As a scotch marine steam boiler evaporates water to produce steam, fresh makeup water must be introduced into the boiler to continue the process. As the boiler water evaporates, solids in the water are left behind. The new water entered into the system is untreated and carries contaminants such as oxygen and other solids that water treatment chemicals try to remove. Dissolved solids are minerals in the boiler water such as calcium, magnesium, silica, sodium, and potassium that have gone into solution with the boiler water and are now in liquid form. Dissolved solids are often called hardness minerals, and the total amount in the boiler water is referred to as Total Dissolved Solids (TDS). As the water evaporates and exits the boiler, the dissolved solids remain in the system and leave a solid deposit called scale. Scale is one of the most severe issues for steam boilers. The scale will cause boiler metal to overheat and result in tube failures. Due to scale-related damage, repairs can be costly and sometimes induce a boiler outage. Suspended solids, however, do not dissolve in the boiler water. Instead, the water carries them until they eventually settle on the bottom of the boiler’s surface. Suspended solids create a coating on the heat exchange surfaces and plug controls and valves. The total amount of suspended solids in the boiler water is called Total Suspended Solids (TSS). The deposits formed from water evaporation result in scale formation, which acts as an insulator for the heat transfer surface. Scale buildup on the waterside of a boiler will increase operating costs as the boiler is forced to burn more fuel. As scale builds to 1/2 inch thick, energy costs rise by 60%. Total Dissolved Solids (TDS) and Total Suspended Solids (TSS) are both commonly measured in parts per million (ppm).
Total Dissolved Solids (TDS)
TDS increases as the boiler water turns to steam, and new makeup water is introduced to the system. An acceptable level of TDS in steam boiler water is between 2,500ppm and 3,000ppm. The TDS should not exceed 3,500ppm. Maintaining TDS near the maximum allowable range will minimize heat and water loss. However, keeping TDS concentrations as low as possible will increase the quality of steam produced and the system’s overall efficiency. High levels of TDS will cause rapid scale formation. Scale in the boiler will affect the heat transfer and results in lower efficiency and overheating tubes, causing tube failure. Removing scale once it has built up is extremely difficult and expensive. The increased scale buildup is usually followed by corrosion. When boiler water contains a higher TDS level concentration, the solids are carried over when the water evaporates, reducing the quality of the steam. Poor steam quality will cause damages to control valves, heat exchangers, and steam traps. Low TDS levels may also indicate a problem with the boiler, such as excessive energy waste. If there are low TDS levels, one cause might be an excessive introduction of freshwater due to significant water loss that may have resulted from a leak in the condensate return line. Performing an extreme amount of boiler blowdowns will also lead to increased freshwater intake and low TDS levels. Excessive blowdowns increase operating costs due to lost energy and water treatment chemicals lost. Freshwater is highly corrosive to steel. Even with proper chemical treatment in your boiler, too much freshwater will wash away the chemical. The freshwater entered into the boiler is corrosive because of its dissolved oxygen content.
Total Suspended Solids (TSS)
Total Suspended Solids are mainly composed of calcium, magnesium, iron, copper, and silica. The TSS in steam boiler water should be around 8ppm to 10ppm and never should exceed 15ppm. Similar to the effects of high TDS, high TSS will cause scale to form and will negatively interfere with heat transfers. High levels of TSS will cause fouling to occur. Fouling is when foreign material resides on the surfaces of the boiler and pipes. Fouling prevents heat transfer and limits the results of scale-inhibitor products such as sodium nitrate with a copper and iron inhibitor, that has been entered into the boiler. As scale builds on the heat transfer surfaces, more fuel is required to convert water to steam, causing the unit’s efficiency to drop and energy costs to increase. When suspended solids settle on the bottom of the boiler surface, oxygen is consumed under the solids, becoming the anode. The cathodic reaction with the bottom metal surface causes galvanic corrosion and oxygen pitting to transpire. Galvanic corrosion is when two dissimilar metals, such as copper and iron, are absorbed in a conductive solution. The greater the concentration of copper and iron minerals in this highly conductive water leads to galvanic corrosion as these different types of metal settle on the steel surfaces of the boiler.
What is a Boiler Blowdown?
Fresh makeup water is brought into the boiler to replace water lost by evaporation. A portion of the remaining water is drained to remove the concentration of solids that the evaporated water has left behind. Boiler blowdowns are a crucial procedure in boiler maintenance to control the TDS and TSS levels in the water and extend the life of your boiler.
Three Different Types of Blowdowns
A control blowdown is typically the most common blowdown performed by boiler operators. Control blowdowns are performed at the low water cut off to verify that it operates correctly. Low water cut off controls are safety devices designed to stop the burner from firing if the boiler is low on water to prevent a “dry fire.” A dry fire occurs when a steam boiler runs low on water, causing the boiler to overheat. To perform a control blowdown on the low water cut off, the blowdown valve must first be opened slowly so the pipe warms up, reducing the chances of thermal shock. Now that the piping is warmed up, the valve should be opened all the way, and the water in the sight glass should begin to lower. As the water lowers, the burner should turn off. Once the valve is closed, the system will return to regular operation. It is important to simulate actual conditions when performing the control blowdowns to ensure the controls are operating correctly. Quickly opening the blowdown valve does not simulate the actual conditions. The low water cutoffs may shut off if the valve is opened quickly but not with a slow test if dirt and scale build-up in the float chamber. The frequency of control blowdowns depends on the boiler operating pressure. If your boiler operating pressure exceeds 15psi, this type of blowdown should be performed once a day. For boilers with an operating pressure under 15psi, the control blowdown should be done once a week.
A top blowdown is often referred to as a surface or continuous blowdown, performed just below the water level surface at the skim valve. A top blowdown aims to decrease the TDS level by removing the dissolved solids on the top of the water surface. Dissolved solids typically reside about 6 inches below the water level and can form a foam on the water’s surface or rapidly lead to scale on the metal’s surface. If not removed, dissolved solids in the boiler water can carry over into the steam system, causing severe damage to equipment. By continuously removing the dissolved solids on the top of the boiler water surface, top blowdowns also provide the advantage of preventing the solids from settling to the bottom of the boiler. The frequency of top blowdowns depends on the boiler water quality and the amount of freshwater being added to the boiler.
Suspended solids and sludge settle at the bottom of the boiler and impede heat transfer. Bottom blowdowns are performed to remove the suspended solids and lower the TSS level in the boiler water. If not removed, suspended solids will accumulate over time as freshwater is introduced to the boiler and will continue to cause efficiency problems leading to costly repairs. To perform a bottom blowdown, the quick valve should first be opened while keeping the slow valve closed to prevent extended flow through the valve. Next, the slow valve should be opened for a few seconds to allow the water to exit. To finish the procedure, the quick valve can then be closed. Keeping the slow valve open for less time is recommended if the water has low conductivity. The slow valve can remain open for about 5 to 10 seconds longer if the water shows high conductivity reading. Performing a bottom blowdown too often can run the boiler low on water. To optimize suspended solid removal, consistent, short bottom blowdowns are favored over periodic, extended bottom blowdowns.
Controlling and maintaining proper TDS and TSS levels in a steam boiler is important to achieve optimal efficiency and reduce costly repairs. Understanding what causes too high or too low TDS and TSS levels is needed to ensure a suitable amount of boiler blowdowns are performed. Scale and corrosion cannot be prevented entirely, but proper and routine maintenance can slow the growth rate. If you have any additional questions regarding blowdowns or the maintenance services we offer at Controlled Combustion, please feel free to contact us.