Can your Steam Boiler handle the pressure?
Fluctuations in steam pressure may have the potential to affect a steam boiler’s performance, not to mention wear and tear to components, but how do we make sure a boiler operates to the correct design pressure at all times? Thankfully, avoiding poor quality steam and carry-over is far from an impossible task.
The first thing to consider is that any impurities found in the boiler feed water, which do not boil off with the steam, will concentrate in the boiler water. As the impurities become more and more concentrated, steam bubbles form which will fail to burst when they reach the boiler water surface. As the steam space within the boiler becomes filled with bubbles, eventually foam will be carried over into the steam main – a potential cause of catastrophic failure or even an explosion.
This can lead to serious issues because not only is the steam wet as it leaves the boiler, but the boiler water contains high levels of dissolved and suspended solid that can contaminate boiler system components such as control valves and heat exchangers.
Water carryover is simply defined as any contaminant that leaves the boiler with the steam and it can occur in three forms; solid, liquid, or vapour. Mechanical causes include boiler design, high water level, firing method, and load characteristics. Chemical causes include high solids concentration (dissolved and suspended), excessive alkalinity, grease, oil, and other contamination.
The occurrence of boiler water carryover can usually be verified by testing the condensate. As a general rule, low conductivity condensate indicates high purity steam is being produced, while high conductivity condensate indicates the presence of carryover.
Water droplets in high-velocity steam can be as abrasive as sand particles in terms of eroding pipe components and valve seats. In addition, if a puddle of water is allowed to accumulate in steam pipes, this water will be propelled by the high-velocity steam, increasing its chances of the water slug crashing into pipe elbows, tees and valves. An often overlooked consequence of boiler water carryover is the efficiency loss due to wet steam. Since water at a given pressure has lower heat content than steam, wet steam has lower heat content than dry steam.
Even the most well-designed and operated boiler will produce minor amounts of water carryover. The key however, is to know how much water carryover can be tolerated and then operate the boiler system as accordingly to ensure the required desired steam purity is achieved.
Matt Walton, Contracts Manager at Bosch Commercial and Industrial