Natural Gas Line Sizing using General Pressure Drop Equation

Line sizing is a part of every engineering activity. Although it is not major part, line sizing is required when we prepare piping and instrumentation diagram. In this post, I want to share you how to do natural gas line sizing (actually the method is applicable for all kind of gases).

I usually use process simulator when it came to natural gas line sizing. Long time ago, I tried to size manually. But the result is not the same as from simulator. But, I tried again and yesterday I got a satisfied results.

Natural gas line sizing calculated in this post will be from manual calculation and from process simulator. API RP 14E is used as reference.

Based on API RP 14E, single-phase gas lines should be sized so that the resulting end pressure is high enough to satisfy the requirements of the next piece of equipment. So, the point is the end pressure. The velocity is also a noise problem if it exceeds 60 ft/s. However the velocity of 60 ft/s should not be interpreted as an absolute criteria. Higher velocities are acceptable when pipe routing, valve choice and placement are done to minimize or isolate noise.

API RP 14E mentioned several approach to calculate pressure drop, but in this post I will use general pressure drop equation. 

The idea of this line sizing is: to guess pipe ID until it met end pressure criteria. We will use spreadsheet to goal seek the equation. It is really simple and easy.

General Pressure Drop Equation

This is general pressure drop equation:

The key to get correct results by using this equation is to prepare all correct data. Last time I put wrong data for gas specific gravity at standard condition, so that the results is different from I got from simulator.

I used Norsok standard as pressure drop criteria in single-phase line sizing. You can use your own standard or you can determine the value of end pressure (P2) by yourself which is high enough to satisfy next equipment as mentioned in API RP 14E.

Example: Natural Gas Line Sizing by using P2 as Criteria

This is an example. I have gas with the following properties. I used Norsok to determine the value of acceptable P2. So that, I have collected all data I need to line size.

By using second equation, I guessed value of pipe ID (d, in inch) so that I have calculated flow rate is the same as target flow rate, which is 0.4008 MMscfd. I got pipe ID = 2.32 in. The nearest value is 2.47 in (NPS 2.5 in). At this pipe ID, gas velocity is 55.54 ft/s, which is almost close to standard mentioned in API RP 14E. The pressure drop is 1.595 psi.

By using process simulator and the same pipe ID, this is the results I got:

• Gas velocity = 55.44 ft/s
• Pressure drop = 1.591 psi

The results are the same! Congratulations everyone.

Example: Natural Gas Line Sizing by using Velocity as Criteria

Example above shows that the velocity is too high. We want to reduce the velocity to about 30 ft/s, for example. Thus we have to increase pipe ID. Let say pipe ID = 3 in. Then we got :

• Gas velocity = 33.18 ft/s
• End pressure = 43.27 psia, or pressure drop = 0.43 psi (use equation 1)

By using process simulator we got:

• Gas velocity = 34.18 ft/s
• End pressure = 43.28 psia, or pressure drop = 0.426 psi

The result is similar too.

BONUS!

This is a spreadsheet I used to calculate line size of gas manually. All the results above are got from this spreadsheet. I hope you find it useful. If you feel confuse, do not hesitate me to contact me.

Reference: API RP 14E

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