# Head loss and pressure relationship

### Pipe Pressure Drop Calculations Formula, Theory and Equations

Pipe pressure drop calculations and how to calculate pressure loss in a pipe. Our Pipe Flow software automatically calculates the friction loss in pipes using fitting losses will be minor in relation to the the overall pressure loss in the pipe. constant v1 = v2 hydraulic loss is equal to the head of pressure drop or head loss An approximate explicit relation for f is given by S.E. Haaland in as. 1. Head loss or pressure loss represents frictional flow resistance. The head loss from friction is related to the velocity energy of the liquid squared.

The liquid in the "box" on the very surface of a pipe does not flow or move. It always remains stationary.

The liquid in the "box" above it has to slide against it and that requires an amount of energy to overcome friction between the two "boxes.

A layer is formed by this non-moving liquid and reduces the inside diameter of the pipe. This increases the velocity of the liquid passing through it. The liquid is not moving at the pipe wall but has a much higher velocity at the center of the pipe.

## Darcy–Weisbach equation

The condition of the inside of a pipe also has a great effect on the head loss of the flow of liquid. The rougher it is, the thicker the layer of non-moving or slow moving liquid near the pipe wall. This reduces the inside diameter of the pipe, increasing the velocity of the liquid. With the increase in velocity comes an increase in friction losses.

Pipe Fittings Any time a liquid flow changes direction there is resistance. Since all liquids have weight, they also have momentum. This means the liquid will always try to continue moving in the same direction. When the liquid encounters a change in direction such as an elbowits momentum carries the flow to the outer edge of the fitting.

### Head Loss in Piping Systems - TechInfo

Because the liquid is trying to flow around the outer edge of the fitting, the effective area of the fitting is reduced. The effect is similar to attaching a smaller diameter pipe in the system. The velocity of the liquid increases and the head loss due to friction increases. Energy Loss Any time a liquid is asked to change direction or to change velocity there is a change in energy. The energy lost by the liquid is converted to heat created by friction.

Since the amount of liquid exiting a pipe has to equal the amount entering the pipe, the velocity must be equal. If the velocity is equal, then the velocity energy head must be equal. This only leaves one place for the energy to come from: The measured pressure entering the pipe will be higher than the measured pressure exiting the pipe.

Friction Loss Tables In an effort to easily predict the head loss in pipes and fittings, there were a number of studies made many years ago. These have been published, as formulas and tables, for different size pipes, fittings, and flow ratings.

## Head Loss – Pressure Loss

The most common used are "Darcy, Weisbach" and "Williams and Hazen. The "Darcy, Weisbach" tables are based on the head loss in clean, new pipe. They are based on the head loss in ten-year old pipe.

Their values must be adjusted for different pipe age and materials. The data is given in table form for the different pipe sizes and flow rates. The loss that a specific pipe fitting introduces is measured using real world experimental data and this is then analyzed to determine a K factor a local loss coefficient that can be used to calculate the fitting loss as it varies with the velocity of the fluid passing through it.

Our Pipe Flow Software programs make it easy to automatically include fitting losses and other local losses in the pressure drop calculation since they come with a pre-loaded fittings database that contains many industry standard K factors for various different valves and fittings, at various different sizes.

All the user has to do is to select the appropriate fitting or valve, and then choose 'Save' to add this on to the pipe, and have it included in the pipe pressure loss calculation.

Pipe Component Loss Calculations There are often many different types of components that need to be modeled in a piping system, such as a heat exchanger or a chiller.

Some components may introduce a known fixed pressure loss however it is more likely that the pressure drop will vary with the flow rate passing through the component. Most manufacturers will supply a component performance curve that describes the flow verus head loss characteristics of their product. This data is then used to calculate the pressure loss caused by the component for a specified flow rate but the flow rate itself will also be dependent on the pressure loss downstream of the component and so it is very difficult to model component head loss performance without the use of appropriate software such as Pipe Flow Expert.

Pressure Loss due to Change in Elevation Flow in a rising pipe If the start elevation of a pipe is lower than the end elevation then on top of friction and other losses there will be an additional pressure loss caused by the rise in elevationwhich measured in fluid head is simply equivalent to the rise in elevation. Between two points, the Bernoulli Equation can be expressed as: In other words, the upstream location can be at a lower or higher elevation than the downstream location.

If the fluid is flowing up to a higher elevation, this energy conversion will act to decrease the static pressure.

If the fluid flows down to a lower elevation, the change in elevation head will act to increase the static pressure. Conversely, if the fluid is flowing down hill from an elevation of 75 ft to 25 ft, the result would be negative and there will be a Pressure Change due to Velocity Change Fluid velocity will change if the internal flow area changes. For example, if the pipe size is reduced, the velocity will increase and act to decrease the static pressure.

If the flow area increases through an expansion or diffuser, the velocity will decrease and result in an increase in the static pressure.