Hi all
How exactly the exhaust pressure for a steam turbine is selected. I understand that depending on the turbine size, there is a sweet spot at which the increase in efficiency by lowering the exhaust pressure is balanced by other issues related to lowering the pressure such as blade wetting and increase in KE losses. What is a bit confusing is the wide variations in exhaust pressure used in plants. I noticed for large plants usually very low pressures are used i.e. 3-10 kPa (0.03 to 0.1 bar); but data sheets for commercial turbines give a max exhaust pressure of much higher values (25 to 100kPa), which makes me wonder why would you choose higher exhaust pressure?
And is there a rule of thumb to select the optimal exhaust pressure based on turbine size?
Exhaust pressure for steam turbine dont directly depend from turbine size.
You usually select exhaust pressure to provide required turbine power.
Turbine power depend from differential entalpy (Hs):
Hs= Io - Iz
where Io - entalpy of steam before turbine
Iz - entalpy of exhaust steam
(Entalpy is function of tempereture and pressure. )
On practice, you usually have rigid values parameters of steam at entrance in the turbine ( Io), so to provide required turbine power you automatically get entalpy of exhaust steam (Iz) and as a consequence exhaust pressure.
(The equation for turbine power: N=G*Hs*nu
where G-mass flow rate; Hs-differential entalpy; nu -efficiency.)
Turbine size is directly depend from number of stages. Number stages depend of differential entalpy. If you have too higher differential entalpy you can not have one stage turbine - you need more stages if you want good efficiency. But it is not necessary, i can make turbine with high entalpy differential and low efficiency, so that turbine will have less stages.
Thats why,in my opinion, you need select exhaust pressure depending on the required power.
I suggest you:
1. The required turbine power defines the exhaust pressure.
2. After selecting the exhaust pressure, you define the turbine.
3. The size of the turbine itself depends on the number of phases. With multi-phase turbine you provide excellent efficiency.
With respect and good luck
dear amir nashed,
the exhaust pressure is primarily fixed by the cold water temperature (in the condensing large steam turbines, for nuclear or thermoelectric plants, for example). The lower the condensing pressure, the better is the cycle efficiency. According to the power level (fixed the rpm) you choose then the number of LP parallel flows.
Thank you all, appreciate your time and share of expertise
In that case, is the condenser pressure controlled based on the ambient temperature if cooling tower or air cooled condenser is used (or water temperature for direct cooling)? Also is the condenser pressure, hence the exhaust pressure, controlled through a vacuum pump?
dear amir nashed,
the "design" exhaust pressure is fixed assuming a right "mean", "expected", temperature of the cooling medium (water, air ...); as environmental conditions change, the condensation pressure adapts itself (according to the refrigerant mass flow, for example). Anyway, the degree of vacuum is guaranteed by extraction pumps (or ejectors).
The condensing pressure is selected according to the average cooling agent temperature of the site and depends of the type of circuit (open, closed or mixt) and also of type of coolant (water or air). The turbine is designed and built for this average temperature. In winter season, the condensing pressure will decrease, while in summer will increase.
The vacuum is established by the condensing process, while the auxiliary systems (pumps, ejectors) contribute to the vacuum maintain, exhausting un-condensing gasses (air, etc.)
Exhaust pressure of condensing turbine of course is decided by temp of cooling media,lower the cooling air /cooling water,higher will be output.In case of closed circuit cooling water temp will be decided by W B temp and approach.Keeping a lower approach(4-5deg C)will reduce cw inlet temp to Condenser.Similarly for air cooled condenser.
Now W B temp will again vary throughout the year.One should collect the db and wb temp from local observatories and by staistical method establish a judicious des temp and based on that select cond pressure.
Dear Amir!
The pressure after the steam turbine is a parameter determined by the purpose of the turbine and the conditions of its operation.
For the main stationary and ship turbines, there are certain combinations of parameters before and after the turbine.
For auxiliary turbines - similar.
So it is for utilization turbines.
Please clarify your question.
What is the power of your turbine?
What are the parameters of the steam at the inlet?
Which installation uses a turbine?
With best regards,
prof. Mykhaylo Tkach
Hi Amir,
To be very specific on reply to your query how to select optimum exhaust pressure I am limiting to condensing turbine as Exhaust press of Back press turbine is guided by what pressure is required by process/other use or how much power you want to generate.
Having said that B P of cond turbine is dependent on cooling water temp you have to keep a check on Dryness Fraction of exhaust steam which is generally limited to 0.85 otherwise moisture content increases.This causes rotor vibration and erosion.If you find DF is lower than 0.85,you may have to increase the condensing pressure slighly to get desired DF.Large plants go for as low pressure as possible otherwise power out will be reduced drastically, also they start with very high inlet pressure and temperature and operate much on right side of mollier diagram and may not reach 0.85 DF. Smaller industrial turbine operate with much lower inlet press and temp and reaches 0.85 at lower condensing pressure when it is desired to operate turbine at slightly higher exhaust pressure.I hope I could clarify your specific query why there is variation in exhaust pressure.Pl let me know if have any further query.
Well if you design steam turbine using H-S chart , then you can plot the drop in steam pressure . H-S chart provide the process visually , and avoid wet steam if the exhaust is for LP turbine....
a question for Mr Kumana.
his two papers seem really interesting and practical.
with reference to the document "steamTurbineAnalysis-Kumana(IETC2017) 3-27-17", what does the DT parameter (in the Table on page five) represent?
Hi All
Your answers were really helpful, i think i kind of got the idea. Thank you so much all
Amir
It depends on the type of cooling system, cooling water temperature and the concept of chock waves at the outlet of the LPT.
The condensing pressure of steam turbine is selected according to the average cooling agent temperature of the site and depends of the type of circuit (open, closed or mixt), the type of coolant (water or air) and condensing pressure. .
Dear Dr Amir Nashed;
Vapor velocity in LP turbine should remain unchanged to avoid droplet nucleation and damage to blades of last stages. LP turbines MUST be designed carefully to avoid sonic and supersonic velocities because of choking of flow and shock waves which can simply tear the blades to pieces. This is the main criterion which should be taken into account in designing minimum exhaust pressure.
Moreover, regardless of the maximum exhaust pressure which leads the turbo-set to trip, there is an optimum exhaust pressure in which the power plant works in highest efficiency (most of the manufacturers provide a diagram for optimum exhaust pressure based on ambient air temperature and load of turbine).
Any pressure less than the optimum point results in sub-cooling of condensates and consequently more fuel is needed to increase the feed water temperature in boiler.
And on the other hand, higher values of exhaust pressure reduces enthalpy difference of LP turbine which is not desired.
So, any attributes of LP turbine should be designed firstly with regards to vapor velocity (caused by exhaust pressure and inlet temp) and secondly efficiency of the plant.
Respects.
Exhaust pressure of Steam turbine corresponds to operating pressure of Condensor. The insolubility of air and associated gases in steam at very low pressures is a big issue. Which will lead to increase in partial pressure and thus the operating pressure of condensor. This will lead to decreased condensor efficiency.
As i know cooling system has own effects on pressure drop, but you have to consider is he cycle is combine or not! for example weather in iraq reach 55 C, the condensers effeciency was unstable, imagin what will happend to the turbines out let.
Condensing turbines belong only in pure power generating stations which are miles away from the nearest steam user, and never in a steam-using process plant. So then the optimum operating policy is to supply your condenser with the maximum amount your cooling system can deliver, and the let the steam exhaust pressure float to its minimum value. If you try to control the exhaust pressure, you will have unstable operation.
Reply to Dr Invernizzi: The table itself defines what DT is. It is the Temp difference between inlet and outlet steam from the BACK-PRESSURE turbine.
Hi dear
The efficiency of the cycle is strongly influenced by the LP turbine exhaust pressure. The back pressure of the condenser sets the saturation temperature at which the expanded steam rejects its latent heat of vaporisation to the cooling water. Consequently, changes in back pressure affect the temperature of cycle heat rejection. The effect of a change in condenser saturation temperature on the cycle work done is shown on a T-S diagram. Generally, a low exhaust pressure is sought as it improves cycle efficiency.
However, there are several plant constraints affecting the selection design exhaust pressure. Consider the effect of lowering the exhaust pressure on the turbine condition line on the Mollier diagram. Expansion to a lower pressure results in increased wetness, and increased specific volume at the exhaust.
To obtain good thermal efficiencies of geothermal power generation, the turbine exhaust pressure must be as low as can be economically achieved with the available cooling water or air temperature. Attached Fig. shows saturation pressures versus temperature for some common working fluids in geothermal power plants.
Advances in Steam Turbines for Modern Power Plants provides an authoritative review of steam turbine design optimization, analysis and measurement, the development of steam turbine blades, and other critical components, including turbine retrofitting and steam turbines for renewable power plants.
https://doi.org/10.1016/C2014-0-03636-2