Can your steam boiler handle the pressure?
Published: 05 April, 2016
With fluctuations in steam pressure having the potential to affect energy savings, environmental credentials, and wear and tear to boiler components, it is vital that the steam boiler operates to the correct design pressure at all times. Matt Walton, contracts manager at Bosch Commercial and Industrial, explains how this is best achieved.
The requirement for steam in industrial applications varies greatly. From power stations through to food and drink manufacturing, the demand for steam spans across both the private and public sectors. What was once associated with the industrial revolution, is now an essential product of a number of sophisticated modern technologies.
One of the main reasons steam is used by many industrial stakeholders as the preferred heat transfer medium is due to the large amount of heat that is released when it condenses into water and how vast quantities of energy can be carried by very little steam. For industrial users to reap these benefits requires precise system control that allows the correct amount of high quality steam to be provided safely, efficiently and at the correct pressure.
Steam quality is a measurement of the amount of liquid water contained within the steam. For example, 90% quality steam contains 90% steam by weight and 10 per cent water by weight in the form of a fog, cloud, or droplets. Serious system issues, such as frequent boiler shutdowns from low-water level or damage to pipework and valves from water hammer, can arise if the correct steam boiler pressure is not maintained due to the production of poor quality steam, also known as “carry-over.
With water droplets in high-velocity steam having the potential to be as abrasive as sand particles, it is vitally important that industrial users are fully aware of the consequences if correct steam pressure isn’t maintained and the measures which can be implemented to maintain pressure levels.
During the steam generation process, a certain area of surface water is required in a boiler from which to release the steam. However, inconsistent steam pressure levels can have a significant impact on the level of water within the boiler, which can potentially lead to operational downtime if not maintained correctly, so it is important to strike the right balance.
As the boiler generates steam, 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 will 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.
This can lead to serious issues because not only the steam is 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.
It is also important to note that foam contained within a steam boiler can affect the water level probe reading. This has the potential to falsely advise that water is present, when in reality it is just foam. Serious issues can occur from this as foam does not transfer heat as quickly as water and if water is below the “2nd low water level”, below the furnace and heat tubes, this can lead a catastrophic failure or even an explosion. The water that should remove the heat is no longer there and this results in the furnace metal reaches softening point and, with the internal pressure of the boiler, can cause lethal results. Therefore it is vitally important that monitor and control measures are put in place to ensure the correct water level is maintained at all times.
Effects of water carry-over
Water carry-over is simply defined as any contaminant that leaves the boiler with the steam and it can occur in three forms; solid, liquid, or vapour. There are a number of consequences that can occur from water carry-over, including contaminated steam, water hammer and the corrosion of vital boiler components.
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.
In mild cases of water hammer, there is generally noise and perhaps small movements within the boiler pipework. More severe cases can potentially lead to more catastrophic failures such as the fracture of pipework or fittings.
In addition, 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.
The causes of carryover are generally classified as either mechanical or chemical. 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.
All process users wants to utilise steam of the highest quality. But it is important to remember that steam quality depends not on the efficiency of the boiler, but on the ability to maintain the correct steam pressure to ensure process steam can separate from boiling water without carrying liquid water particles with it throughout the entire range of boiler operations.
Even the most well-designed and operated boiler will produce minor amounts of water carry-over; it’s unavoidable. However, the key is to know how much water carry-over can be tolerated and then operate the boiler system as accordingly to ensure the required desired steam purity is achieved.
For further information please visit: www.bosch-industrial.co.uk