Water Quality for IST OTSG
Water Quality for IST OTSG
(OP)
We are commisioning a 2x1 combined cycle cogeneration plant. 2 LM6000PD's; 2 IST OTSG's ; 1 siemens Demag Delaval STG.
Sinct the boiler is a once through, the limit on cation conductivity is 0.25 microSiemens. Our demineralizer is producing 0.6 microSimens specific conductivity. We have 304 SS piping from the demin to a 304ss Demin Storage Tank. Makeup is to a 304SS FWHT via 304ss piping from the Demin Storage tank to the FWHT. The feedwater system to the OTSG is 304SS as well. The piping form the condenser to the FWHT is CS.
We are having problems with conduxtivity in the demin tank. In a few days the conductivity in the tank will rise to 0.6 microSiemens.
We have inspected the tank and found ehat apperas to be rust on the bottom brought in from the inlet piping. Tank has been cleaned and repassivated. Conductuvuty cintinues to climb. Inspection of the pipe reveals some rust on the welds. Not sure if this is from incorrect welding rod or just collection of rust where it hits the weld bead. Pipe is ATM A312, but has not been passivated.
We have been talking to other plants with similar configurations. Some have put final bead filter/ion exchange right at the connection point for the feedwater to the OTSG. This is an expensive option. I am not sure we will ever get the conductivity down to 0.25 w/o it.
Any suggestions or insight are appreciated.
Sinct the boiler is a once through, the limit on cation conductivity is 0.25 microSiemens. Our demineralizer is producing 0.6 microSimens specific conductivity. We have 304 SS piping from the demin to a 304ss Demin Storage Tank. Makeup is to a 304SS FWHT via 304ss piping from the Demin Storage tank to the FWHT. The feedwater system to the OTSG is 304SS as well. The piping form the condenser to the FWHT is CS.
We are having problems with conduxtivity in the demin tank. In a few days the conductivity in the tank will rise to 0.6 microSiemens.
We have inspected the tank and found ehat apperas to be rust on the bottom brought in from the inlet piping. Tank has been cleaned and repassivated. Conductuvuty cintinues to climb. Inspection of the pipe reveals some rust on the welds. Not sure if this is from incorrect welding rod or just collection of rust where it hits the weld bead. Pipe is ATM A312, but has not been passivated.
We have been talking to other plants with similar configurations. Some have put final bead filter/ion exchange right at the connection point for the feedwater to the OTSG. This is an expensive option. I am not sure we will ever get the conductivity down to 0.25 w/o it.
Any suggestions or insight are appreciated.





RE: Water Quality for IST OTSG
I think that you should consider reposting your question in a forum more devoted to materials selection and/or steam power plant operations and design:
Consider:
www.steamforum.com
http://www.steamplantengineering.com/
and
www.hghouston.com
Please come back and tell us about your preferred fix and the reasons for it
regards
MJC
RE: Water Quality for IST OTSG
My understanding of IST's feedwater requirements has lead me to conclude that the only way to comply is to have a feedwater polishing system, either powered resin or mixed bed. operating without one will surely result in voiding of the warranty.
You will need the N2 blanket even with polishers to extend times between regenerations or pre-coating because the CO2 absorbed in unblanketed tanks will exhaust the anion resin prematurely.
The rusting you see in the tank and piping maybe due to workers using tools (grinders, wire brushes, etc.) that have been contaminated with ferrous iron or made from ferrous iron. Tools for stainless stell fabrication must be segregated from those on carbon steel.
RE: Water Quality for IST OTSG
Although the CO2 will increase the conductivity, it might not be deleterious to the cycle , and might be permitted by IST. There is another means of measuring conductivity ( de-gassed?)which eliminates the CO2 effect and focuses only on the ions that will damage downstream equipment , and perhaps that conductivity is the governing effect. We have the same issues on drum type boilers, wherein the breakdown of organic amines will result in a high conductivity of the steam due to CO2 formation; to meet steam turbine specs, we would use a degassed (?) conductivity reading to judge acceptable steam conditions.
If the IST unit is their typical design, they have tiny orifices at the inlet of the economizer ( for flow stability) , and they cannot have any traces of iron oxide in the feedwater to avoid pluggage of the orifices. Your comments of using a CS line from condenser to FWHT may be a source of the oxides, espescially if you are using traditional AVT alkalien feeedwater treatment methods ( ie, it forms magnetite , which easily spalls off). I would either replace the CS line, or consider using the combined oxygenated feedwater treatment method.
As far as needing a polsihing system, Alstom had proved with their once-thru HRSG's that a polisher is only needed for the first month of operation, assuming the condenser is tubed with stainless steel tubes. After the first month , the cycle is cleaned up and the rented polsiher is returned to the rental agency. Historic expereince with conventional once thru units , on Rankine cycles with regenerative feedwater heaters and copper tubed condensers , does not strictly apply to the case of a once thru HRSG with a stainless tubes condenser.
RE: Water Quality for IST OTSG
With 0.6 uS/cm coming out of your DI system you need either a polishing strong acid cation or a mixed bed exchanger to make specified water.
RE: Water Quality for IST OTSG
RE: Water Quality for IST OTSG
I had some errors in the post. The mixed beds produce 0.06 to 0.1 microsiemens conductivity.
We did clean the demin tank and re passivate it.
We have come to the same conclusion about the CO2. We are looking at polystyrene balls to reduce the surface area of the tank and limit CO2 absoprtion.
We have installed a degassed cation conductivity analyzer on the feedwater tank.
Our mixed bed demineralizer is sized for 750 gpm. 17 GPM is for the makeup to the cycle, and the rest of the capacity is for polishing condensate return from the steam host. We have also installed a bypass fron the discharge of the condensate pumps to the demin. This will polish the condensate during times of low process steam load.
I think these changes will get us where we need to be.
Since some of you have experience with IST units, we are experiencing strange pressure oscillations at part loads on the units. This occurs even if we drop the feedwater valve into manual. IST is looking into it, but have not come back with anythiong yet.
Anyone else have this issue? I suspect that the the flows to the tubes is not uniform.
RE: Water Quality for IST OTSG
IST has also lately licensed a different boiler technology ( Benson from Siemens KWU), which uses an alternative approach- instead of adding 400psid pressure loss in inlet orifices, they use intermediate mix heders at strategic locations to eliminate the Ledineg static flow instability issue. But it lookslike you are using the IST classic technology, so you may need to find a way to live with it.
RE: Water Quality for IST OTSG
Usually you cannot artificialy increase the boiler pressure simply by throttling at the steam turbine control valve- that valve is not suitable for such drastic pressure drops and it is instead used for speed control prior to synchronization. Instead, you can provide a startup bypass control valve in parrallel with the boiler outlet stop valve, and size that smaller valve for holding a constant full upstream boiler pressure during startup.
This fix will not completely prevent the flow instability , but it will help .