Comment

1 May 2007

Accurate pressure measurement

The pressure scanners have a Transducer Electronic Datasheet (TEDS) chip that identifies each module

By Blair Chalpin, Charles A. Matthews, and Nicholas Sheble

Accurate pressure measurement is vital to the understanding of steam turbine efficiency, reliability, and service condition.

We will look at and describe a safety purge system that isolates and protects PC-based pressure sensors from steam pressure media while providing high accuracy, serviceability, and automation.

Programmable pneumatic configurations sequentially isolate the pressure sensors from the measurement source, purge the scanner lines, and purge the test article pressure input lines of any accumulated condensate with no affect on the properties of the turbine during a test.

Current methods, roadblocks

Typically, steam turbine measurement applications use individual all-media pressure sensors designed for harsh and corrosive applications.

With the all-media approach, differential readings are achieved with either, two sensors measuring and mathematically comparing pressures or by using large, expensive all media differential sensors.

Using two sensors for one differential measurement may result in a reduced accuracy, as well as doubling the instrumentation costs when compared to a single differential sensor.

All media sensors, which may weight up to 29 lbs (1.3 kg), are for a permanent installation.
One must remove them and send them to a calibration lab for calibration, which for a system measuring 128 inputs could require up to 40 hours to complete.

Sensors calibrated in a laboratory environment may experience significant zero and span shift errors when subjected to the high ambient temperatures near steam turbines.

Measurements good at the beginning of a test may deteriorate as the test progresses due to head pressures formed as trapped air leaves the pressure media and forms air pockets in the system tubing.

The permanent installation prevents measurement validations during a test.

Safety purge measurement system

A new method using intelligent pneumatic pressure scanners is available. This system utilizes electronic pressure scanners, incorporating relatively inexpensive piezoresistive sensors, to measure pressures in steam turbines.

These sensors normally could not measure liquid or high-temperature steam media pressures. However, this system has a safety purge feature that isolates the sensors from the harsh media without affecting the accuracy of the pressure measurement.

Two separate purge paths are in the safety purge system, one within the scanner equipment rack, and one at the input lines from the turbine.

The first purge path, referred to as "cabinet purge," uses a valve in the pressure scanner to purge input lines to atmosphere.

The second purge path, referred to as "turbine purge," uses a remote purge valve located near the system measurement connection to the steam turbine to purge those lines to a drain.

Scanners with safety purge

The safety purge system uses an electronic pressure scanner. This module contains 16 thermally compensated individual differential pressure sensors. The sensors couple to a pneumatically piloted valve allowing "in situ" pressure calibrations, isolation, and line purge.

Standard modules connect the reference side of the sensors to a common reference, typically local atmospheric pressure. It is possible to manufacture true differential measurements by adding pneumatic valves to the reference side of the sensors.

Full-scale accuracy of 0.05% is achievable by pressure-temperature characterization of each sensor with nine pressure points at 10 temperatures from 0°C to 69°C. The calibration temperatures used are module-operating temperatures as the sensors are isolated from the high temperature steam.

These modules also have line reference capabilities. In high-line, low differential testing, pressures higher than the normal sensor value interface at both sides of the sensor diaphragm. The best accuracy is when the differential measurement spreads over the full range of the pressure sensor.

When equal high pressures are at both sides of the sensor diaphragm, the differential reading is not zero. The sensor housing does not have a balanced construction, and due to its greater surface area, the reference side of the sensing diaphragm experiences greater forces than the positive measurement side.

After re-calibrating the zero point at this elevated pressure, only slight span errors exist when applying known differential pressures.

System installation, operation

In all cases, steam from the test article will mi-grate through the measurement lines to the scanning module even though there is no flow. Condensate forms and collects inside of the tube due to ambient temperature differences. If water enters the measurement system, head pressure errors will result.

If water reaches the sensor level, sensor die and wire bond corrosion will occur, resulting in sensor failures. To prevent problems, technicians must install the system just so:

• The cabinet should be 4 to 6 feet (1.2 to 1.8 meters) above the measurement points.
• The measurement system should be 6 feet (1.8 meters) or more above the safety purge system.
• A purge air source capable of providing purge pressures 20% greater than the maximum test pressure is required.
• The purge pressure may be dry air, or nitrogen. If air is used, it must conform to the ISA-7.3 Instrument Air Quality Standard.

The purge pressure will move any condensate that might form in the pressure input lines back to the turbine. Purge pressure levels greater than this can paint the inner walls of the tubing with a fine layer of micro droplets, which can reform into large drops.

Purge flow meters may be in each line to monitor purge flow and adjust final purge pressures to the steam turbine. The system uses clear Teflon tubing from the safety purge cabinet to the measurement cabinet. Periodic visual checks of this tubing are necessary to insure condensate has not formed in the measurement lines.

Purge flow requirements

Purge flow requirements may vary with the test requirements. Purge flow de-pends on the number of pressure points, the maximum pressure of the system, the capacity of the purge pressure source, and the test article backpressure.

Here are the possible configurations:

• De-energized mode: All purge air is out. The input lines to the sensors do not connect in the pressure module valves. This mode is for when the steam turbine is not operating.
• Dormant mode: Measurement lines are closed, and the system is in a safe mode. This mode is to conserve the purge pressure source while the turbine is stabilizing and prior to data collection.
• Purge mode: All lines purge. Purge pressure prevents moisture from making contact with the sensors. If electrical power is lost, the system will default to this mode.
• Measurement mode: Measurement lines connect to the pressure scanners. This mode is for short periods of time when data is to be collected. If control pressures are lost, the system will default to this mode.

De-energized mode: In this mode, all valves in the safety purge cabinet and the measurement cabinet vent. No purge pressure is applied. The internal valve in the pressure scanner is set to the calibrate mode, which connects the input line to the module calibration manifold. This mode must not work during tests, or whenever the steam turbine is operating.

Dormant mode: In this mode, valves 2 and 3 have energy, placing the system in the "safe" mode. The pressure inputs are isolated in the safety purge cabinet. The connection from the safety purge cabinet to the measurement cabinet connects to a drain line. The sensors are isolated from the input lines. The input lines connect to the pressure-scanner calibration-manifold.

Purge mode: The purge supply to the safety purge cabinet has energy. Valve 1 has been energized directing purge air to the steam turbine. The measurement cabinet valves 4 and 5 also have energy, directing purge air to the input lines forcing any condensate out the drain line. The sensors are still isolated. This mode is always for testing unless one is measuring pressures. The system must be in the purge mode for a minimum of 10 minutes prior to a test.

Measurement mode: The purge air supply has been de-energized. Valves 1 through 7 are set to vent. The pressure input lines connect to the sensors. Empirical testing has shown a purge time to measurement time ratio of 2:1 is required for the best accuracy.

Measurement system

The measurement system is comprised of one or more intelligent pressure scanner enclosures and a calibrator enclosure. The intelligent pressure scanner enclosure can accept up to eight pressure scanners for a total of 128 pressure input channels. A typical system may include several intelligent pressure scanner enclosures. The enclosure is a mini-data system. It contains a processor, RAM, a hard disk drive, and 16 bit A/Ds.

Each enclosure in a system has calibration data for all system pressure scanners stored on the hard disk drive. The pressure scanners have a Transducer Electronic Datasheet (TEDS) chip installed, which will identify each module to the enclosure.

At power-up, or upon command, the enclosure reads the TEDS chip data from each of the scanners and maps the calibration coefficients into RAM. When a data scan starts, the enclosure scans the input channels and converts the voltage outputs from each sensor to an A/D count value.

The A/D count values convert to engineering units and output to the hard disk or to a "host" computer. This system communicates with a "host" or plant computer via Ethernet TCP/IP. All data transmit in a choice of formats. Pressure data can report in any one of 23 pre-programmed engineering units. Data may be output in Binary or ASCII formats.

The user can configure all of these setup parameters. A 128-channel pressure measurement system consisting of eight pressure scanners installs in a single "intelligent enclosure."

Calibration and validation

A key function of this system is its ability to perform a full system calibration or validate individual sensors "on demand" and "in-situ." The system includes secondary standard pressure calibrators that the system software controls. This feature allows a user to perform a calibration/validation while the system is monitoring the steam turbine. The calibration/validation pro-cess requires only a few minutes for each pressure range.

The system does not have to be permanently located at a turbine. System cabinets can be portable so the system could test several turbines at a power plant. Turbines could be monitored more frequently thus improving overall plant efficiency.

Although the system shown in this paper is a fixed installation, portable systems are in use. Portable systems operate in the same manner as a fixed installation. Portable systems allow a user to test steam turbines in remote locations.

• This method of steam turbine pressure measurement has worked well at six installations worldwide with more systems on the horizon.
• This system has a lower overall cost per channel when compared to a system-using individual all media sensors for each pressure input. Individual all media sensors cost approximately $1,000 per channel including signal conditioning, but not the cost of calibrations. This system will cost approximately $500 to $600 per channel including pressure calibrators.
• Calibration is easier than using individual sensors. Each pressure range may be calibrated in situ, simultaneously, and "on demand" as required during a test.
• Increased turbine efficiency lowers cost of operation. This system allows more frequent tests to improve efficiency.
• The safety purge cabinet ensures the pressure measurement system is safe from harsh media.
• Ethernet communications-any PC or plant network point can monitor the system.

About the Authors

Blair Chalpin (bchalpin@scanivalve .com) is a mechanical design engineer, and Charles Matthews (cmatthews@ scanivalve.com) is a product support manager at Scanivalve Corporation in Washington. Nicholas Sheble (nsheble@isa.org) is the senior technical editor at InTech magazine.