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Steam production is responsible for up to 60% of total energy consumption. As a result, ensuring steam is not unnecessarily lost, and the fuel used to create it not wasted, benefits both production and profitability.
Here, Sam Mawby, Technical Director at Thermal Energy International, takes an in depth look at how some of these losses can be avoided and how effective steam management can further improve processes on site.
Within an unoptimised steam system, just 55% percent of fuel input results in useful heat output. In terms of costs, for every million pounds spent on fuel, £450,000 is lost. Of this wastage, around 18% is classed as stack losses.
This is where valuable heat is lost to atmosphere through the boiler exhaust, stack or flue. It is typically the largest single energy loss in boiler operations, with heat recovery offering the solution.
Whether it’s bolt-on economiser technology, or large open shower and spray towers, heat recovery can be applied to a number of heat applications or ‘sinks’ helping to alleviate fuel requirement, improve batch times or CIP cycles and increase efficiency.
In each case, facilities need to work with suppliers to carefully identify both where the heat is lost as well as how it can be used. This can make the process of improving plant operations through heat recovery much more involved than simply identifying where easily managed, marginal savings or improvements can be made.
Small improvements, big impact
Of the remaining losses in a typical unoptimised system, approximately 5% occur as a result of pipe leakages and standing losses, a further 5% is the result inadequate of pipework insulation, another 5% is caused by condensate loss and flash and a final 2% can be attributed to blowdown and shell losses.
However, accounting for approximately 10% of energy wastage, are steam traps – the second largest contributing factor to losses within a system. While no system is 100% efficient, in the vast majority of cases, this 10% loss is very much preventable.
What makes the majority of steam trapping inherently inefficient?
Due to the large amount of energy released as it condenses to water, steam is the preferred heat transfer medium across a multitude of industries, including the food and beverage and chemical sectors.
As a result, draining this condensate effectively, through well designed stream trapping is vital for the efficient and safe operation of a system.
Yet, steam traps remain a major cause of steam and heat loss across industry. This is because for decades, if not centuries, the methods of steam trapping have remained unchanged.
Traditional mechanical steam traps discharge condensate by opening and closing, losing a little live steam with every cycle. There are various types of mechanical traps including: float, thermodynamic, inverted bucket, and thermostatic.
Most of these rely on a metal to metal seal to avoid steam leakage. As with any mechanism, especially one in such an aggressive environment, moving parts are subject to failure, breakdown, or wear and tear.
For example, floats can become crushed or rupture, buckets dislodged from their hinges, discs can fail to seal and balanced pressure capsules also have weaknesses causing them to fail.
In fact, such failures are so common, and even expected, that the industry generally accepts an average mechanical steam trap failure rate of 5-10% annually.
Depending on the operating mechanism and failure mode, traps can fail either open or closed. When the metal to metal seal is obstructed by debris and dirt, or the mechanism itself is damaged preventing the close action – the trap has failed open. Steam is then lost, wasting valuable fuel and money.
If a system discharges to atmosphere, one of the key ways you may be able to tell if traps have failed open, is a plume of vigorous pressurised steam which discharges following the direction of the pipe. This is caused as the water droplets condense making the clear live steam opaque. Flash steam on the other hand is not pressurised.
It is wispy in appearance and immediately affected by the wind or air flow on exiting the pipe. This makes visual assessment difficult and needs experience. Better methods include a combination of temperature measurements and ultrasonic testing which are usually carried out by maintenance teams.
However, those involved in the process element of operation may be able to notice the audible cues of failed traps such as rattling within the traps, this will be the damaged buckets, floats, or discs moving as steam escapes.
This noise is not to be confused with the sound associated with water hammer, which is often the result of failed closed steam traps. Water hammer occurs when slugs of condensate or liquid accelerate at a high velocity around a system.
These slugs or walls of water create a ‘hammer’ like effect damaging plant equipment and raising serious safety concerns. While improvements in system design and workplace awareness have made it less common, indicators of water hammer range from a gentle ‘pinging’, to an aggressive banging or crashing. Shaking pipes can also be observed, depending on the severity and level of condensate powering around the system.
Impact of failed closed steam traps
In addition to water hammer, failed closed or ‘shut’ traps have other serious implications on production. When steam traps fail closed, condensate collects at low points the system and in the equipment itself.
This sitting condensate can back up and flood heat exchanger tubes causing corrosion and erosion, limiting heat transfer and reducing the overall temperature of the system. Not only does this damage the equipment, it ultimately slows throughput and affects the quality of product.
For example, a common application where temperature is particularly important is rubber curing or tyre manufacture. In this scenario, small variations in the press temperature can affect the end product as can changes in steam pressure in the curing chamber.
Similarly, in the food and beverage or pharmaceutical industries, where steam is directly injected, the presence of wet steam from undrained condensate can ruin a product.
For example, many powdered products, such as instant coffee, go through an agglomeration process to make final product dissolvable. However, if wet steam is used the process will not work as required resulting in a wasted batch.
Failed open process traps are quickly noticeable as processes simply do not get the heat they require. This does however place the onus on process professionals to acknowledge where steam trap failure is at fault and recognise it before it has an adverse effect on production.
Where closed traps cause less noticeable damage is on line drainage and trace heating systems. In these cases, some operators may be inclined to simply isolate failed open line drainage steam traps which, far from solving the problem, is a precursor for potentially dangerous water hammer.
When failure is not an option
Whether they have failed open or close, once steam traps have failed, it is often more cost and time effective for sites to simply replace damaged mechanical steam traps, rather than remove, repair, and re-install them. However, both options cause downtime and have a knock-on effect on production levels.
Having an effective strategy to limit steam trap failure is the only way to reduce steam loss, minimise fuel usage, and improve processes. While water treatment can go some way to decreasing the level of debris and build up in a system, a growing number of businesses are turning to the new generation of steam trap technology in order to permanently solve the issues caused by unreliable mechanical steam traps.
With some simple understanding of a steam trap’s application ‘venturi orifice’ traps, such as the GEM™ Steam Trap, consistently exceed the performance of mechanical traps, with the added benefit of having no moving parts that can seize and fail.
The technology uses an orifice and multi-staged throat design to manage condensate flow rate. The orifice and stage throat are carefully sized to ensure there is always seal of condensate, while allowing the remaining condensate to be discharged continuously through the orifice as it is created.
This sizing process is essential to ensuring effective performance across the process operating window, avoiding condensate back up and maintaining temperature control. As a result, it is best completed in person by a reputable supplier onsite via a full site survey.
Only this way can steam consumption be validated from a variety of sources including HMI systems, secondary side data measurements, process specifications and, where necessary, OEMs.
Once sized and installed onsite commissioning helps ensure venturi traps are operating optimally with thermal imaging used to highlight any traps that may need recalibrating.
How do venturi orifice steam traps work?
As hot condensate flows through the orifice, it moves from high pressure conditions to the lower pressure throat. This sudden drop in pressure causes a known percentage of the condensate to re-evaporate as ‘flash steam’.
Restricting the expansion of this flash steam creates a localised variable back pressure within the throat. This is crucial to accommodating the pressure variability in industrial loads, and can only be achieved through the combination of orifice and multi-staged throat.
This makes venturi orifice traps superior to standard orifice traps – the design of which cannot accommodate variable loads, making them unsuitable for most applications. With no moving parts, these traps cannot fail open, thereby eliminating potential steam loss.
The right solution for you
Within the broad term ‘venturi steam traps’ there are numerous different designs. Often, determining the most suitable design will depend on a variety of factors including, pressure, application and location – even the need for hygienic steam should be considered.
However, regardless of end design, it is important to ensure a trap’s manufacture is consistent with ‘avoiding steam loss wherever possible’. It is therefore advisable to opt for solid body traps, as opposed to insert or nozzle-type designs, which have inherent leak paths and so and increased potential for difficult to diagnose leakage through the trap body.
Can there be a permanent solution to steam trapping?
By design venturi orifice traps cannot fail. This is the reason for their comparatively incredibly long guarantee periods – with many manufacturers offing up to a decade.
However, as with any element of a steam system impurities such as scale, corrosion, welding metals and other solids, present within the pipeline make them susceptible to blockage.
For larger process, which require a larger orifice sized to accommodate flow rates, blockage is not a significant concern. However, for line drainage, trace heating and other small duty applications a small orifice allows the trap to operate effectively.
In these cases, protecting the orifice from blockage and ease of maintenance are important factors. This is because periods of downtime, be it planned or unplanned, impact production regardless of how efficient the trap itself is.
Strainers form the first line of defence; sites will just want to bear in mind that a slightly finer strainer than usual may be required depending on orifice size.
Venturi orifice traps with special features developed to combat the likelihood of blockage, such as our magnetic cap or debris deflector, further help optimise operations. Specialist tools are also available allowing traps to be serviced in line and back in operation within minutes.
It is unrealistic to expect that all steam losses can be eliminated from a system. What’s more, it is typically impossible to identify a single source of inefficiency.
However, for those coming under increasing pressure to improve operations and increase process control, reviewing steam trapping can offer multiple benefits that are all too often over looked alongside the more widely acknowledged payback of improved fuel use and reduced emissions.
