Analyses for liquid pipelines include flow rate, pressure drop, velocity, pumping power and reduction in diameter. Frictional pressure drop (ΔP) is the pressure difference between the beginning measurement point (P1) and the ending measurement point (P2) over a given length (L) of pipe. Pressure drops should take into account equivalent lengths, elbows, valves, fittings and elevation changes.
At Clean Harbors, we use the following rules of thumb and equation: maximum liquid velocity shouldn't exceed 15 ft/sec (4.6 m/sec); minimum velocity shouldn't drop below 3 ft/sec (0.9 m/sec); and flow is determined via:
ΔP = (11.5×10 -6) fLQ2SG/d5
where ΔP is in psi; f is the Moody friction factor, dimensionless; L is in ft; Q is liquid flow rate, bbl/day; SG is the specific gravity of the liquid relative to water; and d is pipe inner diameter, inches.
The Moody diagram (Figure 2) plots ƒ as a function of Reynolds number, NRe, and relative roughness, i.e., absolute roughness, ε, divided by pipe diameter, D. At NRe less than 2,100, laminar flow prevails and f only is a function of NRe while at NRe exceeding 2,100 (flow in the transition and turbulent regimes) relative roughness also has an impact on f, so it's necessary to determine ε. Deterioration over time due to corrosion, erosion and scale buildup considerably increases the roughness factor, thereby reducing the pipe's effective diameter and requiring an adjustment in f.
A plant can take several steps to maintain flow efficiency:
Cleaning. A number of technologies can increase efficiency and safety, and extend pipeline life. These range from closed-loop pigging to onstream mechanical, chemical or ultrasonic cleaning.
Selecting the most effective cleaning strategy requires some knowledge of the scale or residue that may be in the line. Examining a sample of the deposits will allow for a better determination of the types of pigs needed, and chemical cleaning agent(s), e.g., detergents, surfactants or acids, best suited for the pipeline. This also will aid in the planning for collection and disposal of effluents from cleaning. In some cases, chemical circulation or vapor phase cleaning may be the most effective cleaning option.
Internal pipeline rehabilitation and coating. Following inspection and cleaning, coating the inner pipeline surface often provides the most effective approach to increasing pipeline efficiency and durability. This effectively isolates the metal surface from water, hydrogen sulfide and other contaminants. An epoxy coating eliminates corrosion because there's no contact between the pipe wall and the material being transported. The coating increases throughput and reduces maintenance and inhibitor costs at a fraction of the cost of pipeline replacement. It can rehabilitate long sections of existing pipelines and extend the life of new ones.
The in-situ epoxy coating process only requires access at the ends of the pipeline segment being serviced. The particular epoxy coating chosen depends upon the material being transported. Two specially designed coating pigs, operated at a closely controlled driving pressure and velocity, apply a series of thin coats of the specified epoxy over several passes. The process provides a uniform, smooth, homogeneous coating throughout the pipeline, including all field joints (welds) and bends. The coating's surface roughness doesn't increase over time, unlike that of an uncoated surface.
Measuring the improvement. The increase in pipeline flow efficiency should be evident, and can be as much as 15–30% after cleaning and especially after internal coating. The percentage improvement as it relates to fluid throughput can be determined via:
% = (ΔPafter cleaning - ΔPbefore cleaning) × 100
(Note that ΔP is proportional to the square of flow rate.)
THE BOTTOM LINE
With fixed throughput and upstream pressure, a well-maintained pipeline section can achieve a higher downstream pressure by decreasing the surface roughness of the pipe caused by corrosion. A higher suction pressure (and, thus, a lower required head or specific energy) will result in a lower energy cost for pumping. The amount of savings depends upon the degree of the roughness improvement; an internal coating provides optimum results.
Keeping a pipeline in top condition also enables more efficient product movement, and boosts safety and quality.
ANDY SANTALUCIA is Houston-based Manager, Pipeline Services, for Clean Harbors. E-mail him at Santalucia.Andrew@cleanharbors.com.