Pipelines at processing facilities impact overall plant performance and profitability. Their efficiency contributes to greater productivity, lower energy costs and better safety. However, ensuring highest pipeline efficiency demands periodic inspections, cleaning and rehabilitation.
Corrosion can decrease efficiency — increasing the energy required to move product, operating temperatures (which may affect chemical reactions) and pressure. Reducing deposit accumulations, friction and corrosion, along with engineering design enhancements, chemical treatments and internal coatings, can accelerate flow rates in pipelines.
Corrosion also can undermine the integrity of pipelines, causing safety risks and leading to expensive replacements.
A line's corrosion rate depends upon what it's handling. Some pipelines, such as those for water, need frequent monitoring and maintenance while others, designed to carry specific chemicals, only require periodic monitoring for impurities that could affect line integrity.
Corrosion significantly contributes to pipeline inefficiencies, especially in water supply and waste lines. Some chemicals also pose substantial corrosion risks. For instance, sulfuric acid at 80–90% concentration isn't corrosive but should its concentration drop to below 50%, say, because of an upset, the acid becomes quite corrosive. Methanol and jet fuel lines that operate intermittently can suffer corrosion and internal metal loss; the fluid itself isn't corrosive but subsequent introduction of water, hydrogen sulfide or other chemicals causes reactions with the steel.
Plant staff should look particularly for corrosion in lines:
• handling feedwater or wastewater;
• in which the concentration of a chemical has gone down, either
inadvertently or because of deliberate dilution;
• where the chemical has become contaminated;
• carrying steel-reactive chemicals; and
• not in constant use.
Typical indicators of corrosion include:
• greater turbulence (friction factor);
• higher pump energy consumption;
• increased pressure drop;
• decreased flow rate; and
• elevated product temperature.
The surface condition of the inside wall of a pipe also impacts flow efficiency. In a commercially manufactured pipe this wall isn't smooth (Figure 1). The degree of surface roughness is a function of the pipe material, method of descaling and environment to which it has been exposed. In turbulent flow the surface roughness affects the friction factor and, thus, the pressure gradient in the pipe.
Over time, scale and deposits may form, impeding flow — reducing throughput, stressing equipment and increasing the probability of unscheduled downtime.
For a simple, short, straight section of a 12-in.-diameter (9,300-mm) water line, "The Pipeline Pigging Handbook," 3rd edition, notes: "if the inside diameter is reduced by 5% (15 mm) by a smooth deposit, the loss of throughput at a given pressure would be over 10%. To bring the throughput back to its original level would require pressure to be increased by almost 30%. However, if as is more likely, the deposit was uneven, the resulting turbulent flow may cause an effective reduction of 15%. In this case, throughput could be reduced by some 35%, while the pressure to overcome these losses would need to be increased by more than 140%."
Routine inline inspections provide a good sense of the condition and projected life of the line. They can identify potential problems, including safety or fluid-contamination issues, and may give early enough warning so managers can plan repairs during a scheduled turnaround.