Originally published in the June 2012 issue of Pumps & Systems.
Energy companies have performed enhanced oil recovery with supercritical carbon dioxide for decades, typically using centrifugal pumps with noncontacting dry gas seals to handle these pipeline applications. As interest in CO 2 capture and storage increases, however, supercritical CO 2 applications face new challenges. Operating pressures, rotational speeds and temperatures are on the rise, resulting in increased seal leakage and underscoring how essential the correct sealing system is for successful equipment operation.
Because of CO 2’s unique properties at supercritical conditions, end users should consider several factors when choosing the right sealing system for turbomachinery in CO 2 applications. Selecting an appropriate sealing system will help reduce maintenance time and costs and ensure peak performance.
Best Practices for Sealing
Since the late 1980s, end users have sealed their CO 2 applications with noncontacting dry gas seals optimized for fluids at or near their point of vaporization. The American Petroleum Institute (API) recognizes these seals as dual unpressurized seals, or tandem arrangements.
Tandem seals have a pair of seal faces that are each able to handle the application’s full pressure requirements, with a full backup seal for safety reasons. The seal support system may include a nitrogen buffer in the primary vent, the secondary vent or both, depending on several factors — including:
- The climate
- Whether the equipment is indoors or outdoors
- The availability of nitrogen
The most common sealing arrangement includes a heated, filtered flush (Plan 12 under API 682) with a low flush rate and a system to monitor vapor leaks in the primary vent (Plan 76). Filtering the seal flush is critical for seal reliability. End users should consider using a duplex filtration system, which allows for uninterrupted operation if the filter element needs replacing, with a 3-micron nominal filter element. End users should also conduct a detailed fluid analysis to determine the appropriate filter element to use.
Viewing these applications on a pressure-enthalpy diagram is helpful to see whether the state of the application’s fluid is liquid, vapor or supercritical. Traditionally, end users have used pumps for applications with low vapor pressure margins or for low- to moderate-speed supercritical fluid applications. Recently, some turbomachinery operators have begun using compressors for these applications instead. Regardless of the equipment, noncontacting dry gas seals provide the best operating results for this type of service.
In general, CO 2 applications can be grouped into:
- Low-speed applications (less than 4,000 rpm)
- Moderate-speed applications (between 4,000 and 8,000 rpm)
- High-speed applications (more than 8,000 rpm)
While most applications have rotational speeds lower than 3,600 rpm and sealing pressures between 1,900 and 2,500 pounds per square inch [gauge] (psig), the market is beginning to demand solutions for more challenging moderate- and high-speed applications. Test results for these applications so far have been positive, but they require additional measures to ensure smooth operation.
Figure 1. Typical PI&D diagram of supercritical C0 2 pump application
Low-speed applications typically involve pipelines with centrifugal pumps operating slower than 4,000 rpm. A dual, unpressurized (tandem) seal arrangement is recommended for these applications. End users have used this sealing arrangement successfully for decades, accumulating more than 1 million operating hours.
The double-ended pumps in low-speed applications require a relatively simple seal support system with a heated, filtered flush at a low flush rate of one to two gallons per minute and a vapor monitoring system for primary vent gas leakage. Depending on the climate, the system may also need a nitrogen sweep (Plan 72) to prevent primary vent icing.
The tandem sealing arrangement recommended for these applications is reliable and easy to manage and minimizes seal leakage. In comparison, conventional contacting (wet) seal solutions are more complex and costly. They require a very high flush rate to keep the temperature in the seal box at an acceptable level and have higher levels of seal leakage, which may contribute to emission problems.
Pressure requirements for pipeline applications are increasing as end users strive to reach deeper reinjection reservoirs. In response, many centrifugal pump original equipment manufacturers (OEMs) are developing high-speed pumps that can handle these increased pressure demands.
When selecting a sealing system for moderate-speed applications, end users must consider the heat generated by rotating seal components in a high-density fluid. Regardless of the application’s ambient temperature, the losses and heat generated by the viscous drag of the rotating seal components, or churning heat, can raise the temperature in the seal chamber significantly.
By using an empirical formula for dry gas seals in high-density fluids based on the Bilgen-Boulos equations for high Reynolds numbers, estimating an application’s churning heat is possible. For example, applying this formula to a moderate-speed application with a sealing pressure of 3,118 psig and a speed of 7,700 rpm results in an estimated churning heat of 15 to 16 kilowatts. While this figure is significant, testing shows that a sealing arrangement with a low, filtered flush similar to the one recommended for low-speed applications is also appropriate for these applications. For moderate-speed applications, however, the flush would not be heated.
A growing number of end users are looking for OEMs of high-speed centrifugal pumps, centrifugal compressors and turbo expanders to create products for the expanding supercritical CO 2 market. Transportation companies, which have growing pipeline pressure requirements; end users performing high-pressure reinjection for enhanced oil recovery or sequestration; and end users performing carbon capture, or recycling the waste exhaust gas from refineries, are particularly interested in high-speed applications for their turbomachinery.
The high rotational speeds of these applications, along with the high density of supercritical CO 2, create friction that leads to considerable churning heat. For example, one application for an integrally-geared compressor with a suction pressure of 2,300 psig, available seal gas at a temperature of 350 F and rotational speed of 26,000 rpm would produce an estimated churning heat of 50 to 60 kilowatts. As a result, these applications have greater cooling requirements than typical low- and moderate-speed applications, which end users must address with a properly designed sealing system.
Analyzing the rising temperature in the seal chamber shows that end users must increase the mass flow rate of the gas feeding the seals and cool the seal supply gas (typically fed discharge gas) to keep the temperature within the seal design’s limits. Several programs are commercially available for estimating the state of the fluid at the conditions used in this analysis. Based on the results, end users should implement a seal system with a high flush rate of approximately 10 gallons per minute and cool the seal supply fluid temperature to 125 F to manage the temperature inside the seal chamber properly. Since the flush rate is significantly higher than the rate used in low- and moderate-speed applications, end users must take the flush rate into account before designing a seal support system for their equipment.
Ensuring Peak Performance
While end users have handled traditional CO 2 applications successfully for decades with tandem seal arrangements and relatively simple seal support systems, higher-speed applications pose new challenges. As application pressures and rotational speeds rise, end users need to consider the churning heat that these applications generate and choose a sealing system with the appropriate cooling measures. Regardless of the application’s speed, performing a detailed fluid analysis and selecting the correct filter element for the seal system to ensure reliability is important.
By understanding their application’s requirements and designing an appropriate sealing system, end users can trim the time and money that they spend on maintenance and promote peak equipment performance.
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