Feedwater Temperature

If the actual temperature and flow rate is unknown then ‘How can you accurately treat the oxygen content’?

Water with a temperature below 100°C will absorb oxygen, which unless removed by suitable treatment reacts with the metal inside the boiler and steam system causing corrosion. The lower the feedwater temperature the greater the amount of dissolved oxygen it contains and therefore proportionally more oxygen scavenging chemicals are required to prevent damage.

The following table illustrates the quantity of oxygen dissolved in water in a hotwell for various temperatures. 

The oxygen reacts with the iron in the boiler to form red oxide as Fe₂O₃, that is every 3 molecules of oxygen will react with 2 molecules of iron and be converted to rust.

If this is considered in terms of atomic weights (using 16 for oxygen and 56 for iron) then every 3mg of Oxygen can remove approx 7mg of iron from the boiler system.

Think of the damage that 5,000 litres per hour of poorly treated feed water at 80°C can do to a boiler operating for 10 hours a day for say 200 days per year.

                                                                      7x10^-3 x 5000 x 10 x 200 = 70kg

The damage is not uniform however as oxygen damage is characterised as localised pitting so it does not take too much before a tube failure occurs or problems start to appear in the steam and condensate lines.

Neutralising the oxygen before it enters the boiler is obviously vital but this is not without its downside – as some oxygen scavengers raise the Total Dissolved Solids (TDS) of the boiler water for example, Sodium Sulphite, which is commonly used on land boilers requires approx 8mg of product to neutralise 1mg of dissolved oxygen thus raising the TDS of the feed water by at least 8mg/litre so if your philosophy to combat oxygen damage is to overdose the chemical for the worst case condition you introduce other problems like higher rate of surface blowdown with the attendant energy and water loss.

The problem is that even with a high proportion of hot condensate being returned to the feedwater tank, fitted with a direct steam injection heating system, it is difficult to achieve an even feedwater temperature.

Whether you have a daily start-up or operate 24seven the feedwater temperature will vary throughout the course of the day each day and every day. Ascertaining the correct dosage of Oxygen Scavenging chemicals is based on calculation:  on start-up or with an increase in load, the natural balance between steam flow and condensate return will change and the feedwater tank level will fall. Cold make-up water is required to correct the imbalance. The tank temperature will be depressed and dissolved gas content will increase.

The feedwater tank is part of a very dynamic system with a number of variables all of which have an impact on the stored water temperature. Consider for a moment a typical system:-

On start up there is a natural time delay between steam flowing out of the boiler house and condensate returning during which time cold water is used to make up the contents of the feed tank. The temperature drops.

Initially, at start up, the returning condensate would be cooler than normal as the steam warms the plant through, but is at a higher temperature than the cold make up water, again affecting the temperature inside the tank.

After a couple of hours the system settles down and with a combination of cold water, returning condensate and direct steam injection heating system, a steady tank temperature is achieved. If it is a closed loop system with no vents, then all is well.

But most systems collect condensate in vented receivers and pump it back to the feed tank. On a system were all steam traps are working correctly, there will be a flash steam loss of between 10 – 15%, based on operating steam pressure.

This 10 – 15% loss has to be made up by cold water, the flow rate of which is controlled by an on/off control system supplying a relatively high volume of cold water in a very short time. This intermittent cold water addition depresses the temperature of the tank.

 Testing once a day, at about the same time will probably give you similar results. But the results are not representative of actual operating conditions and beg the questions:-

• At what temperature do you set your chemical dosing regime?
• How much reserve do you need to cater for the unknown?

Setting the dosing level on too high a temperature and you will be under dosing for long periods throughout the day, risking oxygen attack. Too low a temperature and you will be over dosing, raising the TDS level inside the boiler, increasing the blowdown rate and wasting money.

At a temperature of 80°C there is 3mg/l of dissolved oxygen in the water being fed to the boiler. At 95°C the Oxygen content will be reduced to 1mg/l reducing the sulphite requirement by approx 2/3^rd.

Furthermore Sodium sulphite greatly contributes to the dissolved solids in the boiler; economies in operating costs can be expected as the reduction in dissolved solids means less blow down, saving energy and water.

Dosing chemicals based on an estimated temperature or one sample per day contributes considerably to problems in the boiler which ultimately reduce efficiency and increase operating costs throughout the lifetime of the boiler.

BS 2486:1997 Section 5.7.2 ‘Oxygen Scavenging Chemicals’ states:-

“These should always be dosed continuously. The feed point should be selected so that it gives the maximum possible time for reaction. This will normally mean dosing the storage section of a De-Aerator or, where there is no De-Aerator, to the outlet of the boiler feed tank/hot well”.

Given, therefore, the uncertainty of the stored feedwater temperature and the boiler demand wouldn’t it make sense to monitor the feed rate and the dissolved oxygen level (or feed water temperature) and to design your dosing algorithm based on these factors.

It would then be possible to accurately adjust the dosing rate in real time, according to the feed water demand. Not too much, not too little but just the right amount.

1. Eliminating tube failure due to oxygen attack
2. Reducing chemical usage
3. Saving energy by reducing blowdown
4. Saving water by reducing blowdown