A Report on one of the First RheoVac installation at Conesville Generating Station in 1994. Celebrating over 30 years of Air-In-Leak Measurement Excellence!

ADVANCEMENTS IN AIR IN-LEAK MEASUREMENT

 J.W. Harpster
R.A. Maston
D.S. Molnar
J.F. Stumpf

Abstract

Air entering the sub-atmospheric side of steam turbine power plants is recognized to add to the operating cost and increase the plant heat rate.  Real time accurate measurement of air in-leakage has been historically elusive.  Current methods to obtain acceptable air flow measurements are clouded by inaccuracies caused by measurement error and a reliance on assumptions regarding the physical state of the fluid.  A new instrument based on direct measurement of gas parameters is described.  It provides a continuous output signal in direct proportion to the mass transport of air and separately, water vapor flowing within the air removal line between the condenser and air removal equipment.  Accordingly, the instrument measurement sensitivity and installation location in the extraction line provides a direct method for measurement of air removal equipment capacity and variations in capacity that may be experienced due to wear or changes in operating conditions.  Historical developments based on results from power plant installations over a two year period will be presented.  Data on the new instrument is presented showing correlation between a simulator system used for instrument calibration and power plant installations.  The measurement method and its importance in improving plant heat rate is described.

Delivered at EPRI Plant Performance Seminar on August 2,1994

Background and Introduction

Power plant steam turbine condensers operate at pressures below atmospheric.  With such large systems having many mechanical seals, air in-leakage is inevitable.  To maintain the desired low back pressure on turbines, exhausters are used to remove this undesirable air which finds its way into the condenser space.  Accurate measurement of this air in-leakage has been historically elusive.  Intek has developed a system, the RheoVac⁷ system, to accurately measure condenser air in-leakage by sensing its flow in the line between the condenser and the exhauster.  The user-installed RheoVac system combines a field proven Rheotherm⁷ mass flow sensor with other primary sensors to yield the air in-leak measurement. At the same time the RheoVac system uniquely provides other valuable condenser system fluid stream properties.

Intek’s line of Rheotherm flow meters have been used world-wide since 1978 in the measurement of both liquids and gases.  They are based on a thermal method patented⁽¹⁾ in 1981 and have an output response related to the temperature differential between two temperature sensors one of which is heated by a constant power heater.  Fluid (gas or liquid) flowing past the heated section has a heat transfer coefficient which varies with the mass flow rate of the fluid.  Unique to Rheotherm technology is the ability to measure the low mass flow rates of air and water vapor which exist in condenser exhauster lines.

Rheotherm flow sensors, responsive to air and water vapor, have been employed since 1990 as flow monitors in condenser air removal suction lines.  During this period they have proven to be a reliable indication of changes in air in-leak.  Through repeated contacts with plant operating personnel, plant visits and trend data evaluation, Intek developed a working level appreciation for the complexity of the operating conditions and equipment associated with this dynamic subatmospheric environment.

A research and development effort was initiated to define the physical state of gases in the vacuum line, measure the constituent parts and construct a cost effective measurement system suitable for use in the harsh power generating plant environment.  It was quickly determined that a simple system consisting of sensors to measure flow rate, pressure and temperature at a location within the vacuum line as described elsewhere⁽²⁾ was not adequate.  This kind of system makes use of equilibrium steam table data⁽³⁾ and the assumption that saturated conditions exist at all times.  Simply, the flow sensor would measure the total flow rate in the line, the temperature measurement would be used to calculate the water vapor partial pressure (from steam tables), and the pressure sensor would be used to define the total pressure.  The air partial pressure is found by subtracting the calculated equilibrium water vapor pressure from the total pressure.  Additional calculations would then be used to derive air flow rate as well as other stream characteristics.

The problem with this air in-leak measurement method is that equilibrium steam table conditions rarely exist in the lines between the condenser and the exhauster.  For example, should heat be added to the fluid, the amount of water vapor would be below that of saturation, thereby giving rise to calculations of air flow rate being excessively low.

This paper presents the patent pending⁽⁴⁾ RheoVac measuring system which is uniquely suited to power plant measurement of air in-leakage.  The inclusion of a direct measurement of water vapor relative saturation at the point of measurement permits measurement of all necessary stream parameters for precise determination of the air flow rate as well as water vapor flow rate, total mass flow rate and other properties of the fluid.  These determinations allow for the performance of power plant vacuum side diagnostics, a capability which has not been available to the plant operator.

RheoVac System Description and Calibration

The measurement system consists of two major components, the sensor head and the electronic control and digital signal processing (DSP) system.  The sensor head contains a Rheotherm mass flow measurement probe, a platinum RTD for temperature measurements, a variable capacitance absolute pressure measurement device and a vacuum compatible relative saturation sensor.   All sensors are housed within a one inch diameter stainless steel shaft located at one end as shown in Figure 1.  The shaft is inserted into the vacuum line through a 1″ ball valve “hot tap”.   The construction is compatible with any measurement line size but which are typically in the range of 6″ to 14″ schedule 40 pipe.

The wall mountable electronic package contains the sensor power supplies, signal conditioning and DSP electronics.  It contains a local air in-leak display, 4-20mA air flow rate signal and a selectable RS232 or RS422 output for control room data display and recording.  A typical display is shown in Figure 2.  The display is real time and is updated each second.  Data recording or archiving would be a control room responsibility.

The RheoVac air in-leak monitor is calibrated in both a static chamber and dynamic system simulating the condenser and vacuum line environment.  The static chamber is designed to calibrate the temperature, pressure, and water vapor relative saturation sensors.  A dynamic system, shown schematically in Figure 3, designed to represent conditions in the power plant, was constructed for instrument calibration.  Major components of this system are:  a temperature controlled water source, air water vapor mixer, a separator, and a 2″ diameter flow stabilizing tube all under vacuum provided by a SIHI (Two SCFM at 1″ Hg) liquid ring sealed vacuum pump.  Air is supplied to the system through a calibrated variable area meter.  Stream velocity range in the flow tube is similar to that found in power plant exhauster vacuum lines allowing for direct flow rate calibrations.  Stream temperature is adjustable from 65 to 140EF.  Relative saturation of water vapor/air mixture is adjustable from zero to 100%.

Field Application

With the cooperation and recommendation of American Electric Power (AEP), the plant at Conesville, Ohio was provided as a test site for the RheoVac test system.

Unit Number 6, a 430 MW unit, was selected since it was scheduled for shut down allowing for hot-tap installation.  The condenser utilizes a Nash, two stage model AT 3004, liquid ring sealed air exhauster.  This Nash exhauster is rated at 25 SCFM at 1 inch Hg absolute.

The site selected for probe insertion was in a 25 ft. straight line section of the vacuum line just ahead of the exhauster suction.  This location provided the lowest pressure point in the line and better represents water vapor/air equilibrium conditions into the exhauster.  The prototype electronics unit was located on a wall nearby.

The test electronics unit consisted of a personal computer (PC), the necessary A/D converters, sensor drivers and power supplies all packaged in a NEMA 4 box.  The computer provided the ability to calculate necessary parameters from the primary sensors and record in memory primary sensor as well as calculated data.  Based on system dynamics an archive rate of 30 seconds was established.  The PC monitor accurately represents control room data available with the RheoVac system using one of its output data ports.

Data from 2 pm (14:00) June 23, 1994 to 2 pm (14:00) June 24, 1994 is shown in the following figures.  Figure 4 shows the dry air in-leak in SCFM.  Figure 5 shows the total mass flow, the dry air mass flow and the water vapor mass flow.  Figure 6 shows the sensor temperature and the total absolute pressure.  Figure 7 shows the relative saturation and Figure 8 shows the Conesville Unit #6 gross power output.  All of this data, except gross power, is available from the RheoVac system.

The main feature of the above Figures is the cool down and pressure reduction from 23:30 to 7:00 which coincides with the Unit #6 power output reduction (Figure 8).   In addition to the power output reduction from 405 MW to 180 MW, a thunderstorm cooled the ambient air at approximately 23:30.  The temperature before and after this event was approximately 117EF and the pressure before and after the event was approximately 3.65 inches Hg absolute representing conditions at full power production.  The graphs show that all parameters in the 14:00 to 23:30 time period are repeated in the 8:00 to 14:00 time period.  The relative saturation sensor averages 93.5% during both periods.  If it was assumed that 100% saturation existed, as would have been using past methods in the absence of the RheoVac instrumentation, a 25% error in air in-leak mass flow rate measurement would have resulted.

From 3:00 to 5:00 the power output was steady at 180 MW.  The RheoVac system demonstrated a very stable response during this period.  During the low power period the water vapor flow rate was substantially reduced from 210 lb/hr to 50 lb/hr.  The dry air mass flow rate went up slightly from 57 lb/hr (13 SCFM) to 72 lb/hr (15.5 SCFM).  The measured lower pressure during this period resulted in a higher air in-leak rate due to a larger pressure difference across in-leak sources.  However, all of the air flow rate increase cannot be explained by the pressure changes.  Plant operation differences which result in equipment and temperature changes which in turn can cause larger gaps (leaks) due to thermal coefficient of expansion mismatches may be the cause of additional air in-leak.

An interesting feature is the two temperature and pressure spikes at 23:00 to 23:30.  The RheoVac system shows an increase in the mass flow rate of water vapor but virtually no change in the air mass flow (see Figure 5).  This shows RheoVac’s instrumentation distinct ability to separate air in-leak from water vapor flow.

There are two events one at 2:00 and one at 5:30.  The 2:00 event shows a decrease in air in-leak as well as small decreases in water vapor flow, relative saturation, and pressure.  An accelerated temperature drop is also visible.  The 5:30 event shows an increase in air in-leak, water vapor flow and pressure but slight decreases in temperature and relative saturation.  The exact cause of these events is unknown.  Detailed power plant operating data is not available to help explain these events.  However, the 2:00 event occurred near the end of power down and the 5:30 event occurred just before power up.  Power output and other operating data was only available on the hour and was not available at 6:00 or 7:00.  The ability to record such events however, can be an aid to the operator having intimate knowledge of other equipment responsible for these occurrences.

Table 1 presents data which highlight the features of the RheoVac system.  All the data show the highly dynamic nature of the condenser.  The RheoVac system enables the plant operator to not only accurately measure the air in-leak, but to understand the performance of the air exhauster and view the dynamic events of the condenser.

Table 1:  Additional Event Identification

Conclusions

The RheoVac system has been shown to accurately measure air in-leak into the subatmospheric side of steam turbine electric power generation plants.  Since directly measured properties of the fluid stream are employed errors associated with assumed equilibrium conditions in this dynamic environment are removed.  Therefore, all defined quantities such as water vapor, flow rate, water vapor to air mass ratio and partial pressures of water vapor and air have a high degree of accuracy.  For this reason, they are worthy of scrutiny to determine if additional cost-controlling features can be added.  Rate or threshold level sensors on air-measured or determined parameter could be installed in the computer to provide an alarm.

System installation was shown to be simple.  There was no need for critical mechanical field adjustment of any sensor including the Rheotherm flow rate sensor.  Exhauster discharge air was independently measured using a DP cell.  This DP cell air flow rate reading agreed with the RheoVac readings within anticipated error of the DP cell.  Further, water vapor flow rate could be measured as Nash pump separator overflow, in the form of condensed water, directed to an open drain.  Using a stop watch and a known volume container, the RheoVac indication of water vapor flow rate was also in agreement with the measured drain water flow rate.  Therefore, factory calibrated instruments are plant installable without a requirement for field adjustment.

The information provided shows the RheoVac system to be a trouble-free vacuum diagnostic instrument perfectly suited to the plant environment.

RheoVac installation report from Conesville Generating Station (1994)

References

1. J.W. Harpster, USPN 4,255,968.

2. J.A. Ferrison, P. Baker, and W. Ray, “An Air Ingress Monitor for Turbine Condensers – Its Development and Validation”, presented at EPRI/ASME Heat Rate Improvement Conference, Birmingham, AL (November 1992).

3. Heat Exchange Institute Inc., “Standards For Steam Surface Condensers”, Eighth Edition, Heat Exchange Institute Inc., Cleveland, OH (1989).

4. J.W. Harpster, patent pending.