How to conduct a plant performance test

Tina L. Toburen, PE, and Larry Jones, McHale & Associates Inc.

Pages: 1 2 3 4 5

Understanding guarantees and corrections

The most common performance guarantees are the power output and heat rate that the OEM or contractor agrees to deliver. Determining whether contractual obligations have been met can be tricky. For example, a plant may be guaranteed to have a capacity of 460 MW at a heat rate of 6,900 Btu/kWh—but only under a fixed set of ambient operating conditions (reference conditions). Typical reference conditions may be a humid summer day with a barometric pressure of 14.64 psia, an ambient temperature of 78F, and relative humidity of 80%.

The intent of testing is to confirm whether the plant performs as advertised under those specific conditions. But how do you verify that a plant has met its guarantees when the test must be done on a dry winter day, with a temperature of 50F and 20% relative humidity? The challenging part of performance testing is correcting the results for differences in atmospheric conditions. OEMs and contractors typically provide ambient correction factors as a set of correction curves or formulas for their individual components. But it is often up to the performance test engineers to integrate the component information into the overall performance correction curves for the facility.

The reference conditions for performance guarantees are unique to every site. A simple-cycle gas turbine's ratings assume its operation under International Standardization Organization (ISO) conditions: 14.696 psia, 59F, and relative humidity of 60%. The condition of the inlet air has the biggest impact on gas turbine–based plants because the mass flow of air through the turbines (and consequently the power they can produce) is a function of pressure, temperature, and humidity. Performance guarantees for steam plants also depend on air mass flow, but to a lesser extent.

The barometric pressure reference condition is normally set to the average barometric pressure of the site. If a gas turbine plant is sited at sea level, its barometric pressure reference is 14.696 psia. For the same plant at an altitude of 5,000 feet, the reference would be 12.231 psia, and its guaranteed output would be much lower.

The relative humidity reference condition may or may not have a significant bearing on plant performance. In gas turbine plants the effect is not large (unless the inlet air is conditioned), but it still must be accounted for. The effect of humidity, however, is more pronounced on cooling towers. Very humid ambient air reduces the rate at which evaporation takes place in the tower, lowering its cooling capacity. Downstream effects are an increase in steam turbine backpressure and a reduction in the turbine-generator's gross capacity.

The most important correction for gas turbine plant performance tests involves compressor inlet air temperature. Although a site's barometric pressure typically varies by no more than 10% over a year, its temperatures may range from 20F to 100F over the period. Because air temperature has a direct effect on air density, temperature variation changes a unit's available power output. For a typical heavy-duty frame gas turbine, a 3-degree change in temperature can affect its capacity by 1%. A temperature swing of 30 degrees could raise or lower power output by as much as 10%. The effect can be even more pronounced in aeroderivative engines.

ISO-standard operating conditions or site-specific reference conditions are almost impossible to achieve during an actual test. Accordingly, plant contractors and owners often agree on a base operating condition that is more in line with normal site atmospheric conditions. For example, a gas turbine plant built in Florida might be tested at reference conditions of 14.6 psia, 78F, and 80%. Establishing a realistic set of reference conditions increases the odds that conditions during a performance test will be close to the reference conditions. Realistic reference conditions also help ensure that the guarantee is representative of expected site output.

Establishing site-specific reference conditions also reduces the magnitude of corrections to measurements. When only small corrections are needed to relate measured performance from the actual test conditions to the reference conditions, the correction methods themselves become less prone to question, raising everyone's comfort level with the quality of the performance test results.

Beyond site ambient conditions, the PTCs define numerous other correction factors that the test designer must consider. Most are site-specific and include:

Generator power factor.
Compressor inlet pressure (after losses across the filter house).
Turbine exhaust pressure (due to the presence of a selective catalytic reduction system or heat-recovery steam generator).
Degradation/fired hours, recoverable and unrecoverable.
Process steam flow (export and return). Blowdown (normally isolated during testing).
Cooling water temperature (if using once-through cooling, or if the cooling tower is outside the test boundary).
Condenser pressure (if the cooling water cycle is beyond the test boundary).
Abnormal auxiliary loads (such as heat tracing or construction loads).
Fuel supply conditions, including temperature and/or composition.