Gross estimation of pipe size (no head calculations)

 Can pipe size be determined without engineering calculations involving the Bernoulli or the energy balance equations?

The short answer is yes; but some experience may be needed and the result may only be point of start or guess for formal calculations.

Here is how to do it, pros and cons.

Some restraints to this approach

Before using this approach you better know the limitations of it:

1. since no pressure gradient nor head losses are being considered the estimated pipe size may not be in agreement with the hydraulic load balance,
2. the pipe size may be larger or smaller than required so that experience is to be used to reduce the margin of error,
3. you will need data sheets for different pipe materials,
4. this approach works better for pipe sections without fittings,
5. most data sheets only have data for water as working fluid (air is sometimes used as well).

Some advantages of this approach

Despite the errors, possibly involved by this method, there are some good things about it:

1. only real pipe sizes are used so that the result is better adapted to already sized equipment characteristics (pumps, heat exchangers, etc.),
2. you can get a good approach to the pipe size in a shorter time than by performing large engineering calculations,
3. pipe material, ranges for flow rate and fluid velocity so that head loss may be balanced are only required.

Gross estimation of the pipe size (no calculations)

The key is in a table having the following data:

• pipe nominal size,
• flow rate,
• fluid velocity,
• working fluid,
• pressure drop (a head loss indication).

A good example for commercial steel pipe Sch. 40 is given in the book Flow of fluids through valves, fittings and pipe. Crane Co. Look in its engineering data appendix. We shall not reproduce the table here since you may find the bok over the internet.

An example of pipe size gross estimation

Consider the a situation in which about $2.5\times 10^{-2}$ m$^3$/s of water are to be transported across 200 m. The pipe to be installed will need a number of bendings and little elevations.

What would be an estimated pipe size for this case?

Solution.

Let us first take a flow rate margin of 5% for the flow rate. Then, we would be looking for pipes capable of transporting about:

$Q=1,500 \pm 75$ L/min of water

Now, if you take a look to the data table from the book Flow of fluids through valves, fittings and pipe. Crane Co several pipe candidates, capable of carrying this amount of fluid, appear!

In fact, pipes of nominal diameters: 3 1/2 inch, 4 inch, 5 inch, 6 inch and 8 inch fit our needs.

What is the best option?

Here is where your engineering experience is important and integration of other information (fittings, elevations, pumps, etc.) help to make a decision.

You may choose one of the pipe sizes by answering the following questions with approximate values:

• How fast will the fluid travel through the pipe?

You should recall that as the internal diameter increases the fluid velocity reduces and vice versa. You may compare different pipe sizes for the same flow rate and see how the fluid velocity dicreases with the size.

• Do you expect the fluid to be highly pressurizsed?

Two options may arise here: to increase the pipe size or to increase the wall thickness (schedule). One way of going around this issue is lby chosing a pipe size whose average capacity matches your needs.

• Economic factors

The larger the pipe size the larger the economic bill. If the extension of the pipe line is short then perhaps economicas may not be a trouble.

• Do you expect head losses to be important?

The parameter $h_L$ is becomes important with the number of fittings, pipe elevation and when pipe line is too long. If you forsee a large $h_L$ in the order of 10 m, for example, then perhaps a larger pipe size would work better. However, the fluid velocity will be reduced too.

Now, back to the question on the most suitable pipe size.

For this case the pipe of ND 4 inch has been selected. The main reasons are:

1. it is large enough to allow a maximum flow rate of 2,200 L/min which is 147 % more than required,
2. being only the second pipe in size it may give a reasonable economic advantage,
3. if fittings are installed this pipe may still give a good margin to keep $h_L$ at reasonable values.

Note: Remember that this is just a gross estimation before more detailed and formal calculations but it may be a good guess.

Any question? Write in the comments and I shall try to help.

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Ildebrando.