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Sensor test in operation

Especially interesting for
  • district heating and cooling
  • network operators

In practical applications, the operating conditions for heat meters in large facilities or networks often deviate from the idealised calibration conditions which are predominant at PTB's test facility. This leads to – partly considerable – measurement deviations. A newly developed procedure now allows heat meters to be investigated under installation conditions and without service interruption.

For the new optical procedure for in-situ measurements of flowrates on large facilities in accordance with the Laser Doppler velocimetry principle, a laser is coupled to the respective pipe system via an optical window.

The greatest uncertainty contribution during the operation of a heat meter is caused by the flow sensor. It has, to date, always been calibrated on a test facility for its specific use. Due to the idealised conditions prevailing there, unknown measurement deviations of the flow sensor occur at the later place of use. Over the years, these can change due to ageing, depositions or abrasive wear, since the devices can have a long service life.

To date, the flow sensors could not be calibrated in-situ. At best, the sensors were recalibrated on a test facility; for that, they had to be dismounted. The service interruption which became necessary for this purpose led to high costs. The optical procedure developed by PTB – in cooperation with the TÜV Rheinland and the companies ILA GmbH and Optolution GmbH – allows the flow sensors to be examined in-situ without service interruption, i.e. without having to dismount the flow sensor and without having to empty the pipe circuit. This procedure has already been patented.

With the aid of a known drilling procedure (hot tapping), an inspection window is mounted into the pipe. Hereby, first of all, a ball valve is welded onto the pipe through which the pipe is perforated. By means of a special mounting bushing, a window can now be mounted which fits the contours of the pipe. After that, a Laser Doppler velocimeter (LDV) is installed, and the flow velocity is determined over a measurement path by means of the Doppler shift of the back-scattered Laser light. After completion of the measurements, the laser can be removed, and the optical access can be closed with a blank flange. By integrating the measured flow velocity over the pipe cross section, the volume flow is determined. This result of the LDV measurement provides the reference for the measurement result of the flow sensor.

Investigations carried out on the heat meter test section of PTB confirm that – under ideal conditions – a measurement uncertainty of less than 1 % can be achieved. Presently, investigations are being carried out to find out which measurement uncertainty can be achieved in the event of disturbed incident flows and how the pipe cross-section – which is important for the integration – can be determined more precisely.

Scientific publication

Guntermann, P.; Rose, J.; Lederer, T.; Dues, M.; Müller, U.; Duckwe, A.: Vorort-Untersuchungen von Wärmemengenzählern im Betrieb. EuroHeat & Power 39 (2010) 44–47