HRSG Feed Water and Low Pressure Steam System in combined cycle power plant production

Process control for the LP system

The feedwater pumps with an intermediate-pressure interstage bleed-off serve to feed the high and intermediate-pressure system of the HRSG with a quantity of feedwater corresponding to the quantity of steam emitted. The feedwater system consists of one pair of 100% capacity feedwater pumps (or 3 with a capacity of 60% each) which supply feedwater to the high-pressure and intermediate-pressure economisers, as well as the high-pressure and reheat spray water, for attemperation purposes. The low-pressure (LP) steam generation system is located between the condensate system and the LP steam system: it generates low-pressure steam from the thermal energy contained in the gas turbine exhaust. The saturated steam from the low-pressure steam drum is supplied to the deaerator - for deaeration, when the oxygen content or conductivity in the feedwater requires this - and also feeds superheated low-pressure steam to the low-pressure main steam system i.e. the LP section of the steam turbine. The condensate preheating system controls the temperature entering and leaving the preheater of the HRSG and supplies the feedwater system and low-pressure steam drum with preheated condensate. The blow down tank system collects and discharges excess water from all HRSG parts during start-up, normal operation, up-set conditions, shut down and while draining the HRSG. This includes steam and condensate from the main steam and reheat sections of the HRSG and its associated piping, to expand and flash it to atmosphere. The blowdown can range from less than 1% when an extremely high-quality feedwater is available to greater than 20% in a critical system with poor-quality feedwater.

The feedwater flowmeters are of high importance for process control and safety and need to cope with the high temperatures and pressures as well as accuracy and redundancy requirements. Typically venturi tubes or flow nozzles installed in meter runs with redundant DP-transmitters according to ISO 5167 or ASME MFC-3M are used. The engineered KROHNE primary DP flow elements and meter runs are hot water factory calibrated at high Reynolds numbers. The state of the art OPTIBAR DP 7060 C pressure transmitters are 100% 3D-calibrated over the entire temperature and static pressure range and available with measurement capsules capable of up to 700 bar line pressure. KROHNE supplies the complete flow solutions with DP-flow element, DP-transmitter, pressure and temperature transmitter, process valves, engineering, calibration and documentation.

A further option here are the OPTISONIC 3400/ 4400 series ultrasonic flowmeters, also suitable for the requirements of temperature, pressure, materials redundancy etc. KROHNE OPTISONIC are drift and maintenance free, have no pressure drop and an extremely wide turndown ratio which enables very good low-end accuracy. Also for Steam, Reheat, injection/attemperation water, condensate and other typical flow applications KROHNE OPTIBAR DP and OPTISONIC Ultrasonic flowmeters are available. Drum level is usually measured with differential pressure transmitters. The 3D calibrated OPTIBAR DP 7060 C transmitter is ideally suited for such applications with high static pressure and the small differential pressures which occur at a steam drum. Every single transmitter is calibrated over the entire temperature and static pressure range.

Requirements
  • High pressure
Requirements
  • High temperature
  • High pressure
  • low conductivity
Requirements
  • Process control
  • Redundancy
Requirements
  • Process control
Requirements
  • Process control
Requirements
  • Dynamic calculations
Requirements
  • High temperature
  • High pressure
  • low conductivity
Requirements
  • High static pressures
  • Small differential pressures
  • Redundancy
Requirements
  • Low conductivity
  • High pressure
  • Low flows
Requirements
  • Dyanmic calculations
  • fast response
Requirements
  • Process control
Requirements
  • Process control
Requirements
  • Process control and performance testing
  • ASME PTC, IEC 60953
  • redundancy, e.g. materials P91/P92
Requirements
  • Process control and performance testing
  • ASME PTC, IEC 60953
  • redundancy, e.g. materials P91/P92
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