Wet compression extended to V-series machines21 September 2001
Wet compression has proved an effective technology for boosting the output of Westinghouse W501 gas turbines. Next year, following tests in Berlin, a prototype is planned for a Siemens V84.2 gas turbine in the USA.
In recent years there has been growing interest in ways of boosting the output of existing gas turbines by conditioning the air input to the compressor. There are two main categories of such technology. The first involves the introduction of water and cooling of the air through the enthalpy of evaporation of the water. In this category are wet compression, inlet fogging and evaporative cooling. The second category involves use of a heat exchanger to reduce air inlet temperature, without addition of water. In this category, which results in a lowering of inlet air humidity rather than an increase, are inlet chilling and refrigeration.
Wet compression was developed in the early 1990s by Dow Chemicals, which holds the patent. It has been applied commercially by Siemens Westinghouse to around 20 installations, all of them Westinghouse W501 type gas turbines. The lead plant has now operated for 25000 hours. The first installation was on a W501A machine, in 1995. The first installation on a W501D5A(DLN) turbine was in 1997, while 1999 saw the first installation on W501D5 and W501D5A machines.
Recently, tests of the wet compression technology have been carried out on a Siemens type V84.3A2 gas turbine at the Siemens test bed in Berlin. A prototype application of the technology on a V84.2 machine is planned for the spring of 2002 at a US plant. Subsequently, the technology will be tested on a V94.2 gas turbine, also slated for 2002.
Siemens Westinghouse believes that wet compression is the most cost effective of the common techniques for augmenting gas turbine output. It can increase gas turbine power output by 10-15 per cent and raise efficiency by 1-2 per cent.
When fitted on engines with conventional combustion systems, NOx emissions have been reduced by 20-40 per cent. In terms of CO2, wet compression increases fuel consumption, but the increase in power is greater – resulting in less CO2 emissions per unit of power generated.
As mentioned in the factfile below, another advantage of wet compression is that it allows power to be increased and maintained independent of ambient temperatures and rerlative humidity – it even works with relative humidity of 100 per cent. It can also be used in conjunction with evaporative cooling.
Wet compression can have advantages over direct combustor water injection for NOx control and power enhancement. In the case of water injection, heat rate is increased (due to vaporisation of the water), whereas wet compression improves heat rate while at the same time intercooling the compressor.
It should however be borne in mind that in combined cycle, wet compression increases heat rate. This is because, with wet compression, gas turbine fuel consumption increases in line with the power increase, but turbine exhaust energy increases to a lesser extent.
The thermodynamics of wet compression is relatively straightforward. The compressor inlet air is oversaturated, with the double effect of cooling at the compressor inlet (dependent on ambient conditions) and intercooling, ie cooling inside the compressor (which is independent of ambient conditions). Thermodynamics dictates that an isothermal compression will consume less work than an adiabatic compression. Once the oversaturated air is inside the compressor, the evaporation of the remaining droplets provides intercooling and thus moves the compression in the direction of an isothermal process. As an additional effect the overall increase in the massflow increases the power output from the turbine.
Before a decision is taken to install wet compression a number of factors must be taken into consideration. Wet compression technology does not suit all turbines because the intercooling effect will change the operating gaps in the compressor. Droplet sizes must be carefully controlled to minimise erosion of the compressor blades. Also, wet compression causes the pressure ratios inside the gas turbine to vary, which results in changes to the cooling air management requirements. These and other effects are influencing the commercial take up of wet compression technology.
A wet compression system includes fitting of the spray atomisation system and spray nozzle rack, modifications to the plant control system (eg to adjust firing temperature as a function of the quantity of water being injected), installation of high pressure pumps, application of protective coatings to the inlet ducts, and installation of water traps in first compressor bleed lines. The injection of water changes the work distribution of the compressor and requires changes to turbine cooling circuits and usually installation of an automated flow control system.
Since its introduction some six years ago the technology has evolved in a number of ways, including improved nozzle design and reduced average droplet size.
Tests on the V84.3A2 turbine at the Siemens test facility in Berlin have aimed to gather basic thermodynamic data for the Siemens V series machines. The tests were conducted for fuel oil and fuel gas premix modes with low NOx combustors. The V84.3A2 had test instrumentation to monitor the following: vibration and natural frequency of the compressor blades; compressor blade tip clearances; compressor casing temperature distribution and deformation; pressures and temperatures in bleeds; and thermodynamic values.
With an injected water temperature of less than 30°C, ambient conditions of 22-23.3°C, 1004 mbar, and 60 per cent rel humidity, the wet compression gave a power gain in the range 10-15 per cent. The tests demonstrated successful application of the technology to the V series.
Recently, Siemens Westinghouse installed a wet compression system in just six weeks. This was at the Calpine Auburndale plant in Florida. Siemens Westinghouse says the installation, which became operational on 11 June, following a contract signing in April, demonstrates the benefits of a new team concept it has implemented in the Gas Turbine Modernisations division. The wet compression system was installed in half the projected time by what Siemens Westinghouse calls an MEP team, comprising marketing, engineering and project management. Another key to success, says Siemens Westinghouse, was “working with a customer who wanted to stay involved.”
|Alternatives to wet compression|
| evaporative-cooling-is-a-widely-used-technology-for-boosting-gas-turbine-output-particularly-in-dry-and-hot-climates-unlike-wet-compression-where-water-is-sprayed-into-the-compressor-inlet-evaporative-cooling-uses-a-stationary-water-saturated-medium-over-which-the-inlet-air-is-passed-because-the-residence-time-is-short-evaporative-cooling-does-not-allow-the-air-to-become-fully-saturated-achieving-humidity-levels-in-the-range-85-95-per-cent-typically-the-hardware-for-evaporative-cooling-is-installedTablesTypical wet compression performance (W501D5A gas turbine) Wet compression installations to date (Factfile) Comparison of power boost technologies for 118 MW W501D5A gas turbine (simple cycle) - 90°F day, 60% rel humid, sea level
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