Slack Line Flow

[Crudelanguage]

I have inherited a project that involves slack line flow
which I do not have much experience with.

It is a a crude pipeline approx 20 miles long that goes over some fairly rugged country with large changes in elevation. It is typically run by gravity flow but pumps are available.

The line is slack and we would like to tightline it to improve our leak detection. The previous Engineer was planning on installing a backpressure regulator on the downstream side before it goes into the tank, and operating the pumps to increase pressure.

Intuitively it seems as if increasing downstream pressure will pressurize the line and prevent slack flow. However after reading this

http://www.psig.org/papers/1990/9505.pdf

I am questioning whether this solution will automatically work.

There are many locations where the elevation profile is steeper than the head curve. I think we have a slack line in multiple locations.

So is the solution simply increasing backpressure on the line..or is it more complicated than this and may involve computer modeling.

Thanks ahead of time for any help.

 

[JLSeagull]

I think that you are looking at the complicated computer models. I was involved in large a crude pipeline over the mountain range in Colombia in about 1995. The contractor and the operating partners each had a pipeline specialist - each with different computer simulations. My involvement included the liquid surge valves on the downhill side of the line.

 

[MortenA]

I second that

Best regards

Morten

 

[BigInch]

Fortunately the article does not say anything about not raising backpressure, because increasing backpressure will stop slack line flow. So, increase the backpressure, if you can. If you can get it high enough, the pipeline will fill and slack line flow will stop. The disadvantages are that tight line operations are more expensive, your velocities and pipe erosion will tend to increase, and waterhammers may be more severe with a tight line.

Slack line flow, as you already know, does make leak detection more complicated, and may increase erosion at the bottom of the pipe, however in a one-product pipeline there are relatively few other problems and slack line flow can be an efficient means of operating the pipeline, especially during the pipeline's early days before full flow capacity is reached. If running at an increased pressure turns out to be prohibitingly expensive, or cause higher velocities than necessary, slack line flow can be a desireable condition. Slack line flow is often associated with downstream pressure control. A tight pipeline running down to the bottom of a valley from a high mountain top might require a backpressure greater than the maximum operating pressure of the rest of the pipeline, so it is not uncommon to find it being used in those locations. Slack flow may also be unavoidable due to high product vapor pressures, although crude typically has low vp, so probably not in this case.

You might have to change your startup and shutdown sequences to avoid forming and operating at slack conditions again. Since increasing the backpressure will change the system curve, and increase power requirements, your operating points at some pump stations will be changed. Your operating costs will rise too. So, I hope you are prepared for those possibilities and your pumps will still be within their operating range and close to BEP. Be sure to check those, the MAOP and any possible effects on surge pressures from running at higher pressures, velocities and eliminating that slack flow region before increasing that valve setting.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

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