Discontinuing Hydrazine and Raising FW pH

[mxd0gia]

We just switched from an mixed metallurgy system to an all ferrous system. However there is still some copper in our SJAE. If we discontinue the use of Hydrazine will we run a risk of corrosion in the SJAE?
Also are there any other risks associated with this?

Thank you,

 

[JimCasey]

Hydrazine is there to scavenge oxygen from the system before it corrodes the ferrous metals. Hydrazine is not far from Ammonia chemically so the logical expectation would be that hydrazine would attack, not protect, the copper-bearing components in the system. Fortunately you dose the system with Hydrazine in the ppm range, and if your dosing is right, the hydrazine is consumed in the O2-scavenging duties instead of being free to attack the copper.

Raising feedwater pH: get water hot and the pH goes up all by itself. That's why it is necessary to use mica shields on glass liquid level gages. At saturation conditions above 250 psig, the pH of the water is so high that it attacks the glass of the level gages.

If your maintenance guys are measuring pH by grabbing a sample in an open beaker and walking halfway across the plant to the lab to measure the chemistry, what they get to the lab with is not very close to what came out of the system. By then it has cooled, and exchanged gases with the atmosphere, and probably dissolved some of the sludge in the beaker. Best way to measure the system chemistry is with an in-situ instrument, real-time. Second best is to measure the chemistry in the beaker as the sample flows in from the sample tap, and then only after the sample tap has flowed long enough to get up to system temp and to have whatever fluid was resident in the sample system completely purged. Wear appropriate protective equipment.