Question about scavenged oxygen system in boilers 2

[mpeck1982]

I have a water tubed boiler at our steam facility. The boiler feed water is oxygen scavenged by a DEHA system. 98% of the oxygen is taken out of the system. Then Phosphate is used to take out the other 2% of oxygen out of the boiler feed water. The way the plant operates the boiler is not what the manufacturer operation suggest. Basically we go from a hot stand by to a cold stand by mode. Cold standby is basically letting the boiler cool down to atmosphere. So during the transition of hot stand by to cold standby the superheated steam in the boiler condenses to condensate and sits in the superheater tubes. If oxygen is scavenged out of the system how does it get back into the system during condensing??? There was severe damage of oxygen pitting in the superheater tubes. So oxygen was in the system. we have weekly reports of DEHA (excess scavenged oxygen residual). Reports are above the recommeneded levels. Any help would be appreciative.

 

[Latexman]

Does this give some ideas:

Oxygen Scavengers

Good luck,
Latexman

 

[rmw]

What are you doing when the boiler goes cold to prevent air from entering the boiler. Either you go under a very very deep vacuum when the steam consenses or you let air get in or you blanket with nitrogen. If you do nothing, that is where the O2 is coming from.

rmw

 

[mpeck1982]

The steam plant is basically letting the boiler cool down and leaving it off (aka cold stand by). Their hot standby basically is a back up for the boiler that is in operation. Just in case if it fails. A Lead-Lag operation. Cold stand by the boiler cools down to atmosphere. Once boiler cools down then shut the incoming feed water coming in to the boiler. This is what operations have been doing. So, if from going from hot stand by to cold stand by (basically turning the boiler off) the steam condensing in the super heater section of the boiler will be in a vacuum. When steam condenses you will be pulling atmosphere (i.e. 21% oxygen in) then the oxygen will be absorbed into the water. Therefore despite scavenging the oxygen, when steam condenses it will pull a vacuum of atmosphere and get oxygenated condensate in the tubes.

Did I understand you right rmw??

 

[rmw]

YOu understood correctly.

rmw

 

[mpeck1982]

If the oxygen is already scavenged out of the Boiler feed water and the steam condenses once boiler is shutoff where is it getting the oxygen from since it is a closed system? Wouldn't it be pulling oxygen from somewheres else? The vacuum process from steam to condensate water wouldn't pull in oxygen from the closed system would it? It would have to get oxygen outside of the system correct?

 

[macmet]

Oxygen can get sucked in through the valves, either through the packings or if you have drains or vents that may not close completely.

 

[davefitz]

You might be able to prevent the oxygen ingress during cold outages by adding a N2 seal system - the N2 keeps a +5 psig nitrogen cap on the piping as it cools down.

 

[FredRosse]

The cold shutdown boiler will form a vacuum and ambient air will almost certianly be drawn into the boiler, with it comes oxygen. The superheater tubes will condense pure steam, and will not have the oxygen scavenger present unless you are using volitale treatment, so the superheater tubes corrode quickly.

Nitrogen blanket is easy and cheap, a nitrogen cylinder, regulator with 1 PSIG setting, a check valve so that when the boiler is fired steam will not wreck the nitrogen regulator. Another option is to draw a high vacuum on the boiler continuously with a vacuum pump, this also works but you have to get the proper vacuum pump, liquid ring and oil filled pumps are not appropriate, use a dry vacuum pump capable of hig vacuum.

 

[davefitz]

FredRosse,

I think you would need to add a leakage relief valve on the LP pipe system if the only other protection is a check valve.

 

[KiwiMace]

Almost all steam systems (those not designed for vacuum) are fitted with vacuum breaker valves that open to prevent vacuum pressures caused by condensing steam from destroying the pressure vessels in the boiler. As rmw pointed out, this valve is letting the air in, and without it your steam drum would probably be crushed like a tin can.

 

[MJCronin]

Ummmm..... Its not leaking valves or vacuum breakers...

The deaerator will admit air/oxygen when it cools down.

Also, you state: "...the superheated steam in the boiler condenses to condensate and sits in the superheater tubes."

Your boiler internal design is not the best....Most modern superheater boiler designs call for a fully draining tubes.