing the flow rate.

When working with piping systems, a common question is if the flow rate is proportional to the pressure, and how they are related. Pressure and water flow are the two components that power plumbing systems.

Plumbing and piping systems completely depend on the water flow and pressure. For example, if the water pressure is too high or too low in your plumbing system, it can damage equipment such as pipes and fixtures.

Flow rate is the amount of fluid that moves through an open channel or closed pipe, and the pressure is the internal pressure inside the pipe. The greater the differential pressure, the higher the flow rate is.

To understand the relationship between flow and pressure, you need to understand what flow and pressure are, how to work out the flow rate from the differential pressure, and what flow meters are used.

## Pressure Sensors

Whether you are monitoring the water flow in a hydroponics system or a large plumbing system, a flow meter is required to measure flow and a pressure sensor* to measure the pressure.

Flow meters accurately measure the flow rate and the amount of water that is flowing through a pipe. For accurate flow monitoring, both a precision flow meter and a well-designed flow meter totalizer are required.

*If you are unsure which pressure sensor your application requires, read out guide on the 7 types of pressure sensors.

## Bernoulli’s Equation

When talking about pressure and flow, it’s not long before Bernoulli’s equation is mentioned.

Bernoulli’s principle came from Daniel Bernoulli in 1726 when he stated: “In a current or stream, if the velocity is low, the pressure is high. If the velocity is high, the pressure is low”.

Referring to hydrodynamics, Bernoulli’s principle is used in fluid mechanics, and it is the essence of fluid mechanical energy. As Bernoulli’s equation was concluded from the conservation of mechanical energy, it can only be used to work out the flow and pressure of incompressible fluids.

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## What Is Pressure?

The general term “pressure” is the continuous physical force exerted against or on a contacted object. But, water pressure is the force exerted on the water to push it through a piping system or larger plumbing application.

Water pressure is affected by altitude and gravity. Water is a denser substance than atmospheric air, so the position and height can change the water pressure. The higher the water source, the greater the pressure. Gravitational force also affects water pressure.

### Does Pressure Affect Flow Rates?

Yes. As there is a very close relationship between water flow and pressure, an increase in pressure also increases the flow rate. Therefore, changes in the pressure will directly change the flow rate.

## What Is A Pressure Drop?

A pressure drop also referred to as a pressure loss, helps to determine the size of the pumps or motors needed in the piping system. It also allows you to work out the pipe diameter required to move the fluid through the piping system.

The greater the pressure drop in the pipe, the more energy is consumed to maintain a constant flow. Therefore, requires a more powerful motor.

On the other hand, the lower the pressure drop in the piping system, the less energy is used, and therefore, less power is needed from a motor.

The pressure drop also plays a key role in the type of pressure pump head required in the piping system. The pressure pump measures the force the pump applies to move the fluid. If a large pressure drop occurs, the pressure head will be larger to overcome the drop, which can have adverse effects on the piping system. Issues with large pressure heads include premature failure of seals and possible over-pressure faults in the piping system.

### What Affects The Pressure Drop?

The pressure drop is dependent on the flow rate, and vice versa. So if the flow rate is higher, the greater the pressure drop will be. Alternatively, if the flow rate is lower, the pressure drop will also be lower.

## What Is Flow?

In plumbing systems, flow is the amount of water to pass through a pipe at any given time. The water flow in plumbing is determined by the pipe width and the pressure. To put it into perspective, smaller pipes supply water to plumbing systems at a lower flow rate than larger pipes.

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The pressure also affects the flow rate. The greater the water pressure, the greater the water flow rate. So, the larger the plumbing system, the harder it is for the pressure to be dispersed evenly, and so the water flow rate will be lower. This is known as the flow rate and differential pressure.

## Flow Meters

If you are not quite a mathematician, a flow and pressure calculator can be used to calculate the average flow rate of water based on the length and diameter of the pipe.

As previously mentioned, the relation between the flow and the pressure is directly proportional. So, as pressure increases, the flow rate increases. This can be seen in a pressure and flow rate chart.

### Converting Flow Rate To Pressure

To convert the flow rate to pressure:

The flow rate is not determined by the rate of flow or pressure in the pipe, but by the pressure drop gradient along the pipe. So, to work out the flow, the length of the pipe and the differential pressure at both ends give the flow rate and the flow rate inside the pipe.

From the perspective of qualitative analysis, the relationship between the pressure in the pipe and the flow rate is directly proportional; the greater the pressure, the higher the flow rate.

The calculation looks like this:

Flow rate = flow rate x pipe inside diameter (ID) x pipe ID x π ÷ 4

When calculating the flow rate, we take the measurement at one end. After all, the pressure only comes from one end of the pipe because the flow of fluid inside a pipe is unidirectional. When the outlet is closed, the fluid cannot pass through the pipe.

When the pipe valve is open, the pressure drops, and then the pump kicks in and the pressure climbs. The fluid then flows depending on the pressure inside the pipe. When the valve is closed, there is no flow, and the pressure climbs.

To understand quantitative analysis, we use hydraulic model experiments. These can be done in your home or at work. All you need is to install a pressure gauge and a flow meter.

Another method to measure the flow is to time yourself filling a bucket with water. This method is commonly used to measure water flow at a faucet or shower.

## Relationship Between Pipe Diameter Pressure And Flow

The pipe diameter takes into account the thickness of the pipe wall. The pipe’s outer and inner diameters, typically have the same thickness, so the pipe diameter is taken from the inner and outer average.

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Pressure refers to the internal pressure of the fluid inside the pipe. The flow rate is also known as instantaneous flow; the amount of fluid that flows through the pipe (it can be closed or an open channel) per unit of time. The amount of fluid that passed through the pipe is called volumetric flow when the fluid is expressed in volume.

Most piping systems use a pipe diameter of 15-22mm*. The smaller the pipe diameter, the less water can travel through the pipe. An increase in friction occurs when the pipe gets smaller, so the pressure decreases.

*Make sure the pipe is adequately sized to allow the flow without exceeding the pressure rating of the pump and system.

## Summary

Fluid passing through a system requires a pressure gradient between two points, such as the inlet and outlet. Within the system, flow is directly proportional to the pressure. So, when high pressure happens, it drives a greater flow rate compared to lower pressures.

If you have any questions regarding flow or pressure, or what flow meter/pressure sensor will best suit your needs, do not hesitate to contact our world-class team at Atlas Scientific.

## Pipe Pressure Types

Pressure is the force that pushes the water through the pipe, which measures the pressure from the inlet to the outlet of the pipe.

There are three types of pipe pressure that you need to know.

The first one is the pressure source to the destination. This pressure is the force required to push the fluid through a piping system. An example of this is pushing fluid into a pressurized tank from a pipe or a hydraulic system.

Next is pressure and head. As you push fluid from a high to a low elevation, the pressure increases. The higher the elevation or the heavier the fluid, the greater the pressure at the pump. You can feel this pressure when you swim to the deep end of a swimming pool, or if your tap water comes from an elevated water tank.

Lastly, is pressure due to friction. This pressure is the largest contributor to pump systems. When friction increases inside the pipe, the pressure and flow are decreased. A frictional loss occurs due to the fluid passing through downstream equipment. When the pump speeds up, the flow increases, and so does the pressure.

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