In my previous blog post, I talked about how pigging can serve many important purposes, from product batching and displacement to isolating a line for repair. But what is one of the top reasons for pigging your pipeline, you might ask? It’s simple. Cleaning!
Cleaning for Optimal Flow
A clean, well-maintained pipeline provides a variety of operational, integrity and asset lifecycle benefits, including flow efficiency. Pigging is one of the most effective ways to ensure that your pipeline is clear of debris, solids, corrosion and other restrictions that can impede flow, make internal pressure rise, increase risk of failure and affect your bottom line.
Cleaning pigs work by scraping the inner wall of the pipeline and pushing debris and loose material ahead to the trap. Most of the time, pigs are propelled through the line by product flow, although they can also be towed by a mechanical device.
Just how much improvement can you expect after pigging? Let’s look at a real-world example. In Figure 1, cleaning a 24-inch dry gas line dramatically reduced friction and improved flow. The blue line represents the effect on flow and the red line represents the percentage improvement. As pigging removed debris – which is a major contributor to friction loss in a pipe – flow was substantially improved.
Figure 1: Effect of Friction on Gas Flow
Of course, cleaning can do more than keep your line running smoothly. It is also a critical part of corrosion control.
Pigging to Control Corrosion
Internal corrosion is common in many upstream and liquid pipelines due to the nature of the products they carry. When trace water and corrosive products, such as hydrogen sulfide (H2S) or carbon dioxide (CO2) mix with the flow, it can cause corrosion that attacks the pipe wall (see Figure 2). But this problem isn’t limited to the oil and gas industry: in the U.S., for example, the EPA states that H2S-associated corrosion is shortening the life of some infrastructure by as much as 80 percent. That means infrastructure with a design life of more than 50 years is failing in as little as 10 to 20 years.
Figure 2 - Corrosion Triangle
Cleaning pigs remove corrosive debris and clear the way for corrosion inhibitors, which are two critical facets in the fight against internal corrosion.
Pigging for Other Purposes
In addition to removing buildup and debris and inhibiting corrosion, pigging can be useful in a variety of other situations:
- Drying onshore pipelines after hydrotesting. Foam pigs can be used in conjunction with dry air, nitrogen or vacuum drying to carry residual water out of a pipeline after hydrotesting. Drying prevents corrosion, freezing and the formation of hydrates in natural gas pipelines.
- Preparing pipelines for in-line inspection (ILI) The Pipeline Operators Forum (POF) recognizes that that effective cleaning before an ILI run has a significant impact on run success. In fact, the line probably needs to be its cleanest during inspection to improve sensor contact for more accurate data. That includes being free of ferrous debris that can be misinterpreted as corrosion.
- When the inspection is critical and assurance of cleanliness is paramount, service providers often deploy a “dummy” ILI tool first. The dummy tool replicates the behavior of the real inspection tool to test line piggability without risking expensive sensors and electronics.
As Figure 3 shows below, nearly one-third of ILI run failure is attributable to problems with or lack of preparation or cleaning
Figure 3: Causes of ILI Run Failure
Source: Pipeline Operators Forum
- Reducing black powder. Although it is sometimes attributed to residual water from hydrotesting, black powder -- which is typically found in gas lines – occurs when oxidants such as water vapor or oxygen react with steel to form sharp, aggressively erosive contaminants. Small in size (< 10 µm) and easily crumbled, black powder occurs in large quantities that are difficult to remove and can carry over into downstream sections of the pipeline system.
- Mechanical cleaning can be effective against black powder, but requires high-capacity filtration using both cyclone- and cartridge-based systems. A better alternative is chemical cleaning, where a chemical slug injected between two pigs loosens debris and contaminants from the pipe wall. Cleanliness can be assessed by the clarity of the liquid returns.
How Clean is 'Clean?'
If you read the title of this article and expected a definitive answer to the age-old question, “How do I know when my pipeline is clean,” prepare to be devastatingly disappointed.
I like to compare this question to the old adage, “How long is a piece of string?”
One answer is, “It is as twice as long as half its length.” Ha!
The truth is, there is no universally accepted standard for pipeline cleanliness. Your definition of “clean” will vary depending on a variety of factors, including the type of debris or pipeline wall deposits you’re dealing with and the kind of ILI tool you’re planning to use: for example, ultrasonic technology (UT) requires a far cleaner pipeline than magnetic flux leakage (MFL) does.
Determining an acceptable level of clean also depends on your reason for cleaning in the first place. For example, an operator whose goal is corrosion prevention may have a vastly different definition of “clean” than an operator who is cleaning to prepare for ILI. For example, in Figure 4 we see the returns from a chemical cleaning of a gas pipeline in Mexico. After cleaning, the solids concentration was less than 0.5% by volume. The pipeline owner -- whose goal for cleaning was to remove black powder to prevent further damage to pipeline equipment and to inspect the pipeline -- agreed sample clarity was sufficient to call it clean.
Figure 4: Chemical Cleaning - <0.5% Solids Concentration
Or, consider this example from the Middle East, where an operator was seeing degrading flows because of hard scale/wax build-up on the pipe wall. The deposits had a very hard consistency, similar to the surface of a tarmac road. Before instituting a cleaning program, the operator wondered how to measure cleanliness. The answer: use a geometry ILI tool before and after cleaning to assess the change in the level of debris. Figure 5 highlights the two geometry runs: in the top chart, the blue spikes show areas of lodged debris – around 150 kg (330 lbs.) worth -- that are missing in the bottom chart after 146 kg (326 lbs.) were removed.
Figure 5: Proving Line Cleanliness
Your pipeline services company can help you define your own definition of clean, based on your unique situation and requirements.
That said, I can offer three basic assessment methods you can use to evaluate your results after a cleaning:
- Do a visual assessment. One way to assess the results of a cleaning is to simply look at the “pig trash” – that is, the deposits, either dry or in liquid suspension, removed by the cleaning runs. Recording this on each run, especially if using a progressive program, can provide invaluable information about pipeline cleanliness.
- Attach a debris measurement tool. This tool can be attached to a cleaning pig to directly measure debris as the pig works its way through the pipeline. As I showed in the example from the Middle East, the use of a geometry tool can provide a direct measurement of line cleanliness.
- Use a data logger. Installing a data logger on one or more cleaning tools can measure such parameters as pressure, differential pressure (front to rear), accelerations and temperature – all of which will be different in a dirty pipe than they are in one that’s clean. Mapping this data along the pipeline length can help assess where there may be built up debris, wax, etc., and where it has been removed.
Part of a Healthy Maintenance Routine
It’s not easy to define what makes a pipeline “clean,” but there’s one thing that we can all agree on: Whether you’re trying to remove paraffin from a crude oil pipeline or get your gas line ready for ILI, a cleaning pig can get the job done!
Next time: We’ll discuss progressive cleaning.