Venturi traps save useful energy in paper mills

Tim Gardner, managing director of GEM, a division of Thermal Energy International, explains efficient steam production in paper mills.

Steam is an essential part of the paper making process with steam boilers, steam distribution systems and other processes accounting for a large percentage of a paper mill's energy costs.

Useful energy is often lost from boilers and process equipment as well as from transporting steam from boilers to different areas of the plant.

Although a mill may well have a cascade steam system on the paper machine, mechanical trap failure can needlessly elevate fuel bills and emission costs by over 20 per cent.

Steam traps have the function of removing condensed steam and non-condensable gases without losing any live steam.

It is anticipated that up to 20 per cent of mechanical steam traps could be malfunctioning in a paper plant at any one time, accounting for massive increases in energy costs.

Where condensate is not returned, the water is lost as well excessive amounts of energy.

The result is a significant economic loss, directly via increased boiler running costs, and potentially indirectly onto production via decreased steam heat capacity.

Steam leakage through traps can account for 55 per cent in high usage processing industries such as paper.

Steam traps need to be working at optimum efficiency with a minimum impact on the environment.

For example, a stand steam system could be losing 22 per cent of energy from the boiler, 25 per cent from distribution and a further 5 per cent from other processes resulting in only 48 per cent useful energy.

If a steam system were working at maximum optimisation, useful energy would be increased to 75 per cent.

Steam traps can have different sized orifices to suit different conditions.

If a trap leaks steam, the amount wasted will depend on the size of the trap and the steam pressure.

The cost of waste will also depend on the number of traps and the operating time.

For example, a paper mill with 200 traps based on an average trap size of DN20 and a stream pressure of 14 bar(g) with 10 per cent failing annually will have steam wastage of 8,900 tonnes.

If the overall cost of steam for this plant were GBP20 per tonne, the direct cost of ignoring these leaking steam traps would be GBP356,000 each year, which is equivalent to more than a million litres of fuel oil.

The cost to the environment would be 3,000 tonnes of CO2 dumped into the atmosphere each year.

Paper mills are looking for ways of reducing overheads and many are cutting maintenance budgets and staff.

The consequence is a spiral of ever-increasing steam loss and escalating fuel bills as maintenance is cut and failed mechanical steam traps remain open blowing live steam.

This has left management with two options: either minimal maintenance and watch the steam plumes rise along with the fuel, water and chemical treatment costs, or regularly test, repair and replace faulty mechanical traps at considerable ongoing cost.

There are numerous steam traps available and selecting the correct type of steam trap is an important element of any steam system.

While thermostatic, thermodynamic and mechanical are extensively used, the fixed orifice condense discharge trap is now becoming the steam trap of choice.

Instead of utilising a valve mechanism to close off steam for maximum energy and water conservation, a venturi orifice design effectively drains condensate from the steam system.

As these steam traps have no moving parts to wedge open or fail, they provide the ultimate in reliability necessitating only minimal maintenance and requiring no spares, testing or monitoring of equipment.

They are available in a range of options for specific applications, manufactured from corrosion resistant stainless steel and are performance guaranteed for 10 years, obviating the need for repair or replacement.

A performance analysis carried out by Queen's University, Belfast, into efficiency of steam traps has shown that, over varying condensate loads and steam pressures, the venturi trap is the most efficient steam trap available.

In the research a variety of steam traps were utilised including buckets, floats, thermostatic and thermodynamic valve arrangements.

Tests were performed on each trap at a constant pressure of 54 psig (3.7 bar) and varying flows from 'no flow' to 20kg/hr.

The results proved that venturi steam traps are significantly more efficient over varying loads than all other types of traps.

Free-floating float trap and the more conventional float trap proved least efficient with losses averaging over 1.5kg/hr.

For example, a major paper company in Kent installed 25 venturi orifice steam traps on a series of coater batteries that resulted in GBP125,000 saving on an investment of just GBP25,000 in just over two months.

GEM's customers in the paper industry have seen an increase in condensate return from just 28 per cent to 70 per cent.

Replacing preheater and end corrugator rolls with 12 venturi orifice designed steam traps resulted in drop of 11 per cent in fuel usage.

Similarly, replacing mechanical traps with the venturi orifice design resulted in steam savings of 30 per cent.

Overall, a paper mill can achieve savings of 1.1 tonnes/hour by converting to venturi orifice steam traps, which equates to an increase in sales of GBP1m.

In many paper mills many of the steam traps are actually steam wasters.

The complacency of ignoring steam traps is costing steam users much more than they realise.

The hard reality of a plant maintaining its boiler and forgetting about the rest of the steam system can be a horribly wasteful proposition.

Losses can include not only wasted energy but also replacement of damaged equipment and misuse of man-hours.

Fortunately, installing low maintenance orifice venturi steam traps can avert much of these potential losses.

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