»
Tube Maintenance
Feedwater heater leak detection
Figure 1 : Shell and tube tubesheet . Source : Dreamstime .
Feedwater heater reliable operation plays a critical role in maintaining low heat rates and high availability . Thermal power plants suffer significant derates and energy loss due to feedwater heater tube leakage annually . This report is intended to provide a brief guideline to diagnose common leaks encountered in a feedwater heater .
By Mohammad Rahman , Corporate Mechanical Integrity & Reliability Engineer - Zeon Chemicals
Feedwater heater operating performance is evaluated based upon the following items :
• Heater drain level
• Terminal temperature difference ( TTD ) = Extraction steam saturation temperature – Feedwater outlet temperature
• Drain cooler approach ( DCA ) = Drain outlet temperature – Feedwater inlet temperature
• Temperature rise ( TR ) = Feedwater outlet temperature – Feedwater inlet temperature
Performance degradation of the feedwater heater is often related to tube failures which increase exponentially with heater age . A leak can be diagnosed by analysis of the abovementioned indicators trend . Some early indications of tube failure are :
• Increase in drain valve position ( to maintain heater drain level , control valve position increases ; concurrently feedwater flow rate and boiler feed pump motor amps also increase )
• Decrease in feedwater outlet temperature ( TTD� )
• Decrease in drain outlet temperature ( DCA� )
• Decrease in temperature rise ( TR� )
• Increase in extraction steam / drain flow at the heater outlet .
Leak discovery Any leak will result in deviation of heater level control valve position , TTD , DCA and TR values from normal operating condition . For example , it was detected that one out of four feedwater heater level control valve positions was increasing in a coal-fired power station . Additionally , feedwater flow rate and feed pump motor amps were also increasing to near maximum , all of which suggested that the high pressure feedwater heater had tube leaks . The plant personnel isolated and inspected the heater and discovered two leaking tubes . Based on an indication of leakage during operation , the heater should be isolated and removed from service as soon as practical to prevent collateral damage from leak impingement . When a leak is discovered , following information are important and should be documented as part of heater operation history : 1 . The location of the leaking tube with respect to the other tubes in the bundle . This can be done by plotting leak tube position on a tubesheet diagram i . e ., tube map .
2 . Length along the tube from the face of the tubesheet where the leak occurred . This can be done by probing with a tight-fitting plug or stopper while the shell side is under air pressure .
3 . Types of the leak found ( pinhole , longitudinal crack , ruptured tube , tube to tubesheet leak etc .)
4 . Types of the plug and summary of repair methods used ( plugged with welded plug , taper plug , mechanical seal plug , stabilizing rod for ruptured tubes and insurance plug around it , etc .) Above mentioned information along with general operating condition of the unit prior to the occurrence of the leak should be part of the heater history , and they are necessary to diagnose future troubles and determine repair strategy . Inspection photos , tubesheet map updated with tubes www . heat-exchanger-world . com Heat Exchanger World September 2023
19