Gas turbines that are installed in areas where icing conditions could exist shall be equipped with an anti-icing system. In general, ambient temperature dropping below 4ºC (39 ºF) with high humidity could be subject to icing problems. Icing could induce very significant pressure drop in the inlet air filter house which leads to gas turbine power output deterioration.
This article is an excerpt from the paper, “Gas turbines and associated auxiliary systems in oil and gas application”, presented by Emmanuel Bustos, Michael Hotho, Mounir Mossolly and Alfredo Mastropasqua of TechnipFMC at the 2018 Turbomachinery & Pump Symposia.
Anti-icing can be achieved by using intra-compressor bleed air, dispersed into the inlet air stream with a bleed rake (specially designed piping to evenly distribute the bleed air over the area of the filter house inlet). Bleed rake design requires finite element analysis (FEA) to ensure uniform flow of warm air into the filter housing. Using bleed air will result in a performance loss to the engine which will have to be determined after analysis of the bleed air requirements and icing conditions. The loss will be a function of the mass flow rate of bleed air and from which bleed system it is taken. As an example of anti-icing on a gas turbine bleed is taken from a higher-pressure compressor section results in around 2% of total engine air flow loss.
Advantages: – Simple design; – Additional pressure drop added by the filter house is small; – It may be possible to use CO turndown bleed as a source of anti-ice air; – No additional systems required; Disadvantages: – Results in a loss of performance, thereby reducing the maximum possible shaft power;
Exhaust Recirculation System
An exhaust recirculation system mixes portion of hot exhaust gas with cold ambient air. A by-pass line installed from the exhaust system to the air intake inlet system disperses evenly through a distribution skid hot exhaust gas directly into the inlet air stream.
Advantages: Heat source is available; – Additional pressure drop added by the system is small;
Disadvantages: – Moisture in exhaust gas can create problems; – Impurities in the exhaust gas can create compressor fouling; – Exhaust system is a low-pressure system which can create difficulties in directing the exhaust gas flow to the inlet system; – Exhaust gas is CO2 rich and will adversely impact combustion and power available;
Slipstream of ventilation exhaust
Depending of heat requirement parts or the entire filtered ventilation air for the gas turbine enclosure can be directed to the inlet of the air filtration housing. The warmed-up air from the enclosure will be mixed with the cold inlet air.
Advantages – Heat source is available; – Little effect on performance; – Easy installation;
Disadvantages – Content of hydrocarbon in ventilation air possible and possibility of hydrocarbon concentration build up; – Safety risk; – Additional gas detection devices might be required; – Hydrocarbon in ventilation air might result in GT trip or auto ignition in combustor;
Ambient air blown through a coil in the Gas Turbine Exhaust Stack
Exhaust gas from the turbine outlet passes through a heat exchanger which is in the exhaust duct. Ambient air flow extracted from a specific point downstream of the inlet filter system and with the help of a fan the air is blown through the heat exchanger. The ambient air will be heated up by the exhaust gas and directed to the inlet air system and heats up the inlet air. Advantages – Little effect on performance; – Amount of warmed up ambient air can be controlled; – Dry heating system; Disadvantages – More complex system, inlet throttling probably required to control flow; – Requires extra space in the exhaust system; – If fan fails anti-icing system does not work;
Hot Water Heating Coils
Anti-icing can be achieved by heating the inlet air as it passes across a hot water heating coil. The heating coils would typically be installed upstream of all filter elements and weather vanes. Gas Turbine manufacturer will provide the coils and interfaces for the customer’s water supply near the edge of the filter house. All other components are typically in the customer’s scope of supply.
Advantages – The design is simple; – Hot water-glycol can be provided from any source or from a standalone system;
Disadvantages – Heating coils will add a pressure loss to the filter house; – Heating coils are exposed to dirty air, as they are installed upstream of the filter elements; – Requires additional ancillary systems (and footprint) to provide the water glycol which is used for a very small portion of total operational time;
Electro Thermal System
Electro thermal system consists of resistance wires which are imbedded in rubber pads which are mounted on the icing surface which must be protected.
Advantages – Heats up only the icing points;
Disadvantages – Locate the critical points where ice can be found; – Safe, no hazardous area; – Hot spot potential, possible non-uniform heat distribution; – Complexity of wiring runs; Compressor Bleeds Air (or Gas Film Heating)
Compressor bleed air is like turbine blades gas film cooling technique. Canals will be drilled in the components which have a great hazard for ice build-up such as filter, IGVs and parts of the inlet duct. Hot air, which can be extracted from the compressor or the exhaust gas, passes through these canals and warm up the surface of these components.
Advantages – Heats up only the components for potential ice build-up and blockage;
Disadvantages – Very complicated manufacturing process; – Very expensive; – Rarely used in stationary gas turbine installations; – Design change of IGVs possible; – Complex spare parts replacement; – Potential to “puff” dirt off filters and quickly clog filters located below;
A pulse filter system will provide the anti-icing function in standard configuration. As ice forms on the filter elements, the differential pressure across the filters increases and upon reaching the programmed cleaning set point, the control system will initiate pulse cleaning of the filters. Any ice and dirt which have collected on the filter elements is knocked off and extracted from the filter house by the ASC (Augmented Secondary Air Circuit) system. The frequency of pulse operation is dependent upon the severity of the icing conditions.
Advantages – Standard design for some gas turbine manufacturers; – Maintains clean pre-filters as well as providing anti-ice function;
Disadvantages – Uses a large amount of compressed air; – System is more complex than other types of anti-ice systems; – Pulse filter system creates a higher-pressure loss in the filter house; – Generally, not used in offshore applications due to high moisture content;
Thermal radiation from an infrared source, typically infrared lamps, may be used to heat surfaces to prevent ice from forming on them. There must be enough thermal radiation to sufficiently heat surfaces above the temperature of incoming air. This type of system may not be suitable for equipment operating in or near a hazardous area, due to the high temperatures created by infrared lamps. The system is composed of the infrared bulbs, thermal safety, and an electrical switching circuit operated by the package or engine control system.
Advantages – The design is simple;
Disadvantages – Infrared bulbs create high heat, which will not be suitable for areas which may contain gas; – Areas of the filter house with irregular or no air flow are subject to overheating;
Weather vane trace heating
Special trace heating tape may be applied to the leading edges of the weather vanes to prevent ice formation on them. As air passes over the heat trace, it absorbs energy and its temperature rises. The heat trace will ensure that ice cannot form on the surfaces to which it has been applied. The system is composed of the heating elements, thermal safety, and an electrical switching circuit operated by the package or engine control system.
Advantages – The system design is simple; – Can be procured for hazardous area duty;
Disadvantages – The trace heating only de-ices the surfaces to which it is applied and would have to be used in conjunction with another type of anti-ice system to protect the entire combustion air inlet.