Gas pipeline: Velocity vs pressure drop, importance

[engr2GW]

Hello,
sometimes during gas hydraulic calculations, one may meet the recommended velocity margin, but the pressure drop may be a bit on the high side, other time, one may get the pressure drop per 100feet just right, but the velocity may be lower than typical recommendations.
Because the source and destination pressure can not change, it seems like if the dp/100' if achieved given the P1 and P2 and length, then we have to make it work using the available velocity? especially if no other pipe size will give us that velocity with the given conditions?

Example: A pipe size at the given source and delivery pressure gives me 0.5psi/100' which is at the lower end of the dp range, given the MCFD. But the velocity is half the minimum required velocity (i.e., 25ft/s instead of 45 or 60 ft/s). Is this one of those cases where we use periodic pigging operation to help with the velocity (I am assuming that the velocity is to keep the pipe from collecting liquid dropout at the bottom of the pipe?)

THANKS FOR YOUR HELP

As much as possible, do it right the first time...

 

[danschwind]

If your P1 and P2 are fixed then why are you bothering calculating dP/100?

Your pressure drop is known and therefore flowrate as well.

For a pipeline you will usually have a wanted flowrate and either a source pressure or a downstream pressure. 3 variables, only 1 unknown.



Daniel
Rio de Janeiro - Brazil

 

[alchemon]

If pipe size is fixed, and your demand doesn't meet the minimum velocity specifications - simply reduce the supply pressure. The fluid will then speed up to meet the same demand.

You note that this can't be done. Why is that?

Otherwise, if upstream and downstream pressures can't change, then yeah the fluid will just flow at the velocity which balances out for a given pipe size. This may or may not be the minimum you need. A velocity which is too slow implies that the pipe was oversized. A velocity too high then of course implies that the pipe was undersized.

If it really can't be changed, you'll need to run cleaning pigs periodically if you are worried about stuff condensing out. Also, you could think about adding drips to low spots. They'll need regular cleaning.

 

[LittleInch]

First off, what fluid are we talking about?

Dry sales gas or something else?

Either way the velocity numbers are really only a guide, not a minimum, but normally what you don't want to exceed for long periods of time.

Pipeline sizing has to look at many things to get the most economic size for your particular system. If flow is variable or may increase in future you might choose a bigger one. If your pressures are in your control then you have more options.

Your 25 ft/sec seems OK to me. Indicates that the pipe is quite big, but if you don't have much pressure drop and your flow is big then so be it. What's the problem?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

You will get different answers to the problem depending on the purpose of the design and the importance of the factors which affect the velocity.

A process pipeline in a CPF is short compared to a transmission gas pipeline, so high velocities, high pressure drops, easy maintenance, liquid holdups due to elevation profile, number of pump or compressor stations, availability and cost of power, minimizing weight and cost of pipe, will not be of high importance, resulting in smaller diameters. Economics is not the priority.

Pipelines several thousand km long need larger diameters to operate economically by keeping pressure drops as small as possible to minimise compressor/pump stations, maintenance and wall thickness and cost of pipe, maximize flow, and yet keep it swept of dirt and liquids and dirt. Economics is of high importance.

Gathering systems are usually not so long, but many Petroleum fields operate with low pressures, yet velocities must remain as high as possible to sweep dirt, sand, water, gas liquids out while minimising erosion, or avoiding hydrate formation. Maximising flow rate for future wells may lower current velocities, but increase them later. The Economics often changes in importance between now and future.

Each purpose of a particular pipeline may change how each variable affects the result of any other. Sometimes velocities are important, sometimes not, as may be all other design variables, and the resulting ranges of all variables will differ accordingly.



Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."