Alkalinity Control Chemical for Boiler: Dosage and pH for Sohar Plants

If your boiler water pH keeps falling, scale is forming on the tubes or the condensate lines are corroding, you need an alkalinity control chemical for boiler water. This is a concentrated liquid alkali that you dose to hold pH and alkalinity in range. We supply RXSOL Alkalinity Control (part number RXSOL-50-5002-025) from our Barka warehouse to plants in Sohar Industrial Zone and to ships at Sohar Port. With correct treatment, boiler water pH normally sits at about 9.5 to 11. Our datasheet sets a desired alkalinity level of 100-150 ppm as CaCO3. You reach it by dosing in measured steps and testing again 3 to 6 hours later.

How an alkalinity control chemical for boiler water works

Boiler alkalinity control products are concentrated liquid alkalis, usually based on sodium hydroxide (caustic soda). Products of this class are typically colourless liquids with a density of about 1.25 to 1.3 g/ml and a concentrate pH of 13 to 14. One high-strength builder of this type contains about 32% NaOH.

Inside the boiler the product does two jobs. It raises and holds alkalinity so that the steel is protected from acid corrosion. It also gives your hardness control the alkaline condition it needs to work. Calcium hardness then comes out as a soft, non-adherent sludge that bottom blowdown removes, so it does not bake onto the tubes. Without this, scale restricts water flow and reduces heat transfer. It also starts local corrosion, so the boiler loses efficiency and components fail.

Our Alkalinity Control boiler alkalinity builder also neutralises acidic gases to support corrosion control. Products of this class also keep silica in solution.

Target pH, P alkalinity and M alkalinity by boiler pressure

Correct treatment normally gives a boiler water pH of about 9.5 to 11. One product sheet for this class sets feedwater pH targets of 9.5 to 11.5 for low-pressure boilers and 10.5 to 12.0 for high-pressure boilers.

As pressure rises, the allowed M alkalinity falls. The general guide bands, in mg/L as CaCO3, are:

• 0 to 150 psi: 100 to 700, with P at 25 to 40% of M
• 151 to 300 psi: 100 to 500, with hydroxide above 150
• 301 to 600 psi: 50 to 300
• 601 to 900 psi: 15 to 100
• above 900 psi: below 15

For shell boilers up to 30 bar, British Standard 2486 guidance puts boiler water limits at about 3000 mg/L TDS and 1200 mg/L M alkalinity. Whichever limit is lower sets your cycles of concentration and how much you blow down. Published bands differ because boiler design, heat flux and feedwater quality differ from plant to plant. Your boiler maker's limits always come first.

Dosage for each tonne of water and where to dose

Our datasheet gives the base figure: 1 milliliter of RXSOL-50-5002-025 in one tonne of distilled water provides 0.5 ppm of alkalinity hydrate. For the product class, a typical dose of about 225 ml for each tonne of boiler water raises P alkalinity by about 100 ppm as CaCO3. Another source gives about 100 ml for each tonne for a 50 to 60 ppm rise. Your working dose comes from a pH and alkalinity test dosage chart, based on your pressure band and your current reading.

Here is a worked example. A package steam boiler at a food plant in Sohar Industrial Zone holds 8 tonnes of water. The test reads 40 ppm as CaCO3 and you want 100 ppm, which is a rise of 60 ppm. On our datasheet figure, each tonne needs 120 ml (60 ppm divided by 0.5 ppm for each ml). Across 8 tonnes, the dose comes to 960 ml. On the class figure of 225 ml for a 100 ppm rise, each tonne needs about 135 ml, which comes to about 1080 ml for the boiler. Start at the lower figure and dose. Run a control test after 3 to 6 hours before you add more.

For best results, dose direct to the boiler with a boiler water treatment dosing unit, or through a bypass pot-feeder installed in the boiler feed line.

• Low-pressure boilers (up to 28 bars): add the product to 5 litre of feed water and pass it through the recirculating or main feed pipe, or into the hot well tank.
• High-pressure boilers (28 to 60 bars): dissolve the required volume in 5 litres of feed water and pass it into the water drum.

Testing P and M alkalinity and what the results mean

Take the sample from the same point every time, after blowdown. Cool it and test it at once, usually daily. Titrate P alkalinity to pH 8.3 with phenolphthalein. Titrate M alkalinity with methyl orange to its end point, which is pH 4.3 to 4.6 depending on the method. Report both as mg/L CaCO3. Hydroxide alkalinity is often estimated as twice the P reading minus the M reading. A P and M alkalinity test kit covers both titrations on board or in the plant laboratory.

Low alkalinity causes acid corrosion and thinning of the steel. Too much alkalinity causes foaming, priming and carryover. A high level of uncombined hydroxide can cause caustic embrittlement. High M alkalinity in the feedwater raises the CO2 in the steam, and that CO2 corrodes the steam and condensate return lines. Read your results like this:

• P alkalinity or pH below target: dose alkalinity control and re-test after 3 to 6 hours.
• pH and P alkalinity above target, or foaming and priming: stop dosing and increase blowdown until readings return to range.
• M alkalinity high in the feedwater: check the make-up water and look for corrosion in the condensate lines.
• TDS or M alkalinity near the limit: increase bottom blowdown, which also removes the conditioned sludge.

Desalinated make-up water in Sohar and the full treatment programme

Many plants in Sohar feed their boilers with desalinated or reverse osmosis water. This water contains very little hardness or alkalinity, so the boiler water has little natural buffering and its pH can fall quickly. You therefore depend more on the dosed alkali to hold pH in range. Captive-alkalinity programmes are typical where make-up water is demineralised.

Alkalinity control usually runs as part of a programme, with hardness or phosphate control, a sludge conditioner and an oxygen scavenger. Our product is compatible with other boiler chemicals, and we supply Hardness Control Liquid (RXSOL-50-5021-025) to run alongside it. After acid cleaning with RXSOL-11-1008--DC descaling liquid or RXSOL-54-1008-SA Safe Acid powder, you can use the same product as a neutralising agent before the boiler returns to service. Our full range of boiler and industrial chemicals covers the rest of the programme.

Storage, handling and PPE in Sohar heat

Caustic solutions of about 30% are classified Skin Corr. 1A, H314 (causes severe skin burns and eye damage), and H290 (may be corrosive to metals). Contact with aluminium, zinc or tin releases flammable hydrogen gas. Keep the product away from acids and from oily or fatty materials. Wear chemical goggles or a face shield, alkali-resistant gloves and an apron. Ask us for the MSDS to confirm the exact classification of our product.

Some liquid builders crystallise at low temperature, and one sheet gives a freezing point of about 7°C. Caustic solutions of about 30% are best stored above about 20°C, which is rarely a problem in Sohar. The Batinah heat does mean you should keep drums in the shade, on a bund, and tightly closed so the product does not absorb CO2 from the air. Store it closed in the original HDPE containers. Stainless steel, polyethylene, polypropylene and PTFE are suitable for tanks and pump parts, and mild steel and PVC are acceptable. Do not use aluminium, galvanised or tinned fittings.

Where to buy alkalinity control in Sohar

Sohar Chemical (RXSOL group, Oman) supplies Alkalinity Control from our Barka warehouse, with deliveries to Sohar and Al Batinah. This includes plants in Sohar Industrial Zone and vessels at Sohar Port. Call +968 99489269 or message us on WhatsApp for boiler chemical orders. Products of this class are typically packed in 25 litre pails, with 50 litre and 210 litre drums for larger plants. We confirm our packing for your order in the quote, and the MSDS is available on request.

The same product appears in our catalogue as RXSOL-50-5002-020 Alkalinity Control for marine buyers. For plants in Muscat and Rusayl, it is also listed on Muscat Chemical's alkalinity control page. Plants in the south and ships at Salalah port can order it through Salalah Chemical for Salalah port deliveries.

To request a quote, send us your boiler pressure, water volume and current P and M readings on WhatsApp at +968 99489269, or use our quote request page.

Frequently asked questions

What chemical is used to control alkalinity in boiler water?

A concentrated liquid alkali, usually based on sodium hydroxide (caustic soda). It raises and holds boiler water alkalinity and pH so the steel is protected from acid corrosion, and it turns calcium hardness into soft sludge that blowdown removes. We supply RXSOL Alkalinity Control, part number RXSOL-50-5002-025, from our Barka warehouse.

What is the ideal pH and alkalinity of boiler water?

With correct treatment, boiler water pH is normally about 9.5 to 11. Our datasheet sets a desired alkalinity level of 100-150 ppm as CaCO3. M alkalinity limits fall as pressure rises, so always follow your boiler maker's limits.

How much alkalinity control do I dose for each tonne of boiler water?

On our datasheet, 1 milliliter in one tonne of distilled water provides 0.5 ppm of alkalinity hydrate. For the product class, about 225 ml in each tonne typically raises P alkalinity by about 100 ppm as CaCO3. Dose in steps and run a control test after 3 to 6 hours.

What is the difference between P alkalinity and M alkalinity?

P alkalinity is measured by titrating to pH 8.3 with phenolphthalein. M alkalinity is the total alkalinity, titrated with methyl orange to about pH 4.3 to 4.6 depending on the method. Both are reported as mg/L CaCO3, and hydroxide alkalinity is often estimated as twice the P reading minus the M reading. Test daily, from the same sample point after blowdown.

What happens if boiler alkalinity is too high or too low?

Low alkalinity causes acid corrosion and thinning of the steel. Too much alkalinity causes foaming, priming and carryover, and a high level of uncombined hydroxide can cause caustic embrittlement. High M alkalinity in the feedwater raises CO2 in the steam, and that CO2 corrodes the condensate lines.