02.01.

02 - 110

Process engineering

Pumps, turbines and compressors

Steam turbines (4)

This lesson examines the operating instructions for steam turbines. These are
general rules, and it is therefore important for you to realise that deviations are
possible, depending on the type and make of turbine, but also on the application
of the equipment. Therefore, the instructions supplied by the manufacturer
should always be consulted.

Modern turbines are fitted with numerous alarm and safety devices. These
devices can never take the place of vigilant staff who know the equipment well;
the devices will only assist the staff.

Keeping the workplace tidy, immediately repairing leaks in steam, air and oil
lines and immediately detecting any changes in turbine noises and pressure-
temperature ratios form the basis of good operational management and can
prevent serious damage.

Contents of the lesson

1 Different systems

2 Starting up the turbine

3 Operational aspects of steam turbine installations

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Lesson
1. Different systems
Before a steam turbine can be started up, it should be ensured that all the
different systems can function properly.

1.1 The steam circuit

- lowest pressure The pipes have to be commissioned starting from the point with the lowest
pressure. The shut-off valves in the steam exhaust pipes are opened first. The
steam passes through a steam filter. The turbine's entire pipe system is then
pressurised up to the start valve - with proper draining. As soon as the system
- steam traps has reached the desired temperature, draining can be taken over by the steam
traps.

The presence of water during pressurisation of the steam pipes can result in very
- damage serious damage to the system. As a result of the high velocities at which the
- hammering of mass of water can start to move through the pipe system, the pipes can start to
pipes hammer and come loose from their brackets. This phenomenon is known as
- water hammer water hammer and should be avoided at all times.
Even small quantities of water that can reach the turbine vanes, carried along by
the steam, can lead to irreparable damage to the vanes. For this reason, it is
- overheating advisable to maintain some degree of overheating of the steam.

1.2 The cooling water circuit

Here too, the general rule applies that operation should start at the point of
lowest pressure and then proceed towards the high-pressure section. First of all,
the valves in the cooling water discharge pipes are opened. The valve in the
cooling water supply line to the oil cooler should be opened slowly after the
turbine has been started up. The oil should be brought to 40 °C as quickly as
- not recommended possible. It is not recommended to keep the cooling water supply closed until the
desired oil temperature is reached. In that case, the temperature changes would
- ∆T too large be achieved with a ∆T that would be too large. This would lead to excessive
thermal stresses resulting in a shorter service life of (components of) the turbine.

1.3 The lubrication oil system

First of all, the oil level in the tank should be checked. If necessary, water should
be drained off initially by means of the drain valve fitted at the lowest point in
the oil tank. The electrically driven auxiliary lube-oil pump(s) must be in
operation before the turbine is started up. They should ensure that each bearing
- provided with oil is well lubricated, which will generally be the case at an oil pressure of approx. 2
bar (gauge). Often the discharge line from the bearings to the drain tank is fitted
withwindows. This enables us to verify the presence of lubricating oil in the
bearings.
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Once the turbine has reached its operating speed, the lube-oil pump driven by
the turbine will deliver sufficient pressure to supply the system with lube-oil.
The auxiliary lube-oil pump(s) can then be taken out of operation and switched
to stand-by.

1.4 The turbine

All drain valves on the turbine should be opened so that the condensate which
forms during heating can be discharged quickly. To open the start valve, the
- reset quick shut-off valve must be reset. The quick shut-off valve forms part of the
turbine protection system and closes automatically in situations that endanger
the turbine. This may, for instance, be the case with excess speed, low lube-oil
pressure or excessive vibration. The turbine can also be tripped manually by
means of this valve.

Question 1
Why is condensate released during heating of the turbine?

2. Starting up the turbine

Once all the systems havecommissioned and function correctly, the turbine can
be started up. In this section we will examine all the steps relating to this
procedure.

2.1 The steam supply when bringing up to speed

Having verified that all the lube-oil pressures and temperatures are as they
- open slowly should be, the turbine's start valve can be opened slowly. The valve position is
adjusted so that the turbine runs at a speed suitable for warming up, referred to
as the warming-up speed.

2.2 Preheating
To prevent warping of the shaft, the turbine must not be preheated while
stationary. Preheating is best done at low speeds.

The time required for start-up and the start-up speed depend partly on the type of
turbine and partly on the oil temperature. If the turbine is started up from a cold
state, the minimum preheating times must be observed. To prevent warping of
- jamming the shaft, deviation from the minimum preheating times is not permitted under
any circumstances. It is necessary, however, when steadily increasing the speed
of the turbine, to pass through the critical speed range rapidly, after which the
speed can be steadily increased further.
- critical speed In the critical speed range, the forces acting on the turbine cause it to be in
range imbalance. Every turbine has a critical speed range. Consequently, a turbine
driving a pump has two critical speed ranges: one for the turbine and one for the
pump.

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2.3 The lubricating oil (lube-oil) system

When starting up the turbine, it should be ensured that the oil pump driven by
the turbine takes over the oil supply from the auxiliary pumps promptly. The
pressure will usually be increased slightly at this point. The turbine must not run
at a high speed until the oil temperature has reached 40 °C because if the oil
temperature is too low, the lubricating oil film may be broken which can lead to
- bearing damage bearing damage.
Once the oil temperature has reached approx. 40 °C, the cooling water supply to
the oil cooler can be adjusted in such a way that this temperature is maintained.

2.4 The steam supply at operating speed

Only after the control valves have beenobserved to tend to close on reaching the
operating speed, can the start valve be fully opened. As a result, the control
valves will close a little more in order to keep the turbine at the desired set
speed. From this moment on, the steam supply to the turbine is therefore
controlled by the control valves.

2.5 Testing the safe guarding

After every period of standstill, the quick shut-off valve should be tested both
manually and by using the automatic overspeed sageguarding . The turbine must
not be fully loaded until these turbine trip tests have been conducted with
satisfactory results. Testing is not started until the turbine is properly heated
through and the oil has reached operating temperature. In addition, the proper
operation of the safefguarding instrumentation should be inspected periodically.

Question 2
Why are these tests conducted only after the turbine has been brought up to
operating temperature?

2.6 Automatic testing

It should be pointed out that at every start-up procedure, the quick shut-off valve
must litterally taken on manual. This way it can be closed immediately if the
overspeed governor is not functioning. The speed is increased to the point where
- release mechanism the automatic cut-out is activated. This will usually be the case at a speed that is
5 to 15 % higher than the rated speed. The occurrence of large stresses in the
turbine is thus prevented.

In the turbine trip test, the speed at which the quick shut-off mechanism is
initiated should be read with care.

Question 3
What is this value used for and for what purpose?

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2.7 Malfunctions
- causes The following causes may result in the quick shut-off mechanism not
functioning properly:
- the spring-loaded pin in the shaft has become stuck as a result of coking.
This can be solved by taking the turbine out of operation and cleaning it;
- although the spring-loaded pin has come loose, it has not touched the catch.
The cause is usually a bent or broken catch;
- the catch has come loose, but the quick shut-off mechanism does not close.
This may happen if the lever fastening is stuck or the spring is no longer at
the correct tension.

Question 4
Why is it a good idea to take the quick shut-off valve litterally on manual when
starting up the turbine?

3. Operational aspects of steam turbine installations

To be able to rely on turbines operating without problems, it is important for the
operator to be thoroughly familiar with the principle of operation of the
installation. Nevertheless, the superviser in charge must know which aspects are
of key importance and which aspects are of minor importance in the operation
and supervision of this part of the installation.

3.1 Recording important process parameters

In order to prevent potential disruptions or even to recognise them at an early
- recording stage, the main process parameters pertaining to the turbine should be read and
recorded regularly, for instance every two hours:
- the pressure and temperature of the live steam;
- the pressure in the turbine casing;
- the pressure in the steam discharge or the back pressure;
- the pressure and the temperature of the lube oil;
- the inlet and outlet temperature of lube oil to and from the bearings;
- monitoring of the axial displacement by means of the axial measuring
device, if one has been fitted.

Question 5
Why must the process variables not only be read, but also recorded
periodically?

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3.2 Pipe system

The (sliding) attachments of the steam pipes must be in good condition. This
will prevent the steam pipes from being subjected to large stresses. Any
attachments that have come loose should be repaired, and it should be ensured
that vibrations cannot propagate to the turbine. The steam traps should be tested
regularly for correct operation, since the narrow passages can become blocked
- blocked with small weld spatter or moulding sand that has come loose. All shut-off
- fittings devices in the steam pipes must be kept in good condition. Fittings that only
move with difficulty or shut-off valves that do not close properly should always
- turbine overhaul be overhauled or replaced as soon as possible.
In cooling-water circuits, fouling will only have a noticeable effect on the
operation after a considerable time. Keeping in mind that to prevent is better
than to cure, during overhaul of the turbine any contamination should be cleaned
from the pipes by thorough flushing. Any deposited sludge or rubble can be
removed by mechanical or chemical cleaning.

3.3 The steam

- turbine protocol The steam conditions as specified in the turbine protocol must be observed
accurately. Higher temperatures put the control devices, turbine casing and
vanes at risk as a result of thermal stress. Lower temperaturescan cause erosion
of the vanes in the low-pressure section of the turbine. In order to facilitate
troubleshooting in the case of malfunctioning of thr turbine, it is advisable to
have the pressure and temperature of the live steam recorded as far as possible
- computer
by recirders or a computer storing all the data.

3.4 The turbine

It can be theoretically shown that the steam pressure measured anywhere in the
turbine is directly proportional to the power.

The turbine pressure curve is therefore a measure of the load, but also of the
internal state of the turbine. It is possible, for instance, to observe a reduction in
steam capacity. This is the result of fouling of the vanes or damage caused by
foreign objects. This can be concluded from the pressure rising downstream of
the first wheels, while the load, steam pressure and steam temperature as well as
the steam pressure downstream of the turbine remaining unchanged.

- causes of A change in turbine behaviour can give an indication of the causes of a

malfunctioning malfunction. The turbine running unsteadilly can, for instance, be caused by:
- a large bearing clearance;
- loose bearing caps;
- damaged vanes;
- rotor imbalance;
- sagging bearings or bearing blocks;
- the installation not being aligned correctly.

In addition, the rotor may have become warped as a result of uneven preheating.

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However, uneven fouling of the vanes may also lead to unsteady turbine
operation. This is noticeable by increased vibration.

3.5 Axial displacement

- adjustment The axial displacement should be checked regularly. The adjustment of the
thrust bearings should be constantly monitored. Wear must not exceed 0.2 mm,
and this amount of wear should only occur over an extended period. If wear
occurs rapidly, or if it amounts to more than 0.2 mm, the turbine must be taken
- touching out of operation, otherwise there is a large risk that the labyrinth seal will touch
other parts, which may damage the labyrinths, top seals and possibly the shaft as
well.

Question 6
Before actual metal-to-metal contact occurs, a labyrinth seal will already be at
serious risk if the clearance falls below a certain minimum. Explain.

3.6 The lubricating oil system

The lube oil system deserves our undivided attention. The bearing temperature
must not exceed 75 °C.
- operating During operation, all the valves in the oil pipes must be kept in the correct
condition operating position.
The condition of the oil should be checked regularly, as oil ages with time.
Maintenance schedules will therefore contain instructions for changing the
lubricating oil at intervals as specified by the manufacturer.
The lubricatingoil must only be topped up with clean oil. Leaks in oil pipes must
always be remedied immediately, as they represent a serious fire hazard.

3.7 Turbine control

The tachometer and the speed controller should be lightly lubricated. Only low
viscosity spindle oil that does not leave resinous deposits should be used to
lubricate the tachometer.

- risk If the turbine runs at constant load for an extended period, there is a risk that the
control mechanism gets stuck in one position. To prevent this, the load is
changed periodically. This tests the operation of the control system. This type of
test is extremely important and should be conducted with care.
The control mechanism may get stuck as a result of using steam which is not
entirely clean or due to formation of dangerous deposits from the oil on the
spindle of the quick shut-off valve.

For this reason it is necessary, circumstances permitting, to close the quick shut-
off valve at regular intervals as far as necessary to actuate the control valves.
Subsequently , the quick shut-off valve must be opened completely again. The
spindle must be kept free of oil.

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Question 7
Why does the position of the control valves change when the quick shut-off valve
is throttled?

- regular The proper operation of the overspeed control, the most important turbine
inspections safeguardingdevice, should also be checked regularly.
The proper operation of this safeguard should be checked every 600 hours of
operation in turbines that do not operate continuously but only at irregular
intervals . These turbines should be inspected at least every three months. The
inspection is carried out by increasing the speed until the safeguard acts.

- testing In the other turbines, the safeguarding device is tested periodically by actuating
thecut-out manually or by disturbing the lube oil circulation. In most cases, the
manufacturer's manual will specify the recommended inspection interval.

Question 8
Why should oil always be filtered before it is put into the tank?

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Summary
In this lesson we examined a number of general operating instructions which
apply to the operation of steam turbines.
We should point out once more, however, that the contents of this lesson should
not be seen as a strict law. The contents of this lesson should be seen rather as a
general explanation of the operating instructions as specified by the engineering
department and / or the manufacturer of the turbine.
We examined the various systems and the way in which they should be put into
operation. We also examined a number of aspects of the operation and we
emphasised a number of issues which require particular attention as part of safe
and efficient operating procedures.

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Test
Exercises
Do not send in your answers for correction

1. When the bearing temperatures become too high, in what way can we
determine whether the cause of this malfunction is a fouled duplex filter or a
fouled suction filter of the lubricating oil pump?

2. Which can be the causes for to the quick shut-off valve not to function
properly?

3. In what way is the proper operation of the overspeed safeguarding device

tested periodically? Does this method pose any risk?

4. What might be the causes of a shorter slowdown time of the turbine after it
has been taken out of operation according to the specified procedure?

5. In what way can we prevent warping of a turbine rotor during preheating,

for instance as a result of differences in temperature.

Answers to the questions in the lesson

1. During turbine start-up, vents and / or drains will still be open. If the turbine
is at ambient temperature, say 20 °C, and the pressure in the turbine is, for
instance, 1 bar, then the temperature of the steam entering the turbine will
drop rapidly in the vicinity of metal surfaces. The temperature will drop to
(about) 100 °C. At this temperature (and at a pressure of 1 bar) the steam
reaches its saturation point. Additional heat removal will cause the steam to
condense. The condensate thus formed must be drained off to avoid damage
to the vanes.

2. If the turbine is at operating temperature, the clearances that occur are

nominal. The lubricating oil will then also have the right viscosity.
Failures which may occur when conducting this sort of test therefore cannot
be caused when these preconditions are met; in this way a number of
potential malfunctions are excluded.

3. The speed at which the cut-out is activated must meet certain criteria. It
must not be too high, with a view to centripetal forces, but neither must it be
too low, with a view to the frequent 'tripping' of the turbine.
The speed at which the turbine trips should in any case be recorded.
The value thusrecorded can be compared to the trip value, and the
safeguarding device can be adjusted if required.

4. With the slightest malfunction observed during the start-up of the turbine,
the person in charge can immediately take action and stop the turbine.

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5. The recorded process data also makes it possible to follow process trends. It
can, for instance, be concluded from a remarkably quick increase in a certain
process value that the turbine will develop problems. This information will
therefore enable preventative action: necessary measures can be taken in
advance.

6. When the clearance between the rotating part and the stationary part
becomes very small, the steam molecules will glide over each other in
layers. As the number of layers involved will be very small, the amount of
heat generated may be considerable. As a result, the various parts will
expand ('thermal expansion') and the clearance will become even smaller. It
is obvious that this cannot continue for any peiod of time; seizure and
enormous damage will be the result.

7. See the figure below. The first valve is the quick shut-off valve (which can
also be operated manually), and the second valve is the control valve.

5539-110-001-P

For stable operation of the turbine, the steam supply to the turbine must
remain constant. The pressure p3 must therefore remain constant. This
pressure is directly dependent on the quantity of steam flowing through the
two valves. When the resistance of the quick shut-off valve is increased
manually, the quantity of steam will be reduced as a result, and with it the
pressure p3. By further opening the control valve,its resistance will decrease.
When the sum of the two resistances remains constant and the supply
pressure p1 remains unchanged, the pressure p3 will also remain unchanged.

8. Contamination in the lubrication oil can lead to the control mechanisms, or,
even worse, the sageguarding systems, becoming stuck. In addition, this
may damage the races.

Answers to the exercises

1. With a duplex filter, there is usually a pressure gauge upstream as well as
downstream of the filter, as well as equipment for the alarm and any trip
circuits. Sometimes a differential pressure gauge is fitted across the filter. If
a specified value is exceeded, this is a signal for the filter to be replaced. The
dirty filter must be cleaned immediately after the filter has been replaced, as
we never know what the nature of the contamination is and how long it will
take for the next filter to become clogged. In addition, the type of
contamination will have to be checked. If any bearing material is found, the
turbine must be stopped and the bearings checked.
It is also possible for the suction filter of the pump to become clogged. This
filter is there to protect the pump. A fouled suction filter is noted by a drop
in pump suction pressure. In the case of a considerable decrease in the pump
flow rate, the discharge pressure of the pump will obviously also decrease.

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2. Potential causes for malfunctioning of a quick shut-off valve are:

- the spring-loaded pin has got stuck as a result of coking;
- a bent or broken pin;
- the lever attachment has got stuck;
- the spring is no longer at the correct tension.

3. The turbine is actually brought into to overspeed. In doing so, we should

keep a close eye on the speed, for if the safeguard does not work, manual
intervention will be necessary to prevent enormous damage.

4. During the preheating of a turbine, it is desirable to slowly turn the turbine

rotor. With somewhat larger turbines, a mechanical device is used for this
purpose. With smaller turbines, turning is done manually.

Problems and assignments

Answer and send in for correction

1. For a given turbine installation, on which factors does the quantity of

cooling water which has to flow through the lube oil cooler depend?

2. Explain why the start valve should only be fully open when the turbine
reaches its operating speed.

3. Give a number cases in which the operator can decide to trip the steam
turbine manually.

4. a. What will be the consequences for a turbine if water droplets are

present in the steam?
b. What causes the formation of these droplets?
c. Where will the consequences of these water droplets be noticeable first?
d. Which measures can be taken to prevent this phenomenon?

5. Which types of draining systems are found in turbines? How might the live
steam be freed from waterdroplets? If necessary, add a sketch (or a copy of
a drawing).

6. What adverse effects does an too highly superheated steam have on the
operation of the installation?
What is the disadvantage if turbine installations are operated at a too lowly
superheated steam?

7. What possible causes are there for the unexpectedlyunsteady rotation of a

turbine?

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