What Do We Really Know About Pipeline Pigging And Cleaning?
Pigging technology today has been advancing exponentially, and what we mean by technology is not so much the hardware side but the application.
This article will address the application side of pigging while discussing rules-of-thumb for liquid and dry cleaning of pipelines in concert with running mechanical cleaning pigs and the effectiveness of the respective results. P&GJ will publish Part 2 in a later issue.
What is it that we really know or understand about pigging a pipeline? If we think that just running any type of mechanical pig through our pipeline at any speed and getting it out in one piece constitutes a clean or good run and that we are now ready for the MFL tool, then the answer, respectfully, is we do not know very much.
Pigging of any type requires planning and assistance from the pig manufacturers and/or qualified pipeline-cleaning service companies. The goal of pipeline cleaning is to minimize or eliminate sensor liftoff of the ILI tool. A side benefit is increased pipeline efficiency. That topic will be discussed in Part 2. Technology today allows for a proven product from the pig manufacturing process to assist companies in achieving maximum results whether they are mechanically dry pigging or liquid cleaning using mechanical pigs. Mechanical pigs have come a long way from bails of rags wrapped with barbwire to today’s formulated polyurethanes.
There are many types of polyurethanes. However, this article will discuss only castable elastomers. The act of mixing and pouring together two liquids - a prepolymer and a curator - makes castable urethanes. There are basically two chemical structure types of polyurethane prepolymers. According to R.W. Fuest, the two chemical structures are 1. MDI (methylenebisdiphenyl diisocyanate) and 2. TDI (tolylenediisocyanate). Both types use a curative and a prepolymer that, when mixed together, cause a chemical reaction forming the castable urethane. Each manufacturer has its own ratio mixture, other additives, dyes, and processes that differentiate them in the market.
Some advantages of polyurethane, says Fuest, include 1. non-brittle, 2. elastomeric memory and 3. abrasion resistant. Some disadvantages, says Fuest, include 1. breakdown in high temperature, 220-225°F, 2. Moist hot environment (hydrolysis in the presents of moisture and elevated temperatures), 3. certain chemical environments dissolve urethane, (very strong acids and bases, aromatic solvents: i.e. toluene, ketones, methanol and esters) and 4. UV exposure greater than six months as a rule is not good (covering and storing inside prolongs life).
A few differences between MDI and TDI are chemical makeup. In general, MDI urethane is a little more expensive but more durable. For example, it is more durable on longer cleaning runs, >75-miles, than TDI. However, TDI has a better compression set than MDI and handles higher temperatures. Various applications will determine which type is better to use.
Polyurethanes are mostly measured by the Shore (Durometer) test or Rockwell hardness test (see www.matweb.com, Material Property Data.). The Rockwell test is usually for harder elastomers such as nylons, polycarbonate, polystyrene and acetyl. Shore hardness uses the Shore A or Shore D scale as the preferred method of testing for rubbers/elastomers (polyurethanes). Durometer Shore test only indicates the indentation made by the indenter foot upon the urethane. Other properties such as strength or resistance to scratches, abrasion and/or wear are not indicated.
Durometer is expressed by a number system. The higher the durometer number the harder the urethane. TDI urethane is good in the range from 50A to 90A, according to Fuest, with MDI in the 70A to 85A range. Combinations of each durometer can be incorporated in a pig design to maximize desired conditions and/or results. The rule-of-thumb is: the harder the durometer the better scraping capability, and the softer the durometer the better the sealing characteristics.
Pig types and functions are as numerous as people’s political opinions. As a rule, most pigs of any type are a standard design with a length-to-diameter ratio of 1.5 times the OD of the pipe, i.e. a 24-inch pig is 36 inches in length. This is why the lowest ell bend of 1.5D is important. If your line has less than 1.5D ells then consideration may be required to replace with greater radius ells if you are trying to make the line piggable for ILI tools or to use specially designed tandem pigs. Pig types are of three basic designs: polly foam, unibody urethane and steel mandrel discs/cups.
Polly Foam. These open-cell polyurethane foam types are usually made in the full OD of the pipeline. Polly pigs have the ability to negotiate short radius ells and bends, miter bends, tees, multi-dimensional piping and reduced port valves. The pigs come in various densities determined in pounds of urethane per cubic foot but most common are ranges from 2-lbs./cubic foot; 5-8 lbs./cubic foot; and 9-10 lbs./cubic foot. These densities are usually color coded: yellow for 2-lbs., red for 5 lbs., and scarlet or blue for 10 lbs., depending on the manufacturer.
The polly open cell is the least aggressive of the pig design family. They are great for sealing and light abrasion removal and can reduce in diameter up to approximately 35%. Length can be increased to allow maneuverability through large tees, some older Orbit valve designs and other type gate valves.
Wire strip brushes, nose pull rope, transmitter cavity and jetting ports can be incorporated in each density and type of polly foam pigs. Assorted selections of various configurations (polly criss cross, polly criss cross wire brush, bi-directional, bullet shape and bare swab), of each density are as numerous as there are requirements, so check with your manufacturer’s representative and pipeline-cleaning service companies for help in designing to meet your requirements.
Unibody. These are popular. They are single-body cast-polyurethane pigs designed to be more aggressive than pollys but more forgiving than the steel-body mandrel type. Uses include 1.) removing liquids from wet gas systems and liquid pipelines, 2.) controlling paraffin buildup in crude oil lines, 3.) separating refined products, 4. commissioning pipelines and 5. evacuating product.
The unibody design can also maneuver in less than 1.5D radius ells and bends and is usually but not limited to a multi-disc cup configuration. The multi-disc shape, designed in a bullet concave nose type or bi-directional type, can have wire brushes attached along with other configurations and add-ons.
The unibody cast polyurethane with hollow shaft can handle up to a 20% reduction in pipe ID, according to Girard Industries. These pigs can be cast from various durometer strengths.
Steel mandrel. This pig is the most aggressive type made. The configuration of the steel body allows for multiple designs for multiple purposes. Steel-body mandrel pigs are built around a steel-constructed mandrel. Three basic designs are usually available: cleaning pigs, batch and gauging and conical cup. This article discusses only the cleaning pig type.
Cleaning pigs can be made with all discs, a disc with scraping cups, a disc with conical cups, with any combination of all, and all types of wire brushes and scraper urethane blades. Any of the cast-polyurethane products can be made from various durometer material strengths.
Polyurethane discs are cast and molded to the desired diameter of your pipeline. There are basically three types of discs - sealing, scraping and slotted. The sealing disc is usually thinner, ≤ 1-inch, and is designed for low to medium scraping characteristics but high on liquid sealing. The scraping disc is usually > 1-inch in thickness and - mpared to the description of the functions of the sealing disc it functions just the opposite. Sometimes a combination of both types is required. Slotted discs or feathered type discs are generally used on multi-diameter pipelines. Special design may be required for each pipeline condition. Considerations of pipeline length and pipe wall roughness to be pigged will also determine the type required for each type. When all multi-type discs are used, the pig can also be used as bi-directional.
Just like the discs, cups come in two basic types: scraper and conical. Scraper cups are as the name implies but the design allows for greater surface forces to be exerted on the pipe walls, especially in less than oval shape pipe while maintaining its ability to seal. These cups can reduce, on average, 15-20% of design diameter. Conical cups allow for maximum sealing with minimum scraping to remove solids. This type is normally seen on gauging plate pigs and multi diameter and out-of-round pipelines. Conical cups can reduce up to approximately 30-35% and maintain adequate seal. Again, conical and scrapper cups can be made in various durometer.
ILI tool companies require that data be known before the ILI tool is run on all ells, bends, wall thicknesses, ovality and pipeline cleanliness. Generally, either the ILI companies or other caliper companies will offer a caliper pig to be run first to retrieve this data. The multi-channel tool gives multiple data points, welds, taps, valves, types of nineties, bends, direction of bends, wall thicknesses and other data - all in the o’clock position with pipeline linear footage location.
The ILI companies have different tolerances for different tools and one will need to discuss required data for each. Once tolerances are known and approved by an ILI company, a date is scheduled to run their dummy tool, then the ILI tool.
Most pipeline companies discover their pipeline is contaminated with solids and debris and needs cleaning during the installation of launcher/receiver and/or block valve replacement.
Once the 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. Offline is done with the pipeline out of service and depressurized. As a rule, offline cleaning can be twice as expensive as online and the cost is compounded by the loss of gas revenues. In general, the extra costs are due to several factors: slower pig runs entailing more man hours, more cleaning runs, the requirement for continuous nitrogen and air to propel the cleaning train, and the cost of the fuel needed to generate that propellant over the duration of cleaning. An exception would be if natural gas at low pressure were used to propel the pig-cleaning trains instead of nitrogen and compressed air. In either option, expect a cleaning program of a pipeline section less than 100 miles long to take four to six of actual cleaning runs. Of course, this depends on the cleanliness of the pipeline.
Online cleaning allows the pipeline company to continue to operate and serve its customers with uninterrupted service. This procedure is quicker, safer and less costly than offline, as a rule. The general rule-of-thumb - velocity for any size diameter pipeline is greater than four feet/sec but less than 15 feet/sec. It is not that velocities greater than 15-feet/sec cannot be used, but experience and studies by pig manufacturers have shown that, at that elevated speed, hydroplaning of the pigs will occur in the presence of liquids, which causes greater blow-by, leaving greater volumes of liquid and entrained solids in the pipeline. This frustrates the objective, which is to remove the solids and minimize free liquids in the pipeline. Special procedures must be designed with your cleaning service company to counteract this concern.
What Is Clean?
First of all, there is NO industry cleanliness standard. Clean can mean internal conditions that minimize or eliminate ILI sensor liftoff. Therefore, to even the playing field among cleaning companies, the pipeline company must tell the cleaning service company bidders to propose a given amount of cleaning runs for all. The author’s experience has shown that three liquid-cleaning runs are the minimum. Usually, the third liquid-cleaning train removes the greatest amount of solids and extra sequential runs are for polishing. Fewer runs can be achieved, but the concern is always the probability of removing too much, too fast, resulting in the possible plugging of the pipeline and/or the receiving equipment used to separate the liquid/solids from the gas or liquid stream.
The rule is to remove the pipeline contaminates a layer at a time by using a combination of the right liquid cleaner in a diluent and the right choice of pig type. Pipeline-cleaning companies, in conjunction with many customers, have set a standard of four cleaning runs with a final run resulting in a solids percent in the solution of 6% by volume or less. Some pipeline companies say 10%v or less and the pipeline is considered clean for smart pigging. However, 6%v or less is the norm.
Other factors, such as pig condition and residual of solids on pigs combined with the field test percent, assist in a combined pipeline company and cleaning service company’s agreed upon satisfactory cleaning performance. Measures are available to lessen the 1-mil volume or less generally left behind after cleaning and should be discussed with the pipeline service company if further pigging is required to remove free liquids.
But you say, “We only have clean-treated gas, therefore, our pipelines should be clean.” Consider this: if glycol dehydration is upstream of your system, it is safe to say you have free liquid triethylene glycol (TEG) in your pipeline not to mention various types of lubricants, scavengers, flow promoters, corrosion inhibitors, methanol, hard hats, wooden skids, pig bars, chill rings, welding rods, and electric grinders.
Dr. John Smart III and this author have discussed the theory that liquids will travel short distances through the pipeline close to the point of introduction but TEG vapor will travel greater distances than originally thought.
According to Don Ballard, the rule of thumb is that you lose one pound of liquid glycol per MMscf of gas treated. According to Huntsman Corp., triethylene glycol weighs ~9.36 pounds per gallon and is usually acidic when it leaves the glycol dehydrator.
According to Manning and Wood, all glycols (EG, DEG, TEG, TTEG) in the presence of H2S, COS, CS2, RHS, CO2, O2 and water, in the gas stream, naturally become acidic. Once acidic, according to Kensell, the glycol starts digesting the dehydrator unit components, causing free iron loss absorbed in the glycol and stabilized foaming, then large amounts of glycol carryover into the pipeline.
In the author’s experience (Roberts, 1984-2009), the iron carryover greatly accelerates the formation of long chain polymers (shoe polish-looking substances), and contributes to black powder fouling.
Using mass balance calculations. H2S at 1 ppm (0.25 grains per 100 cubic feet is 4 ppm) in a continuous gas stream of 10 MMscf/d, if all converted to FeS, will produce more than 800 pounds of iron sulfide in a year. Thus, even pipeline quality gas has the potential to cause internal problems, according to Richard M Baldwin.
Even a 1-mil inch (0.001 inches) film buildup of iron oxides can produce quantum amounts of solids, according to Dave Parnell.
The gist of all this is - less free iron on the pipe walls will ensure greater accuracy from the ILI tool. Part 2 of this article will discuss cleaning a pipeline, liquid cleaning with surfactant base cleaners, other liquid cleaner types and pipeline efficiency.
Randy L. Roberts is with N-SPEC® Pipeline Services, a business unit of Coastal Chemical Co., L.L.C., a Brenntag Company, 6133 Hwy 90 East, Broussard, LA 70518. He can be reached at (337) 261-0796 or firstname.lastname@example.org.
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