Steam Quality - 5 Key Questions You May Ask

Steam is an incredible heat transfer medium. Every kilogram of steam contains around 2200 kj of condensable energy. That’s 26 times more usable energy than a kilogram of water in a typical heating system based on flow and return temperatures with a delta of 20oC.

Steam is easy to distribute as it requires no costly electric circulating pumps and, when its work is done, it is simply water – reassuring to know in our environmentally conscious world.

But do we need to consider the quality of the steam we use?

1. For starters, what is meant by steam quality?

In order for your plant to operate effectively, steam should be delivered:

• In the correct quantity
• At the correct pressure and temperature
• Clean, dry and free from air and other non-condensable gases (NCGs).

The quality measures above will be raised later in this guide.

In addition to steam quality, we need to consider another factor – steam purity.

Steam purity is determined by the needs of your process and is measured by analysing the contaminants in the condensate.

The Purity of your steam can be graded as:

• Plant
• Filtered
• Clean
• Pure, with Pure having the fewest contaminants.

2. Why is steam quality important?

Good quality steam will help you to get the best out of your applications and processes.

Heating water, cooking food, drying garments, curing tyres, or any application where steam is applied, will require steam to perform efficiently and effectively. 

For steam to perform at its optimum level, the steam must be dry with a minimal amount of entrained moisture and free from NCGs. As mentioned earlier, the steam must also be delivered at the optimum pressure and temperature.

Good quality steam ensures correct process control, short warm-up times and maximum productivity. Using good quality steam also minimises the amount of steam used – this will be explained later in the blog. Less steam used equates to lower running costs and reduced CO2 emissions.

3. How do I achieve good quality steam?

The quantity, pressure and temperature of steam

The correct quantity of steam must be available for any heating process to ensure that sufficient heat flow is provided for heat transfer. The correct flowrate avoids product spoilage or a drop in the rate of production. Therefore, steam loads should be accurately calculated and pipes must be correctly sized to achieve the flowrates required.

Steam should always reach the point of use at the required pressure and desired temperature for each application in order to maximise performance. It is important to carry out the correct sizing and selection of the control to regulate the flow of steam into the process.

Air and the effect of other non-condensable gases

Air will cause the temperature of the steam to be lower than expected. As a result, steps should be taken to remove air from the steam system. If allowed to accumulate, the presence of air will introduce a barrier to heat transfer at the process.

Best practice at the boilerhouse is essential to minimise the potential for air entering the distribution system. One area to consider is the correct management of the boiler feedwater. Raising the feedwater temperature in excess of 85oC will help to release the dissolved gases from the water, driving them out of solution and preventing them from reaching the boiler.

It is common to refer to BS EN 285 as a benchmark for steam quality, which stipulates a level of NCG in the steam not exceeding 3.5%.

Steam distribution systems have their part to play

Conventional carbon steel plant steam systems can present with deposits and corrosion.  Therefore, proper control of the boiler water treatment and a preventative maintenance program for steam traps and strainers around the steam system will help to maximise the system’s cleanliness. This helps to keep control valves, steam traps and other components associated with the steam system free of deposits and corrosion and operating efficiently.

Clean and Pure grade steam distribution systems should be constructed of stainless steel and operate with a higher specification of water quality, hence will be inherently cleaner than plant steam systems. This is driven by the needs of the final application, which is why we see the healthcare and pharmaceutical industries using clean and pure steam systems for their critical processes, often for sterilising or drug manufacture; mitigating the contamination risk with increased levels of steam purity.

How dry is ‘dry’ steam?

At the start we made reference to the high energy content of steam. Entrained moisture in the steam will reduce this overall energy content, the result being that you need more steam to do the job in hand. Therefore, the aim is to have dry steam without water droplets. Less entrained moisture also minimises the potential for pipe erosion and waterhammer. These will help to increase system longevity and safety.

Dry steam is desired for virtually all steam applications. Again we can refer to BD EN 285 which states 95% dryness as a minimum. This standard resonates across many applications in a range of industries, and provides an ideal reference for steam quality. 

Four important factors to consider when striving to achieve the desired dryness of steam

• Correct stream trap management
• The use of moisture separators
• Best practice in the boilerhouse
• Correct pipe sizing and system design
  
  

4. What should I do first to improve my steam quality?

An overall assessment of your steam system will help to identify what areas to target first to improve steam quality. 

A steam trap survey is a critical activity to ensure that moisture (in the form of condensate) is removed from the system. This will help you to achieve and maintain good quality steam. The correct operation of the boilerhouse is also fundamental in delivering good steam quality.

Taking a holistic approach will help you build a targeted plan of action and allow prioritisation of what to improve first.

A system audit will help you to identify areas of improvement, including the following important aspects of the boilerhouse:

• The correct control of ‘total dissolved solids’ in the boiler water
•  Accurate boiler control
• Matching boiler plant to site demand
  
  

5. How do I test my steam quality?

Spirax Sarco have a team of trained and certified engineers that are able to carry out not only the steam quality test, but also the auditing and surveying of your entire steam and condensate network. We advise you on areas of the system that could be improved in order to enhance your steam quality AND your process.

 
So, with these 5 questions in mind - Are you looking to discover more about how Spirax may be able to help you achieve better quality steam? Or are you curious about your steam quality score and what this might mean for you? Click the buttons below to find out more.

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