Several options to improve the operating characteristics of gas turbines do exist and, given that those few extra percentage points of operating efficiency are essential to maximizing the value of an installation, they should be taken up. Michael Gabriel reports.
In the present economic environment, owners and operators are seeking cost effective ways to expand gas turbine operability, improve efficiency, gain more output and extend the life of their existing equipment. The regulatory process for permitting new generation sources is slow and more demanding than ever before, making minor turbine improvements to existing equipment a more attractive option.
Many things can be done to keep a turbine operating at peak efficiency. Attention to detail when observing and trending operating parameters can identify degrading turbine performance. Various types of monitoring packages exist to assist in this endeavour. Some are designed to warn of impending failures (i.e. bearing vibration), thereby averting costly forced outages. Other products are aimed at analyzing turbine or plant performance over time. Remote monitoring centers can do either or both.
|Figure 1: Typical plant operational envelope|
An additional challenge to plant owners and operators are the variations in fuel composition needed over time. Combustion systems are designed and built to operate in a relatively narrow range of fuel quality. Significant fuel composition variation can cause considerable operational issues with regards to emissions, flame stability and combustor dynamics. These changes are most challenging to lean, premixed, ‘low NOx’ combustion systems.
|Figure 2: ECOMAX plant operational envelope|
TURBINE EFFICIENCY – DEGRADATION AND RECOVERY
There are many ways a gas turbine experiences performance degradation over time. Some can be corrected or mitigated in an operational manner, while others require significant maintenance during a shutdown. Examples of common causes of turbine efficiency loss are clogged inlet filters, dirty compressor blades, damaged compressor and turbine blading, excessive clearances between casings and moving compressor or turbine blades, and sub-optimal combustor tuning.
Inlet filter cleaning and maintenance
Properly maintaining the inlet filtration system is vital to maintaining overall turbine health. Poorly maintained inlets can result in damage to compressor blades, dirty, fouled compressor blades, and excessive pressure drop across the filters (with the resultant loss in turbine efficiency).
Periodic inspections of the clean side of the filters should be performed – searching for ‘light leaks’ as well as material that is or could become loose and enter the compressor.
Some turbines have a reverse air ‘puff’ cleaning system for the filters, which utilize the compressor discharge air (after cooling it) to reverse flow filters for a brief instant, with the intention of dislodging the contaminants entrapped on the dirty side of the filter. One of the largest drawbacks to this system is that the vast majority of the contaminants can be drawn back into the filters after the short puff, requiring many repeats of the process to significantly reduce inlet filter differential pressure. A simple improvement is to utilize the plant air system to puff the filters when the turbine is shut down. This method significantly reduces the time required and amount of air consumed in the cleaning process.
Routine compressor washing is essential to maintaining turbine output and efficiency. Significant performance degradation can be experienced due to dirty compressor blades, especially if the gas turbine is in, or near, an industrial environment.
Compressor washes can take place with the unit in operation (online) as well as shut down (offline). Some turbines, especially those with low NOx combustors, do not allow detergent use during turbine operation. A typical regime for these turbines is to wash online once a day for a few minutes a day. When the unit can be taken offline, a more thorough wash is conducted, utilizing detergent, with wash and multiple rinse cycles.
As an example, Gas Turbine Efficiency model 600i compressor wash systems were installed on two 9E gas turbines in Malaysia. Implementing a proper routine of online and offline washes, the plant realized a heat rate recovery of up to 1.5%. This resulted in an economic payback period of two months. Additionally, the gas turbine output was increased by 8% to 13% during the online wash cycles.
Typical output recovery achieved after conducting an offline wash can exceed 2% when compared to pre-wash turbine operation.
Other benefits include redution in emissions. Results will vary based on turbine type, usage, and time since last washing.
Inlet conditioning – reducing the inlet air temperature is a very common method for improving gas turbine output. There are three well known methods for achieving this output improvement: fogging, evaporative cooling, and chilling. These are summarized in Table 1.
Fogging involves injecting atomized water into the inlet air stream, downstream of the inlet filters. The system carefully controls the amount of water injected to ensure no large droplets of water are ingested into the compressor.
Evaporative cooling equipment is installed in the inlet ducting, after the filtration system. The system consists of a media bank in the air stream, which is continually wetted by one or more water pumps. As the incoming air passes through the media it absorbs some of the moisture and at the same time loses heat (via water evaporation), resulting in cooler, denser air. This dense air then enters the compressor. Cooler, denser air allows the turbine to produce more output power with improved turbine efficiency.
Chilling the inlet air stream is an effective way to increase the air density and thus improve turbine output. The chiller – typically a large mechanical refrigeration unit – is adjusted to provide a controlled inlet air temperature to the compressor. No matter what the ambient air temperatures are, the chiller ensures that the compressor inlet temperature will not exceed a specified target temperature.
Peak firing a gas turbine (above the target OEM design limit) is another option for producing additional output, but the additional maintenance demands imposed on combustion and turbine hardware make this economically feasible on rare occasions.
However, there can be some opportunity to take a turbine from its current operating state and allow it to operate at the OEM defined design limit. It is not unusual to find a turbine slightly under fired as compared to OEM design limits. Increasing the turbine output in this manner must be undertaken with great care so as to not exceed the OEM limits under any, and all, ambient conditions.
IMPROVING EMISSIONS COMPLIANCE
Emissions compliance is a continual focus of operating plants, with the requirements only becoming more stringent over time. An example is the EU large machine directive, which is driving emissions of NOx and CO to less than 20 ppm for all plants greater than 20 MW. A similar programme in the US is pushing for requiring upgrades of all non attainment zone sited plants to the best available retrofit technology.
Emissions compliance is not only better for the environment, but it will help avoid costly fines. Typically, gas turbine combustion systems are ‘tuned’ twice a year, once prior to the summer and once prior to the winter. Each tuning period is designed to optimize the turbine for the upcoming season while ensuring, using a significant safety margin, that there should not be excessive emissions, excessive combustor dynamics or loss of flame (due to lean blow out).
Unfortunately, these tuning services can be time consuming, preventing the turbine from operating to its full potential during the tuning period. In addition, seasonal tuning inherently leaves ‘operational performance margin’ on the table, as the combustion system must be tuned to ‘safely’ allow for acceptable performance under all projected ambient conditions over the next six months (until the next tuning event). Fortunately, automated tuning systems are available which continually optimize the combustion system.
One such system is Gas Turbine Efficiency’s ‘ECOMAX’, which utilizes real time emissions and combustor dynamics data to continually determine the optimal operating point for the combustion system. The plant operators can select, in real time, to optimize emissions, combustor dynamics, and/or turbine output. This system also allows the operators to safely take the turbine up to OEM design firing limits, as mentioned in the previous section. Additionally, ECOMAX can be configured to optimize the part load and base-load heat rates, in simple cycle or combined cycle configurations. The system is illustrated in Figures 1 and 2.
Operational experience with ECOMAX has shown that it has optimized the combustion system on one unit almost 300 times in a single month. Additionally, the unit had almost a 2% gain in output due to operation at or near the OEM design limit.
Additional challenges arise from the current trend of having more frequent variations in fuel quality/composition. The results can be flame-out turbine trips (lean blow outs of the combustion system), emissions violations as well as combustion dynamics excursions. These negative events are more prevalent in lean, premixed, low NOx combustors. Continually tuning a combustion system can enable the turbine to successfully operate in a much broader window fuel composition variation. This can also be turned into an economic advantage for the plant, allowing consumption of lower BTU content fuel at a significant cost savings. Essentially, why pay for ‘high test’ if your turbine can run on ‘regular’ with no ill effects?
Finally, automated tuning systems can be adjusted to reduce emissions on a continual basis, setting targets below the accepted, standard emissions levels normally achieved.
DOING MORE WITH LESS
Once degradation is noted, the first step is to find the cause, with the logical second step being rectifying the issue. Some issues will be internal to the turbine casing, and require deep turbine knowledge coupled with operating data to pinpoint the likely cause. Other issues may be external to the casing of the turbine, and can be discerned by a combination of operating data analysis coupled with experienced eyes and ears examining the turbine auxiliary components and systems.
One common issue is the loss of compressor efficiency due to air leaking by closed shut anti-surge (bleed) valves or bypass valves. Some plants may have installed instrumentation which show a temperature rise in the associated piping. If your lines are not instrumented, a thermal gun is all that is needed to discern which may be leaking. The same process can be used for the inlet air heating valve.
In the era of shrinking staffs coupled with more regulations and administration, the trend is that fewer eyes and ears are roaming the plant with the intention of finding issues before they become significant. One solution is to have a plant audit conducted by people familiar with the technology, but not too close to the operation of that particular plant. A questioning attitude by the auditors is vital. The most successful audits are thoroughly embraced by management and operational personnel before, during and after the audit.
For those owners/operators with a large fleet of similar units, significant savings can be realized with regards to proper outage scheduling and asset management, especially in the area of capital spares over the life of the turbine. A robust programme which manages hardware life and predicts fallout allows owners to properly allocate funding to purchase capital spares without overspending potentially millions of dollars. These services are commercially available for fleet operators who find themselves wanting to implement this type of program without expanding their already overburdened staff.
In summary, many options exist for owners and operators to improve the various operating characteristics of their gas turbines. They range from inexpensive and simple to costly and complex, and are marketed by OEMs and third party vendors alike.
Michael Gabriel is the manager, Monitoring Centre, with Gas Turbine Efficiency, Orlando, Florida, US.