carryover in boilers

[weorrego]

Dear Members:

We are writing from Colombian Sugar Factory. First of all this is important because in the sugar factory the steam demand is variable, due to there are many crushing cane stops in the daily crushing, that change the steam demand, besides the equipments factory changes its operation state.

We are having big problems for boilers´s solids carryover in the steam. This problem generate valves jam in the turbogenerator. Many months ago, we are taking correctives actions, like that:

Decrease the level in the steam drum. Before we are working with 50% like normal level, now is 40%.
We are working to feed the Caustic Soda (NaOH) directly to the drum, becaus now is feed in the feed water piping to the boilers.

The chemical treatement for the boliers is: Caustic Soda for pH regulation, phosphate for the hardness, and sulphite for the oxigen, and polymer for Iron. The treatment is controled by NALCO.

Now, we would like to know the solutions possible for the steam boilers carryover.

Sincerely,

Wilson Orrego

 

[metengr]

Could you provide some additional information on the type of boiler (design), operating pressure and temperature, and if you have blown down capability?

 

[rmw]

Does your boiler have any type of mist eliminators at all in the steam drum. Chevrons, cyclones, perforated pipe? Anything at all in the drum to try to prevent carryover?

If your drum came with baffling originally, is it still there. (once encountered a similar problem only to find out that the plant personnel got tired of dismounting and taking the baffling out every time they had to go into the drum to fix a leak, so they just threw it away. Found some in a twin sister boiler out of service, and installed them, and the problem went away.)

What type of level controls do you have?

How many boilers do you have, and are all of them priming (carrying over?)

Are your traps in the steam header working correctly?

rmw

 

[hbw]

I have little boiler experience. My guess is that condensate carryover is caused by sudden drop in steam pressure that entrains unvaporized condensate. Try to slow the rate of increase of steam demand from the boiler (slow the application of load to the boiler).

 

[davefitz]

Carryover has many possible causes. See the B+W book "Steam".

The first possible cause is a higher than indicated drum water level. If the drum water level gauges are not temperature or pressure compensated. then the apparent level was not corrected for the true level. Related to this is the need for one of the level gauge legs to remain un-insulated.

The second possible cause is the steam loading of teh driers is higher than they were designed for. The driers were probably sized for max steam flow at full drum pressure, and if you are operating at less than full drum pressure at full steam flow then you will overload the driers. The boiler mfr should have provided a curve of max permitted steam flow vs drum pressure, based on this limitation.

The third possibility is that there is mechanical fault in the way the seperators were mounted or the driers were mounted. During the next outage, inspect the drum internals to ensure that there are no unsealed gaps between the centrifugal separators and the girth baffle plate, and that all driers aremounted securely.

A fourth possibility is that the boiler mfr did not know what he was doing and did not size or the drum internals correctly. You can sometimes fix this problem by retrofitting the driers with drainable driers or using a SS mesh pad in lieu of chevron driers.

 

[GenB]

It may be a chemical imbalance causing foaming as well.
you may try antifoaming compounds which is un-expensive.
GB

 

[crawfishdaddi]

What are your control parameters for TDS levels in the B.F.W.? High TDS levels can contribute to foaming and water/solids carryover into the steam. Ask NALCO about an antifoaming agent, it's probably a good place to start.

 

[smahsan]

Hi,As per my little experience that carry over main causes are the level control,blow down method and the chemical injection as per the boiler capacity but level and steam admission mode to turbine and method is major part of it.pressure and temperature are not the major part of it.U can avoid by blow down which is best method.U must control TDS,DO2,chloride in condensate.
In my knowledge at my boiler we dose N2H4 in our Main boiler via condensate and directly(In Drum) Amonia and phosphate as per chemist recommendations (Drum Pressure 185 bar and steam generation capacity1000 t/h) and also auxiliary boiler(operating pressure 20 bar and T=200 degC.
Regards
Ahsan

 

[Xianglu]

I have dealt with carryover for many applications.
First of all, let's list all possible causes:
1. Solids are carried over mechanically: In which case, steam carries water droplets from the drum, as steam is heated up in supper-heater, solids deposit in the super heater tubes; or saturated steam passes through piping, as pressure decreases, liquids droplets vaporise and solids will jam piping fittings. To eliminate this carry over, one could increase separation efficiency in the drum such as separators and demisters. Another approach is to reduce solids content in the boiler water, which ways include better water quality (Ion exchangers, demineraliser, etc), higher blowdown ration and inside drum steam washing.
2. The second type of carry-over is called selective type, by which way, some solids dissolves into steam at high pressure and come back to solids at lower pressure, these substances include silica. If your application is not high pressure, that will not be the case. Solutions to this issue include increasing water quality and steam washing.
From your case, my guess that the problem is poor feed water quality. you need better treat your feed water for your water system.

 

[davefitz]

There is still another source for solids carryover to the turbine. If there is a desuperheater spray attemporator, the solids in the spray water will directly pass into the steam to the turbine.

 

[deanc]

My guess would be carryover from the juice evaporators foaming. Bet your water walls are also blistered from internal sugar deposits.

In anycase you may find some good material here.

http://www.sugartech.co.za

 

[rmw]

Then the question would be: are you using any evaporator condensate for boiler make-up, or just condensate from the first (or Pre evaporator) effect?

If so, then you have to check the sugar content of the condensate.

rmw