Mixing Chemicals During Wet Lay Up
Mixing Chemicals During Wet Lay Up
(OP)
The firetubes on our boilers are corroding during wet lay ups and the problem has been tracked to a lack of mixing of the oxygen scavenging chemicals.
Currently, site is suppose to go down once a week, test the water and fire up the boiler for a little while to mix, if needed. However, this step gets skipped a lot and so there is insufficient mixing and the fire tubes corrode.
We are looking into placing a circulation line/pump off the blow down line that would circulate the water from the front blowdown drain and pump it back into the rear blowdown drain.
Are there other ways of mixing the chemicals and would this circulation line work? Also, since we are a hydroelectric company, electricity is virtually free while the natural gas isn't (hence why we want to move away from lighting the boiler to mix). Also, the solution would need to be simple from an operational stand point as these are auxiliary boilers and are low on the totem pole of things to get done during the day.
Any advice/input would be much appreciated.
Thanks
Currently, site is suppose to go down once a week, test the water and fire up the boiler for a little while to mix, if needed. However, this step gets skipped a lot and so there is insufficient mixing and the fire tubes corrode.
We are looking into placing a circulation line/pump off the blow down line that would circulate the water from the front blowdown drain and pump it back into the rear blowdown drain.
Are there other ways of mixing the chemicals and would this circulation line work? Also, since we are a hydroelectric company, electricity is virtually free while the natural gas isn't (hence why we want to move away from lighting the boiler to mix). Also, the solution would need to be simple from an operational stand point as these are auxiliary boilers and are low on the totem pole of things to get done during the day.
Any advice/input would be much appreciated.
Thanks





RE: Mixing Chemicals During Wet Lay Up
Most likely possible problems would be lack of flow through all the tubes and in closed-off pipes going to the boiler, so corrosion would continue in the low flow/no flow tubes.
RE: Mixing Chemicals During Wet Lay Up
RE: Mixing Chemicals During Wet Lay Up
On the other hand, the isolated sample station and circulating pumps can be assumed to stay at "near" room temperature (boiler shutdown temperatures), not operating boiler temperatures.
RE: Mixing Chemicals During Wet Lay Up
It is a high pressure boiler and all equipment is being spec'd to above the boiler operating pressure and temperature, matching the exisitng blow down line. Even though the recirc line should never be exposed to the boiler operating conditions, it will be able to withstand them.
Another issue we had was what flow rate to use. Is there a way of determining an adequate flow rate to ensure proper mixing for this type of situation? We selected a flow rate based off circulating one complete volume exchange per hour, giving us a flow rate of 40 GPM. When speaking with the boiler manufacturer, they said this would do but weren't exactly sure.
RE: Mixing Chemicals During Wet Lay Up
But the critical flow will be through each tube - more closely, through the lowest flow (highest resistance) tube at the furthest inlet point from the header. If you have 100 tubes (!) and each had identical resistance that means 40/100 = .4 gpm through that tube.
But real world, two of the hundred get 4.5 gpm, and 2 get .5 gpm, and the rest "share" the remaining 30 gpm. And, as Murphy plans things, it's (of course) those two getting the lowest flow that need the best corrosion resistance.
RE: Mixing Chemicals During Wet Lay Up
The fire tubes are corroding from the outside because the water surrounding them is still and the chemicals eventually settle at the bottom because there is currently no mixing. The recirc line will move the water around to, hopefully, keep the chemicals properly mixed and prevent the oxygen pitting.