The Unloaded Cost of Producing Steam
The unloaded cost of steam production considers the fuel cost, the steam enthalpy (BTU/lb), the boiler feedwater enthalpy and boiler efficiency. The boiler efficiency considerations are provided in ASME PTC 4.1, which is available free in public domain sources, such as Google.
The equation to calculate the unloaded steam cost is:
Sc = Unloaded Steam cost
Fc = Fuel cost
Es = Steam enthalpy (BTU/lb)
Eb = Boiler feed water enthalpy (BTU/lb)
Ƞ = Boiler efficiency is 80% (ASME PTC 4.1)
Example:
Calculate the unloaded steam cost where the fuel cost (averaged over typically 1 year) is US $9.50/million BTUs, the steam enthalpy is 1190 BTU/lb, the boiler feed water enthalpy is 208 BTU/lb.
Sc = US $9.50 (1190 – 208) / 1000 x 0.80 = US $11.67 per 1,000 lb or US $25.65 per 1,000 kg
The Loaded Cost of Producing Steam
The loaded cost is the preferred method of plant accountability and involves serious management determination. Loaded costs include those of electrical power, chemical, water use and waste/sewage disposal, as well as Governmental emission payments, management, operational, and new projects. It can be shown by most companies that the loaded cost can be 50 % to 100% higher than the unloaded costs of steam production.
By way of example we shall assume a 75% loaded cost over the unloaded cost. Now the true cost of raising steam would be US $11.67 x 1.75 = US $20.42 per 1000 lb or US $44.88 per 1000kg
How Accurate, Long Term, Low Cost tekVorx can Help
If we take a typical 4” (100mm) pipe line at 10 bar (145 psig) with a flow range typically 1150 – 28,900 lb/h and a normal flow rate of, say, 20000 lb/h. With a loaded cost of US $20.42 per 1000 lb, the cost at normal flow rates would be US $20.42 (20,000 /1000) = US $408.4 per hour = $3,529,000 per year.
One of the most common flow meters for measuring steam flow is the sharp edged orifice plate, across which the differential pressure (dp) is measured and the square root law of flow rate to dp is extracted. This has a typical range of not more than 4 : 1, and an average system accuracy based over 1 year of ± 3% of full scale. This error accounts for the erosion of the sharp edge due to abrasive steam flow. Since the accuracy is based on “full scale,” at normal flow the actual accuracy would more like ± 3.5% of reading. In loaded cost terms, this would represent a flow reading ambiguity of ± 3.5/100 x $3,529,000 = ±US $ 124,000 per year.
The tekVorx vortex flow sensor manufactured by tekflo sensors has a performance insensitive to the most abrasive of steam. Any erosion caused on its internal bluff body is negligible . The typical accuracy under site conditions is ± 1% of reading over a 25 : 1 turndown and is certified traceable to the USA National Institute of Standards and Technology (NIST) and other international standards. It is custom calibrated to our customers required flow range, making it a No Dispute sensor.
Using the example above the ambiguity of reading would be reduced to US $35,300 per year.
With the ambiguity of loaded cost being reduced by 3.5 times, the tekVorx provides plant management with significant scope to REDUCE loaded costs of steam raising, with a “No Dispute” NIST traceable calibration over a typical 25 : 1 flow turndown to their credit.
For comprehensive details on the tekVorx vortex flow sensor please see the attached data sheet. Request a quote through our exclusive Authorized Distributor, or through tekflo sensors. You will be pleasantly surprised at how tekflo sensors has achieved such high Singapore quality at probably the lowest price available.
For questions relating to the above and other flow applications, please e mail:
Dr Robert Batey
robert.batey@tekflosensors.com
Director of Engineering and Marketing