Seeking advice on Alkali Boil Out 1

[msajjad]

Hello Sir,

We are about to start alkali boil out of our water tube boiler (20 kg/cm2G and 70t/h).
I will be highly grateful if you can comment on my few doubts:

1. Can we use Demineralised water to
prepare chemical solution out side steam drum?

2. We have to first give a flush to the
steam drum before sending chemical into it, what precautions
should i take while filling steam drum by hot Boiler feed
water (105 C and 22 barG)?

3. Actually we want to do pre dry-out
of waste heat boiler refractory along with boil out
opearation. Boiler vendor says that heat steam drum upto
boil out pressure ( he does not mention pressure value in
IOM) and then cool it. repeat this uoto 8 hrs then drain all
chemial solution to complete the boil out. However
refractory vendor says heat the steam drum untill producing
steam of 2-4'barG and temp of 150 C and hold it upto 20 hrs
then cool it to achieve pre dryout along with boil out. Do
you think any problem with this?

4. Any valuable suggetsions on
preserving boiler after boil out, actually right after boil
out inspection, we will be doing complete dry out of
combustion chamber refractory by heating up to 1200 C ?

Thanking you.
sajid

 

[KoachCSR]

Can we use Demineralised water to
prepare chemical solution out side steam drum?

Yes. In fact, many vendors can provide a pre-mixed slurry which you can pump in, although the cost does go up. You can pre-mix it yourself; be mindful that you follow appropriate safety measures, watch your concentrations, and use appropriate PPE.

We have to first give a flush to the
steam drum before sending chemical into it, what precautions
should i take while filling steam drum by hot Boiler feed
water (105 C and 22 barG)?

I would recommend you perform the initial flush with something besides boiler feedwater, simply because it will be much easier and you are going to chemically clean it directly after. If a demineralized water source isn't easily available, open the manholes on either side and simply hose down the inside thoroughly.

If you simply have no choice but to use boiler feedwater, be careful not to thermally shock the steam drum. Most manufacturers recommend a heat-up rate not to exceed 100°F / hr (37°C / hr). Confirm your manufacturer's recommendation and follow it to the letter. With your boiler feedwater being that hot, fill the boiler VERY slowly - using a manual bypass just cracked open (but never more than 10% open).

Actually we want to do pre dry-out
of waste heat boiler refractory along with boil out
opearation. Boiler vendor says that heat steam drum upto
boil out pressure ( he does not mention pressure value in
IOM) and then cool it. repeat this uoto 8 hrs then drain all
chemial solution to complete the boil out. However
refractory vendor says heat the steam drum untill producing
steam of 2-4'barG and temp of 150 C and hold it upto 20 hrs
then cool it to achieve pre dryout along with boil out. Do
you think any problem with this?

Most refractory I've worked with dries out in two or three stages. One can usually dry out the first stage (say, hold at 300°F for 12 hours), before beginning the boil out. You don't want to hold the boil-out chemicals in the steam drum too long before bringing the boiler up to pressure. I would expect the duration of your boil-out would have some acceptance criteria based on the samples you pull, I'd confirm that before you begin. 8 hours seems like a short boil-out to me. Do not try to proceed into final refractory cure with boil-out chemicals still in the drum; the dry-out of the refractory will be much hotter than your boil-out.

4. Any valuable suggetsions on
preserving boiler after boil out, actually right after boil
out inspection, we will be doing complete dry out of
combustion chamber refractory by heating up to 1200 C ?

Sounds like you are trying to rush the commissioning; however, you should cool the boiler down after the boil-out and perform another rinse of the internals to ensure all of your boil-out chemicals have been flushed out. Refill the boiler, then begin your refractory cure. Rushing things will cost you in the long term, whether by giving you a water chemistry headache or worse.

Some general questions of my own:

What chemicals are you using for boil-out?
What is the refractory you are drying out?
What is the fuel being burned by the boiler?

 

[msajjad]

Thanks a lot KoachCSR for your detailed and very informative reply.
The chemcials proposed to be used are NaOH, NaNO3 and sodium phosphate.
The cobustion chamber and waste heat boiler have castable refractory as well as refractory bricks.
We will use Natural Gas, mostly methane for boil-dry out purpose, however in normal operation there will be liquid waste stream and process gas streams along with natural gas.

I will definetly check with vendor about acceptance criteria and duration of boil out, although IOM mentions about freeing the surface from oil or grease as completion of boil out.

sajid