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Steam quality


filtered steam, which (as the name implies) has additionally been filtered in some manner to remove physical debris down to a defined size; clean steam (with which many readers should be familiar); and, finally, pure steam which is raised in a similar process to clean steam, but using a water source meeting WFI (water for injection) quality. Steam quality is different. It goes to the


heart of why steam is being used – in order to move and utilise large amounts of heat energy with precise control. Steam quality is about defining the physical characteristics which are fundamental to its ability to perform consistently in this way under all load conditions. The good news is that there is already a standard that can help us with this definition. In fact, HTM01-01 Part C contains many ‘cautionary notes’ regarding issues that may affect heat transfer but, in order to summarise and define these clearly, it directly references this standard – BS:EN285 : 2015 +A1 :2021 (commonly shortened to ‘EN285’). There are three key properties of steam essential to effective heat transfer and defined in EN285 - we should (and do) therefore assess all steam quality data against them. These are: air and other non-condensible gases (NCGs) of ≤3.5% by volume, superheat of ≤25°C and steam dryness of ≥ 95%. (There is also a water conductivity measure, which we will not deal with in this article). All can be measured through recognised methods of steam quality testing using dedicated equipment for this specific purpose. However, what do they each mean and what can we do to ensure we are meeting the standards? Steam dryness is probably the most


challenging of these to understand clearly and to measure accurately under dynamic load conditions. Using our extensive steam knowledge and expertise, along with state-of-


the art research and test facilities at one of our UK manufacturing sites, we have covered both issues and are happy to share these insights. One of the fundamental aspects of steam, that we should firstly remind ourselves of, is that it gives up the large amount of heat energy that it carries by transferring this heat directly to cooler surfaces it comes into contact with. While this heat transfer occurs, steam remains at a constant temperature but begins to condense back to a liquid state (water). When we generate steam we are effectively doing the opposite of this and, by adding more and more heat energy at our steam generating plant, we are aiming to reach the point where our steam is 100% vapour with no water (condensate) content. This would be steam dryness of 100%, sometimes expressed as a ‘dryness fraction’ of 1.0, and would give us steam with an energy content (in kJ/Kg) exactly as stated in dry saturated steam tables – in other words, a ‘known quantity’ of heat energy available for use in our process. So, we can clearly see from the information


above, that knowing only the pressure and temperature in our steam space is not enough to be able to confirm steam quality. Steam condenses in order to give up its heat energy to the process so that, at a given pressure and temperature, we may have steam or we may simply have hot condensate (water) at the same temperature. This can be illustrated in Fig.1 ‘The temperature / enthalpy curve diagram’.


In this diagram temperature is shown on the vertical (‘y’) axis and energy content, or enthalpy, is expressed in kJ/kg on the horizontal (‘x’) axis. The ‘origin’ point where the axes meet is zero energy content and 0°C - the gradient line on the left of the curve therefore representing increasing temperature and energy content.


The descending vertical lines on the right of the curve indicate various values of pressure, with the right hand boundary of the curve representing the point at which 100% dry saturated steam occurs. The area to the right of this line represents the superheated steam region. The horizontal lines within the curve therefore are evaporation lines and represent the amount of energy that must be added to water at a given pressure and temperature to achieve dry saturated steam with no water content remaining. For the purposes of this example, we have


chosen steam at 2barg (bar gauge) which will have a temperature of 133°C as shown at the dotted line on the left. Looking at the three values indicated along the horizontal evaporation line, we see that at this pressure and temperature it is possible we could have steam at any number of levels of dryness. We have highlighted 10%, 50% and 90% to illustrate the point (remember that EN285 calls for ≥ 95%).


Is there an answer to the quality challenge? If maintaining pressure and temperature alone are insufficient to guarantee steam quality, what is the answer? There are some mechanical ‘best practice’ steps that can be taken within both our plant steam and clean steam systems that will help us to ensure resilient steam quality. We can take steps to eliminate condensate build-up in our plant steam systems (strainers, traps, separators all correctly selected, installed and working) - any dead-legs or poorly isolated/ drained areas of our system can also be addressed. Our steam generating plant can be properly sized, configured and controlled to eliminate any risks of “carry-over” of boiler water.


Air must be allowed to escape (through proper venting) but must also be driven out of


Figure 1. What is steam quality and why is it so hard to monitor? 82 www.clinicalservicesjournal.com I April 2024


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