Optimize Your Steam Plant
The operation and economics of the steam circuit are amongst the least well understood features of sulphur recovery plants. Operators will often add auxiliary boilers to make up for diminishing steam production with declining sulphur rates without stopping to ponder why the steam system was put there in the first place.
At the design stage the exothermic reaction from a sulphur plant can usually generate saturated steam at pressures of 250 to 400 psig. On the other hand, the endothermic portion of the plant (usually the amine reboiler) generally requires steam in the range 40 - 50 psig.
There is therefore good economics to generate high pressure steam at 250 - 400 psig, run it through turbines to generate power and condense the exit (50 psig) steam in amine and (possibly) other reboilers. In the simplest of plants, steam is then recovered and pumped back from 50 psig to steam generation pressure in the water phase.
The complete circuit is best understood using the pressure/enthalpy (p-h) chart as illustrated below.
Condensed water is pumped from 50 psig to 250 psig using boiler feed pumps. This process takes us from point A to point B. Water is then converted to steam using heat generated by the sulphur plant. This process takes us from point B to point C. Theoretically we could superheat the steam (which would take us to the right of point C) however for most sulphur plants this requires unnecessarily complicated controls. Saturated steam at point C is then run through single stage impulse turbines which takes us from point C to point D. At point D the steam exiting the turbine is generally in the range 5 - 9% wet. This means that over 90% of the steam's latent heat at the 50 psig level is still available for release to a process requiring heat. This 50 psig steam is then run through an amine reboiler. This takes the process from point D to the starting point A.
The great advantage of this circuit - when the exothermic and endothermic process requirements are in balance - is that the steam circuit is 100% efficient. That is to say all the enthalpy is recovered in the steam process moving from point C to D and back to A. Although only about 7% of the high pressure steam energy is actually converted to power in the turbines, the remaining 93% is usually consumed in the amine reboiler and other heat loads. So far so good.
The bad news occurs later in the life of the plant when the exothermic reaction from declining sulphur recovery becomes inadequate to provide the heat energy required at the endothermic (reboiler) end of the process. This also means that the steam turbines become short of required steam to generate the power required for whatever services they are connected to.
The mistake often made at this point is to assume that, because this steam circuit was such a good idea in the first place, therefore we must continue to supply the steam turbines with steam - from auxiliary boilers if necessary.
This may be correct logic if the extra steam generated by the auxiliary boiler is actually required at the low pressure end for endothermic services. If, however, this is not true and the operator has to install excess steam condensers to return the boiler feed water to a condensed condition, then the addition of auxiliary boilers is likely a mistake.
To see the logic we refer again to the p-h chart above. If extra steam is generated in auxiliary boilers solely to provide energy for the steam turbines and then this exhaust steam is condensed in excess steam condensers it is seen from the chart that approximately only 7% of the energy put into the auxiliary boilers is usefully converted into real work in the steam turbines. About 93% of it is wastefully rejected in excess steam condensers!
At this point, taking steam turbines out of commission and replacing them with electric motors should be a serious consideration.
Equipment Sound Levels: Truth or Fiction?
When undertaking noise control design the most desirable information is to have manufacturer's equipment sound power levels based on measured test data. As acoustical consultants we often request this data, and may or may not receive it. Upon receiving the data, we then review it try and determine the validity of the data.
We often receive a dBA sound level at a certain distance from the equipment with no octave band data. This information is not very useful for engineering design as noise control measures are frequency dependent. With only an overall dBA number, it is not known in which frequency bands the sound energy is concentrated.
We can also receive sound power levels or sound pressure levels. There is a significant difference between them. If sound power or sound pressure octave band information is provided, we then look to determine if this information was gathered with an actual measurement undertaken according to an acoustical standard, or if there is a description of the test or measurement set-up. If the equipment was tested according to a standard then a copy of the test report should be available. If the measurements are not undertaken to a standard then it is necessary to find out under what conditions the sound level measurements were taken.
When using sound pressure levels for engineering noise control calculations it is necessary to know if the sound pressure levels were taken in a reverberant field, semi-reverberant or free field condition, and how far away from the equipment the measurements were conducted. Calculations can then be made to adjust for the measurement condition. Care must be taken when converting sound pressure to sound power or when undertaking sound propagation calculations. Simple point source calculations can lead to inaccurate results if sound level measurements have been taken close to equipment having large physical dimensions, such as large cooling fans.
Information for fans can be particularly confusing as manufacturers can provide total fan sound power levels, in-duct fan sound power levels, fan sound pressure levels with a correction for a theoretical room and correction for end effect. When obtaining fan information it is again necessary to know how the information was obtained and if the information is total power or ½ power and if an end effect has or has not been applied to the data.
We often receive data that states the sound level for a machine is 85 dBA at 3 feet, because of Occupational Health and Safety noise regulations. Often this data has little bearing on the actual sound power levels or sound pressure levels produced by the piece of equipment. The manufacturer has merely indicated that this level will be met as this may have been indicated in the request for quotation.
When obtaining information from manufacturers of noise control devices such as silencers, it is again important to know how they have obtained their information, whether it is based on test data or theoretical calculations, and if they have applied any acoustical weighting to the numbers. We often receive muffler attenuations that have been A-weighted. When taking engineering noise control calculations, it is important not to apply an additional A-weighting correction or the result will seem very good but is actually wrong and very misleading in the low frequencies.
In summary, it is important to know what sound level information has been provided, how it was determined and the correct method to properly apply the sound levels to your specific situation.
