Economizer By-pass

[NewBern]

We would appreciate comments or shared experiences for a significant problem we are dealing with. Thanks.

The pulp mill I work at has a boiler with an economizer that has developed tube leaks (boiler capacity is 200,000 lb/hr at 850psig/825F steam - fed with 305F dearated feedwater). The tubes have thinned due to external corrosion (burning #6 oil + sulfur waste gases -- 12 years in service).

We plan to replace the economizer in 14 weeks, but must continue to operate it until then. The boiler is critical to the operation (no production when it is down!).

We are considering by-passing the feedwater around the economizer (feedwater directly to the boiler steam drum).

Because of the downstream scrubber system, the boiler exit gases (700-800F) must be cooled to <400F. The plan would be to run water from a cooling tower through the economizer. Thus, the economizer would act as a carbon steel heat exchanger cooling the flue gases (cooling water in at 70F and out at 160F).

The water would not be deaerated and it is colder than normal feedwater so corrosion would be greater. On the positive side, the pressure is significantly less (100 psig vs. 1000 psig).

This is a bit unusual -- we would obviously would not do this for the longterm. The question is: will the economizer survive 14 weeks operated in this manner? Perhaps someone has some direct experiences with something like this.

Thanks again.

 

[MJCronin]

New...

Your cooling tower water will be evaporating under the new configuration (~100 psig with ~ 700 F on the gas side)

You will be making steam in the economizer


-MJC

 

[NewBern]

Thanks for the reply MJC. Boiling in the economizer would not be good.

The economizer needs to remove ~30 MMBtu/hr of heat from the flue gases. The idea is to flow at least 700 gpm of cooling water through the economizer. This should heat the water to ~160 degF.

Is there a way to estimate what flow rate would need to be to suppress boiling at a moderate back pressure (<100 psig)?

Thanks.

 

[chief]

If you check out the Steam tables, you will see that at 100 psi the water will evaporate at approx 340 F . At a pressure of 850 psi the evaporation temperature is 530 F.
If possible , split the feed water supply to the boiler, with just a partial flow going thru, the economiser, but of course maintaining the same feed pressure. You can regulate the split to maintain your exhaust at circa 400 Fahrenheit.


Offshore Engineering&Design

 

[chief]

I presume there is a by-pass and isolation valves to isolate the Economiser should it eventually fail. A means of reducing your exhaust gas temperature to the required 400F, would be to rig a temporary draft air supply into your exhaust trunking either from the existing boiler air supply or other source. A back draft flap (check) in the air supply would be required and another flap to regulate the air supply.
This rig could be prepared in readiness for a quick coupling,if required.

Offshore Engineering&Design

 

[eyec]

if you are in a Code State it would be wise to run this by your State's AI as well as your Insurer's AI.

 

[chief]

My code state was in the centre of the Pacific and a get me home situation.

Offshore Engineering&Design

 

[davefitz]

The feeding of cold feedwater directly into the steam drum may cause high thermal stress at the feedwater nozzle- confirm that the boiler mfr has supplied a thermal sleeve at the drum feedwater nozzle.

The boiler firing rate will increase for a given final stam flow, due to the lower boiler efficiency, so you will need to limit the burner heat input to the original max expected firing rate .

The cooling tower water has lots of nasty chemicals in it , and will cause big corrosion and fouling problems in the economizer.

 

[toothless]

I imagine you will get a fast build-up of scale in your tubes, along with high corrosion rates due to the oxygen.

We would not consider operating in this manner.

 

[hacksaw]

Hold on, you cannot valve-off this economizer

you might be able to plug the offending tubes

or shut the boiler down...

CW in the boiler is asking for a much greater problem and one that will require its complete replacement

 

[chief]

We need some more details of your plant set-up. We are presuming that it is a standard steam plant with alternative means of feed water heating apart from the Economiser, such as a de-aerator and intermediate feed heaters.

Offshore Engineering&Design

 

[NewBern]

Thanks for the discussion. I liked the idea from "chief" of mixing ambient air with the hot flue gas to cool it down. That is thinking "outside the box". Calculations show this could work, but the boiler would have to be run at a much reduced rate to keep the volume of air reasonable.

Again, the thought is to by-pass the feedwater around the economizer. The feedwater is deaerated and at 305 degF. This would go to the steam drum (currently the feed water picks up heat in the economizer and enters the steam drum at 420-460F).

The economizer would be used as a heat exchanger to cool the flue gases from 700-800F to 350-400F. A high flow rate of water (700-800 gpm) would be put through the economizer (hopefully boiling would not occur - exit water temperature should be 160F).

We understand the oxygen in the cooling water would be corrosive, but it would only need to last 10 weeks until we can replace the economizer.

Has anyone actually done anything remotely similar to this?

Thanks for the comments.