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.

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.

Durometer