Control valves. Photo courtesy of Mokveld Valves BV
Government of Alberta has funded studies on life cycle assessment of bitumen derived fuels and SAGD energy efficiency study, thermal in-situ water conservation, assessing the impact of increasing water recycling, zero liquid discharge on energy usage, green house gas emissions (GHG), waste generation in thermal in-situ production of bitumen etc. Water recycling is a component of in-situ bitumen production by SAGD. Certainly there is tradeoff between emissions and water recycling and this depends on water quality and water treatment methods. The value of emissions compared to water use and air quality influence the decision of technology selection by operators along with safety, plant reliability and operability including capital cost.
A few technologies currently used in De-Oiling (SAGD) are:
- Single, multi and nut shell SAGD filters
- Single cell & quad cell induced gas flotation and walnut shell oil removal filter
- Produced water treatment with ceramic membrane
- High quality distillate with silica removal
- Evaporator technology
- Warm lime softening
- Blowdown evaporation
Valves used in SAGD plants go through slurries (bitumen+water mix). Slurrying is a two-phase system with small solid particles suspended and carried along in a liquid. Medium particles range in size from fly ash to lumps of solids. The shape and hardness of the solid particles are significant in relation to damage. Slurry- particle velocity, usually very close to liquid velocity causes damage in piping systems. A slurry piping designer adopts a maximum allowable average velocity, perhaps 5 to 10 ft/sec, for the lines but the valve cannot escape the danger as restrictions and throttling are in the valve design and operation. High velocities tend to erode valve components whereas low ones can cause the solids to settle out within the valve.
Gas compressor station. Photo courtesy of DFT Valves, Inc., USA.
The impact angle of slurry particles is also important. It is common experience that the outer-radius line of an elbow in a slurry line wears faster than does the straight pipe run; similarly a sharply curved passage or target-like step in a valve body will wear faster than the wall of a large straight waterway.
Materials choice, although important in determining a valve's resistance to slurry damage, is not the only factor. The design of the valve is also vital, and the concept of design includes not only the general type or configuration of the valve but also the smallest details of every internal area and component of the valve.
Another determinant of valve life on slurry service is the location of the valve in proximity to restriction and changes in direction of the piping.
Valves that can be used for slurry services are: knife gate valve, plug valve, ball valve, butterfly valve, diaphragm valve, and pinch valve. The knife gate valve in essence provides a tight closure on lower pressure slurries and prevents collection of solids in interior spaces. The simplest plug valve, with PTFE lining inside a metal plug valve, will function both, i.e. as bearing and sealing surface. The lining has good resiliency, it will seal a cylindrical plug and keep particles out of top or bottom cavities.
Knife Gate Valve PACKING CROSS SECTIONCourtesy of DeZURIK Valves.
Calculation of pressure drop (Cv value)
Cv is the basic and most widely used industry standard for determining the fl ow capacity of a valve. It is defined as the number of gallons per minute (gpm) at 60°F (15.55°C) that will pass through a full open valve with a pressure drop of 1 psi. Pressure losses are the result of:
- Pipe friction losses due to roughness of pipe interior finish, velocity, and inside diameter of the pipe.
- Changes in the direction of the flow.
- Other obstructions inside the pipe.
- Change of shape of the fluid path
KNIFE Gate VALVE CROSS SECTION-Courtesy
of DeZURIK Valves.
Cavitation occurs because the pressure in a fluid is converted to kinetic energy due to restriction at the valve closure member, causing an increase in velocity. In addition, as the temperature of the liquid increases, vapor pressure increases and that causes cavitation.
When a valve is exposed to fire conditions, it will allow minimal leakage through the seat and stem, and continue to provide effective shutoff during fire or exposure to excessive temperatures for a certain period of time.
The following key points should be kept in mind when choosing a valve for use on slurries:
- A full-port valve which streamlines turbulence and erosion, due to no restrictions in the flow path.
- Valve seats are not in the flow path which will reduce seat wear.
- All sealing members must be hardfaced.
- The closure element should move within its own geometry, as for instance a ball does when rotating about its own axis.
- The coating on the valve and seat is also important – it must be harder than the process particulate matter. Coatings may also be applied through the body flow parts.
KITZ CF8 - 3-300# Gate Valve. Photo courtesy of KITZ Corporation of Americas.
Choosing a suitable valve for application is not enough – the functioning of the total system must be considered to achieve optimum reliability, such as:
Stagnant areas, particularly valve-body cavities, should be purged with clean fluid to prevent particle build up or adhesion.
Valve cavities should be free from debris in order to drain during shutdown when the valve is in a vertical installation.
Flanges and elbows must be located 5 to 10 times pipe diameter away, in order for easy removal of valves from pipe sections for cleanout.
The valves must be designed so that they can be easily disassembled and cleaned. A two piece / three-piece body design is attractive for this purpose.