TDS - Total Dissolved Solids. What is it and why does it matter?
TDS is the name given to the impurities found in your boiler's feedwater, that accumulate inside your boiler over time. Typically, household drinking water alone contains between 50 - 1000ppm (parts per million) TDS, and as such is typically unavoidable. As the feedwater is introduced into the boiler, the water becomes pure saturated steam but the impurities remain behind, the more steam the boiler produces, the higher the TDS will become. TDS can cause issues with efficiency, steam quality, fuel usage, tube cooling and carry-over.
What happens if the TDS becomes too high?
Depending on the size, style, pressure and load of the steam boiler a typical TDS target is between 2000ppm & 3000ppm. As the TDS gets higher outside of this range, the boiler water becomes more stable. This means that the bubbles that occur on top of the water layer that burst and release steam, are becoming more and more resistant to bursting. Eventually as the TDS gets higher, the bubbles can become so resistant that they occupy the entire steam space and carry over into the steam piping system. When steam leaves the boiler it is very pure. When steam bubbles (foam) leave a boiler, it carries with it impurities and contaminates such as chemicals and solids. These can contaminate controls valves, steam traps, pressure reducing valves and can cause cycle failures on autoclaves. Scale can also plate out and deposit throughout the boiler.
What is scale?
Scale is the name given to the TDS that becomes concentrated and plates out on the boiler tubes, walls and drums.
Video: Scale in fire tube boiler. Concentrated around the sparge tube (where the water enter the boiler)
The scale acts as an insulator, reducing the heat transfer efficiency as well as disrupting wall flow through the tubes, which absorbs heat and at the same time, cools the metals from the furnace heat. In the best possible case, the scale will cause inefficiencies, in the worst possible case the scale can cause boiler tube failures and/or shell failures due to over heating of the metals. This is particular important when operating off centre fire tube boilers. The main blast tube (where the burner fires in to) is subject to the most heat, due to housing the burner and facilitating the combustion process. When scale is allowed to build up it can eventuate to a point where the scale can insulate the bottom of the blast tube resulting in minimal cooling. This can cause the metal to become red hot and the main blast tube yield to the pressure of the steam exerted upon it. The attached video shows a scale build up approximately 50mm from the bottom of the main blast tube of a horizontal fire tube boiler with an offset blast tube to the 6 o'clock position.
Video: Scale build up in bottom of fire tube boiler.
Photo: Ruptured water tube, located during hydrostatic test.

Photo: Water tube, blocked with extremely hard silica scale.


What do we fix it?
Once scale build up becomes so much, the only way to remove it is to conduct an acid clean. This can pose many other risks to the boilers and steam system and is a last resort. Acid cleans are also most effective where the acid can get flow. When a tube is completely blocked the acid cannot break down the scale.
How do we avoid it?
Typically there are 3 common methods for controlling TDS in steam boilers.
Operator testing and manual blowdown.
Automatic blowdown system.
TDS Controller.
Operator testing and manual blowdown;
- While it will always be necessary to carry out your daily boiler checks and do a manual mud drum (bottom drum, water drum) blowdown, using this method to control TDS can be tedious, time consuming and often only effective for a short period. For example; you complete your daily checks at 8am, by 4pm the TDS may be exceeding the recommended operating range and begin affecting your system. Automatic blowdown system;
- This system uses a timer and a control valve to automatically and periodically blow down the boiler. These are either scum blowdowns (taken from the top of the water level) or mud blowdowns (taken from the bottom of the boiler) and must be manually programmed, monitored and adjusted to keep the TDS just right. With varying loads, multiple boilers, load sharing and faults, this system can often over do it. So what? The TDS will be lower right? Correct, the TDS will be lower, the wasted energy will be higher, Feedwater usage will be higher and the boiler chemical consumption will be higher, all leading to higher operating costs. That's why its so important to "Control" the TDS.
TDS Controllers;
- This system uses a programmable controller to sample and test the feedwater using only minimal amounts of blow down. Based on the results of the sample, the controller with then schedule blow downs to bring the TDS into the correct operating parameters. Once the TDS is tested within the parameters, the controller returns to idle (a programmable period usually 30mins to 60mins) before sampling again. If the sample remains within the limits, the controller will resume back to idle, with no blowdowns. This is a very efficient and accurate way to "Control" your TDS levels. Every boiler we maintain that is fitted with a correctly serviced TDS controller, has minimal to no scale build up on annual inspections. Some of these boilers even operate on bore water! In addition to the sampling and blowdown schedule capability. The TDS controllers are also equipped with programmable alarm points for TDS too low and TDS too high, further mitigating the reliance on operators to ensure the system is working correctly.
What do we recommend?
We recommend using a TDS controller as a scum blow down. We also recommend a pneumatic blowdown valve that operates periodically to remove mud and loose scale from the bottom of the boiler without operator intervention. The alarm output on the TDS controller can also trigger to blow down valve to assist with the lowering of TDS.
What's the cost?
A typical acid clean can range between $15000 and $65000 for the procedure and the supporting labour. It typically takes at least 3-4 days from start to finish depending on boiler size and scale type.
A typical TDS controller supplied, installed and commissioned usually range between $4500 - $7500 depending on the size of the boiler and location of installation components etc.
A typical pneumatic blowdown valve system supplied, installed and commissioned usually range between $2000 - $4000 depending on valve selection and location of installation components etc.
What's the payback period on a TDS controller?
This will depend on the individual case of each plant, however, these systems focus on maximising the efficiency of the two most costly components of steam boiler operation, fuel and water treatment costs. These savings along with minimising operator intervention, reduced down time, decreased risk to plant and personnel and protection of your investment prove to be well worth it for our customers time and time again.
Where can we buy them from?
See our web store to order yours now!
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