Changing heat exchange management

The importance of an appropriate approach to O&M cannot be over-emphasised
The importance of an appropriate approach to O&M cannot be over-emphasised
Credit: MAN Diesel & Turbo

A strategy equipped to deal with the rigours of operating an on-site heat plant or CHP system has always included boiler and heat exchanger maintenance as a key element of safety. But in today’s turbulent energy markets, how plant managers adapt to changing commercial requirements can have a big impact on both operational efficiency and commercial profitability. David Appleyard reports

Effective operations and maintenance strategies and their implementation are obviously critical to any operation where failure can have catastrophic consequences. Naturally, boiler and heat recovery equipment used in on-site and combined heat and power (CHP) installations fall within this group and the importance of an appropriate approach to O&M cannot be overemphasised. But aside from the obvious and obligatory health and safety considerations, there are also significant economic drivers pushing efficient operations and maintenance strategies to the fore.

It seems pretty obvious that, for example, when heat exchanger tubes are coated in oil, combustion products or other deposits, the heat transfer characteristics change, and not for the better. The system loses efficiency, exhaust temperatures rise and energy is wasted.

By way of illustration, Derry Carr, Technical Manager and Group Gas Manager at Veolia and Chairman of the Combustion Engineering Association (CEA), explains the importance of effective water treatment in boilers to maintain performance and minimise maintenance requirements. He says: ‘It’s down to the quality of the water treatment, the pre-treatment, etc, which has a massive impact on the boiler’s operation.

‘If it starts scaling up you can lose efficiency, if it gets too bad you can get a boiler explosion. One of the biggest causes of boiler failure is water treatment: you get heat spots underneath scale,’ he says,

Deposits also inevitably impede gas flow on the heat exchanger side, increasing pressure in the exhaust stages of the engine.

This is a point noted by Industrial Cleaning Europe’s Klaas van der Meulen, manager at a company offering heat exchanger cleaning using high pressure (500″600 bar) water lances.

Explaining the potential impact, Van der Meulen observes: ‘When you have too much back pressure for the engine it will reduce the power and oil consumption goes up and so the engine reduces more in power and so you have more back pressure and more oil consumption, and in that way the heat exchangers pollute very fast in the end.’

Steel heat exchanger tubes, which typically have a bore of around 18 mm, can see their inner diameter decrease to as narrow as 10 mm with deposits of oil and oil combustion products. ‘In that case heat exchange is almost impossible,’ Van der Meulen says, adding: ‘And then your exhaust temperature goes sky-high.’

He suggests with a typical exhaust temperature of 500à‚°C, a post-exchanger exhaust temperature of 110à‚°C could be expected when the system is running normally. This could double to 200à‚°C where exchanger tubes are heavily contaminated.

Changing operational patterns are a big factor in increasing the challenge of an effective maintenance programme
Changing operational patterns are a big factor in increasing the challenge of an effective maintenance programme
Credit: Veolia

Addressing operational challenges

Inevitably, the operational regime can also have a significant impact on the maintenance requirements for components, including heat exchangers and boilers.

As Van der Meulen notes: ‘When you run an engine for 18″19 hours a day, maybe sometimes 24 hours in a day, there’s almost no oil consumption. An engine needs to go at 95% of its power [for optimum performance] and when it does less, then the oil consumption goes up. When you have many start-stops it also takes lots of oil.’

He points to changing operational patterns as a big factor in increasing the challenge of establishing an effective boiler and heat exchanger maintenance programme. With operators running plants starting in the morning at, say, 7 am, stopping at 11 am and starting at 5 pm, running until 8 pm and so on, ‘then you have three times a day to start and stop, and maybe more,’ says Van der Meulen, adding: ‘That way you use a lot of oil because the engine has cooled down, the turbo will use more oil from a cold start, it’s a problem.’

Increased oil consumption inevitably suggests increased contamination of the heat exchanger components.

Carr also points to management approaches which can help reduce the requirements for maintenance. For instance, he cites a typical example of a hospital with three boilers, two running at maximum and one running in so-called hot reserve mode, ready to start up rapidly if required. He argues that it would be far better to run all three boilers at, say, 75% of maximum output where efficiency is still reasonably high and without any boilers having to transition through startup in the event of failure. ‘I always advocate, don’t have a boiler on standby, have it on, have it operating,’ he says. Confirming Van der Meulen’s point: ‘In actual fact,’ he says, ‘because you’re stopping and starting it every day and it’s cooling down and warming up, the increased maintenance load on doing that is really quite expensive; you might as well leave it running. An added advantage to this approach is that the water chemistry within the boiler remains in a stable condition, further reducing the risk of damage.’

He argues that while profit margins are indeed lower outside peak hours, net savings on fuel costs from a shutdown are substantially lower when factoring in increased maintenance requirements, and suggests it may even be negative, though the evidence for this is currently limited.

Carr concludes: ‘The thermal stresses on an engine/boiler combination stopping and starting rapidly decreases the life of both the engine and boiler or increases the chance of failure ” it’s like a lotto.’

Nonetheless, as Van der Meulen also notes, maintenance is expensive, and given the narrow commercial margins at which such units operate, owners are sometimes tempted to ‘stretch the limits’ on maintenance. He explains that while back (exhaust manifold) pressure of some 40 mbar might be expected to be produced where a newly cleaned heat exchanger is used, this could rise to as much as 100 mbar where exchange tubes are heavily contaminated. A clogged heat exchanger can reduce engine performance to below 50% of rated output, he says, adding: ‘Now you get a phone call: “You have to come now, because the engine don’t want to run anymore.”‘ Indeed, with heat exchangers in such poor condition, the engine management system should automatically shut down the unit as the engine temperature will typically exceed safety limits.

Carr also picks up on the important safety implications of poor boiler plant management decisions. For example, in a combination boiler with heat supplied by both burners and exhaust gases from, say, a 2 MW reciprocating engine, a low water alarm in the boiler could suggest the immediate dumping of all heat sources. While this makes sense for the burners, such a decision can have a dramatic impact on the engine.

Carr explains that there is a period of some minutes between the triggering of a low water alarm and water hitting the top of the boiler tubes when furnace collapse could occur (known as the sinking time). This provides ‘more than enough time to shut down the engine safely’. Emphasising the potential outcomes of a wrong operator decision, he says: ‘There have been incidences where engines have suffered catastrophic failure where they have suddenly stopped, rather than take the load off the engine in a controlled way and then shut down’.

Reducing the load in a safe way before shutting down is now a CEA operational guideline in such a situation. ‘You’re not only looking at just a boiler, you’re also looking at the engine as well, so this management of the whole system is vitally important,’ he notes.

Monitoring is also evidently key to effective operations and maintenance in CHP and on-site applications, but not all plants are equipped to a high standard, as Van der Meulen points out: ‘It’s important that you can see on the monitor what is the exhaust gas temperature and back pressure of the engine, but not every engine is fitted with a sensor for back pressure and a sensor for exhaust temperature behind the heat exchanger.’

However, he adds that retrofitting such a system need not necessarily be particularly costly and may well yield potential operational improvements. ‘It can be very simple,’ he says: ‘It doesn’t have to be electronic, you can have something that you can check visually, so you can see the [post exchanger] exhaust gas temperature is 100à‚°C and maybe half a year later you see it’s at 140à‚°C, and then you know it’s not good and you have to take action.’

OEMs are addressing the challenges of changing regimes
OEMs are addressing the challenges of changing regimes
Credit: MAN Diesel & Turbo

Building in good design

In a competitive market, engine and turbine designers are understandably focused on factors such as cost and efficiency. However, Karim Saidi, Engineer Gas Turbine Development & Construction at MAN Diesel & Turbo SE, outlines the challenges engine designers and manufacturers must overcome in order to satisfy a changing operational profile while maintaining a clean burn through repeat ramping cycles.’

So far, there is no standardised and certified solution available that is sensitive to operation modes [i.e., many start-stop cycles] and is able to automatically decrease deposit formation, says Saidi, adding, ‘The best way to minimise the effects of deposits from the constructional or design side is to increase the surface [area] of heat exchange, which means increasing the steam or hot water production and also the thermal efficiencies (boiler and CHP). This is done by modifications to the tubes and to their configuration or geometry.’

Nonetheless, Saidi also echoes both Carr and Van der Meulen in advocating a strategy of minimal startup and shutdown cycles: ‘From an operational point of view it makes sense to decrease the engine’s or turbine’s starts and stops by keeping the machines operating under low loads. That way the stationary state, once established, is sustained and the deposits are better controlled.’

Good overall design can also play a major role in supporting effective O&M. For instance, Van der Meulen cites examples where heat recovery equipment is mounted high above the roof, hanging from the ceiling or directly behind the engine within a cramped noise compartment, making access, disassembly, cleaning and replacement far more challenging than may be necessary. This point is also suggested by Carr, who argues that good design can effectively mean maintenance requirements can be reduced: ‘If you’ve got something with exceptionally peaky demands and you’re throwing the boiler from very high powers to low powers very quickly, that does have an effect on the boiler. You might think, when you’re building it, just to iron out those things, you might put an accumulator in.’

However, Carr also emphasises the importance of correctly scaling the project to match heat demands. ‘If you’ve nothing to use that heat on, then you may as well take it off the grid. You’ve got to have that energy/heat load balance,’ he says.

Good design can effectively mean that maintenance costs can be reduced
Good design can effectively mean that maintenance costs can be reduced
Credit: Veolia

Designing future plant for future needs

Saidi identifies a number of key avenues of future development by equipment OEMs, including primary energy savings, shorter payback times and, of course, nitrogen oxides (NOx) limits. ‘The objective here is to improve exhaust parameters while increasing electrical efficiency,’ he says. This means that the firing temperature of the combustion systems has to increase, which also increases the efficiency of the heat process.

‘For suppliers the main challenge following this path of optimisation is the need to use more expensive material and a decreased lifecycle. A gas turbine’s usual life cycle is 40,000 hours, [but] over five years it may be reduced to a little less than 30,000 hours.’

Saidi also points to future trends that are likely to add further complexity to effective management, operations and maintenance of on-site power and CHP facilities. These include multi-fuel systems able to operate not just with natural gas or diesel but with other fuels such as ethanol, biofuels and syngases; solar hybrid combined-cycle systems, replacing supplementary firing with a solar thermal field; and fuel cell-driven combined cycles, which Saidi notes ‘are an interesting future option and can be very efficient for small or intermediate capacities. Thermodynamically an electrical efficiency between 70% and 74% is possible.’

He adds that although catalytic converters reduce emissions, they also have some effect on the pressure losses of the complete cogeneration plant and therefore ‘their use implies a little decrease in power output, and also a slight variation to the heat process capacity. The overall thermal efficiency of the cogeneration will also decrease slightly. The use of catalytic converters is still in a phase of development with room for improvement.’

With emerging technologies on the horizon and rapidly changing operational profiles a phenomenon of relatively few years, it seems that OEMs, plant designers, system integrators, owners and operators will all wield significant influence over plant maintenance costs when considering how they design, build and operate their installations, now and in future.

Effective O&M strategy in a nutshell? ‘It’s automation, monitoring and manning,’ says Carr.

David Appleyard is a freelance journalist focusing on energy matters

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