Saving Energy in Steam Systems

Opportunities to lower operating costs for Steam systems using energy efficiency improvements – there are plenty opportunities to improve industrial energy efficiency for steam systems in China, and elsewhere in Asia. And some projects may also qualify to earn extra income from a carbon credit (officially known as CER – Certified Emission Reduction) under the Clean Development Mechanism (CDM).

I see the potential for the wider application  of CDM AM0017, which is the official CDM methodology for calculating the Steam system efficiency improvements by replacing steam traps and returning condensate.

System Systems
Steam is still a marvellous high density medium for transporting heat energy, and an essential part of industrial process needs, however a high pressure fluid, at temperatures up to 500 Deg C needs to be respected.  Twenty years ago I cut my teeth on steam projects in the United Kingdom, a typical hospital project demonstrates the utility of steam, where it is used for autoclaves, sterilising, catering, cleaning, domestic hot water, humidification, and also heating systems.

A steam system, consists of four main elements:

  1. Steam Generation
  2. Steam Distribution
  3. Steam Traps
  4. Condensate Return

energy efficient steam and condensate systems

Energy Audit Opportunities
An energy audit should examine the whole steam system, from generation through to point of use to identify wasted energy, and identify any cost effective improvements. You’ll notice immediately that unlike other piped systems, the steam flow and condensate return have to be handled separately.

Steam Generation
Steam generation means creating steam using fuel typically coal, oil, or gas, although electricity is sometimes used also.  Water is heated from atmospheric pressure to the designed steam pressure for use in the facility.  Operating boilers at maximum efficiently, including monitoring air flow, improved firing controls optimise the use of fuel and can yield good results. Power stations often use coal fired boilers, and naturally have a low thermal efficiency thirty percent is common, so there are opportunities to utilise that wasted heat energy for an local industrial process.  Opportunities for energy savings would include recovering any waste heat energy for example from flue gases, or blowdown to pre-heat the any fresh (raw) water. For large industrial plants it could be possible to use higher pressure steam to drive electricity generating turbine, and use that lower pressure exhaust for process purposes.

Steam Distribution
Steam distribution is the transfer of your steam now under high pressure from the boiler to the point of use with minimising losses, Steam is not mechanically pumped, its movement driven from the inherent pressure difference, high to low pressure.

It is important that the steam distribution system does not reduce or lower the quality (dryness) of the steam because that lowers the heat energy. Unlike other piped systems the steam can travel at high velocity, upto 30m/sec, and the self drainage of the steam pipework is critical to effectively deliver dry steam, and is air vented for start up conditions.

Piping configurations that dip under obstacles such as other services and beams would create a natural low point where condensate will accumulate impacting the steam quality, and provide a source for damage by water hammer. Particular care is required for the configuration of expansion joints to ensure they are self draining.

Opportunities for energy efficiency improvements in the steam distribution system include minimising heat losses, reducing piping routes, where possible design out low points, and economic insulation.

Steam Trapping

Although Steam trapping could be considered as part of Steam Distribution, or Condensate Return, the problems are so common and distinct Steam trapping deserves a separate section.

The steam trap is the gateway between the process outlet and the condensate piping system, very often the traps leak due to internal blockage.  Most steam traps have a small orifice that can easily become blocked by debris and fail in the open position. A failed steam trap wastes energy due to causes increased heat-up time, and lengthened the product cycle times because the potential latent energy in steam passes straight though the process and is lost in to the condensate system.

Condensate

Bad design or maintenance panic (just to get production running again) causes another common problem, the wrong type of steam trap, and facility operators are unaware that the wrong type of trap is wasting energy. In some circumstance, poor management can cause injury to operators.

Traditionally,  condensate steams were fitted with a special type of fitting known as a sight glass so operators had the opportunity to visually check that water, and not steam, was flowing in to the condensate line.

However, the sight glass had many disadvantages.  Over time the “glass” viewing port become obscured and unusable. Also some sight glasses were installed in such a location that the operators couldn’t physically access the sight glass to check it.  To overcome these shortfalls a different type of steam trap monitor was invented to provide remote monitoring of condensate or steam flow, for example, the Spira-Tech manufactured by Spriax Sarco, other companies provide similar systems.  This type of trap monitoring system immediately alerts the facility operator that they have a faulty trap, and importantly its exact location.

Condensate Return
After the steam has been used in the facility process to heat a product, what remains is the Condensate (hot water). It must be noted that still many industrial steam plants don’t have any condensate return system! Why is that a problem? because it millions of litres of hot water are wasted, in additional “cold” raw water needs to be purchased to replace it.

Where uncontaminated condensate can be captured, it can be sent through insulated piping back to to the boiler for reuse. In my experience, next comes the most commonly asked question “What percentage of condensate should be returned to the steam boiler plant?” In a perfect world 100%, yes, all the condensate should be returned to the boiler, since the condensate contains up to 20-30% of the heat energy used to create the steam, returning it to the boiler saves both fuel and raw water.

However, there are no targets written in stone, 100% is an ideal goal but it is simply not practical in the field, any system that returns less than 70%-80% condensate warrants investigation.  It is worth noting that condensate flow varies, during start-up approximately twice the flow rate of normal operating conditions is experienced so condensate handling must account for higher loads at start-up. Opportunities for energy efficiency improvements include increasing the quantity of condensate returned to the boiler, eliminating leakage, and economic thermal insulation for the condensate piping.

Carbon Credits
Energy efficiency improvements are driven by the economic imperative, lower facility operating costs. In addition to the lower costs, saving fuel also reduces the demand for finite fuel resources such as oil and gas.  Another potential income stream from energy efficiency improvement projects in developing countries is provided by the Clean Development Mechanism (CDM).  AM0017 is the CDM methodology for calculating the Steam System efficiency improvements from replacing steam traps and returning condensate. That means the saved energy can be translated into a carbon credit which has a real monetary value, and can be sold on the carbon market.

by John A. Herbert, Consultant

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