Preparing for Inline Inspection
By Randy L. Roberts
When the decision is made to run an inline inspection (ILI) tool, the following questions are asked: What preparation do the ILI companies require before running the tool? Do we dry-clean the pipeline using mechanical pigs only or liquid (chemical) clean using both cleaners and mechanical pigs together? Do we clean online or offline? Does the pipeline need to be cleaned? How clean is clean? Pipeline cleanliness is what?
Let’s start by saying ILI tool companies require data on various pipe bends, wall thickness, ovality and pipeline cleanliness before running the inspection tools. Generally, either the ILI companies and/or other caliper companies will offer a caliper pig to be run first to retrieve this data.
ILI tool companies have different tolerances for different tools and you will need to discuss required data from each company. Having this information will help in your decision on how to clean your line. Single diameter lines require — or we should say allow for — less stringent cleanliness than multi-diameter lines. Once tolerances are known and approved by an ILI company, a date is scheduled to run the ILI tool. This all sounds simple and it should be. However, unexpected internal conditions on most pipelines may open a Pandora’s Box even before the caliper tool is run. Most pipeline companies discover their pipeline is contaminated with solids, debris and various liquids (lubricants, inhibitors, glycols, condensate, etc.) while doing pipeline modifications and/or after running a mechanical cleaning pig.
One of the amazing findings that this author has experienced is that, in general, everyone thinks their pipeline is clean. In more than 15 years of internally cleaning all types of pipelines, that statement has proven to be profoundly untrue.
What Makes a Pipeline Dirty?
If glycol dehydration is upstream of your system, it is safe to say you have free liquid triethylene glycol (TEG) in your pipeline. Also, in transmission gas pipelines, one may experience various types of lubricants, scavengers, flow promoters, corrosion inhibitors, methanol, hard hats, wooden skids, pig bars, chill rings, welding rods and electric grinders. Another possible contaminant is hydrogen sulfide (H2S) at 1 part per million (ppm) in a continuous gas stream of 10 million cubic feet per day (MMcf/d), if all converted to iron sulfide (FeS), will produce more than 800 lbs of iron sulfide in a year. Thus, even pipeline quality gas has the potential to cause internal problems. Even a 0.001 in. of film buildup of iron oxides can produce quantum amounts of solids. Also gas velocity plays a big role in the buildup of solids along the pipeline.
After a decision to clean a pipeline is made, you need to evaluate whether this line is to be cleaned online or offline. Online is defined as operating the pipeline under normal conditions while cleaning, while offline is with the pipeline out of service and depressurized using other propellant sources. In either option, expect a cleaning program of a pipeline section less than 100 miles long to take four to eight days of actual clean-ing runs, depending on the cleanliness condition of the segment.
Online cleaning allows the pipeline company to continue to operate and provide service to customers with minimal interruption while cleaning the pipeline. The general rule of thumb for pigging velocity in any size diameter gas pipeline is more than 4 ft per second but less than 15 ft per second. In liquid lines, flow rates greater than 3 ft per second will not allow water to phase out, therefore minimizing corrosion products to form and settle at the six o’clock position in the pipeline.
Pig speed on liquid pipelines usually is of minor concern other than the limits of the cleaning company’s portable separation equipment. Since the liquid line cannot be stopped while receiving cleaning pigs, the separation equipment must be capable of dumping spent cleaning solution at a greater rate than the pipeline flow rate.
It is not that velocities greater than 15 ft per second cannot be used. However, experience and pig manufacturers’ studies indicate that at that elevated speed, pigs will hydroplane if liquids are present, causing greater blow-by and leaving greater volumes of liquid and entrained solids in the pipeline when the object is to remove those solids and minimize free liquids. Special cleaning procedures must be designed with the cleaning service company to counteract this concern.
This brings us to one of the most asked questions in our industry: What does “clean” mean? A more realistic question would be, “What degree of cleanliness is required?” Right up front, we must say that today there is no known specified level of cleanliness, only a suggested level, depending on pipeline diameter and type of ILI tool to be run. Service pipelines can be cleaned to a level of better than mill grade new pipe steel and internally coated pipelines.
Randy L. Roberts is applications manager for N-SPEC Pipeline Services, a Business Unit of Coastal Chemical Co. LLC, a Brenntag company. Contact him at firstname.lastname@example.org.