Pressure Drop In Pipe Lines And Fittings | Part 2

Hassan Ahmed — Thu, 22 Apr 2021 20:25:56 +0000

Pressure Drop Caused By Valves and Fittings

In part 1 the tutorial goes over pressure drop in pipelines. Just to recap pressure drop arises due to skin friction and forms friction. Non-separated boundary layers causes skin friction (the roughness of the pipe causing shear within the boundary layer of the fluid) whereas separated boundary layers cause form friction (geometrical characteristics of the piping system are piled up causing localised losses).

Simply put, pipe pressure drop is due to skin friction. But in a pipeline with valves and fittings pressure drop is mainly due to form friction. The pipelines may be connected to equipment and again pressure drop occur due to the specific piece of equipment. This tutorial is focused on pressure drop in valves and fittings only.

In order to account for pressure drop due to valves and fittings, various methods are used The most common among them is:

K values or Head Loss Coefficient.
Equivalent length method.

1. K values or Head Loss Coefficient Method

Where:

Hf = Head Loss

K = Head Loss Coefficient

V = Average Velocity of fluid

g = Acceleration due to gravity

= Density of fluid

Earlier K values were static constants but in recent applications (Industrial applications) they are seen to be dynamic and change with pipe diameter.

Depending upon the type of valves and fittings, various K values are available, which can be seen in the table below:

K values that are subject to change, then the pipe diameter can be calculated as follows:

The Km, Ki, and Kd values corresponding to the type of fittings and valves that are given in the table below.

Once determining the appropriate K value for your pipe, it can be substituted back into the first equation to determine the pressure drop or headloss

2. Equivalent Length Method.

Where:

f = Darcy’s Friction Factor

D = Internal Diameter of the pipe

Another method that is widely used in calculating pressure drop is the equivalent length method. For every valve and fitting particular L/D, equivalent length value is available in the previous table. To find the equivalent length, multiply the L/D value with the diameter and insert this value in the given formula, in place of Leq.

Similarly, for contraction and expansion losses you need to consider the k value as per the following formula.

Hassan Ahmed

Hassan graduated with a Master’s degree in Chemical Engineering from the University of Chester (UK). He currently works as a design engineering consultant for one of the largest engineering firms in the world along with being an associate member of the Institute of Chemical Engineers (IChemE).

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Pressure Drop In Pipe Lines And Fittings | Part 1

Hassan Ahmed — Wed, 21 Apr 2021 21:46:38 +0000

Pressure plays a prominent role in driving and arresting the fluid flow from one place to another.

What Pressure Is Involved In Driving Fluid Through Pipes And Fittings?

One classical definition is

Pressure is force acted per unit area

Pressure in a pipeline may be due to pumping, vaporization, compression, etc. The fluid should travel the entire pipeline without losing its pressure otherwise we need to spend extra for pumping the fluid to compromise for the loss in pressure.

Why Is Pressure Lost Through Pipes?

The reason is due to friction, wake formation, separation of the boundary layer by fittings, pipe roughness, etc. In order for the pump to work efficiently, the pump will be designed to accommodate any extra pressure that is required.

Pressure drop due to unseparated boundary layers (Skin friction) can be calculated by the following classical formula,

Pressure drop varies with length and diameter of the pipe, velocity, and density of the fluid, and Fanning friction factor. Don’t confuse this fanning friction factor with Darcy’s friction factor.

The Fanning friction factor varies with the nature of the flow. So if we know the nature of flow i.e., laminar or turbulent we can calculate the ‘f ‘ value accordingly.

If the flow is laminar the fanning friction varies with Reynolds number as follows,

If the flow is turbulent a lot of equations are in practice and you can use any of those equations at the expense of accuracy. the equations are mentioned as follows.

Colebrook equation (1938)

Swamee and Jain equation (1976)

Haaland equation. (1983)

The ‘ ε ‘ symbol corresponds to the roughness factor which depends on the material of the construction of the pipe, whereas D and Re have their usual meaning.

After calculating the friction factor, we can find the pressure drop due to skin friction in a pipeline by substituting the ‘ f ‘ value into the pressure drop formula stated earlier.

In the Second Part pressure drop due to fittings will be covered.