Efficient steam production in paper mills

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Courtesy of Thermal Energy International Inc.

The paper and pulp industry is an energy and emission intensive industry. 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 such as heat exchangers, dryers, evaporators etc, 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(s), mechanical trap failure can needlessly elevate fuel bills and emission costs by over 20%.

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% of mechanical steam traps could be malfunctioning in a paper plant at any one time accounting for massive increases in energy costs, and in poorly maintained plants this can be much greater.

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 an astonishing 55% in some 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 standard steam system could be losing 22% of energy from the boiler, 25% from distribution and a further 5% from other processes resulting in only 48% useful energy. If a steam system were working at maximum optimisation, useful energy would be increased to 75%.

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% failing annually will have steam wastage of 8,900 tonnes. If the overall cost of steam for this plant were £20 per tonne, the direct cost of ignoring these leaking steam traps would be £356,000 each year, which is equivalent to well over a million litres of fuel oil. The cost to the environment would be 3,000 tonnes of CO2 dumped unnecessarily 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. Whilst Thermostatic, Thermodynamic and Mechanical are extensively used, the Orifice Venturi 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, the 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, or monitoring 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 (3.7bar) and varying flows from ‘no flow’ to 20 kg/hr. The results proved that venturi steam traps are significantly more efficient over varying loads than all other types of traps with the free floating float trap and the more conventional float trap the least efficient with losses averaging over 1.5 kg/hr.

For example a major paper company in Kent installed 25 venturi orifice steam traps on a series of coater batteries that resulted in £125,000 saving on an investment of just £25,000 in just over 2 months. Other customers in the paper industry have seen an increase in condensate return from just 28% to a staggering 70% which has been continued unchanged to the present day.

Replacing pre-heater and end corrugator rolls with twelve venturi orifice designed steam traps resulted in drop of 11% in fuel usage. Similarly replacing mechanical traps with the venturi orifice design resulted in steam savings of 30%. Overall a paper mill can achieve savings of 1.1tonnes/hour by converting to venturi orifice steam traps, which equates to an increase in sales of £1 million.

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.

Additionally, paper mills can improve the fuel efficiency of boiler operations using technology that converts an Industrial boiler into a condensing boiler. Products like TEI’s FLU-ACE effectively recovers up to 90% of the heat normally lost through the boilers exhaust flue. This is possible because of a unique direct contact (gas/liquid) design and enables optimal recovery of both sensible and latent heat, even in widely varying operating conditions.

Conventional heat recovery technologies require dedicated equipment for each boiler exhaust. However a single FLU-ACEÒ can handle a varying flow of flue gases emitted from multiple boilers.? This results in a lower initial investment, lower operating costs and a higher return on investment. A full payback is achieved, usually in less than two years with continuing savings being maintained throughout the unit’s 20-year plus operating life.

Today in the paper industry, efficiencies are being driven higher and emissions lowered due to innovative technology, which is now being developed, tested and installed.Plant efficiency, fuel use and capital cost are critically related and with higher plant thermal efficiency reductions in annual fuel costs can be made.

Choosing state-of-the-art technology that will provide improved plant efficiency and short-term payback are obviously a win, win scenario and necessary to the continuation of the industry.

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