Don't be distracted by anything but the high pressure steam boilers. Their fuel is the only cost element to consider. If you letdown high pressure steam through a valve and not via turbine extraction, you must generate more high pressure steam. Thus using more boiler fuel.
Bobby
Dear Sir, thank you for you comments,
Howerver, following the text is quoted from the above attached by Mr.Breizh:
" STEAM TURBINE VERSUS PRESSURE REDUCING VALVE OPERATION
A question arising frequently in steam systems relates to the benefits and drawbacks associated with passing steam through a pressure reducing valve or a steam turbine to supply a low pressure steam demand. The most appropriate analysis of the economic benefits of operating the steam turbine utilizes an incremental systems approach. The information of primary importance to the analysis is:
• Incremental electric cost
• Incremental fuel cost
• Boiler efficiency
• Steam turbine efficiency (or the properties of steam entering and
exiting the turbine)
• Steam flow rate (or process demand)
The term “incremental electric cost” relates to the rate structure or tariff applied to electrical purchases at a facility. In particular, the actual
economic impact of any change in electrical consumption is the incremental cost. Many times the price of electricity is dependent on the amount of electricity consumed, the rate of electrical consumption, as well as the time of use. Most electrical tariffs for industrial sites carry fixed charges, which do not change with respect to electrical consumption. A change in electrical demand will typically not incur the “average” electric cost for a facility but the “incremental” electric cost.
To compare the operation of a steam turbine to a pressure-reducing valve, an example is investigated. The example focuses on a boiler producing high-pressure steam, which is operating in support of a site that demands low-pressure steam and shaft power (or electricity).
The investigation considers a facility capable of operating under two different scenarios.
In the first operating scenario, the system receives fuel to produce high-pressure steam. The high-pressure steam passes through a
pressure-reducing valve to supply the site’s low-pressure steam demand. Electricity is purchased to meet the site electrical power demand. In this scenario, both fuel and electricity are purchased to support the activitiesof the site.
In the second scenario, fuel is also consumed to produce high pressure steam; this high-pressure steam is passed through a steam turbine to produce shaft power. The turbine is connected to an electric power generator, which supplies a portion of the site’s electrical demand. Low-pressure steam is exhausted from the turbine and is utilized in site operations. In this scenario, fuel is the only purchased utility for the site.
The example system consists of a boiler, a low-pressure steam demand and a steam pressure reduction component (pressure reducing valve or turbine). The steam turbine drives an electric generator. Figure 1 is a simple schematic of the system.
The analysis procedure first determines the cost of fuel supplied to the facility when the pressure-reducing valve is in operation. Electricity
purchased during this operating mode is not considered until the amount of electricity produced through the turbine-generator set is determined in the second scenario. The second analysis scenario determines the cost of fuel supplied to the boiler when the turbine is operating and allows the electricity produced in the generator to reduce the total site electrical consumption.
One primary factor is held constant in the analysis; the thermal energy supplied to the low-pressure steam demand. This is not to say the steam flow rate supplied to the steam demand is equal in both cases. In fact, the steam mass flow rate will change because the turbine converts some of the steam’s thermal energy into shaft energy. Therefore, the steam exiting the turbine will have a reduced energy content when compared to the steam exiting the pressure reducing valve. As a result, when the turbine is operating, the mass flow rate of steam must increase to supply the same thermal energy to the steam demand. This additional steam flow is provided by the boiler, which requires additional fuel.
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Many times the electric power savings ($112,580/yr) is reported as the potential savings associated with operating the steam turbine because this is the avoided electrical purchase. However, recall the fact that fuel consumption increased to maintain the thermal energy supply to the site.
This resulted in an increased fuel consumption of $43,050/yr. Therefore, the actual savings potential is $69,530/yr or approximately $70,000/yr."
In my case, we use a steam turbine-compressor instead of the turbine-generator. And right now my turbine is operating with 110% designed load and over, so any effort to take steam extracted from it without any letdown valve's support, it means if we try to keep the letdown valve closed, the turbine would be overburdended to play 2 roles: to drive the compressor with a certain power and to supply extraction steam for lower steam system.
Thank you, hope to see more comments from Sir and all of you experts,
Regards,
Nguyen
Edited by zavtranguyen, 09 October 2022 - 09:23 AM.
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