Hydrazine

[pollok]

Can anyone tell me where the best sample point for hydrazine would be on a HRSG. We had been sampling on the condensate, but I thought it would be better to sample on a drum. Any other ideas.

 

[davefitz]

The best place would be immediately upstream of the water feed into the drum.

If you sampled the drum water or downcomer liquid, it would not provide a true indication of oxygen ingress because the LP steam drum acts as a dearator, and strips the O2 off and the O2 passes out with the LP steam .

The steady state setpoint for O2 would be about 2-3 ppb O2, in order to avoid overfeed of hydrazine.

Most forms of hydrazine or carbohydrazide are n ot active until 300F, so any sample point upstream of the LP preheater/economizer is useless.

 

[pollok]

The sample point we have is downstream of our chemical addition, so if we are dosing hydrazine, we get erroneously high results. It sounds like that is still the best sample point, but we need to get operators to pay attention to chemical adds. Does that sound like a fair assessment?

 

[yulinios]

Our cycle is: Condenser Pump - Heater 1, 2,3- Dearetor- Feed Water Pump -Heater 5, 6 , 7 - Boiler. Our inyection of hidrazine is regularly after the condensate pump (which is proportional to the condensate flow in auto), we check the concentration after the Dearetor and before the Boiler. We got some bad experience at begin, we preservate the boiler whit water with hidrazine at 170 ppm, but in the starts all the hidrazine desapear by the high temperature, at 315°C begins his descomposition resulting high values of hidrogen in the steam (experimental data). Actually we are controlling 25 ppb in feedwater before the boiler.

I hope that it can help you.

 

[pollok]

Thank both of you for the help. We do not have a deaerator, so your numbers may be different. We are supposed to maintain about 5-20 ppb residual, but I don't see the need if our oxygen level is within specification. Our only choices for sampling points are condensate, feed (after chemical addition), steams and drums. It sounds to me like feedwater is the place to look.