An Experimental Investigation of Offshore Gas Turbine Intake Air Filter Performance

Abstract

Efficient intake air filtration is a key element for limiting fouling, erosion, and corrosion in the compressor section and hot corrosion in the hot section of offshore gas turbine installations. Current intake air systems on the Norwegian Continental Shelf (NCS) are normally able to prevent serious corrosion and erosion problems in the compressor section and ensure acceptable service intervals for the hot section of the gas turbines, but compressor fouling remains a challenge. This compressor fouling occurs despite the presence of “state-of-the-art” intake air filtration systems. The performance deterioration caused by compressor fouling increases fuel consumption and emissions and has a particularly severe economic impact when it reduces the maximum available power from the gas turbine and consequently reduces oil and gas production.

The main objective of this work is to determine the performance of current intake air filters offshore and their influence on gas turbine performance. The work is empirical, and the empirical data have been obtained from offshore installations, from tests performed on a GE J85-13 jet engine at Kjeller, Norway, and from experimental tests on filter elements in a test rig at the Norwegian University of Science and Technology (NTNU). The filter test rig was designed and built as part of this work.

The work has confirmed that considerable amounts of contaminants build up in the compressor section of offshore gas turbines despite the presence of state-of-the-art filtration systems, causing deterioration in gas turbine performance. It has been determined through analysis of compressor fouling in different offshore gas turbine engines that sodium-based salts are the dominant component of the fouling, and the influence of the observed contamination levels on gas turbine performance has been determined.

The extensive test program on different types of deteriorated filter elements from offshore service has revealed that accumulated salt and other contaminants are shed from the filters and re-entrained into the airflow on the downstream side of the filters when they are exposed to wet/humid operating conditions. The relationship between ingested amount of water and shed contamination from the filter elements as well as the influence on differential pressure drop over the filters have been quantified.

The review of offshore filter performance and experimental testing have documented that the current practice for condition monitoring of intake filter performance by monitoring the total pressure drop over the intake filters is inadequate. The re-entrainment of accumulated particles into the downstream side of the filter elements causes a significant change in the pressure drop characteristics for some types of filter elements. After such incidents, an apparent improvement in filter performance in terms of reduced pressure drop will be accompanied by an increase in compressor fouling. The work has also revealed significant variations in the level of accumulated particle mass and differential pressure drop among filter elements from the same installation. Such variations may cause some filter elements to operate outside of their design criteria and will normally not be discovered by monitoring the total pressure loss over the intake filters.

A thorough review of current international standards and available filter manufacturer specifications for intake air treatment systems has documented important shortcomings in how offshore-specific challenges pertaining to intake air filtration for gas turbines are handled and has revealed a need for new or improved standards for gas turbine intake systems offshore. No international standard for evaluating and documenting the performance of the complete gas turbine intake air systems in offshore applications exists. Applicable heating, ventilation, and air-conditioning (HVAC) air filtration standards for filter elements fail to address the offshore-specific challenges related to salt removal and moist and wet operation and cannot be used to accurately predict operational performance or life. Some suppliers of filter elements for offshore gas turbine application also choose not to test their filters in accordance with these standards.

An international standard for offshore gas turbine air intake systems is needed in order to ensure that the specific challenges that stem from operating in an offshore environment are handled in a responsible manner. Such a standard should take a holistic approach to the intake air system rather than solely focusing on some specific components or conditions.

Experimental performance evaluation of used filters from offshore service and filters exposed to accelerated deterioration in the laboratory has increased the knowledge of intake filter deterioration in offshore applications and created a basis for establishing guidelines and requirements for future standards for offshore gas turbine intake filters and systems.

The accelerated saltwater deterioration method developed and applied in this work was effective in revealing significant variations in how new filters performed when exposed to this challenge. Saltwater ingestion caused filter performance to deteriorate within a short time frame, and the recorded increases in differential pressure drop were within the range of differential pressure drops recorded for used filters from offshore operation.

The major contributions of this work are presented in five international papers contained in the appendices.