Hydrotransport Maintenance: Pumps and Pipelines
Maintenance planning is a critical component of any oil sands operation. Abrasive silica sand and entrained oxygen in the process water make oil sands slurry lines one of the most high-wear process fluids in the industry. The key to a good maintenance plan is preventative maintenance - repair or replace components before they unexpectedly fail. Calculating the interval between failure is part art and part science. Design, operating conditions and material of construction for slurry pumps and pipelines all factor in to the overall reliability of the system.
HYDROTRANSPORT: MAINTENANCE, MAINTENANCE AND MORE MAINTENANCE
Designing and operating a Hydrotransport line is relatively easy. However, maintaining it in good working condition is a seemingly endless task. Slurry lines require constant inspection and maintenance. In fact, many oil sands mining facilities have full time maintenance crews, dedicated solely to the repair and replacement of slurry pumps and pipelines.
REDUNDANCY: WHY MORE IS BETTER
Oil sands slurry lines are subjected to some of harshest conditions in the mining industry. Oil sands deposits have a high concentration of silica sand which is extremely abrasive. Process water has a high dissolved oxygen content, making it very corrosive. Oil sands slurries also contain angular fine clay particles, which slowly wear down even the toughest of materials.
Hydrotransport lines have the added complexity of very large lumps and rocks, sometimes up to 5” in diameter. These lumps get chewed up by the slurry pumps and can quickly wear out elbows and bends in the slurry piping. In fact, maintenance of slurry lines is so onerous, most operators have spare Hydrotransport installations, enabling them to be maintained without disrupting production.
SLURRY PUMP MAINTENANCE
Centrifugal slurry pumps used in the oil sands are some of the largest and toughest commercially available pumps in the world. The two most common brands of heavy-duty slurry pumps used in Hydrotransport are Warman HTP pumps (by Weir Minerals) and GIW TBC pumps. Both are quite similar in design and construction. The pumps have very large open-faced impellers, which enable any large chucks or rocks to pass through without pump damage. The pumps are designed to be disassembled in place so that individual components can easily be replaced on-site.
Each pump vendor offers a variety of wear materials to increase the time between maintenance intervals. Despite great improvements in metallurgy and design, most slurry pumps still require maintenance at best every few months. Components such as the suction liner and impellers suffer the most abuse and require the most frequent replacement. However, wear rates can vary greatly depending on metallurgy, pump design and operating conditions. Some of the most important factors affecting pump wear include:
- Pump operating head and rotational speed: The higher the pump duty, the faster the rotational speed of the impeller which translates into higher pump wear rates. Since wear is an exponential function of speed, working the pumps a bit harder can translate into significantly more wear. Therefore, monitoring pump duty is just as important as measuring run time.
- Fixed speed versus variable speed: Since fixed speed pumps are much less expensive than variable speed, some operators opt to install a combination of fixed speed and variable speed pumps on slurry lines. By definition, fixed speed pumps run at full speed all the time, including periods of lower flow. Pipeline flow is controlled using the variable speed pumps. Fixed speed pumps therefore have much higher maintenance requirements since they continuously operate at their maximum duty.
- Slurry operating conditions: Pumps that have to handle large chunks of oil sands and debris obviously see more impeller wear than pumps that process a more homogeneous slurry. In the case of Hydrotransport lines, pumps at the front of the line tend to see more wear than the pumps at the end of the line. Apart from being subjected to bigger chunks of oil sands, more oxygen is available at the beginning of pipeline, resulting in more corrosion. As the slurry travels down the pipeline, more of the oxygen is consumed, reducing the corrosion rates at the back end of the line.
SLURRY PIPELINE MAINTENANCE
Slurry pipelines in the oil sands have very high wear rates, particularly for coarser slurries. Since wear is localized at the bottom of the pipeline, the pipes are normally rotated every 4-6 months and completely replaced every 4 rotations (normally about every 2 years). Elbows and bends are also high-wear points. Operators tend to avoid sharp 90° bends in order to reduce impact wear and instead opt for long sweeping bends wherever possible. Bends and fitting are normally overlayed with chrome carbide (CCO) to improve wear-resistance. Overlayed elbows and bends still need to be replaced on a yearly basis.
The exact maintenance interval is a function of slurry velocity and pipeline thickness. All oil sands slurry piping is made of carbon steel, but pipe thickness can vary from as little as 3/8” to as much as 3/4”. Obviously, thicker piping is more expensive to install but will not need to be rotated/replaced as often. Some owners opt to line their slurry piping in order to further extend its service-life. Rubber and neoprene liners have had limited success in the oil sands. Duplex stainless steel and Iracore® liners have shown much more promise and are becoming increasingly popular. Material selection for slurry piping is a trade-off between higher capital expenditures versus higher maintenance costs.
Note that although there are 3 Hydrotransport slurry pipelines (carbon steel lines shown on the right), only 2 are normally in service; the third line acts as an installed spare.
Water piping (far left) have sharp 90° elbows as they turn the corner. However, the Hydrotransport lines are equipped with a long sweeping elbow (typically 2 x 135° bends). This helps reduce impact wear and extends the service life of the elbows.
(PHOTO COURTESY SUNCOR ENERGY)
GLAND SEAL WATER EXPLAINED
Another important requirement of any Hydrotransport system is a clean and reliable gland water supply. Pressurized gland water is pumped into the seals of slurry pumps in order to keep them clear of bitumen, sand and fines. This gland seal water serves three important functions:
- It enables the pump shaft to rotate within its sleeve with minimum friction,
- It prevents the slurry from backflowing into the seals and damaging the shaft,
- It allows for a small amount of cooling of the pump shaft, which heats up as it rotates at high speeds.
Maintaining sufficient gland water flow and pressure is critical to the proper operation of any slurry pumping system. Operating without gland water, even for only a few seconds, will cause the slurry to backflow into the pump’s sleeve and damage the rotating pump shaft. Worn seal packing is far easier to repair than a damaged slurry pump.
The most important factor in designing a gland water system is to ensure the system is reliable and has sufficient pressure. If gland water pressure is too low, it will not be able to overcome back-pressure from the pump and cause the slurry to backflow into the sleeve. However, excessively high gland water pressure is also not recommended since this will also reduce the life of the packing.
The amount of gland water required depends mostly on pump design and how worn the packing is. A new packing will take less gland water. Water requirements will slowly increase as the packing gets worn out and eventually needs to be replaced. Normal gland water flow rates for slurry pumps can be as low as 5 m3/hr to as much as 25 m3/hr if the pump packing is really worn-out.
Ideally, gland seal water is sourced from a clean, cool water supply, such as filtered river water. However, since clean water is such a precious commodity in the oil sands, many operators use water recycled from the tailings pond. Any reliable water source is suitable, however, operators should note that dirty water will obviously deteriorate the packing more quickly. Slurry pump manufacturers have also been working towards optimizing the design of their pump seals in order to reduce gland water consumption rates.