Apr-2010

Optimising steam systems: part I

Simple techniques to reduce the cost of ownership of a refinery’s steam distribution system and condensate return using steam traps and separators

Ian Fleming
Spirax Sarco

Article Summary

So far 2010 is proving to be another challenging year for the refining industry. Not only is it currently experiencing spare oil production capacity of over 6 million barrels/day, leading to a fall in refining margins, but the US Environmental Protection Agency (EPA) has confirmed its stance, that greenhouse gases (GHGs) are a threat to health and welfare. This has led to the Mandatory Reporting of Greenhouse Gases Rule, requiring all US oil and petrochemical companies, as with all sectors of the economy, to monitor and report their GHG emissions. In Europe, the EU has committed to cutting its CO2 emissions by 20% by 2020 from 1990 levels.

This article, and part II to be published at a later date, looks at ways of optimising the steam system, to reduce energy costs (lowering GHG emissions), water consumption and boiler chemicals. In addition, ensuring the steam is of the correct quality, quantity and pressure when it arrives at its point of use can improve process performance.

It is estimated that steam generation accounts for approximately 50% of the total energy consumption in a typical refinery, with energy costs accounting for more than 50% of the total operating expenditure.

The US Department of Energy estimates that steam generation, distribution and cogeneration offer the most cost-effective energy efficiencies in the short term, with potential energy savings of more than 12%. Table 1 estimates the typical savings that can be achieved for the steam distribution system and condensate return of a US refinery.

Further savings can be achieved in the powerhouse where steam is generated. However, this article will examine only steam distribution and condensate return.

Before looking at potential improvements and ways of optimising the steam system, it is worth understanding the basic properties and characteristics of steam. These can be outlined in a temperature enthalpy diagram (see Figure 1).

When energy is added to water, the temperature rises until it reaches the point of evaporation (point B in Figure 1), which varies with pressure. The energy required to reach point B is sensible heat (hf). Any additional energy will convert the water to steam at a constant temperature. At point D, all water has been completely converted to steam, which is known as dry saturated steam with a steam quality (dryness fraction) of 100%.

The energy added between points B and D is the enthalpy of evaporation (hfg) and is the energy steam gives out as it condenses back to water. It is the enthalpy of evaporation that is used in refining.

If further energy is added, the steam’s temperature will increase, creating superheated steam (E). Superheated steam is used in a typical powerhouse (at approximately 100 barg and 450°C) as part of the cogeneration or combined heat and power (CHP) system.

For heating purposes, superheated steam offers very little extra energy and, in fact, the steam has to cool to saturated temperature before the enthalpy of evaporation can be released. Therefore, using superheated steam instead of saturated steam at the point of use actually slows down the heating process.

For the process to achieve maximum efficiency, steam needs to arrive at the correct:
• Quality: target dryness fraction of 100%
• Quantity to allow the process to meet demand
• Pressure, which determines saturated steam temperature and specific volume, so affecting thermal transfer.

Steam quality is a measure of dryness fraction. If the dryness fraction is lower than 100% (say, point C in Figure 1), the available energy/kg of steam is less. Steam quality can be improved by ensuring the mains are well insulated and condensate is removed effectively using steam traps and separators.

The quantity of steam required will depend on the process energy requirements. Effective deliver relies on correct sizing of the steam distribution lines and control valves serving the application. This can become an issue when processes are upgraded or additional assets are added, as it increases the steam load beyond the steam mains original specification. This results in increasing velocities within the steam system, causing higher pressure losses through the distribution system. If the steam pressure is lower than the acceptable design pressure, the process is de-rated, as the steam is at a lower saturation temperature, reducing the energy transfer rate.

Several key areas have the greatest effect on reducing energy costs and improving efficiency: steam system insulation, water hammer and steam trapping.

Steam system insulation
Steam mains and ancillary equipment must be effectively insulated, in particular valves, strainers and separators, which have large surface areas. After any maintenance work on the steam system, the insulation must be replaced properly; good insulation reduces heat losses by up to 90%.

To put this into context, 1m of an uninsulated 100mm steam main operating at 10 barg emits approximately 1.0 kW, which is equivalent to wasting nearly 16 tonnes of steam a year. This assumes that the pipe is dry and there is no wind chill. Good insulation reduces these loses to approximately 1.6 tonnes of steam a year.

But even when insulation standards are good, a certain amount of steam condenses out during distribution. This needs to be removed to maintain steam quality and prevent the possibility of water hammer.

Water hammer
As steam begins to condense, condensate forms droplets on the inside of the walls. These are swept along in the steam flow, merging into a film. The condensate then gravitates towards the bottom of the pipe, where the film begins to increase in thickness.