Topics Feb 11

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Issue 13 - 2011 $5.95 ALSO IN THIS ISSUE:

Steam Purity Page 2Steam Quality Page 3

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Steam is Essential to Modern Technology Page 5Steam Audits – Sustainability Page 6

VHT vent heads Page 7

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Steam PuritSteam Purity

A water treatment perspective

Steam Purityvs. Steam Quality

“Steam Purity” refers to the amount of solid, liquid, or vaporous contamination in the steam. High-purity steam contains very little contamination. Normally, steam purity is reported

as the total solids content expressed as ppb (parts per billion).

“Steam Quality” refers to the amount of moisture in the steam, usually expressed as the percentage weight of dry steam in the mixture of steam and water droplets.

Let's review “Steam Purity” as it relates to chemical water treatment.

Ensuring adequate Steam Purity is important in 2 key areas:

1. Monitoring carryover of any solid, liquid, or vaporous contaminant that leaves a boiler steam drum with the steam. Both ASTM and ASME provide standard methods for sampling and testing for steam purity, but most steam purity specifications are expressed as TDS or conductivity in steam. ASME or ABMA can be consulted for max impurity in steam guidelines depending the boiler type, pressure and intended use of steam. Entrained boiler water is the most common cause of steam contamination. The entrained boiler water contains dissolved solids and can also contain suspended solids. With higher operating pressures, higher superheat temperatures and the need for pure steam in certain processes; greater emphasis is

placed on controlling the factors that minimize carryover. The chemical causes of carryover are as follows:

Foaming. Caused by excesses in alkalinity and solids content.

Organic Contamination. Oil and other organic contaminants in boiler water can react with alkaline boiler water to produce a type of soap. This will lower water surface tension and cause carryover. Measuring organic contamination can be difficult, but the use of boiler antifoams are often an effective counter measure.

2. Ensuring good operation of superheated steam turbines: Solids in the steam leaving a boiler can deposit in the super heater and turbines, causing costly damage. For this reason, close control of steam purity is critical in these systems. One of the most well documented contaminants for causing deposits on turbines is silica. Under the right conditions silica will become volatile in the boiler and pass through with the steam and deposit on turbine blades causing serious balancing problems. Volatile carryover of silica can occur at pressures as low as 25 bar. However, silica vaporization is not usually a problem below 60 bar. For this reason it is imperative your chemical treatment supplier routinely checks for silica in the boiler water, and is aware of the steam purity you require for your steam application.

Ensure you contact your Spirax water treatment representative if you have questions about your steam purity.

Howard Davis Aus / NZ Water Treatment Manager

QLD Seminars:TWO DAY INSTRUCTION COURSE & SEMINAR – STEAM AND WATER TREATMENT

BrisbaneThursday 31st March – Friday 1st AprilGladstoneWednesday 4th – Thursday 5th MayNorth NSW Wednesday 8th – Thursday 9th JuneBrisbaneThursday 14th – Friday 15th JulyBrisbaneThursday 3rd – Friday 4th November

VIC Seminars:Location Vic Office:

Advanced Seminar (covered over two day period) Wednesday March 30th & Thursday March 31st

Basic Steam Training (one day) Wednesday May 11th

Advanced Seminar (covered over two day period) Tuesday August 16th & Wednesday August 17th.

Basic Steam Training (one day training) Wednesday November 16th

3 Country Sessions: Latrobe Valley Region General Steam Training Wednesday April 13th

Albury/Wodonga Region General Steam Training Wednesday July 20th

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Instant KnowledgeSteam PuritSteam Purity

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Soon after steam first came into use, it became clear that it could often be contaminated with water – and that too much water would have damaging effects on steam engines and, when used for heating, seriously downgrade heat output. So dry steam came to be referred to as “good quality steam” and wet steam was of course “poor quality”. And as steam engineering became better understood, methods of determining how much wetness there was in steam were developed, as were means of ensuring the generation and supply of dry steam.

So originally, a Steam Quality Test only determined the proportion of water contained in a given mass (weight) of steam. For example, if 10 kg of steam actually contained 1 kg of water (and therefore only 9 kg of steam), its “dryness fraction” is 0.9, or 90%.

Since those early days, we’ve learnt that other liquids, solids, gases and vapours can contaminate steam, and steam purity is the technically correct term when considering them.

However, over the years Steam Quality Testing has come to include many of the contaminants that come under the banner of steam purity, plus other parameters such as steam pressure and pressure drop, steam temperature, etc. In other words, a Steam Quality Test today covers anything about the generation and supply of steam that could adversely affect the process for which it is being used.

In this issue of Topics, the pull-out technical article discusses all aspects of steam quality today, and in a separate article Howard Davis discusses steam purity from a water treatment perspective.

CONTACT SPIRAX SARCO ON 1300 SPIRAX (774 729) TO LEARN MORE!

Steam quality or purity?

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Welcome to spiraxsarco.com/au

Searching on the SxS website is a completely different experience...

What our website can do for you

It allows you to easily navigate through the vast amount of data that is available. It is user friendly (steam experts like computers to be simple!) and regularly updated.

Spirax Sarco website has all the tools, tutorials and downloads for the consultant, plant engineer, fitter or visiting student. From valve sizing calculators and installation guides to in-depth steam distribution tutorials – it’s all easy to find.

Sometimes too much information is a good thing!

The focus of this article is the two valuable areas of spiraxsarco.com/au:

'Industries & Applications and 'Resources'. Both cover a great deal of content to satisfy a visitor.

'Industries & Applications'

This section highlights our expertise in offering solutions to customers from a wide variety of industries. In particular, Pharmaceutical, Oil & Petrochemical and Food & Beverage industries are highlighted to showcase the products and services we can offer, and case studies illustrate how we’ve solved seemingly insurmountable problems for customers with a tailored solution.

For example, Spirax Sarco recently saved a Petrochemical plant over $7 million in energy conservation by way

of a plant-wide system audit and on-site training and support of customer personnel.

Of course, these are just examples of some of the industries where we can apply our expertise – ‘example of typical solutions’ show that Spirax Sarco can offer customers from any industry the expertise they need.

'Resources'

In its long history, Spirax Sarco has amassed an unmatched level of knowledge of the steam engineering industry, and the resources section offers this knowledge to website visitors for free!

The ‘Steam Engineering Tutorials’ explain the principles of steam engineering and heat transfer, discussing virtually all major applications and products – perfect for those new to steam or the seasoned expert who wishes to brush up on steam engineering knowledge.

Spirax Sarco also provides a wealth of extremely useful tools for use, for example, our extensive 2D and 3D CAD libraries provide models of our products for creation of plant designs and schematics, and our online calculators help steam practitioners and designers select the right product for the job.

And for people who want to know everything there is to know about Steam, ‘The Steam and Condensate Loop’ book offers a best practice guide to energy saving solutions for everyone working within the industry. Seen by many as a ‘bible on steam’, the book can be ordered online.

Coming Soon

We are constantly working on ways to improve the Spirax Sarco website and in the coming months we will be redeveloping sections to make the site easier and more user-friendly.

Stay tuned!

spiraxsarco.com/au

Website Vision

To be the steam users’ first online choice for engineering and product information.

By Snezana Novakovic, Marketing Assistant Australia and Scott Dean, Digital Communications Executive Group Marketing

‘STEAM QUALITY’ 2011 SPIRAX SARCO

Originally, steam quality referred only to how much moisture there was in the steam, ie how dry it was. But these days, particularly in relation to steam quality testing, it has come to encompass anything that can degrade the maximum possible heating potential of pure dry steam. So when assessing steam quality today, the following factors should be considered:

• The steam system must be able to deliver the maximum flowrate of steam required at the point of use.• The steam system must be able to supply the total quantity of steam required by the process. • Steam must be supplied at the required pressure and temperature.• It must be free of air and other incondensable gases.• It must not be wet. The generally accepted minimum dryness figure is 97%.• It must be clean and uncontaminated, ie effectively pure.

The following sections discuss the above, and related issues, in detail.

Correct quantity and flowrate of steam

All steam users will consume steam at a certain maximum flowrate (kg/h). Pipes, valves, fittings and steam traps must all be sized on this flowrate, otherwise excessive pressure drop will occur, the steam may become wet, the required flowrates will not be delivered, and plant output and performance will suffer.

If the boiler cannot match this total max. required flowrate, excessive pressure drop will occur, and foaming and priming (rise in water level) will occur in the boiler. Carry-over of boiler water will then occur, resulting in wet, dirty, contaminated steam. This is because the water in the boiler will of necessity be high in pH (alkaline) and TDS (total dissolved solids), this quality being maintained by the water treatment and boiler blowdown regimes.

If the boiler can match the total max. required flowrate, the water should cleanly boil off into virtually pure steam, which when condensed, will have a neutral pH of about 7.0 and a TDS close to zero.

Conventionally, the boiler is sized for this total max. required flowrate, but this will often result in a boiler that is oversized for much, and even most, of the time. Often, such sizing will necessitate two or more (oversized) boilers which will always be more difficult to control and operate satisfactorily than a single boiler. An oversized boiler will obviously be needlessly expensive and will operate with a reduced overall efficiency.

If all steam users operate together all the time at their max. rate of consumption, the boiler will indeed need to be sized on the total kg/h figure, but it is more common for the operation of individual steam users to be staggered throughout the day or hour, and their max. rates may only last for a short time, being followed by longer periods of much lower flowrates. This is perhaps best understood by way of an example:

Example 2.4.1 – Boiler sizing based on max. flow rate (kg/h)Consider two hospital sterilizers, each with a maximum steam consumption rate of 200 kg/h. If both may be started together, the boiler must be sized to supply at least 400 kg/h. If their operation is always staggered, such that the max. rate on each is never coincidental, the boiler need only be sized on the 200 kg/h rate.

However, what is often overlooked is the steam consumption when the plant is not drawing steam at the max. rate. Sterilizers commonly have a duty cycle of about one hour, during which the max. rate prevails for about 5 minutes, so for 55 minutes in every hour, steam usage is only at the base rate, which for sterilizers is about 10 kg/h. This means that the boiler will be grossly oversized for most of the time, in this example (assuming one boiler and one sterilizer) supplying only 5% of its rated output for 92% of the time.

An oversized boiler is obviously more expensive to purchase, and its efficiency when suppling the small load will drop significantly, making it more expensive to operate. So whenever there’s a big difference between max. and min. flowrates, and especially when the min. flowrate can prevail for a significant period of time, the boiler should instead be sized on the total steam consumption figure and used with an accumulator.

The expected or typical steam usage rates of all steam users should be closely analysed and plotted, if necessary down to the flowrate required every minute or second, to determine the actual total max. flowrate (kg/h) at any moment in time, and to determine the total quantity of steam (kg, not kg/h) consumed in every hour. The total steam consumption figure in an hour may be different in successive hours, and if so, the max. figure is the one to use. Again, this is best explained by way of an example:

Case for Steam Steam Quality


Example 2.4.2– Boiler sizing based on total steam consumption (kg)Assume that there are 2 sterilizers identical to the one described in Example 2.4.1, and that they are both operated together, at exactly the same time, every hour. The max. required flowrate is then 400 kg/h for 5 minutes, which works out to 33.3 kg of steam consumed (over 5 minutes).

For the other 55 minutes of the hour, steam is being used at the rate of 20 kg/h, which works out to a steam consumption of 18.3 kg (over 55 minutes). And as the sterilizers operate on a repeating hourly cycle, the steam use in every hour will be the same, and we do not need to consider different hourly cycles. So the total steam consumption in any hour is 33.3 + 18.3 = 51.6 kg, and the max flowrate is 400 kg/h.

In practice, allowances must be made for all other steam users (heating of the boiler feed tank is often forgotten), and heat losses from the steam distribution system, but for simplicity in this example, they will be ignored here.

The boiler is thus required to supply 51.6, say 52 kg of steam every hour, ie its output rating must be at least 52 kg/h, at whatever pressure is required. A readily available standard boiler is unlikely to have a rating of exactly this figure, so the next one up, with an output of say 60 kg/h, would be chosen.

Steam accumulatorsThe last example showed the selection of a 60 kg/h boiler for an application with a max. steam demand rate of 400 kg/h. Clearly this boiler cannot supply steam at the rate of 400 kg/h. This is where a steam accumulator comes in, and this too is best explained by way of an example:

Example 2.4.3 – Sizing a steam accumulatorUsing the figures from the above examples, although the selected 60 kg/h boiler cannot supply steam at the max required flowrate of 400 kg/h (which only lasts for 5 minutes), it has more than enough capacity to supply the base rate of 20 kg/h, ie 60 – 20 = 40 kg/h of excess steam for 55 minutes. A steam accumulator is sized to store this excess steam and make it available for the next instance of the max. 400 kg/h demand rate.

First, we need to work out how much steam (in kg) needs to be stored. The max. flowrate is 400 kg/h for 5 minutes, ie 33.3 kg of steam. The boiler can supply steam at the rate of 60 kg/h, so in these 5 minutes it will supply 5 kg of steam. The shortfall is

33.3 – 5 = 28.3 kg, ie, the accumulator needs to be able to store 28.3 kg of steam (which would usually be rounded up to 29 or 30 kg).

Second, we need to check that after this steam has been discharged from the accumulator, the boiler will have the capacity to re-charge this amount in the time available. After the max. flowrate of 400 kg/h has passed, the flowrate drops to 20 kg/h. The boiler has an output of 60 kg/h, so it has a spare capacity of 60 – 20 = 40 kg/h, for 55 minutes before the max. flowrate of 400 kg/h next occurs. 40 kg/h for 55 minutes = 36.6 kg. As we only need to store 30 kg max. in the accumulator (see above), the selected boiler has more than enough capacity to

re-charge this accumulator. If the boiler did not have enough capacity to re-charge the accumulator in the time available, the quantity of stored steam would need to be increased until it did.

Steam is not stored in the accumulator as steam because you’d need a ridiculously large and totally impractical vessel. Instead, it is stored as condensate water, at the same pressure (and temperature) as the boiler pressure. Water has a volume several hundred times smaller than the steam it boils into, so the size of the accumulator with water in it becomes eminently practical and economic. In the above example, it would be no bigger than the boiler associated with it, and less expensive than another boiler.

The final sizing of the accumulator is based on the pressure drop that the water in it will be subjected to when the max. flow rate occurs. This is how the accumulator supplies the max. flow rate – the (slight) pressure drop that automatically then occurs causes the water to flash off, ie boil, into steam at the slightly lower pressure. How much pressure drop must be allowed for, and how this affects the overall sizing of the accumulator, is covered in separate documentation on accumulator design and sizing.


Fig. 2.4.1 Example of a steam accumulator application



Other reasons for considering a steam accumulator A boiler + accumulator installation, rather than a boiler(s) only, should always be used when the steam demand rate (kg/h) peaks suddenly and then drops away. This is especially relevant when the peak is significantly higher than the base load, occurs suddenly, and lasts for only seconds or a minute or two (eg, pre-vacuum hospital sterilizers). This is because after a period when the steam demand rate is low, it is likely that the boiler’s burner (or heating element) will have turned off, and once off, it cannot be instantaneously turned back on to max output.

This is especially applicable to gas burners, because when they get the signal to turn on, they must first go through a purge cycle when air (usually cold ambient air) is blown through the combustion chamber to ensure that any unburnt gas (which could cause an explosion when ignited) is cleared out. It can be up to a minute or more before this purge cycle has completed and the gas fire is fully on and generating steam at the max rate.

Until the burner or heating element is fully on and generating steam at the max. rate, the instantaneous peak steam load is being met by pressure drop and flashing water in the boiler (often aggravated by the cold purge air which will be cooling the internal heating surfaces in the boiler, causing further steam pressure drop). Pressure drop and flashing water in the boiler results in carryover, ie carryover of boiler water with the steam. This means poor quality wet steam contaminated with all the dissolved and undissolved solids in the boiler water – see later sections.

An accumulator installation usually involves passing all the steam from the boiler through the accumulator, ie all the steam and any wetness it is carrying is injected into the mass of water in the accumulator where it either condenses or bubbles through as steam to the users. Consequently, any wetness, together with any contamination in that wetness, is fully absorbed into the water in the accumulator. The steam leaving the accumulator is thus guaranteed to be virtually 100% dry, regardless of any carryover from the boiler, or other wetness it came in with.

Virtually 100% dry steam leaving the accumulator can be guaranteed for another two reasons. As a consequence of the sizing principles involved in designing an accumulator, the water surface within it will always be many times larger than that in a boiler with the same output. This means that the steam is being released form a much larger area and the velocity with which it leaves the surface is much lower, so low in fact that regardless of flow rate and pressure drop, the velocity will never be enough to carry-over any water droplets. This is further guaranteed because the water in the accumulator will be condensate, ie almost pure water, with a pH close to neutral and a TDS close to zero. Such water boils cleanly without any chance of foaming, unlike the water in the boiler which will have a high pH and TDS and is always prone to foaming, priming and carryover.

In summary, it may be said that an accumulator can always guarantee the supply of dry, clean uncontaminated virtually pure steam regardless of varying and peak flowrates and any boiler-water carryover (and in practice, even in the best controlled systems, carryover will at times occur).

Correct steam pressure and temperature Steam should reach the point of use at the required pressure and provide the desired temperature for each application, or performance will be impaired. This requires the correct selection and sizing of the boiler (and accumulator if used), pressure and other control valves, and pipework and pipeline ancillaries. This will be based on the boiler working pressure, the pressure required at the user point, and the required max. flowrate of steam.

Undersizing will result in pressure loss and reduced steam flow rates, and often wet steam which may also be contaminated from boiler water carryover.

It is usually assumed that the pressure of the steam at the point of use will determine its temperature (its “saturation temperature” according to Steam Tables), but this will not be the case if the steam is mixed with air – see next section. It will also not be the case if the steam is superheated, which can occur as a result of significant pressure reduction, not only through Pressure Reducing Valves, but also through any other valve or orifice through which a notable pressure drop occurs. This can be predicted if other parameters of steam quality are known and it can be easily checked with a thermometer. Wet steam and the presence of incondensable gases can also give the impression that the steam is not as hot as it should be, as discussed in later sections.



AirAir is always present within the steam supply pipes and equipment at every start-up. When the system is shut-down, the residual steam within it condenses and the pressure decays to below atmospheric pressure, and then of course air is sucked in. It’s always surprising how valves, joints and seals which are steam-tight when under pressure, so easily allow air to get drawn back into the system when there’s even just a slight negative pressure inside.

When steam is again turned on, it will force the air ahead of it, through all the pipework and plant, towards drain points and the remotest points of the system. Consequently, steam traps with sufficient air venting capacities must be fitted to the drain points, and automatic air vents must be fitted to all remote points (usually the end of steam mains).

However, if there is any turbulence the steam and air will mix and a steam-air mixture will be carried to the steam users. With a mixture of air and steam, the effective temperature will be lower than that of pure steam. This is because the total pressure of a mixture of gases is made up of the sum of the partial pressures of the gases in the mixture. This is expressed by “Dalton’s Law of Partial Pressures”, where the “partial pressure” is the pressure exerted by each component gas if it occupied the same volume as the whole mixture. This is more easily understood by studying the relevant equation:

Equation 2.4.1 Effective steam pressure (bar a) = indicated pressure (bar a) x proportion of steam in mixture (by volume)Note – absolute pressure units must be used in this calculation.

Example 2.4.4Consider a steam/air mixture made up of ¾ steam and ¼ air by volume. The indicated pressure is 4 bar a. Determine the temperature of the mixture:effective steam pressure = 4 bar a x ¾ = 3 bar a

The temperature of this mixture will therefore be that of saturated steam with a pressure of 3 bar a, and not that of steam at 4 bar a. Steam Tables tell us that 3 bar a has a temperature of 133°C instead of the expected 144°C.

This phenomena is not only of importance in heat exchange applications (where a lower temperature will decrease the heat transfer rate), but also in process applications where a certain temperature may be required to achieve a chemical or physical change in a product. Large autoclaves and retorts which are filled with product and then filled with steam, must provide adequate air venting otherwise the temperature being achieved inside will be lower than that suggested by the pressure gauge. Successful processing of the product will either not be achieved, or the processing time will need to be extended. This is especially important in sterilizing applications, where a temperature of just 0.1°C below the statutory requirements is considered unacceptable.

In heat exchange applications, an air-steam mixture has another drawback – as the steam flows to and condenses on the heat transfer surface, it carries with it the air mixed with it. There, the steam condenses and the condensate will freely drain away, and instead gradually accumulates as a thin film on the heat transfer surface. Air is a very good insulator, so this film of air acts as a significant barrier to heat flow, and the heat output of the plant will be reduced.

If the air is not properly vented from the system, it can dissolve in the condensate, especially in drain lines from plant, in steam traps, and in the condensate return system. The gases that comprise air can make the condensate corrosive, so this is another reason for proper and effective air venting, and is also why the discharge from automatic air vents should be direct to atmosphere or waste, and not into the condensate return system.

So long as proper and effective air venting is done at start-up, and then steam pressure is adequately and consistently maintained within the system after start-up, air should not be a problem again until the system is next shut down. But individual pieces of steam plant are often shut down for periods of time, when air will again be drawn in, and temperature controlled plant can experience what is called “stall”, ie, the control valve throttles the steam supply to such an extent that the steam pressure it is endeavouring to maintain is less than atmospheric, and air will again be drawn in. In such cases, extra attention must be paid to proper air venting.

Figure 2.4.2 An automatic Air Vent fitted at a remote point


Incondensable gases Incondensable gases (also known as non-condensable gases, or NCGs) can enter the steam system via the boiler feedwater. Make-up water is usually natural water which will always have air dissolved in it, and any other water, eg condensate, when exposed to the atmosphere will readily absorb air. When the water is heated in the boiler, these gases are released with the steam and carried throughout the distribution system.

Atmospheric air consists of 78% nitrogen, 21% oxygen and 0.03% carbon dioxide, by volume, and much lesser amounts of some other gases. The solubility of oxygen is roughly twice that of nitrogen, whilst carbon dioxide has a solubility roughly 30 times

greater than oxygen. This means that the gases dissolved in the boiler feedwater will contain a lot of oxygen, and a much larger proportion of carbon dioxide than suggested by its proportion in air – and both oxygen and carbon dioxide are major causes of corrosion in the boiler and steam and condensate system.

The solubility of these dissolved gases decreases with increasing temperature, approaching zero at 100°C (at atmospheric pressure). This is why the water in boiler feedtanks should be properly and thoroughly heated to as high a temperature as possible, to drive of most if not all of the dissolved gases before the water is fed into the boiler. A temperature of no less than 80-85°C is required, and up to about 95°C is recommended when possible. The tank needs to be adequately vented so that the gases can freely escape back into the atmosphere.

De-aerators allow the water to heated to 100°C or more, and at these temperatures NCGs will be virtually eliminated.

The concentration of dissolved carbon dioxide may also be reduced by demineralising and degassing the make-up water at the external water treatment stage.

It should be noted that when dissolved gases are released within the boiler, they will also be heated to the same temperature as the steam, so unlike air (which will be at ambient temperature), they will have the same temperature as the steam, and automatic air vents will not be able to detect them and expel them from the steam system (automatic air vents operate at a temperature slightly lower than steam temperature). However, unlike steam they do not condense and contain no enthalpy of evaporation (latent heat). Their heating potential is therefore effectively nil compared to condensing steam and if they accumulate anywhere, they will have an insulating effect on the transfer of heat from the steam into the process. It is virtually impossible to eliminate NCGs from flowing and condensing steam, so the only way to control them is to eliminate them from the water before it goes into the boiler.

It should also be noted that some waters contain dissolved solids which can degrade on heating within the boiler to release NCGs, and that some water treatment chemicals and regimes can have the same effect. These should clearly be avoided or dealt with appropriately.

Cleanliness of steamLayers of scale, dirt and/or corrosion deposits found on heat transfer surfaces, inside pipes, and especially inside steam traps, valves and other fittings, will be due to corrosion (especially in older systems), carbonate deposits due to hard water (all water supplied to boilers should be softened), and most usually, carryover of boiler water (boiler water will be very high in pH and TDS). These will often accumulate as powdery or fine gritty deposits on all inside surfaces, and especially in, and immediately downstream of, openings where significant pressure drop or flow throttling occurs, especially through and around steam trap and other small orifices such as pilot valves. They can also form as larger gritty lumps which get carried along with the steam or condensate flow, together with other types of dirt and debris which may be left in,

or can get into, the steam and condensate system, such as swarf, filings, welding slag and badly applied or excess jointing material. Such fragments can also have the effect of increasing the rate of erosion in pipe bends and the orifices of steam traps and valves etc, and they may also clog or block them.

For this reason it is good engineering practice to fit a pipeline strainer (as shown in Figure 2.4.4). This should be installed upstream of every steam trap, flowmeter, pressure reducing valve and control valve.


Figure 2.4.3 Solubility of oxygen in water

Fig. 2.4.4 A pipeline strainer



Steam and water flow freely though the large perforated screen in the strainer while dirt and debris larger than the size of the perforations will be arrested. The cover or cap on the basket of the strainer is removable, to allow the screen to be withdrawn and cleaned as necessary. Mesh screens have a much smaller opening size than perforated screens, and 100 mesh screens are generally recommended in steam lines. Note that steam strainers should be fitted “on their sides”, so that the basket does not form a pocket in which condensate will collect (see Figure 2.4.10).

When scale and deposits build up “Steam Filters” have much smaller openings than strainers and are recommended for critical applications, such as when the steam is directly injected into or onto food, drink and pharmaceutical products, and for sterilization applications, for which a pore size of 5 microns is generally recommended. When scale and deposits build up on heat transfer surfaces, they inhibit heat flow, acting as a barrier to heat transfer. The most common root cause of dirt, scale and deposits in steam and condensate systems is carryover of boiler water and this in turn is caused by incorrect control and operation of the boiler and the water treatment regime. And as discussed below, carryover also results in wet steam.

Wet steam Ideally, steam would be 100% dry, ie it would be 100% steam, as a gas or vapour, and contain no free water (as a liquid). In practice, this can only be guaranteed if the steam is deliberately superheated (to above saturation temperature) but for various reasons this is not normally done, mainly because for virtually all applications other than power generation, dry saturated steam will do the job more effectively and efficiently than superheated steam, and because saturated steam is easier and cheaper to generate and supply than superheated steam.

In practice, saturated steam will never be 100% dry. Carryover of boiler water, as shown in Fig 2.4.6, with the steam out to the process, is all too common, and this of course results in wet contaminated steam. And even if the boiler and water treatment regime are all always fully under control, and 100% dry steam is always released from the boiler as soon as the steam passes into the supply pipework, where it is surrounded by ambient air at a much lower temperature, it starts to lose heat, and of course starts to condense. So dry steam starts becoming wet as soon as it leaves the boiler.

The wetness in steam comes in three forms. The condensate that forms around the inside of pipes, valves and fittings, due to heat losses from them, will be in the form of a thin film. This gets dragged along by the

steam (whose velocity can be up to 100 km/h and more), so the further it travels, the thicker it tends to get, but as it gets thicker, it will tend to drain down to the bottom of the pipe, where it forms more of a river of condensate running along the bottom of the pipe, which also gets dragged along by the steam flow which also gets dragged along by the steam flow, as depicted in Figure 2.4.7.

Water carried over from the boiler will usually be in the form of water droplets (a bit like rain). Water droplets can also be sprayed-off from anywhere that condensate accumulates (or puddles) within the steam system, and over which steam flows. This especially applies to sagging pipework, strainers with their baskets underneath them, low points that are undrained, and anywhere where a weir is formed, such as a reduction in pipe size, and the internal porting and body shapes of valves and fittings (notably globe-type valves, which impose much less of a weir effect when their spindles are horizontal rather than the conventional way, vertical). Water droplets have sufficient mass (weight) such that, given the opportunity, they will fall out of suspension (like rain) and get absorbed into the river of condensate running along the bottom of the pipe, and they also get readily absorbed into the film of condensate around the inside of the pipe whenever they hit a change of direction or other obstruction. Hence over distance and time, and so long as no new water droplets are sprayed off, they tend to dissipate and wet steam can naturally become quite dry.

Fig. 2.4.5

Fig 2.4.7 Steam pipe showing film and river of condensate inside getting thicker over distance and time.

Fig. 2.4.6 Wet steam caused by carryover



However, water droplets will continue to get blown along with the steam at even quite low velocities, and the higher the velocity, the more likely it is that they’ll add to erosion and corrosion problems at changes in pipe direction and through valve orifices.

There may also be a mist of water in the steam, ie, water droplets that are so small that they effectively have no mass, and unlike water droplets, will not naturally fall out of suspension. When present, this mist simply gets carried with the steam wherever it goes (much like mist and clouds in the atmosphere). Because this mist has virtually no mass, it’s insignificant to the dryness fraction of the steam (which is based on mass), but if its source is carried-over boiler water, it will be contaminated with the high TDS and pH of boiler water, and may therefore contribute to the deposits etc that form as a result of carryover.

Steam dryness (or wetness) is simply expressed as a fraction or percentage by weight (mass). So for example, if we have 10 kg of 90% dry steam (= 10% wet steam), what we will actually have is 9 kg of the gas steam, and 1 kg of the liquid water. (Dryness may also be expressed as a “dryness fraction”, eg 90% dry is a dryness fraction of 0.9). The presence of water in steam reduces the effective enthalpy of evaporation (latent heat) of the steam by the same amount as the dryness fraction, so when Steam Tables tell us that the enthalpy of evaporation (of dry steam) is say 2048 kJ/kg, we can easily calculate that when this steam is 90% dry, the enthalpy of evaporation will be 2048 x 90% = 1843 kJ/kg. In other words, wet steam has a lower heating effect than dry steam, so more of it will be required to do the same job. Also, the extra water in the steam gets carried to the heat transfer surface with the steam, where it makes the condensate film there thicker, thus slowing down the rate at which heat is transferred from the steam to the process. And if the wetness is contaminated by the high pH and TDS of boiler water carryover, a heat resistant film of scale and/or other deposits is likely to be deposited on the heat transfer surface. And if this steam is directly injected onto or into the product, it will not only receive extra water, but contamination too.

Dry steam Dry steam is achieved by preventing, reducing, or eliminating the wetness.

Carryover wetness is best prevented at source, ie at the boiler and via the water treatment regime, before it gets into the supply pipework.

The inevitable wetness as a result of normal condensation within the pipework is addressed by proper design and installation. In particular this means correctly sized pipework, headers, valves, traps, and fittings, installed with the proper slopes in the direction of flow, with all low points and other places where condensate may accumulate or puddle properly equipped with automatic drains (steam traps sets). And all pipework and headers, and as much else as possible, should be insulated.

The above will eliminate most of the river and film of condensate mentioned earlier, but for the droplets, a Steam Separator will be most effective. It will also eliminate all of the river and film. In the Separator shown in Figure 2.4.8, the steam’s velocity is dramatically reduced when it enters the Separator’s body, because its cross-sectional area is far greater than that of the pipe. This allows the smaller water droplets to freely drop out of suspension, whilst the heavier ones impinge on the baffle plate, lose their greater momentum, and drain down. The river and film lose their contact with the pipe wall when they enter the Separator, so they too drain down to the bottom, where an automatic steam trap releases the accumulating condensate without the loss of any steam.

A Separator should also be considered as permanent insurance against carryover. Even in systems where carryover prevention is believed to be fully under control, evidence of some carryover often shows up, usually as brown cocoa-powder-like deposits in sight glasses, steam traps and other valves and fittings, and downstream of orifices where a notable pressure drop occurs. This provides clear evidence that carryover has definitely occurred, in spite there being no other signs of carryover and no known cause. In other words, there can be no absolute guarantee that carryover will never occur. But if a high efficiency separator, or separators, are fitted, if and when any carryover does occur, a guarantee can be given that the carried-over water will not get past them.

Fig. 2.4.8 A steam separator

Air and incondensable gases vented

Moisture to trap set

Wet steam in

Dry steam out



WaterhammerIf condensate is allowed to accumulate inside steam pipework, it can build up to the extent that it gets suddenly blown down the steam pipe in a large mass, resulting in waterhammer. This is depicted in the Figure 2.4.9, in which a slug of water forms in the pipe.

This slug of water is dense and incompressible, and when travelling at high velocity (remember that velocities in steam pipes can be up to 100 km/h and more), has a considerable amount of kinetic energy. The laws of thermodynamics state that energy cannot be created or destroyed, but simply converted into a different form. When obstructed, perhaps by a bend or tee in the pipe, the kinetic energy of the water is converted into pressure energy and a pressure shock is applied to the obstruction.

Condensate will also collect at low points, as depicted in Figure 2.4.10. At the very least, these will make the steam wet as it

flows at high velocity over them and sprays-off some of the accumulating water. At worst, a mass or slug of condensate can be blown-off and hurled downstream at valves and pipe fittings. Such low points also include sagging mains, perhaps due to inadequate pipe supports or broken pipe hangers, and globe-type and similar valves fitted with their spindles vertical.

The noise and vibration caused by the impact between the slug of water and the obstruction, is known as waterhammer. Waterhammer can significantly reduce the life of pipeline ancillaries. In severe cases the fitting may fracture with an almost explosive effect. The consequence may be the loss of live steam at the fracture, creating a hazardous situation.

Assessment of Steam QualityAny experienced and knowledgeable person, after closely inspecting the steam system and operating conditions, should be able to provide an approximate estimate of the likely quality of the steam supply, but much experience of actual testing at hundreds of sites around Australia over recent years has shown that apparently good installations may be supplying poor quality steam, whilst bad installations may be providing steam of acceptable quality. Consequently the only way to be sure of the actual quality of the steam supply is to conduct a comprehensive steam quality test, such as is detailed in several international Standards.

It is possible to do this yourself, but it will require considerable resources and investment in money and time. Sophisticated tools and equipment are required, eg calibrated instrumentation and datalogging equipment with an accuracy of 1 kPa pressure, 0.1°C and 0.1 gram. Much of this equipment is specific and unique to, or will need to be modified for, steam quality testing, so it will need to be specially made or sourced and purchased. Then much knowledge and skill will need to be acquired in the use of this “steam quality test kit” so that it can be used effectively and with confidence, and especially so that the test results can be properly interpreted for the site in question and recommendations made for attending to any problems identified. It should also be noted that the instructions given in the Standards, and the equipment described, are lacking in some respects and even contain some factual errors, and that these may be repeated in commercially available Steam Quality Test Kits. These can cause inaccurate and unreliable test results which are only likely to be fully appreciated after much trial and error and experience.

In practice, the best way to find out whether or not your steam is of the required quality, is to have it professionally tested by a specialist service provider. Spirax Sarco have been carrying out steam quality testing for several years now in Australia, and have carried out hundreds of steam quality tests, mostly in places where steam quality is highly critical, such as for hospital sterilizers. The testing we do covers all the above mentioned parameters and more, and fully complies with, and exceeds, the steam quality testing requirements detailed in Australian Standard AS1410.

For further information and to book a test, contact your local Spirax Sarco representative or office.

Fig. 2.4.10 Potential sources of wet steam and waterhammer slugs

Steam

Condensate

Condensate

Condensate

Steam

Steam

Incorrect use of a concentric reducer

Incorrect installation of a strainer

Inadequate drainage before a rise

Fig. 2.4.9 Formation of a “solid” slug of water

Steam

Steam

Steam

Condensate

Slug

5

spiraxsarco.coSteam is Essential to Modern TechnologyWhere would we be without it?

spiraxsarco.com/au

The last issue of Topics posed the question: Can you think of anything that you can eat or drink, wear or apply to yourself, sit in or on, or otherwise use or consume that has not used STEAM at some stage of its processing or manufacture? There were some very inventive replies. We particularly liked the one from the guy who suggested his own bottom, which he said he sits on a lot!

Most of the suggestions concerned fresh fruit and vegetables and other natural products such as free range eggs. But such items are effectively excluded from consideration if you read the question carefully. It specifically refers to anything that has been processed or manufactured, ie, it excludes all naturally occurring or growing products and anything that does not need man’s intervention in order to form, grow and survive. This definitely rules out the guy’s bottom!

A few suggestions were arguably valid answers, eg hand-crafted bamboo chairs. But such things could only be

considered if made from wild bamboo cane by primitive villagers using hand-made tools and without electricity and the other benefits of modern society. And even then, it could be argued that such products are not being processed or manufactured in the manner anticipated by the question.

One interesting suggestion was potable drinking water, ie the water supplied to our homes from reservoirs. This water has certainly been processed (unlike a similar suggestion, tank water). However, the processing of this water (filtering, chlorinating, testing, pumping, etc)

uses electricity, and this is the “Get out of jail” card for anything that you do come up with that does not have a direct steam input somewhere in the processing of its raw or component materials and its manufacture into some product. Most of our electricity comes from power stations in which steam drives the generators, and electricity is used pretty well everywhere today, so steam has an indirect input into virtually everything.

The same goes for crude oil. It’s processed using steam into petrol, diesel, refined oils, and many other products, including plastics. And these products are directly and indirectly used in, and often essential to, everything we eat or drink, wear or apply to ourselves, sit in or on, or otherwise use or consume.

…And getting back to the natural products such as fruit and vegies etc… Most of these are farmed, ie they’re not wild anymore. And where would farms and modern agriculture etc be today without electricity and the products of the oil and petrochemical industries?

Steam – where would we be without it?

By Graham Smith Business Development Manager, Steam Quality

6

If you would like more information about Steam System Audits, please contact Spirax Sarco on 1300 SPIRAX (774 729)

Conducted by experienced Spirax Sarco technicians with the latest digital modelling tools, the audit culminates with a detailed and comprehensive report presented to you, typically covering some or all of the following specific areas:

• Inventory of equipment audited

• Description of problems identified

• Photographic evidence (where applicable)

• Site drawings (if specified)

• Planned preventative maintenance schedule (if required)

• Recommendations for improvements and savings

o Energy/emission losses

o Steam generation and distribution

Energy users face tough challenges, from rising fuel prices, to tightening safety and environmental legislation, to shortages of specialist skills. Spirax Sarco can help address your every challenge through our Steam System Audits. Tailored to your individual facility and available budget, an audit can include the complete steam distribution loop, starting with the water treatment plant, right through to process applications and condensate return.

Audit to SustainSteam Sy

Audit to SustainSteam System Audits

o Engineering practices/correct applications

o Health & safety

o Heat recovery/return of condensate

o Water/effluent costs

• Cost of implementing report recommendations

o Equipment

o Installation costs

The audit report will highlight recommended actions in order of priority, together with an objective analysis to help you decide on further actions.

Take the initiative to ensure that your steam loop is working at its most efficient best. Contact us to set up a free consultation about your steam system.

By Trevor Peeling, Product Manager

7

If you would like more information about Steam System Audits, please contact Spirax Sarco on 1300 SPIRAX (774 729)

If you would like more information about VHT vent heads, please contact Spirax Sarco on 1300 SPIRAX (774 729) or email us at: [email protected]

Product Release

VHT vent headsfor safety and environmental protectionSteam SySpirax Sarco VHT vent heads are suitable for vertical open-ended steam vent pipes. The vent head is designed to safely discharge dry steam to atmosphere at low velocity, protecting personnel from injury, buildings from damage and minimising the nuisance of water spray on the surroundings.

VHT vent heads

How the VHT vent head worksSteam flow entering the vent head is directed over an internal disc and forced into a toroidal vortex, causing entrained water droplets to be flung outwards and 'wetting out' onto the internal surface of the vent head.

As these droplets coalesce they are driven towards the internal drain by the ‘downward’ rotation of the toroidal vortex.

Environmental protectionEnvironmental concerns drive the need for innovative products that save valuable resources whilst achieving reductions in carbon and CO2 emission.

The new VHT has been designed with an internal drain feature to recover valuable condensate that would have otherwise been discharged to an external drain, with the possibility of hot condensate exceeding temperature limits for ground drainage pipework.

InstallationFor sizing and selection and range available see the Technical Information sheet for VHT vent heads. All products ordered will be supplied with a copy of our Installation and Maintenance Instructions to ensure the safe installation and operation of this equipment.

Typical applications

Electrical condensate recovery unit

Blowdown vessel

Efficient separation minimises moisture carryover.

Internal drain, no external pipework required.

Recovery of condensate separated from steam.

No moving parts, minimal maintenance.

Stainless steel body for long life. Lightweight for easy installation.

USER BENEFITS

The new VHT vent heads are ideal for:

Blowdown vessels Hot water storage tanks

Condensate pump receivers Exhaust vent condensers

Boiler feedtanks

Also suitable for all other similar venting applications where there is a need to separate water from exhaust steam flow.

APPLICATIONS

If unclaimed please return to:Spirax Sarco Marketing Department PO Box 6308 BLACKTOWN NSW 2148

PRINTPOST

PP255003/04601

POSTAGEPAID

AUSTRALIA

You can now subscribe or unsubscribe to Australian Spirax Sarco Topics on our website at www.spiraxsarco.com/au

We hold seminars and training courses Australia wide.

Call 1300 SPIRAX (774729) and find out when we are holding one in your state!Training Courses

• This great book is prepared by experts from Spirax Sarco, a world leader in steam engineering.

• The ‘Steam and Condensate Loop’ book explains the principles of steam engineering and heat transfer.

The Steam and Condensate Loop Book

'The Tracer'spiraxsarcoAustralian TOPICS

in focus

Issue 13 - 2011 $5.95 ALSO IN THIS ISSUE:

Steam Purity Page 2Steam Quality Page 3

Searching on the SxS website is a completely different experience... Page 4

Steam is Essential to Modern Technology Page 5Steam Audits – Sustainability Page 6

VHT vent heads Page 7

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MAIN PRIZE WINNER: James Mullins, Oakey Abattoir, QLD

SEMINAR WINNER: Kazi Al Mahmood, Victoria Wool Processors Pty Ltd, VIC

Keep entering our competition for your chance to win and remember

ALL WHO ENTER WILL RECEIVE A PROMOTIONAL GIFT FROM SPIRAX SARCO!

Snezana Novakovic Marketing Assistant Spirax Sarco

James Mullins, (Oakey Abattoir, QLD)

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Contacting UsNEW SOUTH WALES14 Forge Street(PO Box 6308 Delivery Centre)BLACKTOWN NSW 2148Ph: 1300 SPIRAX (774 729)Fax: (02) 9831 8519E-mail: [email protected]

VICTORIA4A/9 Jersey Road(PO Box 353)BAYSWATER VIC 3153Ph: 1300 SPIRAX (774 729)Fax: (03) 9720 5224E-mail: [email protected]

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WESTERN AUSTRALIA1/14 Bannick CourtCANNING VALE WA 6155Ph: 1300 SPIRAX (774 729)Fax: (08) 9455 4809E-mail: [email protected]

TASMANIASteam Plus13 Ferguson DriveQUOIBA TAS 7310Ph: 03 6424 6202Fax:03 6424 6214E-mail: [email protected]

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EnterourCompetitionfor

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MAIN PRIZE:XBOX 360 4GB CONSOLE WITH KINECT brings games and entertainment to life in extraordinary new ways—no controller required. Easy to use and instantly fun, Kinect gets everyone off the couch moving, laughing, and cheering!

SECOND PRIZE:SPIRAX SARCO ENGINEERING SEMINARSpirax Sarco offers customised steam engineering seminars. Enter the competition for a chance to broaden your steam knowledge.

Name ...................................Company .....................................

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1. How long ago did your water treatment supplier test your boiler water specifically for both, alkalinity and silica, to minimise the risk of steam contamination?

3 months ago

6 months ago

12 months ago

Never

2. Does your boiler feedtank:

a) Run smoother and quieter than a Swiss watch?

b) Occasionally/constantly ‘shake’ and ‘hammer’?

c) Emit plumes of live and flash steam?

d) b) and c) ?

3. Would you like more information on Steam Quality?

Yes

No

4. Would you like to find out about Spirax Sarco’s latest method of checking steam traps?

Yes

No

TERMS AND CONDITIONS: Enter the competition by answering a few simple questions on an insert placed in our quarterly Topic publication. The competition starts 1st March 2011. Closing Date of the Competition is 2nd May 2011. Date of the draw will be 9th May 2011. Time will be 10.00am. Draw will take place at the Spirax Sarco Head Office at 14 Forge Street, BLACKTOWN NSW 2148. The total prize value is: $1,000.00. Main prize is the XBOX 360 4GB CONSOLE WITH KINECT valued at $449.00. Second Prize is a place in the next Spirax Sarco ‘Steam Engineering’ seminar when held in your state valued at $450.00. Winners’ names will be announced on the Spirax Sarco website on 13th May 2011 and published in the Issue 14 of Topics. All prize winners will be notified by phone call and mail from their local representative who will personally deliver the prize. Website address is: www.spiraxsarco.com/au in the News section. The Promoter is Spirax Sarco with registered office at 14 Forge Street Blacktown (PO Box 6308, Delivery Centre), NSW. ABN 52001 126 601. Redraw will be held on 9th Aug 2011. Redraw will be held at the same address and time as original draw. Prize winners will have their prizes delivered by our Spirax Sarco staff member for that sales territory and all prizes will be delivered before 27th May 2011. ACT residents excluded. NSW: LTPS/10/10821

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