What are the ​Most Prominent ​Methods to ​Handle Hard ​Scale Formation ​in Plant Boiler ​?

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Hi,

We have a water treatment plant that is designed to treat APO wells water through ion exchange unites to produce water with total hardness < 1PPM.

The treatment water is  feeding two types of boilers- One Through Steam Generation & Heat Recovery steam Generation. 

Hard scales had been collected during the mechanical cleaning with approximately 1- 1.5 ton per each unit. Based on the scale analysis by XRD & EMS the majority of scale are Sodium-calcium silicate – Pectolite ( NaCa2Si3O8(OH)) - Calcium silicate – Wollastonite ( CaSiO3). The BFW Specs as below:-.

1) Chloride 3000 ppm
2) Silica 110 ppm
3) Dissolved O2 2 ppb
4) pH OTSG – 7 - 8, HRSG – 8. 5
5) Total Iron OTSG – 1 ppm, HRSG – 0.25 ppm
6) TSS 1 ppm

What are the most prominent methods to handle Hard scale Formation in plant boiler?

Looking for your valuable feedback.

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3 Answers

  1. Prepare your water of > 10 MegOhm*cm @ 25 degC. To avoid complicated MixBed technology use Cation- Anion- Cation Polisher technology as exploration. from Lanxess. BTW your silica will go down to < 20 ppb. Don't save in NaOH regeneration as your silica level is very high, please check if it means 110 ppm, this is like Phillipne groundwater near volcanic area.

    Please provide as minimum analysis TDS or conductivity. It looks brackish/seawater water with this high Chloride level of 3000 ppm. It's a high risk of corossion issues, 

    Best will be a copy of your feed water analysis may without laboratory and company names. Do what government is doing by blacking information.

    Best Regards Dieter

     

     

  2. The chloride levels need to be reduced, which will reduce the total dissolved solids and RO will help with this.  A scale which contains iron and silica and calcium is hard and very heat resistant.  It means that you are wasting energy heating the scale and reducing boiler efficiency.  I have come across boilers with silica scale, in which the boiler tubes have overheated and in one case the metal has formed a bubble and in the other case it forced the tube out of the back plate and caused water to leak into the fire space.

    Silica is resistant to hydrochloric acid.  Hydroflouric acid will work but this is so hazardous and toxic, it should only be carried out by a specialist contractor who knows the correct safety procedures to use.  It could be that after a de-scale you find you have water leaks from the tubes.  This would not be the fault of the contractor but the scale was masking the leaks.

    By using a double RO plant after the softenor, it would reduce the chloride to to 30 and the silica to 1, which would reduce the risk.

    In waters such as this, the free caustic level should be 2.4 times the level of silica to maintain it in solution.

    Specialist water treatment companies such as Suez, may be able to assist.  There are ways that using the correct mineral caustic levels, a chelant, phosphate and high levels of polymer can cause the scale to crack and it will be separated from the boiler tubes.  You have to be careful that the scale, which comes off does not bridge and block the tubes because that in itself can damage them.  But a reputable specialist company could better advise you analysing the water and the plant conditions.

  3. OTSG and HRSG require better quality feed water than 1 ppm hardness.  Typically, mixed bed IX is required to reduce all salts to very low levels, in particular hardness and silica.  If your current IX system is sodium cycle softeners, I would suggest adding 2-pass RO downstream.  If your current IX system is strong acid cation followed by strong base anion, then mixed bed IX should be added downstream.  Better quality boiler feed water will significantly reduce boiler scale.