My question is regarding alkalinity. I'm operating a boiler at 200 psig. The boiler application is for Central plant hot water heating by direct steam injection using a cascade heater (similar in operation to a dearator). As such, I have 100% condensate return using the high temp hot water as my feedwater source. Makeup is due to system leakage.
I adjust my blowdown conductivity setpoint to maintain my OH alkalinity between 300 and 600. Operators test for P and M alkalinity and use the 2P-M formula for obtaining my OH alkalinity. Conductivity is typically maintained @ 3100 mmhos.
My predecessor set the limits for M alkalinity at 400-700 (700 being the ASME "guideline") and P alkalinity 300-600. Using the 2P-M method, I have frequently exceeded my P and M limits but, fell within the control range of my OH alkalinity. The boiler water pH is within my limits of 10.8-11.5. As I understand boiler water chemistry, OH alkalinity is your control parameter. If that's the case, why does ASME reference total (M) alkalinity? If not, why bother calculating OH?
My question is, Am I harming the boiler (scaling or caustic embrittlement) by exceeding the P & M alkalinity yet maintaining the OH within range? Should I be going by P,M and OH with the high reading being the limiting factor? I have asked my chemical rep but, his answers aren't making sense and he cannot show me a source to justify his answer. I know, it's time to get a new rep. In the meantime, I'm still looking for an answer. Any references that I can read?
Oh... fyi... Makeup water alkalinity is ~110, conductivity ~220 but again, my makeup is fairly low.
I adjust my blowdown conductivity setpoint to maintain my OH alkalinity between 300 and 600. Operators test for P and M alkalinity and use the 2P-M formula for obtaining my OH alkalinity. Conductivity is typically maintained @ 3100 mmhos.
My predecessor set the limits for M alkalinity at 400-700 (700 being the ASME "guideline") and P alkalinity 300-600. Using the 2P-M method, I have frequently exceeded my P and M limits but, fell within the control range of my OH alkalinity. The boiler water pH is within my limits of 10.8-11.5. As I understand boiler water chemistry, OH alkalinity is your control parameter. If that's the case, why does ASME reference total (M) alkalinity? If not, why bother calculating OH?
My question is, Am I harming the boiler (scaling or caustic embrittlement) by exceeding the P & M alkalinity yet maintaining the OH within range? Should I be going by P,M and OH with the high reading being the limiting factor? I have asked my chemical rep but, his answers aren't making sense and he cannot show me a source to justify his answer. I know, it's time to get a new rep. In the meantime, I'm still looking for an answer. Any references that I can read?
Oh... fyi... Makeup water alkalinity is ~110, conductivity ~220 but again, my makeup is fairly low.
