Pipeline Pigging and Cleaning

Pigging technology today has been advancing exponentially, and what writer means by technology is not so much the hardware side but the application.

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.

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)
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
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)
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.

Durometer

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

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.

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