An oil and gas refinery in northern England was experiencing poor reliability of its steam turbine resulting from the failure of its existing seal system.
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By Richard Smith
What are the main problems regarding the use of steam turbines in the oil and gas industry?
The main problems arise from the high levels of steam leakage.
This can cause numerous issues, which can be divided into three groups:
Equipment (reliability and operation)
- Bearing contamination with steam (AESSEAL LabTecta® can be a preventative measure) causing breakdown of bearing lubrication oil.
- Higher bearing housing temperatures as hot steam leakage jet blasts the adjacent bearing housing – causing thermal breakdown (oxidation) of bearing oil.
- Approximately 80% of premature turbine overhauls are due to bearing failures.
- Carbon ring replacement – typically every two years.
- Steam undercutting of shafts, requiring major overhaul.
- Corrosion to structures around the steam turbine.
- Carbon ring housings are often corroded and need replacement.
Safety considerations
- Many users manually drain the water from bearings and perform regular oil changes. This is work carried out in the field. All work in this type of hazardous environment exposes personnel to a degree of risk which the user would clearly rather avoid.
- Corrosion to mechanical overspeed trip mechanism. The failure of this safety feature can cause the turbine to run out of control – giving rise to what is potentially a highly dangerous situation.
- Overspeed test failure
- Injury from steam leakage
- In cold climates, injury due to ice forming as a result of condensed leakage
- Noise from steam leakage jetting
Thermal Efficiency
- Thermal losses as a result of steam leakage are significant. When steam is used in a plant, the more steam that is returned to the boilers for reheating, the less energy is used (and the greater the thermal efficiency). Any loss of steam from the steam turbines reduces the amount of steam returned to the boilers. This shortfall has to be made up from cold ‘make-up water’. Heating this water and turning it into steam requires considerable additional energy, causing a reduction in thermal efficiency.
- Maintenance costs associated with gland condensers, steam extraction and evacuation equipment.
Why do these issues arise?
The traditional method of sealing general purpose steam turbines has changed very little for over a hundred years.
The use of floating carbon rings running on the machine’s rotating shaft involves a ‘controlled leakage’. When the equipment is new, this leakage is relatively small, giving rise to what the industry regards as ‘acceptable losses’.
However, with wear to the carbon rings and shaft, leakage rates will rapidly increase. Fifty years ago process industries such as refineries had more maintenance staff, so these machines would have more regular maintenance intervals. The carbon rings would be replaced and repairs would be carried out to the shaft during major planned overhauls. The number of machines in use was also a lot higher, so there was more expertise available in maintaining, repairing and commissioning. So-called carbon ring ‘run in’ or ‘bedding in’ was one of those black arts talked about back in the day. Some of these skills have been lost in the modern era. Historically users of the machines accepted these losses as part of a plant’s thermal efficiency. What are the main reasons for a reluctance to upgrade to newer and more efficient technology? With many decades of use the industry has learned to live with and manage the problems associated with sealing these machines. In such a risk-averse industry many machinery engineers perceive upgrades as a risk, and the ‘do nothing approach’ or ‘repair as before’ is a comfortable position. The problem is that the financial benefits and the return on investment will fall across multiple disciplines, including operations, process engineering, and maintenance. With responsibility for savings spread across a range of departmental budgets, no one department has complete ownership of the costs. However there are several best in class users where there is a business vision to improve performance. These users are more ready to adopt new technology, and are enjoying the benefits . Can you give any estimate for the percentage of steam turbines in use in the oil and gas industry which are in need of an upgrade? From our observations around the world we would estimate that over 99% of the industrial general purpose steam turbines used in the Oil and Gas industry are still fitted with traditional technology. How do you try to convince companies of the need to invest in this area? With energy prices increasing globally, the cost of steam generation has also significantly increased. Steam loss historically was considered just an acceptable thermal inefficiency. Today that view has changed. Most steam is generated from the burning of fossil fuels. The by-product of this combustion process is CO₂. Many oil and gas corporations are making bold public sustainability statements about ways of reducing their scope 1 and 2 GHG emissions. The deployment of steam turbine seals will assist corporations in reaching their targets, while also making sound business sense. AESSEAL intends to appeal to business leaders to do the right thing for their bottom line, their environmental pledges, and for their biggest stakeholder - the planet. What sets AESSEAL products apart from those of its competitors? Many of the general purpose steam turbines working in refineries across the world are over 50 years old, and there are many design variants. AESSEAL’s range of flexible manufacturing and modular components means that many of these machines can easily be upgraded with little or no modifications. Rapid delivery means that conversion can take place in the normal maintenance cycle. Are there examples of how AESSEAL products have made a difference to companies in terms of turbine efficiency and in protecting the environment? One major user has a rolling program of upgrading steam turbines as they go through the maintenance cycle. The first steam turbines were converted over eight years ago, and steam savings over that period are estimated to be worth over $2 million. With 30 machines now converted, annual CO₂ savings are estimated at over 800 tonnes.
What convinced this company to install AESSEAL products?
This user has used AESSEAL for pump sealing applications for many years, originally choosing us and staying with us because of our technology, flexibility and lead times. One project has involved upgrading from single to dual mechanical seals. This upgrade has made a major contribution to the company’s ability to meet its goals in the areas of environmental compliance and reliability. In addition, we believe that the upgrades saved the company around $20,000 per pump, which over the course of a year would amount to a saving on purchases of more than $2 million.
