Peter Sandberg reports on the importance of keeping gas turbines clean and working efficiently in the toughest of conditions.
Gas turbines operating in an offshore environment are often exposed to the toughest possible conditions. Beside the harsh, salt laden, marine environment they are also exposed to the industrial environment with pollutants such as various dusts particle as well as hydrocarbons.
On top of that, in areas like Middle East and West Africa fine dusts loads from land occur during specific wind conditions.
Faced with all this challenges safe operation of offshore gas turbines is critical since they often are the prime power source necessary for the production and revenue of the large investment an offshore platform represents.
With proper function of the gas turbine being so critical, reliability is of outmost importance. This is not a new situation.
Reliability has always been the prime target and will be so also in the future. But with constantly higher oil and gas prices and environmental issues such as global warming becoming more and more important it's not only important to keep the gas turbines running, it's also important to do it as efficient and with as little environmental impact as possible.
Gas turbines are exposed to pollutants through two possible sources: combustion air and fuel. The latter is normally defined and content is most cases known when the installation is selected. Air inlet combustion air quality however varies from site to site and many inlet filtration systems fail to protect the gas turbine due to incorrect selection and/or sizing.
The Gas Turbine Inlet Filtration System therefore has two main missions:
- Reliability assurance.
- Efficiency assurance.
Reliability assurance means that the Inlet filtration system must be able to protect the gas turbine from penetration of particles that could over time be a danger to the gas turbine. This is typically particles that could cause wear, or fine particles that could plug the holes in vanes and blades for cooling air and consequently cause damages due to over heating.
In addition the inlet system has to prevent the entrance of salt which could cause corrosion and in combination with sulphur in the fuel and high temperature, hot gas corrosion in the power turbine. The higher sulphur content - the lower the acceptable salt ingestion.
Efficiency assurance means that the Inlet filtration system efficiently must remove the entrance of small particles in order to avoid compressor fouling resulting in degradation of the gas turbine performance.
Axial compressor fouling is primarily caused by airborne submicron particles. Fouling changes the shape of both rotating and stationary vanes and results in a reduction of both mass flow and pressure ratio generated by the compressor.
The net result of fouling is a reduction of power output and an increase in heat rate for a given combustor outlet temperature - conversely if an engine is not running at its temperature limit a fouled engine has to run hotter to produce a given output.
The costs of fouling far exceed the cost of eliminating it but since capital costs tend to be weighed heavy in equipment purchase decisions, many gas turbines are equipped with inadequate inlet air filtration and the operator ends up paying many times more in extra operating costs.
Many of the air inlet filtration systems today operating in offshore environment are unable to offer gas turbines the required protection mentioned above. Their failure is mainly depending on the following:
- They are often designed mainly for a marine environment based on a theoretical marine aerosol with a particle spread consisting of course particles which only can exist in sea spray conditions. Being designed for marine environment their ability to handle medium or large quantities of industrial and natural dust is limited.
- In order to minimise equipment size filtration velocity is increased to a level only allowing inertial filtration techniques which only is efficient on large/heavy particles. Efficiency is limited to F5-6 (EN779). Consequently, with small filter area, dust holding capacity becomes low and pressure loss becomes high.
- Inability to handle salt in wet and dry phases. With changing humidity salt changes in size and characteristic. Wet salt that dries out have a tendency to re-entrain due to the high velocity. As a result many operators are faced with an inlet filtration system that doesn't give the reliability and efficiency assurance one can expect and consequently becomes very costly to operate. The major costs consist of the actual and potential loss of revenue. The filter maintenance costs, often looked upon as high, is in reality marginal compared to mayor ones such as:
- Poor efficiency gives rapid engines degradation resulting in loss of production/revenue and/or higher fuel & environmental costs.
- Stopping a gas turbine for offline compressor washing and/or filter change out means in many cases stop of production. With the current oil price this off course must be avoided.
- Gas turbines exposed to salt and fine dust are faced with high risks of serious breakdowns caused by hot gas corrosion, cooling hole plugging, etc. Again potentially very high costs for loss of production and equipment.
Even if above description may seem discouraging it doesn't represent the experience of all operators. Alternative inlet filter systems based on low to medium velocity technology has been on the market since many years and has generally been able to handle the challenges of salt and dust load in a better and more cost efficient way.
We can conclude the following. Firstly, it is extremely important to keep the gas turbine clean to assure good economy and reliably. Secondly, a clean gas turbine operates efficiently and with less environmental impact. Thirdly, The cost of clean air is not the cost of replacement filters - it is how much energy the filters use during their working life at the desired efficiency.
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Peter Sandberg is CEO, Camfil Farr Power Systems, Boras, Sweden. www.camfilfarr.com
