Topic: Steam turbine report

Investigation of losses in a Steam TurbinePower Unit
PWJ   2008,  revised  CD and GP  2011

Introduction

The emphasis in MIET2135 course is on energy quality, particularly as addressed by the Second Law of Thermodynamics.  The course includes consideration of the thermodynamics of steam power plant.

For this experiment, a small, and therefore inefficient, steam turbine has been coupled to an electrical generator so that the turbine can be loaded and its performance measured.   By operating this equipment and measuring and analyzing the performance of the turbine and its associated components, you have the opportunity to learn more about steam power and about thermodynamics.  At the same time, you can develop your experimentation, analysis and report writing skills.

This particular turbine was originally designed to drive an electrical generator to power the lighting system of a steam locomotive.  Don’t be deceived by the simple appearance of the generator, as the steam reaches supersonic velocity and passes through the turbine rotor twice (velocity compounded impulse type).  This velocity compounded configuration enables extraction of more of the kinetic energy than in a simple single turbine stage.

The steam is supplied by an electrically powered boiler and the exhaust steam is condensed in a water cooled condenser.  The test facility and the condenser were all designed by students of this program.

Objectives

1.    To become familiar with the layout of a small steam power plant.

2.    For a loading condition set for you on the turbine, you will measure the operating parameters of the turbine and the associated steam properties

3.    You will then calculate energy flow through the system.  How much energy do we start with, in terms of the rate of steam energy generation by the boiler ?  How much electrical power is delivered by the generator driven by the turbine ?  Where did the majority of the energy go ?

4.    As an exercise in experimental report writing, each pair of students in a lab group is required to submit a report for assessment. (Each individual in the pair must be able, if requested, to explain all aspects of results submitted under their name. Whilst measured data will be the same within a designated lab group, discussion and conclusion sections of the report must be written by each pair of students independently of the other students in the designated lab group.  Any copying that is detected will attract penalties.)

Procedure

The turbine will be started for you.  You will observe several important issues in testing a steam turbine.  They include:
    Recognising that the energy input to the boiler is electrical, the main consideration for the boiler in an emergency is that the electrical power supply is shut down by closing the main power switch.  The boiler is mainly automatic and has good monitoring and automatic shutdown if a normal shutdown is required. The boiler heat input is controlled to maintain a steady saturation pressure/temp. If these controls fail a safety valve will to prevent excessive pressure build up. This vents to the atmosphere outside the building. The water level in the boiler is controlled via control of the feed pump. If the water level is getting too low the heat supply will be stopped.
    Avoiding touching hot surfaces with bare hands. The demonstrator will use protective gloves when he has to operate hot valve handles.
    There is need to keep steam lines free of liquid, as it is desirable that the steam entering the turbine is as dry as possible. Remember that after shutdown the system will be cold and any residual steam will condense to water. Hence the provision of drains and blow-down facilities in order to remove that water.
    The turbine is surrounded by metal guards.  All high speed machinery should be guarded so that people do not get tangled in spinning shafts.
    A governor prevents turbine speeds from exceeding the safe speed limit of 3600rpm. This centrifugal governor senses rotational speed and cuts back steam supply if speed is becoming too high. To provide protection in the event of centrifugal stresses in an overspeed condition causing turbine failure, the turbine wheel is shrouded in a thick metal casing. Recognise how to stop the turbine, if any undesirable operating conditions arise.  The main control is the valve controlling steam admission to the turbine and that should be closed if problems arise.
    The need for a reliable supply of cooling water for the condenser.  The condenser liquefies the exhaust steam. Otherwise large volumes of steam would fill the laboratory.

When the turbine has warmed up, the operating condition will be set by the demonstrator.  When readings have reached steady values, all of the listed parameters are to be measured and recorded.

Wait ten minutes and take all the readings again.  If they differ significantly from the first set, you did not originally reach a stable condition, so take a third set of readings, or more until you are satisfied that the condition is as stable as possible.  When you are satisfied that a good set of readings has been obtained, the turbine should be shut down, with the aid of the demonstrator.

Whilst the readings are taken, other members of the group should prepare a flow diagram showing the main components of the system, the steam and water flows and the location of measuring points.

Other members of the group should carry out provisional calculations of the required parameter values

The Throttling Calorimeter

The steam generated by the boiler will be close to dry saturated steam.  As it flows through the pipes to the turbine, some heat is lost and some of the steam condenses.  Consequently wet steam enters the turbine.
We need to know the state of the steam entering the turbine, but because the steam is wet, its temperature and pressure will not be independent of each other.  Somehow we need to find the degree of wetness (or quality) of the steam.
A well known method of determining quality of steam is to use a throttling calorimeter.
If a sample of the wet steam is expanded through a valve, it becomes superheated.  When it is superheated, temperature and pressure are independent and can be used to determine the enthalpy.
The throttling process is described as constant enthalpy (you can see why by using the steady flow energy equation). Hence you know the enthalpy at the turbine inlet and can use it to find the quality of the steam.

Parameters to be Measured

1st set of readings    2st set of readings    (3rd set of readings)    (4th set of readings)
Temperatures €“ Channel
Turbine inlet oC €“ 2    173    173.2
Turbine outlet oC €“ 3    103.9    104.0
Collected condensate oC €“ 4    65.2    65.5
Calorimeter outoC €“ 5     105.1    105.3
Cooling water inlet oC €“ 6    21.7    21.7
Cooling water outlet oC €“ 7    36.4    36.4
Ambient oC €“ 8    21.1    21.2
Pressures
Calorimeter in €“ kPag (at boiler outlet)    850kPa    860kPa
Calorimeter out €“ kPag    0    0    0    0
Turbine inlet €“ kPag    790kPa    800kPa
Turbine outlet €“ kPag    0    0
Flow rates
Condensate volume €“ mL    1230    1310
Cond. collection time €“ sec    62    60
Cooling water €“ gall/min
(1 gallon = 4.55 litres)    9.8       83%    9.82        82.5%
Speed / Torque
Turbine/alternator €“ rpm    3000    3025
Scale force reading €“ g    480    480
Voltage / current
Voltage output / phase to phase €“ V    22.8    23.2
Current output / phase to phase €“ A    5.59    5.65
Rotor excitation current €“ A    0.549    0.549
Rotor excitation voltage €“ V    7.53    7.61

Note that the lever arm of the torque balance is 250mm.

On this page prepare a flow diagram showing the main components of the system, the steam and water flows and the location of measuring points.
Calculated Results (for average readings)

Calculated value
Condensate mass flow rate €“ kg/s

Turbine inlet specific enthalpy €“ kJ/kg
(by using the throttling calorimeter)
Inlet steam dryness fraction
(from the throttling calorimeter)
Turbine inlet specific entropy €“ kJ/(kg.K)

Enthalpy supply rate at the turbine inlet €“ W

Condensate specific enthalpy €“   kJ/kg

Enthalpy departure rate of the condensate€“ W

Rate of heat removal by condenser cooling water –    W

Enthalpy arrival rate at the condenser inlet €“ W

Turbine outlet specific enthalpy €“ kJ/kg  (deduced from condenser heat balance)

Turbine outlet specific enthalpy €“ kJ/kg  (if superheated)

Isentropic turbine outlet specific enthalpy €“ kJ/kg

Turbine isentropic efficiency –  %
Rate of drop of enthalpy through the turbine if isentropic€“ W     =

Rate of drop of enthalpy through the turbine €“ W     =  OR

Alternator Input Torque €“ Nm
Alternator input mechanical power €“ W
Three phase alternator electrical power output €“ W
= ˆš3 x Vpp x Ip x Cos φ     or = 3 x Vpn x Ip x Cos φ
( for resistive loads Cos φ = 1)
Alternator efficiency €“ %
Isentropic efficiency of combined turbine and alternator €“ %
= alter’r elec power / rate of drop of enthalpy through isentropic turbine
Specific enthalpy of boiler feed water at ambient temperature €“ kJ/kg
Enthalpy arrival rate at the boiler inlet €“ W
assuming feed pump work is negligible

Rate of heat addition to boiler €“ W

Overall efficiency of plant, €“ %
([alternator elec power output]/[boiler heat rate assuming feed pump work is negligible] )
Carnot efficiency (1-TC/TH) €“ % (temps TC and TH in K)
taking the best possible Tc to be the cooling water inlet temperature
Ideal power output if Carnot efficiency was obtained €“ W          =

Content of Laboratory Report

Your report should be professional and show a technically literate reader that you are technically literate.  It may help to focus your thinking if you imagine that you are going to take this report with you to a job interview in order to show the interview panel how good you are at thermodynamics and at report writing.

It should of course make clear what you have done in this experiment and the significance of the experimental results.  You are not asked to sanitize the results in any way.  You are asked to report on your test as clearly and honestly as possible.  If the results are not consistent, do not fudge them.  Instead report any inconsistencies and offer your explanations for them.

The report should be formally presented, with appropriate headings, language and formatting.  The report can include this laboratory guide and the results tables.  In addition the report should contain:

    A Title Page including the date of the experiment, the course name and number and the names and student numbers of the two people writing the report.

    A Summary €“ (aka Abstract) stating in a concise manner the basic objectives of the experiment, the basic procedure and the main findings from the experiment that relate to those objectives

    A schematic sketch of the  steam flow system, showing the relationship of the measuring points to the main system components

    Calculations €“ sample calculations must be provided showing how the calculated results were obtained from the measured results.  Answers should be to an appropriate number of significant figures.

    An enthalpy €“entropy (Mollier) diagram (see Figure A10 in prescribed text) showing
the throttling calorimeter outlet
the inlet to the turbine (and throttling calorimeter)
the outlet from the turbine
the predicted outlet from an isentropic turbine

    Discussion €“ Indicating any problems with the experiment, assumptions made in the analysis, and any other information which would make the report more meaningful to the reader.  Include in your discussion consideration of the following:  If energy is neither created nor destroyed, what has happened to the energy lost in this steam power plant ?  Can we still use that lost energy ?

    Conclusions €“ What are the main results from the experiment in relation to its stated objectives ?   Also, what is the significance of these results and how do the results relate to your expectations ?

Your report is to be submitted at the SAMME reception Building 57 level 3 by 4pm, 14 calendar days after you did the experiment.

Marking Schedule:
Pasted below are the published RMIT marking guidelines that we are using to grade your reports.  The marking guidelines indicate that error free (competent) work deserves around the 60-70% range, and then marks are allocated above or below that according to the overall quality of the work €“ particular insight / synthesis / application or otherwise.

It should be noted that a mark of less than 100% does not necessarily indicate you have done something wrong.  It simply means that there are ways that you could make your submitted work even better.  For example, in a lab such as this, those reports that offer discussion of interesting aspects of the lab (e.g. why we need a calorimeter and how it works, or how the alternator fan affects torque readings) move towards and into the HD range.  Those reports that simply answer the question by presenting numerical solutions and little else, (the do the least amount of work asked for philosophy), tend to be in the 60-70% range.

As you move into your senior years at university you should become acquainted with this marking style as you will find it being used more and more often.  Our intent of course is to make you the most professional and ready-to-work graduate engineers that we can.

Course Grades Available
HD High Distinction 80-100%
Requirements: Exceptionally clear understanding of subject matter and appreciation of issues; well organised, formulated and sustained arguments; well thought out and structured diagrams; relevant literature referenced. Evidence of creative insight and originality in terms of comprehension, application and analysis with at least some synthesis and evaluation.

D Distinction 70-79%
Requirements: Strong grasp of subject matter and appreciation of key issues, perhaps lacking a little on the finer points; clearly developed arguments; relevant and well structured diagrams; appreciation of relevant literature. Evidence of creative and solid work in terms of comprehension, application, analysis and perhaps some synthesis.

C Credit 60-69%
Requirements: Competent understanding of subject matter and appreciation of some of the main issues though possibly with some gaps; clearly developed arguments; relevant diagrams and literature use, perhaps with some gaps; well prepared and presented. Solid evidence of comprehension and application with perhaps some analysis.

P Pass 50-59%
Requirements: Some appreciation of subject matter and issues; work generally lacking in depth and breadth and with gaps. Often work of this grade comprises a simple factual description (i.e. basic comprehension) but little application or analysis. Work of this grade may be poorly prepared and presented. Investment of greater care and thought in organising and structuring work would be required to improve.

N Fail 0-49%
Requirements: Unsatisfactory. Evidence of lack of understanding of subject (minimal or inadequate comprehension and little or no application) and inability to identify issues. Often inadequate in depth and breadth. Sometimes incomplete or irrelevant.