Auto-purge metering solution for a coal-fired power plant

Application Note | Power Generation

  • Long-term stable flow measurement of dust-laden gases, virtually maintenance-free
  • Pre-assembled, tailored automatic self-purging system with DP transmitter, unique purge valve, intelligent control unit and averaging pitot tube
  • Constantly good combustion quality and low NOx emissions through precise balancing and air flow measurement


An electric utility company operates an ultra-supercritical (USC) lignite-fired combined heat and power plant in Slovenia. The modern 600 MWe power station has an efficiency of 46%. The plant was updated with a new unit in the recent past to further cut the plant’s carbon footprint.

Measurement requirements

As many other modern power plants that burn solid fuels, this power unit uses an air preheater (APH) to heat up the combustion air and increase efficiency. This often causes a carryover of fly ash into the combustion air. Low NOx burners and a staged combustion are therefore utilised to keep NOx emissions on a very low level to comply with environmental regulations.

As a rule, the air flow rate in the large-size ducts is measured with Venturi tubes as part of the duct design or by averaging pitot tubes installed in the ducts. The air flow rates in the different combustion zones must be kept within narrow limits depending on the boiler load. Differential pressure measurements in those applications commonly work with pressure tapping and impulse lines which, however, are subject to clogging by the fly ash contained in the combustion air. In conventional installations these measurements need constant cleaning and servicing on a regular basis to keep the measurements operating properly.

In order to improve the carbon footprint while at the same time reducing maintenance of their differential pressure instrumentation, the customer requested a field-proven and less maintenance-prone technical solution to measure the flow rate of the dust-laden air flows to the furnace. The typical duct concentration can vary from 100…1000 mg/m³ at temperatures from +160…190°C / +320…374°F. Flow velocities range from 1…10 m/s. Duct sizes can have diameters up to 6 m / 236”.

KROHNE Solution

The power plant operator decided in favour of an auto-purge metering system. This industry-specific solution is tailored to the needs of flow measurements of dust-laden air and gases. It consists of a primary flow element (OPTIBAR PT 2000) as well as a fully assembled and tested auto-purge cabinet with a motor-operated valve, a customised KROHNE control unit and the OPTIBAR DP 7060 differential pressure transmitter.

The averaging pitot tube is fitted to the duct system. The system purges the high pressure and the lowpressure ports consecutively one after another. This is done with a pulse of compressed air in defined cycles. Purging is carried out very effectively, saving a lot of air this way. During purging, the reading is held, and the transmitter is checked for drift so there is no special programming required for the control system. An optional auto-zeroing function for the DP transmitter can also be provided.

Customer benefits

Using KROHNE’s auto-purge metering solution, the customer stands to gain from significantly higher process efficiency and reduced emissions. This solution allows long-term stable flow measurement through efficient purging, flow control and balancing of the pre-heated air. In this way, an optimal air-fuel ratio can be obtained. NOx regulations are reliably met without regular maintenance of the flowmeters.

The utility company benefits from a tried and tested industry solution particularly designed for power plants and combustion processes containing dust. Unlike conventional technical solutions, this compact and pre-assembled KROHNE solution saves installation and commissioning time. It also consumes only little air during operation. Given that it is virtually maintenance-free, operating costs can be minimised significantly. Being a field-proven system, this auto-purge metering solution requires only very little project engineering for installations.

Products used

Averaging pitot tube flowmeter

DP flowmeter for applications with liquids, gases and steam to high-temperature applications with dust-laden gases

  • Fast and easy installation without additional engineering, impulse lines and supports; the best retrofit capability
  • -200…+1150°C / -328…2102°F; max. 320 bar / 4641.2 psi (line pressure)
  • For line sizes DN50…2000 / 2…80" (larger on request)
  • 2-wire 4…20 mA/HART®, FF, Profibus-PA


Flow computer for process control of liquids, gases and steam

  • High performance flow and energy calculation with DP flow elements, volumetric or mass flowmeters
  • Compact design, quick and easy-to-operate via TFT touch screen
  • Enables split-range operation with two pressure transmitters for increased measuring dynamics
  • HART® communication with all common DP transmitters for loss-free measured data transmission and possibility of automatic zero point adjustment


Differential pressure transmitter for flow, level, differential pressure, density and interface measurements

  • High performance DP transmitter with integrated line pressure measurement
  • Measuring range: 30 mbar…16bar / 0.44…232 psi; Line pressure: up to 700 bar / 10,153 psi
  • Reference accuracy of ±0.065%
  • 2-wire, 4…20 mA/HART®, FF, Profibus-PA, Bluetooth®


Averaging pitot tube with multiple impact-sensing ports

  • Low pressure loss design; from basic to engineered solutions
  • Cryogenic to high temperatures possible (-200…+1000°C / -328…+1832°F); max. 160 bar / 2320.1 psi
  • For line sizes from DN50…2000 / 2…80" (larger on request)
  • Material: 1.4404 / 316L