Hydrotransport Pipelines: Basic Design Principles
A well designed Hydrotransport system is a trade-off between the length required to maximize bitumen recovery and the costs incurred for both installation and on-going maintenance. There are four variables that greatly impact design of the system:
- Plant throughput: the maximum and normal tonnages received from the mine, as well as the minimum turndown
- Required pipeline distance: dictated by both layout and minimum distance required for complete lump digestion
- Elevation changes: the difference in elevation from the slurry preparation pumpbox to the top of the Primary Separation Cell (PSC) in Extraction.
- Coarseness of mine feed: Particle size distribution of the solids within the oil sands are a function of both geology and the Mine's blending ability.
Higher throughputs, longer distances and greater elevation changes all result in more pumps and more installed horsepower. Longer lines and more pump stations proportionately increase capital and maintenance costs, so layout optimization is a critical first step.
DESIGN CRITERIA: DEFINING THE BOUNDARIES
The first step in designing a Hydrotransport facility is to define the requirements of the plant and establish the design criteria. There are four inputs required before the system can be designed:
DESIGN CRITERIA 1. Plant Capacity
Determine the minimum and maximum tonnes of oil sands per hour the facility should be able to process:
- Obviously, the higher the maximum throughput, the larger the pumping requirement and the higher the installed capital costs. Typical nameplate processing capacities in the oil sands mining facilities are 6,000 to 10,000 tonnes of dry oil sands per hour.
- Maximum throughput dictates the power requirement for the pumps.
- The minimum throughput defines the plant’s turndown capacity. A lower minimum (relative to the maximum) is more difficult to accommodate in the design but offers better operating flexibility. Turndown ratios in the oil sands are typically two-thirds of nameplate capacity.
DESIGN CRITERIA 2. Slurry Flow Velocity
Define the minimum and maximum flow velocity of the slurry in the Hydrotransport pipeline:
- Since pipeline wear is an exponential function of slurry velocity, the owner must decide how fast is too fast and what maximum speed will be tolerated in the design. An absolute maximum flow of 6.0 m/sec is normally tolerated, but only on a very infrequent basis.
- Since slurry lines are prone to sanding, guidelines for minimum slurry velocities are normally established in order to prevent sanding. A typical normal minimum is 3.0 to 3.5 m/sec.
- Plants that process coarser oil sands will need to operate at faster speeds, sometimes as much as 4.0 to 4.5 m/sec in order to prevent the sand from accumulating on the bottom of the pipeline. It is therefore critically important to know the average coarseness of the oil sands as well as the maximum volume of coarse material that the mine expects to put through the plant.
DESIGN CRITERIA 3. Pipeline Length
Determine what length of pipeline is required to accommodate the plant layout and meet the desired minimum residence time:
- Pipeline length is dictated by the distance between the Slurry Preparation Plant (SPP) and Extraction. Longer pipelines offer more ablation and mixing, but obviously require more pumping stations and are therefore more capital and maintenance intensive.
- If the pipeline is too short, more bitumen losses can occur, particularly in winter, when ice can bind to lumps of bitumen and clays, which can be difficult to breakdown.
- Retention times greater than 30 minutes are generally not recommended due to concerns of bitumen degradation, although this has never been proven conclusively on a commercial scale.
- If the SPP is too close to Extraction to meet the minimum pipeline length required, the Hydrotransport pipeline can be lengthened by adding extension loops.
- Hydrotransport lines vary in length from as little as 1,000 meters for plants that use very hot water to as long as 5,000 meters for lower temperature operations.
- It is not uncommon for minimum Hydrotransport line length to dictate the location of SPP.
DESIGN CRITERIA 4. Slurry Pump Duty
Determine the normal operating load and maximum duty (head) that can be tolerated for each pumping stage:
- Since pump wear is an exponential function of rotational speed, higher operating heads increase the pump maintenance frequency. The trade-off, however, is fewer pumps installed, which reduces capital costs.
- Industry norms for oil sands slurry pumps range from 40 to 50 meters of head per pumping stage. Pump vendors typically recommend no greater than 60 meters per stage in order to avoid rapid wear of the pump's internal components.
- In order to save money on Variable Frequency Drives (VFDs), some operators opt for a combination of fixed speed and variable speed. In this case, the fixed speed pump operates a fixed duty regardless of throughput.
FIVE STEPS TO DESIGNING A HYDROTRANSPORT LINE
Hydrotransport systems are usually designed around the "normal" conditions, but tested at the extremes. Once the design criteria and operating window is defined, there are five steps in calculating the process parameters required for equipment design.
1. Calculate the maximum and minimum slurry flow and water requirements for the SPP
Slurry flowrates are calculated from the minimum/maximum tonnes of dry oil sands to be processed and some basic assumptions for Hydrotransport slurry density:
- In general, a higher operating density is preferable. This reduces water consumption and improves break-down of the oil sands in the pipeline.
- The theoretical maximum slurry density that can be pumped is 1600 kg/m³. However, operating too close to this maximum greatly increases the chances of plugging the pipeline.
- Operating at densities that are too low is highly inefficient, requires excessive amounts of water and also increases the probability of sanding the pipeline. A typical operating window for Hydrotransport slurries is 1550 to 1580 kg/m³.
2. Calculate the diameter of the slurry pipeline
Once the maximum slurry velocity is set and the maximum flowrate is established, the ideal pipeline diameter can be calculated and rounded up to the closest commercially-available pipe size:
- Minimum operating flows and velocities should be verified to be acceptable. A pipeline which is too large will have limited turndown capacity and will be far more prone to plugging when operating at reduced rates. However, if the pipeline diameter is too small, velocities will be faster than normal, which greatly increases wear rates and requires more installed pumping capacity, increasing both capital and maintenance costs.
- Although the most widely available nominal pipe sizes are 24 and 30 inches, 26 and 28 inch piping is also very common across the oil sands industry. It is very important to correctly size the diameter of the pipeline and not worry too much about pipe availability.
- A general rule-of-thumb for Hydrotransport piping diameter is 24 inches for 6,000 tonnes per hour (t/h), 28 inches for 8,000 t/h and 30 inches for 10,000 t/h of dry oil sands fed to the processing plant.
3. Calculate the pressure drop across the pipeline:
Once the diameter of the pipeline is established, the minimum, maximum and normal pressure drop across the pipeline can be calculated:
- Since Hydrotransport slurries are non-homogeneous, traditional hydraulic models typically underestimate the pressure drop of the system and are inadequate for coarse oil sands slurries.
- The most commonly used hydraulic model for oil sands slurry was developed by the Saskatchewan Research Council (SRC). The SRC Pipe Flow Model (sometimes referred to as the Two-Layer model) takes into account the average diameter of the coarse sand particles and the fraction of lumps in the slurry.
- A slurry with very coarse sand is far more prone to sanding and therefore requires a higher minimum speed, greatly increasing the pressure drop across the pipeline. Conversely, a high-fines slurry with little coarse material more closely resembles a homogenous slurry which is less prone to sanding and results in a much lower pressure drop.
- This proprietary hydraulic model is only available through the Saskatchewan Research Council Slurry Pipeline Design Course (click here for more information).
4. Calculate the Total Dynamic Head (TDH) or total duty of the system
The TDH or total system duty is simply the linear pressure drop (calculated in Step 3) × the total pipeline length + the elevation gain:
- TDH is calculated under both "normal" and maximum operating conditions.
- Maximum TDH is defined as the highest throughput of the coarsest material.
5. Calculate the total number of Hydrotransport pumps required, pump capacity and power draw:
The total number of slurry pumps required is the total TDH (calculated in Step 4) divided by the duty per pump (Design Criteria #4):
- Calculations are normally done for normal and maximum operating conditions.
- The size of the pump motor is governed by the desired duty for each pump, the slurry flowrate, density and the desired safety margin.
- Motors used in Hydrotransport pipelines can be quite large, typically varying from 2,400 to 4,000 hp.
Once the total number of pumps is established, the pumps can be positioned along the length of the pipeline. Pressures should be verified against the various operating conditions to ensure there are no high-pressure or low-pressure spots along the line.
There are 4 basic types of instruments commonly found on every Hydrotransport line:
- Flowmeters: Obviously, the most important instrument for any slurry line is a flowmeter, used for both slurry velocity calculations and performing material balances. Traditionally, wedge or Venturi-type flowmeters were commonly used on Hydrotransport lines. However, the use of sonar technology has become increasingly more popular, since they are non-intrusive and do not contribute to localized wear.
- Densitometers (or Density Meters): Nuclear densitometers are commonly installed on Hydrotransport lines, typically near the front end of the line (closer to the SPP). Although there is no control of density within the Hydrotransport system, slurry density is very important to the operation of the SPP and for mass balancing purposes.
- Pressure Transmitters: Pressure transmitters are the most critical instruments on any slurry pipeline, particularly Hydrotransport lines. Pressure transmitters are typically installed on the suction and discharge of each slurry pump. Pressure readings provide valuable information on how the pumps are functioning and if the slurry is flowing normally down the pipeline. Pressure transmitters installed on slurry lines are normally equipped with membranes to prevent contamination of the instrument from the oil sands slurry.
- Autosamplers: The Alberta Energy Regulator (AER) requires each oil sands operator to regularly report bitumen recovery rates (defined as the percentage of bitumen produced divided by the total mass of bitumen mined). Although measuring bitumen production is quite easy, measuring the volume of bitumen mined is not so straight-forward. In order to help support material balances across the plant, autosamplers are typically installed on Hydrotransport lines, which collect slurry samples at various time intervals. These samples are then analyzed for bitumen content in order to give an approximation of the amount of bitumen sent to the processing plant.
Apart from these 4 types of instruments, the individual slurry pumps and motors are typically equipped with numerous instruments which gauge individual pump performance, such impeller rotational speed, motor amperage draw and bearing temperature. These instruments also provide valuable diagnostics for both the Process and Maintenance teams.
AUTOMATION AND CONTROL
The 2 most commonly used variables for control of the Hydrotransport line are:
- Liquid level in the SPP pumpbox feeding Hydrotransport, and
- Suction and discharge pressures for each Hydrotransport pump.
SPP pumpbox level is the primary control variable for the Hydrotransport pumps. The level in the pumpbox is controlled by adjusting the speed of the pumps:
- As the level in the pumpbox rises, the Hydrotransport pumps speed up to bring the level back down to normal.
- If the level in the pumpbox gets too low, the pumps slow down to allow the pumpbox level to recover.
- If the pumpbox level continues to fall, water is added in the SPP area to help prevent the pumpbox from emptying and keep the system in operation.
All pumps across a Hydrotransport line typically operate at the same speed under normal conditions. However, each pump speed can also be adjusted depending on the pressure signals across the pipeline. Both high pressure and low pressure spots can be dangerous for slurry lines.
High discharge pressures signal a downstream blockage and can eventually lead to full stoppage of the line. As the pipeline begins to sand, flow is restricted, which increases the level in the SPP pumpbox. As the level rises, the control logic tries to increase the speed of the Hydrotransport pumps, hopefully increasing the slurry velocity and clearing the blockage.
Low pressure spots on the pump suction can signal loss of feed. Pumps which experience low suction pressure should automatically slow down in order to prevent complete loss of suction. As a protection mechanism, pumps are designed to shutdown on loss of suction, eventually leading to a complete shutdown of Hydrotransport.
Also note that as a pump wears, it needs to operate at a higher speed in order to generate the same head.
LINE SANDING EXPLAINED: WHY FLOW VELOCITY IS CRITICAL
Slurries produced in the oil sands are non-homogenous streams, typically consisting of two distinct phases:
- a water carrier phase, which includes bitumen, clays and fine solids (typically solids less than 44 microns in diameter), and
- a coarse sand phase, which also includes any rocks or lumps of oil sands.
Fine solids remain suspended in the water phase. These fines do not require a high velocity to keep moving down the pipeline and do not pose any threats to the flow of the system.
However, the coarse sand phase acts very differently. Coarse sand does not remain suspended in the water phase and instead gets dragged along the bottom of the pipeline. This coarse sand phase requires a much higher velocity to keep moving.
Plugging occurs when the velocity in the pipeline is not fast enough to keep the coarse sand moving. The sand will settle in the pipeline and can accumulate fairly quickly. If left unresolved, the pipeline may begin to block, ultimately causing the flow to come to a complete stop. Stopping a slurry pipeline can cause serious downstream and upstream process disruptions and can lead to lengthly production outages.
On average, a normal oil sands slurry should flow at a minimum of 3.0 m/sec to keep all particles suspended. Mines that process coarser oil sands should operate closer to 4.0 m/sec. As a general rule, 5.0 m/sec is fast enough to keep even the coarsest slurries moving. However, operating too fast for long periods of time is not recommended since this greatly increases wear of the pipeline.
THREE WAYS TO PREVENT PIPELINE PLUGGING
So how can you prevent sanding and plugging of slurry lines? Here are a few suggestions:
- Don’t oversize the diameter the pipeline: A larger diameter pipeline requires more oil sands feed and more water to keep the flow velocity sufficiently high. This can be a problem during feed disruptions or when the plant needs to operate at turndown rates. When selecting the diameter of the slurry pipeline, it is critically important to verify the slurry velocities at the minimum flowrates.
- Blend, blend and more blending: Mine Planners should always attempt to blend very coarse ore benches with pockets of high-fines material. This helps average out the particle size distribution of the slurry and prevents wild swings in performance of the processing plant. Failing that, Mine Planners should always advise the processing plant when mining in an area with lots of coarse sand. This signals to the plant operators that slurry velocities should be higher than normal during this mining period, which usually translates into increased water addition.
- Make sure the pipeline control logic is well tuned and all instruments are working: Sanding can quickly be detected by changes in operating pressures. The pressure downstream of the blockage will suddenly drop, while upstream pressure suddenly begins to rise. A well tuned control logic can detect these changes faster than the control room operator. The control logic responds by adding water and ramping up the pumps upstream of the blockage. Pumps downstream of the blockage automatically slow down to prevent complete loss of suction. If the control logic is working properly, many sanding events will clear themselves up before the pipeline blocks completely.