Oil Sands tailing ponds have received huge media attention with a couple of environmental issues such as:
- How they impact ecosystems while active.
- When the land they occupy will be reclaimed.
The media hype and distortion has had serious consequences for the industry’s image and has made it difficult for the general public (outside of the oil industry) to gain a realistic insight into the importance of the environmental issues linked to the industry.
Tailing ponds are an integral component of oil sands surface mining operations, but they are not required for in-situ(SAGD, CSS). They comprise leftovers of fi ne clay, sand, residual bitumen and water from the extraction process. The water is further recovered at 80% from these and recycled and re-used in mining production. The bitumen accumulates and floats to the surface from where it is skimmed off and recovered. The toxicity of clay and water decreases over a period of time in tailing ponds as organic compounds degrade.
The government takes huge safety deposits from producers to safely and carefully have active tailings management systems in place (at the time of approving application for mining). A few technologies have evolved since production by mining in the 1960s to reduce the environmental impact of tailing ponds such as mechanical centrifuges and thickeners. Alberta Energy Regulator has directive 074 on tailing ponds management with performance criteria and clear enforcement actions in place should a producer fail to meet. Oil Sands Learning Institute, Alberta Innovates, CERI are some of the governmental research agencies who monitor the management of these.
In this article we will continue our discussion on upgrading process on mined Oil sands where slurry is a predominant factor. Valves for slurry are known as two phase flow, i.e. small solid particles suspended and carried along in a liquid. Particles range in size from microscopically small fly ash to lumps of reaction products. The shape and hardness of solids are significant to damage. Some particles will shatter into small rounded or chunky bits and other fracture products may be jagged. Slurry-particle velocity is similar to liquid velocity which is another vital term in the damage equation.
A slurry pipeline engineer will design for maximum average velocity of 7ft/sec to 15ft/sec, however the valve specifications cannot escape the danger of damage so readily. Restrictions and throttling are inherent in the valve design and operation. High velocities tend to erode valve trim and internals, and low velocities will tend to settle solids within valve trim and seat pocket areas. The impact angle of slurry particles is important. It is common experience that the outer-radius of an elbow in a slurry line wears faster than it does in a straight pipe run; similarly a sharply curved passage or target like reduction in a valve body will wear faster than the wall of a large straight water flow.
The abrasion or wearing of surface by small particles through scraping or impact is a major mechanism of damage. At valve trim, seats and restricted internal bore (reduced bores), where velocity is higher and impact angle closer to perpendicular, damage is higher. Settled-out solids may protect sensitive valve elements but can block the operation of throttling and closure elements. The material selection is important in deciding a valve’s resistance to slurry damage, however valve design, type of valve, Cv value, concept of design and general configuration of the piping and other smaller details on components also play a major role.
An important part is played by the location of a valve in proximity to restriction and change in direction of the piping. The fluid jet action and vortices that can result will dictate the speed of wear and impair the valve’s control action. For several SAGD plant services, due to high temperatures, the exotic material in valves is important for at least the surface in contact with the slurry.
Piping layout feature
Flow-path configuration, both for the partly closed valve as well as for the open valve, is more important for slurry valves than for any other service valves.
A smooth flow with gradual or no change in direction is advisable. Some valves are inherently better than others i.e. a globe valve is not recommended as flow through the valve makes directional changes and each change causes a scouring action as particles strike the wall and accelerate in a different direction. In order to stand, a vortex can establish itself near an obstruction and rapidly erode through the body wall. Slurry valves should be selected keeping in mind that trim, sealing surfaces and internal valve components are out of the flow. Any obstruction or trim facing the flow shall be made of resistant material. Any rising stem valves are useful so long as the slurry contact to stem area is protected.
Image courtesy Pickford-RedPoint Alloys BV
Internally threaded connections, non-rising valve stems, and close sliding contact are not desirable valves to be used in slurries.
Criteria for choosing slurry valves:
- A full port, streamlined bore minimizes turbulence and erosion.
- Valve with no trim components in the flow path.
- The closure element such as ball in a trunnion mounted ball valve moving at its own geometry/axis is preferred – this avoids displacement of fluid.
- The coating selection to meet process requirement/s.
- Hardfaced sealing components to provide reduced or no maintenance.
- Seating to move in shearing motion rather than contacting each other with significant compressive force to minimize damage.
- Internals designed to generate enough load to maintain shearing or rubbing contact.
- Purging of body cavities to prevent particle build up or adhesion.
- Orientation, cavities to be free of drain during shutdown in vertical run.
- Easy to disassemble for any cleanout or replacement.
Excalibur. Image courtesy DFT Valves Inc, USA
Valves used in slurry service are knife gate, plug, ball, butterfly, pinch, diaphragm etc. However, the knife gate valve in principle provides a tight closure on lower pressure slurries and prevents collection of solids in internal spaces. An important characteristic of most knife gate valves is the complete, or nearly complete, removal of the knife/blade from the valve internals upon opening. The exit may be through an elongated packing box or elastomer seat ring or sleeves. The erosion of the seat happens in knife gate/s either due to profile irregularities near the seat or due to bad flow pattern approaching the valve (i.e. a knife gate installed immediately after an elbow). The plug valve with tapered plug and metal to metal seat is not suitable for slurry application however, some changes such as a tapered plug with PTFE linger functions, are excellent in both bearing and sealing surfaces in slurries, as the lining has good resiliency and the plug seals like a cylindrical style. The butterfly valve with lining and coating on body and disc can rely on fluid resistance to erosion. The fact remains, however, that the disc in the flow path will erode. The diaphragm valve shows relatively low contact pressure on the seat as the characteristic of the diaphragm allows elastomer to envelop particles and still seal. Pinch valves envelop lumps and particles gently and provide a comparatively undistributed fl ow path for slurries, so that impact on surfaces is reduced greatly. Pockets for collection of slurry particles are also practically non-existent in pinch valves. The flexing action during opening of the valves tends to break up agglomerations of particles, helping to get flow restarted.
Demonstrating HIPPS. Similar is being used
in SAGD offsites. Image courtesy Mokveld Valves BV.
Valve actuators for slurry systems are not as simple as those for more prosaic services. For slurries where corrosion usually presents no more of a challenge to an actuator than clear and relatively inert liquids, the uncertainty of torque/ thrust requirements needs attention. The type of valve, the valve location in the line, and the pattern of operation are significant factors in the need to torque/thrust and in the relation of valve setting. Sometimes sizing for pressure dictates the requirement of large actuators on slurries rather than clean processes. Ball and plug valve manufacturers occasionally recommend a higher breakaway torque due to slurry build up of particles in pockets and clearance crevices.
Feature image: GLC Monel Oxy line. Image courtesy DFT Valves Inc, USA