Case Study #1: A Small Problem with Reductant Feed?
The following example concerns a common occurrence at many power plants: a relatively small increase in DO concentration. Most plants might consider this a minor "blip" that can be ignored, but the corrosion environment at temperature tells a different story.
In this system (Figure 6), scavenger chemical feed rate was slaved to steam flow off a primary boiler. A secondary boiler was brought online, which placed a higher load on the single deaerator. Oxygen scavenger was added to the deaerator for the primary boiler, but the feed of scavenger was not adjusted during this event because its control was from the primary boiler steam flow only.
6. Peak performance. A corrosion stress event illustrates that a relatively small increase in DO can lead to large corrosion stresses. Source: Nalco
Note the conductivity decrease during the event, signifying the lower scavenger concentrations in the DO as more makeup water was added but scavenger feed was not adjusted upward. Also, notice that AT ORP data show the lack of feedwater corrosion stress and corrosivity control.
The AT ORP sensor response clearly shows that returning the system to a lower ORP space (lower corrosivity) can take a considerable amount of time and that some resulting damage might not be reparable. Note that AT ORP level does not decline with DO but remains elevated longer as ongoing system oxidation (corrosion) keeps the ORP elevated. This is an indication that the effects of active corrosion are still being sensed even though DO has decreased.
This example demonstrates the need to minimize the magnitude and duration of feedwater corrosion stress excursions because even relatively small events can have a large impact on corrosivity. An inline AT ORP sensor identifies such problems more rapidly and helps manage the impact of such problems automatically.
Case Study #2: A Tale of Two Events
This example illustrates another problem common in power plants: air inleakage through condensate pump seals. In this example, the AT ORP technology initially monitors the event (the "monitoring phase" of the study). Later, the AT ORP system attempts to control corrosion stress in the face of this same corrosion stress event.
The unit has two condenser hotwell pumps (pump A and pump B). During the AT ORP monitoring phase, it was clear that the pumps had different implications for corrosion stress creation in the feedwater system. Figure 7 shows the low-pressure heater #3 (LP#3) and the gland steam condenser (GSC) AT ORP and DO values with the use of two condensate pumps. Figure 8 includes data taken from the two RT ORP probes during the same time period.
7. Leaking air. AT ORP and dissolved oxygen (DO) values on switching condensate pumps indicate that Pump A has a seal that allows air in-leakage. Source: Nalco
8. Pump preference. AT ORP and RT ORP values on switching condensate pumps indicate that Pump A air in-leakage significantly increases the corrosion potential in the condensate system. Source: Nalco
The AT ORP probes reacted immediately to the change in feedwater corrosion conditions as soon as the switch to pump A was made. Some of the corrosion stress was caused by the increase in feedwater DO due to air in-leakage from condensate hotwell pump A.
The passivator consumed some of the DO inventory, but some of the DO was involved in corroding the metallurgy of the feedwater heaters. As mentioned before, the AT ORP probe also picks up the production of soluble corrosion species.
The AT ORP readings indicated that operating with pump A was more damaging to the LP heaters than pump B. The AT ORP probe was more sensitive and responsive to this pump change and was able to detect and react to the real feedwater corrosion stress events better than an RT ORP probe. The RT ORP probes were found to be significantly less sensitive or unresponsive in detecting real-time feedwater corrosion stress events.
Also note that the LP#3 feedwater heater exit AT ORP level was higher than the AT ORP level at the GSC with pump A, even though DO was lower at the LP#3 sample point. The reason for this is that extra scavenger had been consumed (raising AT ORP levels) across the feedwater heaters, and soluble, oxidized corrosion products had been released, thereby raising the AT ORP level as well.
Figure 9 illustrates control of the AT ORP levels at the LP#3 sample point to –350 mV versus the EPBRE (271F) when the different condensate pumps were in operation. Note the increase in feedwater corrosion stress when condensate pump A was operated with its air in-leakage problem. With current scavenger feed limitations at this plant, even controlled feed of carbohydrazide at 100% pump speed could not reach and maintain the AT ORP setpoint of –350 mV versus the EPBRE when the plant was operating with condensate pump A. When the plant operated under constant load (195 MW) using condensate pump A, the lowest AT ORP reading that could be achieved and maintained under current plant operating conditions was –325 mV versus the EPBRE.
9. Pick your pump. Controlling an AT ORP setpoint of –350 mV levels in pump A that was experiencing air in-leakage wasn’t possible even when adding carohydrazide at 100% scavenger pump speed. Source: Nalco
The LP#3 AT ORP probe sensed a change in feedwater corrosion stress and adjusted the carbohydrazide pump speed when the plant switched from condensate hotwell pump A to pump B. AT ORP controlled feed of carbohydrazide was then able to maintain the –350 mV setpoint when condensate pump B was operating. AT ORP readings indicated that operating with condensate hotwell pump A was more damaging to the LP heaters than using Pump B.
Case Study #3: Obtain and Maintain Low Corrosion
This example illustrates how the AT ORP level varied during the monitoring phase and how it was controlled during the control phase. Prior to installation of the AT ORP system, operators at this plant were unaware of the dramatic variations in the corrosion space of their feedwater heater system, even though they had RT ORP monitors.
The goal of this particular program was to obtain and maintain reduced conditions to limit feedwater corrosion in the boiler. AT ORP monitoring at the low-pressure feedwater heater outlet showed how the AT ORP level varied greatly, and there were many excursions to the elevated, more oxidizing regimes.
In Figure 10, the percentile plots show that during the monitoring phase, 90% of the AT ORP data fell within the blue rectangular box. However, when the system switched to controlling the AT ORP space, the average AT ORP level was reduced and then maintained in a much tighter control range, thereby significantly reducing feedwater corrosion and lowering corrosion product transport. The plant now has far greater confidence that its boiler assets are much better protected than they were prior to the AT ORP program introduction.
10. Tight process control. Percentile plots showing how the AT ORP space was controlled after switching from monitoring mode to control mode with AT ORP technology. Source: Nalco
—Daniel C. Sampson (dcsampson@nalco.com) is a power industry technical consultant and Peter D. Hicks (phicks@nalco.com) is a research scientist for Nalco Co.
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