All fired up01 October 2005

Fortum O&M (UK) operates and maintains the Grangemouth combined heat and power plant (CHP), which supplies power and steam to BP's Grangemouth complex, Scotland's largest industrial site and one of the biggest, most integrated oil and gas complexes in the world. Fortum supplies the site with 230t of superheated steam every hour, generated by the CHP plant. The plant is a combined cycle, with a Siemens V94.2 gas turbine and a waste heat boiler that generates 140MW of electricity and 230t per hour of superheated steam, and a standalone Foster Wheeler boiler (B8) which is also able to supply 230t of superheated steam per hour. The B8 is normally run on low load to provide headroom for the case of the CHP trip and it is very important either the CHP or B8 is always available.

However, Fortum was experiencing some problems with the B8 boiler. Ultrasonic flow meters had been installed to measure the airflow being fed into the burners - these were in place to provide a signal to control the combustion air fan, so that the correct fuel to air ratio was occurring. The meters were also supposed to provide a low flow alarm and burner trip, should this be required.

The readings being generated by the existing flow meters were erratic and inconsistent in some conditions. This resulted in poor control of the burners which, in turn, impacted on the efficiency of the boiler and, at one point, caused it to trip out. Fortum decided that before the next planned shutdown - which would require the B8 taking sole responsibility for steam generation - an alternative flow meter needed to be installed.

"We simply couldn't afford for there to be any issues with the B8 and, because we didn't have confidence in the readings coming from the existing flow meters, we decided to look for an alternative," says Ilkka Toijala, Fortum's asset manager. Fortum contacted Mobrey Measurement, a specialist in the design and manufacture of process measurement and control equipment. Mobrey recommended averaging pitot tube technology, because of the minimum downtime required for installation and negligible pressure drop in the system, making it highly energy efficient.

Mobrey's KB series averaging pitot tube flow meter also offers a 1% accuracy of flow measurement and can provide an immediate reading of any liquid gas or steam flow in pipe sizes from 12mm to 250mm.

On visiting the site, it became apparent that one pitot was not going to be sufficient for a reliable measurement. The duct section was 0.6m x 1.5m and, due to a short straight run, the velocity profile would not have fully developed. So, a pitot tube array was built to ensure an accurate representation of the flow. The array consisted of inserting four bars across each duct, with the high and low pressure tapings fed into common manifolds for each duct. With this arrangement, a true average of the flow across the duct could be obtained. By feeding the pressure tapings into common manifolds, Fortum was also able to use only one differential pressure transmitter per duct, thereby keeping it a cost-effective solution.

Once the boiler was brought back online and put through load tests, the meters immediately gave accurate and stable readings of the air flow. "This resulted in improved control of the combustion air flow, flame stability and overall better control of the boiler," says Toijala. Fortum now has full confidence in the boiler to provide the continuous power and steam essential to its operation.

Authors Iain Robertson and Tim Chettle are part of Mobrey Measurement

MEASURING UP TO THE FLOW CHALLENGE

Pitot technology uses a primary flow element inserted into the flow to create a differential pressure (DP). This DP is converted to a fluid flow reading by an associated DP pressure transmitter.

The Mobrey Measurement KB series averaging Pitot tube comprises two vertical plenum chambers or tubes, with a series of small holes along the length and oriented in the flow, such that one tube faces upstream and one tube faces downstream. The flow is forced to separate and travel around the vertical element, thus creating a differential pressure in the plenum tubes, which is measured by the DP transmitter.

The very narrow cross-section of the KB element means that there is only a small pressure drop in the pipeline, making this a very low cost of operation flow measurement device.

SOE

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