This article discusses five steps for evaluating a gas turbine inlet air filtration system. The first step to evaluate an inlet filtration system is to study the operating environment. It is important to understand what must be removed from the air by the filtration system before evaluating the existing system. Once the operating environment is defined, the existing filtration system can be evaluated. Third step is the maintenance, which is an important part of any system. It ensures that the system will stay in an operable condition and perform as required. There are several tasks which, if performed consistently and correctly, can ensure that the filtration system operates properly. Fourth step is the upgrades, which should be considered if the operator finds deficiencies in the current filtration system. The fifth step is completing a lifecycle cost analysis. This analysis quantifies the cost and performance of a system in monetary terms to obtain a lifetime cost of the system. The lifetime costs between two different system options can be directly compared.
A typical gas turbine (GT) ingests millions of pounds of air every day. Therefore, even a small concentration of debris in the air can correlate to a large amount of debris in the GT. For example, 10 ppm of debris in 400,000 lb/hr of air is equal to 4 lb/hr of debris. A GT inlet filtration system is used to protect the turbine from harmful debris which can lead to reduced efficiency and power, component performance degradation, and blade failures. In this article, five steps are suggested for evaluating a current GT inlet filtration system and determining the need for any improvements.
Step 1: The Operating Environment
The first step to evaluate an inlet filtration system is to study the operating environment. It is important to understand what must be removed from the air by the filtration system before evaluating the existing system. The environment, type of debris, and amount of debris dictates how the filtration system should be configured and maintained. Operating environments can be classified into nine main categories. These categories and a list of common debris are summarized in Figure 1. A GT may operate in one or more of these environments throughout the year.
In addition, there can be local, seasonal, and/or temporary debris in the air. Plant emissions are one example of a localized source.The layout of the plant site with respect to the turbine inlet will influence how much soot from exhaust, cooling tower aerosols, or other emissions enter the inlet filtration system. Other examples of localized sources are mining operations or agricultural sites. Seasonal changes and weather patterns (wind, humidity, precipitation, and temperature) will also affect what must be filtered. Lastly, temporary sources such as construction sites will affect air quality.
When defining the operating environment, the first step is to complete a visual survey of the operating site and surrounding area. Next, an air quality survey can be completed near the turbine to obtain information on debris size and concentration. It is also valuable to complete compositional analyses on used filters and samples of deposits from the first stages of the compressor. This will give insight into what is and is not being removed from the air with the current system.
Step 2: The Existing System
Once the operating environment is defined, the existing filtration system can be evaluated. The configuration of the filtration system should already be documented or it can be determined from visual inspection. The majority of filtration systems in operation have multiple stages and each stage should be defined. An example of documentation of a filter configuration is shown in Figure 2.
The operator can evaluate the system by comparing the debris in the air and the type of filtration system the GT has. A brief description of different filter components and their purposes is provided in Figure 3. During the evaluation, it is important to consider filtration efficiency, volumetric flow rate, and pressure loss. In addition, a visual inspection of the filtration system should be performed before filters are replaced. Some specific items that should be noted are:
Filters are wet or dry (wet filters are an indication that water is entering the system),
Rust in filter housing (this indicates that water is entering system and housing is degrading),
Loading of filters across entire filter bank (look for even loading and if filters need replacement),
Leaks in the filtration system (leaks defeat the purpose of the filtration system and should be sealed),
Installation of filter elements (ensure filters are being installed correctly),
Damaged filter elements (determine root cause of damage: FOD, material defect, filter overloaded, or wet filter), and
Deposits on housing downstream of the last filter stage or deposits on first stages of compressor (indication of what is not being removed by filtration system).
A review of these items will provide the operator with a comprehensive assessment of the current state of the inlet filtration system.
Step 3: Maintenance Practices
Maintenance is an important part of any system. It ensures that the system will stay in an operable condition and perform as required.There are several tasks which, if performed consistently and correctly, can ensure that the filtration system operates properly.
The largest maintenance item of a filtration system is the replacement of the filters. Filters are designed for a certain lifespan which is quantified by the pressure loss across the filter. This should be monitored in the filtration system. Each filter is prescribed an initial and final pressure loss and a maximum lifespan by the manufacturer.The filters should be replaced when they either reach the final pressure loss or maximum calendar lifespan. Filters operating past this will have reduced filtration efficiency and high pressure losses. Leaving fully loaded filters operating will lead to reduced performance and increased degradation of the GT, and the possibility of complete filter failure/collapse. This will result in bypass of the system and thus GT contamination.
In addition to filter replacements, maintenance needs to be performed on any auxiliary systems. This can include drainage systems, self-cleaning systems, and/or anti-icing systems. Also, inspection should be performed periodically including: filter condition, filter to frame seal leaks, seals on filter housing joints, inspection ports and doors (closed and sealed properly), drainage points, water drains for plugging, flexible connections in draining system.
If maintenance practices are found to be inadequate, a primary focus should be placed on correcting filter replacement intervals and inspecting the filtration system for any leaks. After these items are improved, an inspection plan should be implemented. This will help to ensure the filtration system performs properly in the future.
Step 4: Upgrades to the System
During the evaluation, the operator may find deficiencies in the current filtration system. If this is the case, then upgrades should be considered. The system can be upgraded on several levels. A filtration system, housing and all, could be completely replaced, or just the filter elements could be upgraded. When considering an upgrade, several items should be evaluated.
What are the requirements for inlet air filtration?
What are the weaknesses of the existing system?
What debris is not being removed by the system that should be removed?
Could the filtration system perform as needed by changing out the filters more often or is a different modification required?
Does the system have sufficient weather protection (snow, ice, rain)?
What is the expected performance of the GT while using the existing system? Is this performance acceptable for the future operation?
The operator must determine the benefits of upgrading the filtration system. These can be realized in several areas: increased filtration efficiency, reduced degradation, improved gas turbine performance, or decrease in pressure loss across filters. When evaluating upgrades to the filtration system, several different upgrade configurations should be evaluated for their cost and benefit.
Step 5: Overall cost of system
One of the most straightforward methods for comparing the cost and benefit of different filtration system options is by completing a Life Cycle Cost (LCC) analysis. This analysis quantifies the cost and performance of a system in monetary terms to obtain a lifetime cost of the system. The lifetime costs between two different system options can be directly compared.
An LCC analysis for a filtration system has seven main components: initial costs, maintenance costs, availability and reliability considerations, GT degradation losses, compressor washing effects, pressure loss effects, and potential failures or events costs. The initial cost is the main cost that is typically considered when looking at upgrading a system. This cost is important for an LCC analysis, but not necessarily the most important or dominant cost. Costs associated with maintenance (i.e. replacement parts, labor, and downtime) and those associated with GT performance (i.e. efficiency) are also important components of an LCC. The effects of the inlet filtration system performance (pressure loss and GT performance degradation) are quantified by placing monetary values on lost power, increases in heat rate, and reduced efficiency.
Once all of the costs of a filtration system are quantified, a lifetime cost of the system in terms of present value is found. Since inlet filtration systems do not produce a profit, the system with the least negative value will have the best lifetime cost. The advantage of an LCC analysis is that it provides a method to perform an objective analysis of different filtration system upgrades.
By completing the five steps described above, the operator can evaluate the current filtration system. This includes determining what debris must be removed from the air, verifying the configuration of the current system, identifying weaknesses in the filtration system, evaluating the current maintenance practices, considering possible upgrades, and analyzing the cost and benefit of changes to the system. The results of these tasks will provide direction for improvements in the operation and performance of the inlet filtration system and GT.