Mixing Chemicals During Wet Lay Up

[rconnor]

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

 

[racookpe1978]

It sounds like a good idea: If the corrosion is on water side of the tubes, and is not starting on the fire side, or from chemicals deposited in thick layers on the fireside. Your description, though, makes it seem like inside corrosion is the case, so you should be OK.

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.

 

[cjw81]

Are these high pressure boilers(over 15 psi. or over 500 sq/ft/heating surface? Putting a pume on the line could get dangerous as any connections added must be able to withstand 1-1/2 times the operating pressure. If you are using sodium sulfite for oxygen scavenging you will be testing for residue of sodium sulfate. If you have residue of that chemical you may have other reasons for teh problem. Take a real good look at teh conditions as it could lead to catasthropic tube failure. Do not treat this as a Secondary problem"

 

[racookpe1978]

True, and a good point: Mechanically and by design, the complete added system (pipes, pumps, seals, sample station and chemical addition pots and flanges) must be capable of withstanding system pressure; or be fully isolated from potential system pressure by double valves & safety reliefs, spool pieces and blind flanges, etc.

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.

 

[rconnor]

Thanks for the advice racookpe and cjw.

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.

 

[racookpe1978]

Well, your 40 gpm is from the suction, through the pump back into the header (the system).

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.

 

[rconnor]

racookpe I should clarify, the pump will pump the water in the drum and not in the fire tubes themselves. The water will be sucked out through the front blow down (or blow off) drain and pumped back into the boiler through the rear blow down(/off) drain. So it will be a single path flow.

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.