Recent developments in heavy oil extraction technology for the Canadian oil and gas industry have created a need for new flow control solutions in steam assisted gravity drainage (SAGD). SAGD uses a pair of horizontal wells drilled from a central well pad to recover heavy oil.
In a plant nearby, steam generators transform water into steam which is injected to the wells. The steam heats the oil where the steam injection and oil production happen continuously and simultaneously. The resulting oil and condensed steam emulsion is then piped from the producing well to the plant, where it is separated and treated.
In this environment, all components including valves, pumps and production chokes must be able to handle the severe service of highly abrasive materials, high temperatures and a corrosive environment. Canada’s severe cold adds to the challenges for the materials and technology which must stand up to oil, gas and produced/brackish water and multiphase fluids with entrained sand.
Working closely with a flow control partner that understands every stage of the process is essential, and has resulted in development of coatings and alloys that have insured the stability of many specially engineered valves.
Advances in Metallurgy
Metal seated valves are used in all of the services associated with SAGD and the valve industry has developed various valves including pressure peal gate valves for steam production and injection. This has led to the evolution of exotic materials.
Some of the metal seat hard facing and coating technology includes cobalt-based overlay materials for abrasion resistance and tungsten carbide for improving seating surface hardness. Recently, proprietary ultra-hard tungsten carbide versions have found a place in the industry as has nitriding (a heat treating process that diffuses nitrogen into the surface of a metal to create a case hardened surface), and molten salt bath ferritic nitro carburizing (a range of case hardening processes that diffuse nitrogen and carbon into ferrous metals at sub-critical temperatures). Depending on a variety of factors, boron carbide and ceramic coating have also proven successful.
To address seat leakage and valve integrity concerns, different types of valves have also been re-engineered. Metal seated ball, butterfly, gate, globe and check valves have all been modified to handle particular situations. The following is one example.
Condensation Induced Water Hammer and Pressure Seal Valves
With Canadian winters, entire piping systems are insulated, including valves. Removing insulation from a live steam system is more hazardous than one might think. In addition to the danger of being burned by brushing up against hot pipes— bare pipes carrying 100 psig steam have a surface temperature of 334oF—there’s also the peril of condensation induced water hammer.
During a major expansion of a SAGD system, valve manufacturers worked closely at the early stages of the project with the engineering company to design special, high pressure gate valves in large diameters for the steam injection line. The main challenge was to develop an on/off metal seated gate valve with zero leakage to be used on the steam line at temperatures up to 420°C (790°F) and elevated pressures. Due to the high pressures involved, the system had to accommodate over-pressure devices that would release pressure in the body area closed to the bonnet.
The pressure seal valve uses internal process pressure to dynamically energize the seal. The greater the pressure the greater the sealing force. In this valve, the initial seal compression is made with draw bolts, and during factory hydro test the draw bolts are re-tightened to specifications. It is recommended that during initial commissioning and at plant turn around the draw bolts should be retightened.
For example, there are two types of seats in Pressure seal valves.
1. Wedge Gate Style or type: It is simple and torque seated. Easy to operate, while closing tighter closes and provides better seal, its disadvantages are it is prone to thermal binding at high temperatures and it is not a double block and bleed valve.
Operation: Both Upstream and downstream seats engage the body seats with equal force. As process pressure increases the wedge is further forced against the downstream body seat , the advantage is when adequate pressure is applied the upstream side of the wedge will flex and move away from the body seat.
2. Parallel Slide Gate Style or type: It is a complicated design with lower torques, seat wipes tight to the surface to and that helps to have less prone to particulate damage, this valve cannot be thermally locked, the seats are dynamically energized hence higher the pressure higher the seating force., this valve is easily pressure locked in cold or hot temperatures. Once again this valve has poor shut off during start-up when differential pressures are low.
Operation: Upstream pressure moves the upstream disc away from the seat and then energizes the downstream disc against the seat.
The typical pressure seal designs comprises following:
- Dissimilar seal and bonnet acute angle 2-3deg variance.
- Contoured seal to facilitate deformation.
- Graphite reinforced with Stainless steel or Inconel.
- Compound bonnet angles.
Key Troubleshooting on Pressure Seal Valves at Plant Level
a) During start-up, a leak develops at the pressure seal ring and the user immediately thinks it’s a defective valve. Leakage during start-up does happen, mostly because the seal ring can only be adjusted by the manufacturer at ambient conditions. Once the valve is in start-up at operating conditions of temperature and pressure the seal ring may need re-loaded by some tightening of the jack-up bolts or bonnet retainer. If this leakage is allowed to continue, the seal ring and/or sealing area can be permanently damaged and require major repair. A lot of times this happens with Class 600 valves as the operating pressures are not high enough to help effect a seal.
b) If the valve gasket and valve body neck does not have sufficient corrosion resistance, then corrosion can take place between the body neck and the gasket thus making removal very difficult. This is also true for the other gasket retaining parts in the body neck, i.e. threaded retaining ring, segmented retaining ring, etc.
c) The use of graphite seal rings wherein the gasket design has no provisions to prevent graphite extrusion and/or the diametrical tolerances/clearances of the retaining parts are not sufficient to prevent graphite extrusion.
Resolving Thermal Binding
Thermal and/or pressure locking of the gate valve wedge in the closed position
This happens when valve is heated and process media is entrapped in the center cavity of valves, the fluid expands and increases the pressure and the valve may not open this is also called as pressure locking and it is common in parallel slide gate valves due to the effective area on which the entrapped pressure acts. This is prevented by drilling a tap hole in upstream side of disc to relieve pressure in bonnet and between the discs. Additionally an external relief valve to connected to bonnet helps relieve pressure also an external bypass valve and relief system can be installed.
Over-pressurization of gate valve bonnets
This happens due to thermal binding effect when disc gets locked in a closed position due to pressure locking the actuation of motor will result in locked rotor current which will rapidly increase the temperature of the motor internals. Within 10-15seconds the head build up can degrade the motor capability to deliver a specific torque, damage the motor or both. This can be eliminated again by providing thermal relief valve connection to bonnet or can be eliminated by having external 2 or 3 valves by pass piping system.
In a business where time is money, there is considerable value in avoiding unscheduled downtime and lost production. Working closely with technologically advanced valve manufacturers has allowed us to overcome issues like thermal binding and water hammer in the challenging and severe conditions of the steam assisted gravity drainage in the oil sands.
Gobind Khiani received his BSc from University of Poona India, Masters of Engineering in Material Science and Project Management from University of Calgary and is a registered member of APEGA. He has worked in the Oil, Gas and Nuclear Industry since 1992. His career started with Transmark Group in UK and Dubai. After 10 years he moved to Canada and worked with VELAN, Proquip, Crane Energy and Cenovus Energy Inc. as Project Manager/Engineer and currently works for Enerplus Corporation. He has managed various projects on pipelines, facilities, asset and project management.