Over the past few years, Siemens Industrial Turbomachinery Ltd. (SITL) has developed a number of package retrofits for upgrades to control systems, complete package systems or smaller package improvements to provide improved reliability in specific areas. One particular area of focus pertains to the options available for the gas fuel control valve retrofits for Siemens’ mature gas turbine fleet, which includes the TA and its variants, TB and its variants, the SGT100 (Typhoon) and the SGT200 (Tornado).
These engines, originally fitted with either a hydraulic, pneumatic or mechanical governor plus sleeve-type throttle valve, were sensitive to gas supply stability and cleanliness issues. However, for operators with clean gas supplies, the valves have operated and continue to operate at satisfactory and acceptable levels.
Siemens has developed a number of fuel system upgrade opportunities, including the Small Turbine Advanced Retrofit (STAR) fuel valve, for use when a good standard of fuel is not available or greater starting reliability is required. The STAR valve, which is NACE MR-01-75 approved as standard, was introduced in 1995 as a retrofit upgrade for the Siemens small gas turbine fleet. Recent reports have mistaken the older Rustronic sleeve valve for the STAR valve and erroneously reported its purported lack of functionality.
The STAR fuel valve system was initially retrofitted on gas turbine packages at sites where arduous operational conditions were experienced and where fuel control valve failures had occurred due to poor-quality gas fuel, with varying levels of solid and liquid contaminants. To resolve this problem, the STAR fuel valve features a rotary ball valve with metal wiping surfaces, which effectively keeps the working surfaces clear of contaminant debris and significantly reduces valve failure rates.
The small turbine advanced retrofit (STAR) valve provides a solution where greater starting reliability is required and is also capable of handling poorquality fuels.
This single moving part in the gas stream plays a major part in the STAR valve’s reliability. Where most fuel control systems only use a valve angle for fuel control, the STAR fuel control system automatically and continuously compensates for any pressure and temperature fluctuations, adjusts fuel delivery to meet demand and has a fully networked connection for diagnostic feedback. The STAR fuel valve has been the main solution for gas fuel system upgrades over the last 14 years on all SITL products, including dry-low emmissions (DLE) units. It has been supplied in over 700 units to both existing and new fleets.
As part of the ongoing development programs and from the experience gained with these valves, Siemens has further developed the fuel system to produce a new integrated fuel valve (IFV). In addition to retaining the single moving part in the gas stream, the IFV has the advantage of incorporating the electronic control unit (ECU), valve and transmitters into one single unit, allowing quicker and easier installation. The revised system is composed of a fuel valve, actuator, pressure instrumentation and digital controller, packaged into a single assembly, to provide a gas flow controller for the gas turbine.
The integrated fuel valve is now fitted as standard on all new SITL small gas turbine packages.
Several systems may be employed to provide multiple flow streams to the gas turbine, each IFV acting on its own as a standalone device. Other key features include:
• dual redundant pressure transmitters with “fault tolerance” on most functions;
• digital signal “from the speed probe to the valve” — no analogue components to drift.
By packaging the components in a single assembly, all wiring, configuring, testing and calibration is completed in the factory at the point of manufacture. This minimizes any setup time required when installing it into the gas turbine package. Calibration details for all integrated fuel valves are stored at the factory and therefore can be reproduced whenever requested.
The IFVs are now fitted as standard on all new SITL packages and are under technical evaluation for retrofitting to previously installed packages. The final designs for retrofits on the older packages are still in development,although a full release is expected in late 2009. When replacing control valves, it may be possible to retain the existing on-/off-skid block and vent valves. However, from a safety perspective Siemens recommends that these beupgraded at the same time to the latest standard. This will allow the operator improved response times, sealing andcorrosion resistance.