Controlling Total Dissolved Solids (TDS) is crucial for efficient and safe boiler operation, preventing issues like scaling and carryover. Based on industry practices for conventional shell boilers, recommended operational TDS levels vary depending on the boiler's size and operating conditions.
Understanding Total Dissolved Solids (TDS) in Boiler Systems
Total Dissolved Solids refer to the total weight of all solids that are dissolved in a given volume of water. When water is heated and turned into steam, the dissolved solids do not evaporate with the steam; instead, they remain in the boiler water. This process concentrates the solids over time. High concentrations of TDS in the boiler water can lead to several problems:
- Scale Formation: Dissolved minerals can precipitate out of the water and form hard deposits (scale) on heat transfer surfaces, reducing efficiency and potentially causing tube failures.
- Carryover: High TDS can cause foaming and priming, leading to boiler water droplets (carryover) being entrained in the steam. This can damage downstream equipment like turbines and heat exchangers.
To prevent these issues, the concentration of TDS in the boiler water must be controlled, typically through a process called blowdown, where a portion of the concentrated boiler water is drained and replaced with fresh feed water.
Recommended TDS Ranges in Conventional Shell Boilers
For conventional shell boilers, the target TDS levels during operation are based on factors like boiler size, operating pressure, and steam demand. According to common guidelines:
- Very small conventional shell boilers are normally operated with TDS in the range of 2,000 ppm.
- Larger conventional shell boilers can operate with TDS levels up to 3,500 ppm.
These recommended ranges are applicable provided that the boiler is operating near to its design pressure and the steam load conditions are not too severe.
Here's a summary:
| Boiler Size | Recommended Operational TDS Range | Key Conditions |
|---|---|---|
| Very Small Conventional Shell Boilers | 2,000 ppm | Operating near design pressure, steam load not severe |
| Larger Conventional Shell Boilers | Up to 3,500 ppm | Operating near design pressure, steam load not severe |
Note: ppm stands for parts per million, a common unit for measuring concentration.
Important Considerations
The specified conditions—operating near design pressure and mild steam load—are important because higher pressures and rapid steam generation (severe load) can increase the risk of carryover even at lower TDS levels. Boiler manufacturers often provide specific water quality guidelines for their equipment, which should always be followed.
Clarification: TDS in Boiler Feed Water vs. Boiler Water
It is important to distinguish between the TDS levels in the boiler water during operation (discussed above) and the TDS levels in the boiler feed water that enters the boiler.
- Boiler Feed Water TDS: The water supplied to the boiler (feed water) is typically treated to remove most dissolved solids, among other impurities. The aim is to introduce water with as low a TDS level as practical to minimize the rate at which TDS concentrates inside the boiler. Recommended feed water TDS limits are significantly lower than the operational limits inside the boiler, often in the low hundreds or even tens of ppm, depending on the boiler type, pressure, and water treatment system used (e.g., softening, demineralization, reverse osmosis).
- Boiler Water TDS: The values of 2,000 ppm and 3,500 ppm mentioned are the target maximum concentrations within the boiler water itself, controlled by blowdown.
In summary, while feed water quality is critical for controlling the rate of TDS buildup, the operational TDS limits for conventional shell boilers during operation are the values maintained within the boiler drum through proper water treatment and blowdown practices.
