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In this project entitled “Variable Speed Turbo Coupling for Boiler Feed Pump” we have studied
the basic principles of hydraulic coupling, various types of hydraulic coupling particularly of
Voith make, areas of application of fluid coupling, types of couplings used for motors and their
alignment. We have also analyzed the B.F.P. regulation, i.e. throttle control and speed control
and comparison between them and the energy saving and economic aspect of V.S.T.C. We also
studied the details of Voith make V.S.T.C. installed in 100 MW Unit II of Hirakud Power, its
design, operation, maintenance, trouble shooting features and finally make certain improvement
in the present V.S.T.C.

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Fluid coupling belong to the hydrokinetic group in which the power is transmitted from input to
output shafts by transfer of kinetic energy of a circulating fluid.
In the Simplest form a fluid coupling can be considered as two hollow bowls having radial vanes
The lower bowl can be considered to be connected to a prime mover such as motor, while the
upper bowl is connected to a load such as B.F.P. If the lower bowl is filled with oil and rotated
by the prime mover, then under centrifugal force the liquid is flung outwards and upwards
constrained by the vanes and the bowl periphery. It will pass across the gap between the two
elements and impinge on the vanes of the upper bowl. The forces located by this action appear as
a torque at the output shaft and will drive the load. After circulating through the vanes of the
upper bowl the liquid returns to the lower element and the process will be repeated.
In Fluid Coupling:
? There is no mechanical interconnection between the input and output members.
? The power is transmitted by a ring of oil continuously circulating between these input and
output members.
? If sufficient load is applied to the output shaft to bring it to rest, the fluid coupling
connecting to impose the torque on the output shaft and on the relieving the overload
condition will again smoothly accelerate the driver machine. When a fluid coupling is
running with its output shaft held stationary, it is said to be stalled.
? The torque capacity of a fluid coupling with a full load slip is about 2 to 4% is:
T = 0.09 n2D5
n = Impeller speed (in rpm)
D = Outside diameter (in mm)

The output torque is equal to the input torque over the entire range of input-output speed ratios
Thus, the prime mover can’t be stalled by the application of load and there is no transmission of
shock loads/torsional vibration between the connected shafts.

Hydraulic coupling are used for driving the B.F.P. and serves the following functions:

? Regulation of B.F.P. by speed variation.
? Stepped up of speed from motor speed to B.F.P. design speed.
? Supplying lubricating oil for B.F.P. bearings and motor bearings by operation of ADP /
Shaft driven pump.

Hydraulic Coupling Slip:
The element driven by the prime mover i.e. motor is termed as impeller and the element
connected to the load i.e. B.F.P. is termed as the runner and the circulation of oil between the
impeller and the runner is the working oil circuit. A difference between input and output speed is
essential in a fluid coupling in order to enable it to transmit torque, which is expressed as
percentage of input speed and referred to as slip.
Slip % = (Input RPM?Output RPM)
Input RPM x 100
Fluid couplings are selected so that the slip under normal full speed working condition is usually
between 2 to 4%.
The torque developed by a fluid coupling is related to the mass of oil circulating between
impeller and runner. Therefore, by adjusting the filling of the fluid coupling, a wide range of
torque output and hence output speeds are available. The mass of oil circulation is adjusted by
means of a sliding scoop tube arrangement in hydraulic coupling. The degree of oil filling with
the fluid coupling can be varied by scoop tube operation from 0 to 100% i.e. complete filling or
discharge. This makes the transmitting capacity of the coupling adjustable and permits stepless
(load dependent) speed regulation of B.F.P. operation against the load characteristic curve.

Propeller’s Law of Centrifugal Machine:

P ? N3
where, P = Power, N = Speed
A variable speed control will ensure lower power consumption over running at fixed speed at
throttled control.

Figure 1: Position of Scoop Tube

Figure 2: Schematic Diagram of Motor, Coupling & Pump

The variable speed turbo coupling in one casing contains a Voith turbo coupling, which provides
regulation of output speed in conformity with the varying requirement of the feed pump. The
turbo coupling is incorporated in a housing and the lower section of the housing serves as an oil
Flexible connecting coupling are applied for power transmission from motor to the V.S.T.C. and
from V.S.T.C. to B.F.P., as flexible coupling are designed to connect shafts which are misaligned
either laterally or angularly and to absorb impact due to fluctuations in shaft torque / angular
speed. Torque is transmitted from primary wheel to secondary wheel hydrodynamically by the
working oil.

The primary wheel and secondary wheel combined to form the working space, which
communicates with a circulating scoop space rotating with primary side and surrounded by the
outer shell. The height of the liquid level inside the working space and thus the transmitting
capacity of the coupling is determined by the radial position of a sliding scoop tube. The scoop
tube is operated by the positioned through electro mechanical actuator. The absorption capacity
of scoop tube is considerably higher than the pump delivery and so that minimum reaction time
can be achieved for control and regulating operations.

Hydraulic Coupling Losses:
There are two types of losses in power in hydraulic coupling, hydraulic losses and mechanical
i. Hydraulic Losses:
Since the regulation principle is based on slip regulation, evidently the V.S.T.C., hydraulic losses
occur which heat up the coupling oil and therefore must be dissipated by heat exchanger. The
amount of losses depends on the run of characteristics and slip required to attain the desired o/p

ii. Mechanical Losses:

In V.S.T.C., mechanical losses occur due to friction in the bearing, ventilation losses and losses
in the oil circulating system which usually do not exceed 1% and are therefore of little


1) Hydraulic Coupling Types:

(i) Constant Filling, constant speed coupling (type T):
? Type T
? Type TV and TVV
? Type TVVS
? Type TRS and TVRI
(ii) Variable speed scoop controlled coupling (type S)
(iii) Geared variable speed coupling (type R-K)
(iv) Highly flexible elastomeric coupling.

2) Types & Sizes of Couplings:

? Constant speed coupling 206T -1150 TV
Maximum power = 1.6 MW
? Variable Speed Coupling 3668VTL – 133OSVNLG
Maximum power ? 2.5 MW
? Geared Variable Speed coupling RlSK-2, R16K-1, R17K-2 and R16K550-1
Maximum power > 6 MW

3) Areas of application of fluid couplings:
In the drive system of:
? B.F.P.
? I.D., F.D., P.A. Fans
? Belts and chain conveyors
? Coal Mills, Crushers and Ball Mills Centrifuges
? Bucket elevators
? Other material handling equipment

4) Requirements to be met by the drive units:
? Unloaded motor shaft.
? Utilization of economically priced, low maintenance squirrel-cage, induction motor,
? Smooth build up of starting torque for controlled acceleration of fan/pump
? Uniform acceleration torque throughout start-up.
? Non-stop continuous operation for longer period.
? Easy handling and compact design.
? Low wear and low maintenance cost.
? High availability.

5) Specification of Motor and Pump of 487SVTL22.2 V.S.T.C.:
? Type: 3 phase I.M.
? Make: Kirloskar Electric Co. Ltd., Bangalore
? Frame: MPW450 H.C
? Output: 1.3 MW
? Speed: 2979 rpm
? Voltage: 6.6 KV
? Current: 131 A
? ENCL IP 55 lC81W
? Efficiency: 95%
? Bearing: DE Sleeve
? NDE Sleeve
? Duty: S1
? Connection: Star
? Temperature rise in bearing: 70 °C

Heat Exchanger of Motor:
? Make: Mysore Engg. Enterprises
? Volume of air circulated: 3.67 m3/s
? Volume of Water circulated: 150 lpr
? Heat Load: 42 kW
? Weight: 275 kg

? Pump type and Size: CHTC 4/12
? Make: KSB
? Liquid Handled: Feed water
? N.P.S.H. pump / plant: 7.6/21 m
? Liquid Quantity: PH = 9.0 9.4 n (hvt) : – 81
? Rated temp: 165
? Specific Gravity: 0.9058
? Kinetic Viscosity: 0.183
? Vapour Pressure: 7.15

? Suction Pressure: 8.52 kg/cm2
? Discharge Pressure: 124.5 kg/cm2
? Diff. Pressure: 115.9 kg/cm2
? Rated flow: 265 m3/hr
? Rated speed: 2875 rpm
? Rated power: 1033.6 + 54 kW
? Driving rating: 1300 kW
? Minm flow (thermal): 65 m3/hr
? Minm flow (continuous): 91 m3/hr
? Shut off need (+/5%): 1664 mt
? Driver make: KEC
? Frame Size: MPW45OHC
? Casing: Barrel type
? Casing orientation Size
Suction: V.UP DN150
Discharge: U. UP DN150
? Bearing house: Standard
? Bearings: Pump: Plain
? LOD: Segmental Pad Type.
? Flanges: ASME RF
Rating Hyd. Test Pressure
ASME 816.5 # 300 15 kg/cm2
ASME B 15.5# 1500 250 kg /cm2
? Stuffing box: HWD
? Feet: CF
? Conn: G
? Hydraulic: 8 Hyd – 3 st + 8.13 hyd = 9st
? Balancing Live Back to: Feed water tank
? Impeller Dia.: 328 x 15.8 / 3W x 17 mm
? Driver: Motor + Hydraulic coupling
? Coupling: Flexible gear type with spacer
? Bearing lubrication: Pressurized oil
? Direction of pump rotation as seen from driver’s end: CW

6) Coupling between motor ; V.S.T.C. and between V.S.T.C. ; Pump:
There are normally three types of coupling used for motors:
? Rigid coupling
? Flexible coupling allowing torsional flexibility
? Flexible coupling allowing longitudinal, lateral and angular flexibility.
The type of couplings to be selected depends on:
? Load variation
? Vibration
? Torque pulsation
? Type of bearing used
? Speed

(i) Rigid Coupling:
These are used to make a solid connection between the shafts, shock, effect of misalignment and
vibrations are directly transmitted without any damping. Even with minor angular or parallel
misalignment the coupled shaft will result in high additional loads on the couplings, shafts and
bearings. Rigid coupling can thus be used only for driving and driven shafts are supported on
journal bearings.
Rigid couplings are not permissible for drives with antifriction bearing or drives using different
type of bearings.

(ii) Flexible coupling allowing torsional flexibility:
This coupling permits a limited torsional deflection of the coupling shafts the amount of torsional
deflection increases with torque transmitted e.g. are coupling with rubber bushed studs, steel
springs coupling or coupling with solid rubber transmission parts. When subjected to a torsional
load the connecting elements suffer an elastic deformation and are thus capable of clamping
shock loads. In addition these couplings compensate for minor axial displacement of the shafts.
This coupling system stabilized while the torque is being transmitted, resulting in the
transmission of considerable axial forces e.g., due to thermal expansion of shafts to the bearings
Couplings giving torsional flexibility are used for drives having high shock loads. Driving and
driven shafts should be supported in journal bearing (if end float is limited) or anti friction
bearings. The use of this type of coupling for shafts supported on bearings differing in the type
only practicable for restricted members of applications.

(iii) Flexible coupling allowing longitudinal, lateral and angular flexibility:
This type of coupling gives flexibility in all direction. It compensates for axial movement as well
as radial and angular deflections with certain limits. Due to the use of rigid elements the torque
and vibrations are transmitted directly and without damping. Oil lubricated gear type couplings
are recommended as these couplings transmit higher torque via a large number of gear teeth for
this reason and because of their small size; they are especially suitable for high-speed drives of
high rating. They can be used for all pole motors. 4 pole motors of rating 2000 KW and higher,
all motors requiring a coupling with high flexibility.

Coupling between Pump and V.S.T.C.:
? Application: Between pump and V.S.T.C.
? Rated Power: 1300 kW
? Rated Speed: 2875 kW
? Rated Torque: 4.31 kNm

Coupling Capacity:
? Coupling rating: 2558.75 kW at 2875 rpm
? Continuous torque: 8.49 kNm
? Weight (mk2) = 0.4079 kgm2 (In solid hubs)
? Dynamically balancing to be GR 2.5 JSO 1940

? Maximum axial deflection = ± 0.5 mm
? Maximum angular deflection = ± 1.5° per gear mesh
? Maximum radial misalignment = 4 mm

Coupling between motor and V.S.T.C.:
Motor Side:
? Continuous rating: 1300 kW at 2980 rpm
? Continuous torque: 4.166 kNm

Coupling capacity:
? Coupling rating: 2652 kW at 2980 rpm
? Continuous torque: 8.495 kNm
? Weight: 36.5 kg
? GD2: 0.97 kgm2

? Maximum axial displacement: ± 0.5
? Maximum angular deflection: ± 1.5° per gear
? Maximum radial misalignment: 1.3 mm

Features of Gear Flex Couplings:
? Fewer backlashes.
? Compact assembly.
? Accommodates angular, parallel and axial misalignment
? Generally used up to 120 °C can be used for higher temperature using proper grade of oil
or grease.
? Can be dynamically balance to the required grade as per ISO 1940
Full flexible type LFG: 101 to 119 (19 sizes)
Rating range:
Type LFG: 157 kW t 232 808 kW @ 1500 rpm
Torque range:
Type LFG: 100.0 Nm to 149000 Nm

Bore range:
Type: LFG 20 mm to 710 mm
Angular Misalignment: 1.5° per gear mesh
7) Alignment of directly coupled shafts:
A flexible coupling is recommended for all direct coupled machines and is essential where the
driven machine has no end play to take up the thermal expansion of shafts due to temperature
rise on load. The shaft locating bearing of foot mounted motors is not capable of taking any best
nominal thrust in the axial direction.
Misalignment unfailingly results in damage to the coupling, bearings and if sleeve to the shaft as
well, beyond repairs. Accurate alignment therefore is of the utmost importance and involves
three basic operations.
(i) Axial positioning
(ii) Paralleling
(iii) Centering of shafts

Axial positioning of Shafts:
Ensure that the individual shaft couplings if firmly secured to the corresponding shafts. The
motor should then be so positioned that the gap between the faces of the two halves of the
coupling as specified by its manufacturer is obtained. In case of motor fitted with plain bearing
having a nominal endplay, this gap should be obtained with the motor shaft fully drawn out in
direction of the driven apparatus. This operation requires a further refinement where the shaft of
the driven axial displacement such as by wears of the valve is a balance valve located pump.

Paralleling of Shaft in vertical and horizontal planes :
The condition for paralleling is that if the disconnected half couplings are rotated together the
measured coupling gaps at two diametrically opposite ends are equal or in any case the algebraic
difference between the two are equal in all the angular position of the couplings.

Centering of the shafts ends:
Centering of the shaft ends should ideally be done with the driving and driven machine at their
normal working temperature but if this is impracticable, an allowance must be made when the
centering is done with both machines at the same temperature, any ambient temperature to
account for the differential thermal expansion during normal operation.
Assuming the motor (driving machine) to be hotter of the two machines, an average excess of the
shaft centre height of the motor over that of the driven apparatus of about 0 03% of the normal
shaft centre height may be assumed a rise of 50°C measured on the frame of the motor.


Pump capacity i.e., flow can be regulated either by pump speed variation or by throttling the
discharge valve. There is a definite saving is power if regulation is achieved by speed variation.

Figure 3: Feedwater System Block Diagram Figure 4: Feedwater Flow Diagram

Figure 5: Boiler ; B.F.P. Characteristics

Types of B.F.P. Regulation:
There are two types of regulation of B.F.P.:
i. Throttle Control
ii. Variable Speed Control

Throttle Control:
In throttle control the feed water control valve must be selected for maximum pressure i.e. 160
bar and minimum flow i.e. 30%. The feed water control valve is used with fixed speed B.F.P.
High fluid velocities through the trion, which appear at higher-pressure drop, are a principal
source of severe service control valve problems.
In throttle control, the damaging effect of cavitations, erosion, high noise and pipe vibration are
typical signs that velocities are not being controlled. These problems are eliminated by using
variable speed B.F.P.

Figure 6: Throttle Control of Feedwater Flow

Figure 7: B.F.P. Characteristic- Throttle Control

Variable Speed Control:
In variable speed Control, the working oil fitting can be varied between fully filled and drained
while in operation. In this way stepless speed regulation of the driven machine over a wide range
is achieved when the coupling operates against the load characteristics. This regulating range is
4:1. The working oil circuit is governed by a system, which can continuously extract or supply
the working compartment fluid. This enables precise adjustment of the driven machine speed to
be achieved. The working circuit is charged by continuously running pump, which delivers oil
from the integral sump below the coupling into the working compartment. The working
compartment is the chamber between the primary (impeller) and secondary (runner) wheel,
which is connected to a rotating scoop chamber consisting of an inner and an outer shell. The oil
level in the working compartment determines the speed at the output side of the coupling and
depends upon the radial position of a scoop tube located in the scoop chamber. The flow capacity
of scoop tube for exceeds the pump delivery; thus with respect to the control and regulation
reaction times are at a minimum.

Figure 8: DRUM BOILER CONTROL with Variable Speed B.F.P.

Figure 9: B.F.P. Characteristic- Speed Control


Figure 10: Power Savings due to Speed Regulation

The operating point is the intersection of the Q-H Curve and the system resistance curve
indicated by point 1. If the operating point is to be shifted to point 2 on the system resistance, the
pump speed has to be varied such that it develops a Q-H characteristic which intersects the
system resistance curve at point 2, or in other case the system resistance valve such that the new
system resistance curve intersects the constant speed Q-H characteristics at Point 2.

When the operating point is to be shifted to point 2, the pump generates more head given by
(H21-H2) that the required head H2 which is being throttled in the discharge valve, thus resulting
in some wastage or’ power, or in other words this results in saving power if the pump speed is
varied rather than throttling and this is achieved by using a hydraulic coupling.

Feed water control with variable speed B.F.P.:

The feed water control to the boiler is achieved by two independent control loops,
? Feed water control
? Differential Pressure Control.

The combined action of feed control value (feed regulation station) and differential pressure shall
be such that correct feed flow is maintained to match the turbine load with constant level in the
boiler drum with no hunting at the feed control valve or scoop tube movement of hydraulic

i. Feed Water Control:
This control loop automatically control the feed water flow to the boiler corresponding to turbine
load requirement and fluctuating in the drum level. Steam and feed water flow signals are
compared and resulting signal is compared with the pressure compensated drum level signal. The
final error signal is used to control the feed regulating valve. The pressure compensated drum
level signals acts as overriding signal for feed water control.

ii. Differential Pressure Control:
A preset differential pressure (5 to 6 kg/cm2) across the feed regulating valve at all loads is
maintained by controlling the speed of the B.F.P. The pressure drop measured across the feed
regulating valve is compared against an adjustable preset value (5 to 6 kg/cm2) and the resulting
error signal is used to control the speed of the pump through the scoop tube of the hydraulic
coupling to maintain the differential at the preset value. Rise in differential pressure reduces the
pump speed and any fall in differential pressure increases the pump speed.
The differential pressure transmitter signal is fed to a master control which has a set value
positioning. The master control in turn distributes the error, signals to the respective individual
slave controllers. The slave controller output signal is fed to a pneumatic power cylinder through
a current signal to a “E to P" converter.

Scoop Tube Synchronisation:
The scoop tube of the hydraulic coupling of the standby pump should follow the running pump.
For there is a provision of feedback transmitter, which also gives a signal for remote position
indicator. The o/p signal of the feedback transmitter is fed into the master control/comparator
that compares the scoop tube position of the individual pumps to synchronize the scoop tube
positions of all the B.F.P.s.

TECHNICAL DATA OF V.S.T.C.: (Type: 4875VTL 22.2)

A. Machine Data:
Rotation seen in direction of the power flow : CW
Scoop tube stroke : 127 mm
Power requirement of driven m/c(Pa) : 1150 kW
Motor Speed (Ne) : 2980 rpm
Minm Slip (approx.), S : 3.5%
Maxm o/p Speed (Na max) : 2875 rpm
Regulating range : 4:1(downwards)
Oil Tank filling capacity : 250 ltrs.(approx)
Required oil viscosity : 150 VG46
Weight (without oil) : 800 kgs.

Noise emission:
Measuring surface sound : 85 dB

B. Attaching Parts:
i. Pump Insert:
Pump: 1 filling pump for working oil & lube oil circuit
Type of filling pump: TZP280B
Drive: Jointly by the input shaft by V.S.T.C.
Type of A.O.P.: RTBP 40 of Tushaco pumps

ii. Scoop Tube Actuator:
Type: Electro-Mechanical Actuator
Accessories: Positioner, Position Feedback Transmitter
Model: GF63.2/SAR6 E22 of Auma (India) Ltd.

iii. Heat Exchanger:
Type of cooler: Duplex cooler

Heat Exchanger Shell Chamber Tube Chamber
Working Fluid Oil Water
Medium Oil (ISOVGG6) Water
Pressure Drip 0.74/0.8 0.28/0.3
Nominal Flow, m3/hr 16.8 44.2
Inlet Temp. oC 72.3 36
Outlet Temp. oC 55 40
Design Temp. oC 120 70
Design Pressure, kg/cm2 10 10
Test Pressure, kg/cm2 15 15

iv. Connecting Coupling:
Input: RLFG-104, Semi-flex coupling
Output: LFG-104, Semi-flex coupling

C. Operating Data of V.S.T.C.:


Working Oil Temp. Operating Range 100 oC
Lube Oil Temp. of Oil Cooler Operating Range < 90 oC
Working Temp. downstream of Heat
Operating Range 2.3 bar
Shutdown at < 1.4 bar
Differential Pressure across the Double Oil
Non-Pressure 0.6 bar
Shutdown at > 0.8 bar
Oil Pressure at Test Connection Operating Range as per measuring points

Oil Flow Rates:

Working Oil Flow Rate 280 l/min
Lube Oil Flow Rate to V.S.T.C. 8 l/min
Lube Oil Flow Rate to External Oil at 1.5 bar 20 l/min

Actuating Speed : 11 sec/90o

Figure 11: Assembly (Vertical Section)


Figure 12: Assembly (Horizontal Section)
D. Parts List of V.S.T.C. (487SVTL 22.2):
Pos. No. Description Qty.
101/010 Shell 1
101/011 Shell Cover 1
101/012 Soc. Head Screw M10 x 30′ 48
101/013 Spring Washer 10.5×16.5×0.8 48
101/020 Bearing Ring 1
101/030 Hex. Screw M16x35-8.8 12
101/040 Spring Washer B16 12
101/050 Cyl. Roller Bearing NU1022 1
101/060 External Circlip 110×4 1
101/070 Primary Wheel 1
101/070/020 Heli. Coil M18.1.5×12 2
101/070/030 Fusible Plug M18x1.5-160C 2
101/070/040 Sealing Ring A18x24 2
101/080 Socket Head Screw M10x30-8.8 48
101/090 Spring Washer 10.5×16.5×0.8 48
101/100 Bearing Retaining Ring 1

Pos. No. Description Qty.
101/110 Hex. Bolt M16x70 16
101/130 Socket Head Screw M10x40-8.8 4
101/140 Spring Washer 10.5×16.5×0.8 4
101/150 Retaining Ring 1
101/151 Soc. Head Screw M10x20 6
101/152 Spring Washer 6
101/160 Primary Shaft 1
101/170 Fitted Key 1
101/180 Spacer Ring 1
101/190 Cyl. Roller Bearing NU316E-TVP2 1
101/200 Cover 2
101/210 Tab Washer MB16 2
101/220 Lock Nut KM16 2
101/230 Secondary Shaft 1
101/240 Secondary Wheel 1
101/241 Screw Ring 1
101/250 Hex. Bolt M16x70 16
101/260 Spring Washer B16 16
101/270 Ang. Con. Ball Bearing 2
101/275 Shim Ring 1
101/280 Tab Washer MB12 1
101/290 Lock Nut KM12 1
101/300 Ang. Con. Ball Bearing 2
101/310 Fitted Key AS 20x12x120 2
101/500 Isolating Disc. 1
201/010 Scoop Tube Housing 1
201/020 Hex. Bolt M16x35 20
201/030 Shaft Sealing Ring BA 42x56x7 1
201/040 Cover 1
201/050 Socket Head Screw M8x20-10.9 4
201/060 Bellows 1
201/070 Labyrinth Cover 2
201/080 O-Ring 2
201/090 Flat Seal Ring 16
201/100 Hex. Bolt M8x40 16
201/110 Circlip: 170×4 1
201/120 Threaded Plug AM 10×1 1
201/130 Flange 1
201/140 Socket Head Screw M8x20 4
201/145 Screw Plug G 1/2″ A 1
201/146 Flat Seal Ring A2x26 1
201/150 Pump Cover Housing 1
201/155 Hex. Screw M16x35 20
201/160 Pump Cover 1

Pos. No. Description Qty.
201/170 Socket Head Screw M8x20 6
201/180 Cover Sheet 1
201/190 Socket Head Screw M8x20 14
301/010 Flange C40x48.3 1
301/020 Sealing 92x49x2 4
301/040 Weld Neck Flange 1
301/050 Flat Sealing Ring 40PN 40 1
301/055 Flange with Connector 1
301/060 Flange with Connector 1
301/061 Socket Head Screw M8x20 4
301/070 Flat Sealing Ring 1
301/090 Flat Sealing Ring 40PN 40 1
301/100 Hex. Bolt M16x80-8.8 4
301/110 Filling Pipe 1
301/120 Hex. Screw M16x35 4
301/300 Vent Filler 1
401/010 Housing 1
401/020 Cover 1
401/030 Hex. Screw M12x25 10
401/040 Screw Plug G1 1/4” 2
401/050 Seal Ring A42x49 2
401/060 Suction Pipe 1
401/070 Oil Sight Glass 1
501/031 Distance Ring 2
501/040 Bearing Sleeve 4
501/060 Pinion (Drive) 1
501/070 Driven Pinion 1
501/080 Socket Head Screw M16x180 1
501/090 Roll Pin M4x24 2
601/010 Spur Gear for Primary Shaft 1
601/020 Spur Gear 1
701/010 Scoop Tube 1
701/020 Sleeve 1
701/030 Roll Pin Ø 8×55 1


Figure 13: Parts of V.S.T.C.

Constructional features of V.S.T.C.:
(i) Housing:
The V.S.T.C. is designed with tunnel housing and accommodated in one-piece, closed sheet steel
housing. This housing forms the oil tank at the same time.

(ii) Coupling:
The coupling consists of:
? Primary Shaft and Primary Wheel
? Secondary Shaft and Secondary Wheel
? Shell (Flanged on primary wheel enclosing the secondary wheel)

Primary Shaft and primary wheel are rigidly connected with each other; the same applies to
secondary wheel and secondary shaft. The primary shaft is connected with the driving machine
and the secondary shaft with the driven machine.
1 2 3 4 5
1. Pump Wheel
2. Turbine Wheel
3. Shell
4. Scoop Tube Housing
5. Scoop Tube
6. Cooler
7. Oil Pump
8. Oil Tank

Primary wheel, secondary wheel and shell form the working chamber in which the working oil
circulates. The scoop tube, with scoop tube housing is integrated in the V.S.T.C. house. The
secondary shaft is supported in the scoop tube housing.
(iii) Bearings:
The primary and secondary shaft of V.S.T.C. is supported by ball and roller bearings. The
primary shaft is guided axially via a relative bearing between primary and secondary shaft.
(iv) Oil Pumps:
A filling pump in oil tank delivers the operating oil for the working oil and lube oil circuit. The
filling pump is mechanically driven by the primary shaft and V.S.T.C. An electrically driven
aux. Lube oil pump during start-up and shut-down.
Power Transmission:
The V.S.T.C. transmits power wear free from a driving m/c. to a driven m/c,
Power is transmitted as follows:
? Between driving m/c and V.S.T.C. through a connecting coupling.
? Hydrodynamically between primary wheel and secondary wheel through the working oil.
? Between V.S.T.C. and driven m/c through a connecting coupling.
? Infinitely variable speed control of driven m/c is possible by means of scoop tube control.

Energy Conversion: (From mechanical to kinetic energy)
The power of driving m/c is transmitted through the primary wheel (function: pump) into
working oil; the working oil is accelerated in the primary wheel and the mechanical energy is
converted into kinetic energy. The secondary wheel (function: turbine) absorbs the Kinetic
Energy and converts back into Mechanical Energy. This energy is transmitted to the driven m/c.
The same torque applies to primary and secondary wheel.

On power transmission the speed of secondary wheel is smaller than that of the primary wheel.
This speed differences is called SLlP. The power loss due to speed difference heats up the
working oil. It is necessary to cool down the oil to dissipate this heat.

Working Oil Circuit:
Through the working oil Orifice Plate, the oil flows into the coupling working chamber and due
to centrifugal force forms a rotating 0:1 ring in the scoop chamber.
The scoop tube position determines the thickness of oil ring in the scoop chamber and thus also
the filling in the working chamber. The scoop tube scoops up the warmed up working oil in the
working chamber and directs it back into the oil tank from where the filling pump supplies the
working oil to the oil cooler. Then the coded down working oil returns into the coupling chamber
through the working oil orifice plate.
The scoop tube is adjusted. if it is necessary to enlarge the working oil filling in the coupling.

Working Oil Temp:
The working oil temperature depends on the power losses (Slip) and the working oil flow rates
and is monitored by the temperature measuring instruments.

Fusible Plugs:
If the oil temp rises to 160 °C due to failure, the solder of fusible plugs in the coupling melts and
oil is thrown into the housing of V.S.T.C. The coupling drains its oil, thus interrupting the power
flow and the driven m/c comes to a standstill.

Speed Control by Means of Scoop Tube:
? Actuator
? Scoop tube rod
? Scoop tube
? Oil Ring
? Scoop tube position 0%
? Scoop tube position 100%

The speed of driven m/c is controlled sleeplessly. For this purpose the coupling oil filling is
changed during operation by means of the movable scoop tube.
? Scoop tube inserted as far as possible in the scoop chamber of coupling (0% position):
Minimum oil ring, minimum o/p speed.
? Scoop tube moved as far as possible out of the scoop chamber of coupling (100%
position): Maximum oil ring, maximum o/p speed.

Variable Speed Turbo Coupling (V.S.T.C.) can be started at any scoop tube position. Starting at
0% scoop tube position is preferred, since the driving m/c can run nearly without load.
Bearing and gears of V.S.T.C. needs to be lubricated prior to and during operation.
Lube Oil Circuit:
During operation the filling pump delivers oil out of the oil tank from which the lube oil is
branched off to the lube oil circuit through the lube oil orifice. Through,
? Heat Exchanger
? Double Oil Filter and
? Lube Oil Orifice

The filtered and cooled oil gets to the lubrication points.

Lube Oil Flow Rates:
The lube oil flow rate required by bearing and gears is set by the lube oil orifice or bores
provided in the spray nozzles. Only on indirect influence of lube oil flow rate is possible by
charging the lube oil pressure.
Lube Oil Pressure:
The lube oil pressure is set at the lube oil orifice and monitored by pressure measuring
instruments (Pressure gauge, pressure switch).

Lube Oil Temperature:
Temperature measuring instruments monitors the lube oil temperature.
Lubrication of External Units:
The oil from lubrication of driving m/c and driven m/c is taken from the lube oil circuit of
V.S.T.C. through the external oil orifice and feed back into the oil tank of the V.S.T.C. Lube oil
pressure and lube oil from rate are set by the external oil orifice downstream of double oil filter
and set by the working oil orifice plate. The lube oil pressure is monitored by pressure measuring
Oil in sump (approx): 250 ltrs.
Oil in rotating parts: 24 ltrs.

Machine Alignment:
Alignment of the unit components with each other at operating temperature the shaft must be in
optimal alignment.
Normal Alignment:
1. Align V.S.T.C. with driven machine
2. Align driving machine with V.S.T.C.
Shaft Misalignment ; Alignment Tolerance:
For alignment of V.S.T.C. any dimensional changes must be observed resulting from:
? Warming up and expansion of housing during operation; and
? Rotation dependent shaft misalignment due to tooth force, bearing clearances and
hydrodynamic lubrication.

Radial and Axial Displacement:
Radial displacement horizontal: ?H
Axial displacement: ?L1
Axial displacement: ?L2

This means that the shafts, connected with connecting couplings, shall not be in alignment, but
must be provided with a radial / angular / axial offset. These offsets compensate the
misalignment to be expected at start up and during operation.

Radial displacement horizontal: ?H=0.40 mm
Axial Displacement:
Input end: ?L1 = 0.49 mm
Output end: ?L2 = 0.45 mm
Assumed housing temperature on installation 20 °C and during Operation 176 °C.
Alignment Tolerance at Operating Temperature:
When turning the shaft initially by 360°

Speed (RPM) Radial Offset (Radial Measurement) Angular Displacement (Angular
100 °C
Shutdown at ; 110 °C

Working Oil / Lube Oil:
Filling pump pressure at operating temperature 2 to 4 bar
Lube oil pressure at pressure gauge (16) Operating range: 0.3 bar
Lube oil pressure at pressure switch Operating range: 1.2 bar
Measuring point 17 AOP Off ; 2.3 bar
Measuring point 17.1 AOP On 1.5 bar
Measuring point 17.3 Main Motor OFF ; 1.4 bar
Lube oil pressure at the differential pressure
visual maintenance indicator
Operating range 0.6 bar
Shutdown ; 0.8 bar
Oil Pressure at the test connection Operating range as per measuring points 16, 17

Oil Level:
Oil Level Checks:
Start the V.S.T.C. at 0% scoop tube position and vent it, dependent on the cooler arrangement.
Ensure that oil level in the middle of fill level range h1 at this operating state.
H1 = Fill level range at 0% scoop tube position
H2 = Fill level range at 100% scoop tube position.
With warm operating oil and scoop tube position at 0%, the oil level must not exceed the
maximum mark L1 or must not fall below the minimum oil mark L2 at 100% scoop tube position.
The oil level is reduced between 0% and 100% scoop tube position since the coupling runner
fills with oil. Even when switching of the V.S.T.C. at 0% scoop tube position, part of the
operating oil remains in the coupling runner.
An exact oil level check is possible only when the V.S.T.C. is running and at 0% scoop tube
Double Oil Filter Change Over:
Differential pressure at change over and
Double oil filter clean filter at ; 0.6 bar.

The double oil filter cleans the lube oil. It has two filtering jugs which only one is always
supplied with oil during operation. The filter is provided with a differential pressure indicator
and/or a differential pressure switch initiating an alarm, in case of a two high differential
between lube oil pressure upstream and downstream of the filter.

Checking the Oil For Water Content And impurities:
Water content
0.05%: Change the oil; separate the old oil / water mixture by centrifuging, depositing or

Elimination Of Condensation Of Water:
If a V.S.T.C. is running frequently, but always only for a short time, the warming up and cooling
down process forms condensation of water which no longer evaporates. To prevent that the total
oil filling needs to be centrifuged.
? Mineral oil (water is heavier than mineral oil and deposit at bottom). Let the coupling rest
for 1 to 2 days. Drain the condensation water out of the oil drain valve provided at the
housing bottom.
? Synthetic fluid with a density of ; 1 (water floats on top) : remove the condensation
water by suction.
Fusible Plugs:
The fusible plugs respond at a working oil temperature of 160 °C in the working chamber. Thus
preventing an overheating of coupling.
Reasons for a temporary thermal rise of working oil temperature might be:
? Cooler does not function properly
? Overload of coupling

Consequences of melted fusible plugs:
? The control behavior of the coupling changes slightly.
? Maximum output power is almost reached.
? The oil temperature in the oil tank rises slightly.

Speed Control:
The actuator positions the scoop tube between 0% and 100%. The scoop tube position is
determined by the speed of the driven machine.

Figure 14: Adjustment of Scoop Tube

Adjusting the Scoop Tube To 0% Position:
? Move the scoop tube by means of the scoop tube actuator towards the scoop tube housing
until the sleeve has a distance of 1 mm to 3 mm to the scoop tube cover.
? Align the stop for stroke limitation (minm o/p power) with the scoop tube actuator and
Adjusting the scoop tube to 100% Position:
? Move the scoop tube by means of the scoop tube actuator towards the scoop tube housing
so that the scoop tube stroke H for the required direction of rotation is reached according
to the following table:
Coupling Size and type Coupling Direction of
Scoop tube stoke ‘H’ (mm)
487SVTL 22.2 C.W. 127
C.C.W. 120

? Align the stop for stroke limitation (max. o/pt power) with the scoop tube actuator and
1) Start the unit
2) Operate the scoop tube over the whole positioning range (scoop tube positions 0% and
100%) by means of scoop tube actuator. Adjust the scoop tube in case of axial scoop tube
vibration / high sound pressure level.
3) Move the scoop tube to the position with maxm vibration.
4) Insert the torsion rod into the bore hole
5) Unscrew the fixing bolt and turn the scoop tube to the most favorable position by means
of the torsion rod.
6) Repeat step 3 to 5 at other scoop tube positions until there is mini vibration over the
whole scoop tube positioning range.
7) Stop the unit
8) Remove the scoop tube, bellows and scoop tube actuator.
9) Bore the removal scoop tube together with the sleeve and pin with the roll pin.
10) Stamp in the direction of rotation of V.S.T.C. on the sleeve
11) Reinstall the scoop tube, bellows and scoop tube actuator.
12) Check free movement of scoop tube over the whole positioning range by slowly turning
the V.S.T.C.
13) Check the scoop tube stroke and adjust the scoop tube stroke.

Figure 15: Scoop Tube Actuator

Operating Fluids:
It is necessary to meet the characteristic values of oil for V.S.T.C. The most important
characteristics are:
? Viscosity from 46 nm2/s (cst) at 40 °C or 104 °F (lSO V646)
? Air separation property 3 5 minm At 50 °C on 12°F to DlN 57381
? Aging stability neutralization
No new oil: NZ (s) ? 2 mg KOH/g Oil
? No corrosion or deposits even on water entry.
? Starting viscosity under site condition not exceeding 250 nm2/s (cst) for gear pump with
sufficient power upto 400 nm2/s.
Supplier Designation
I.O.C.L. Servo System HLP 46
Bharat Shell Shell Tell US 46

Functional and Measuring point diagram for 487 SVTL 22.2:
Set point for item number 17 and 17.1
Auxiliary lube oil pump Off ; 2.3 bar (17)
Auxiliary lube oil pump On 1.5 bar (17.2)
Main Motor OFF 110 ;0.8 Melting
– ;105

Measuring Point Point Number
Lube Oil Pressure 17, 17.1, 17.2, 17.3, 16
Speed Output 38
Double filter differential pressure 41
Bearing Temperature 38, 32.1, 31, 31.1, 30, 30.1
Oil Inlet (Cooler) 30
Oil Inlet (Cooler) 34
Oil Level 35
Oil Temperature at Scoop tube outlet 18.1

Position Number Description
01 Rotating Parts
05 Coupling Housing
06 Gear Pump
07 Vent Filter
08 Oil Level Indicator
09 Oil Drain Plug
10 Scoop Tube
11 Sequence Valve (Working Oil)
12 Orifice (Lube Oil)
20 Fusible Plug
24 Heat Exchanger
25 Scoop Tube Actuator (electro-mechanical) with positioner ; power supply
unit suitable to 230 V A.C., 50 Hz
42 Double Filter
46 A.O.P.
46.1 N.R.V.
46.2 Orifice
55.1 Shut-Off Valve

Lube Oil Header Pressure
Low Set Point AOP START
Lube Oil header pressure
Normal set point ; 1.7 bar
Oil level in tank above Start Pressure
Minimum operating level
Scoop tube of hydraulic
Coupling at minimum position

Lube oil header pressure high ; 2.5 bar
Lube oil pressure low 0.5 kg/cm2 Clogging in strainer
Differential pressure across
hydraulic coupling filter high
?P ; 0.5 kg/ cm2 Clogging in filter


Trouble Causes Action
Start up:

Driven m/c can’t start after the
driving m/c has reached its
rated speed=
Scoop tube at 0% position=Move scoop tube to 100%
Filling pump not depleting any=
Oil temp. in the oil tank 400 nm2/sec
Warm up the oil to > 45 oC
Close the cooling water supply
at the cooler.

Oil level too low
Check the Oil Level & top-up
the valve between minm &
maxm mark
Oil foaming (Oil temp. too
low, oil contains water, poor
air release property, wrong oil
Check the fusible plugs

Pipe clogged
Check oil for impurities
(centrifuge/separate) in the oil.
Change the oil, if necessary

Incorrect direction of rotation
of main motor
Check the filling pump at the
pressure measuring points
Check the tripping, remove
Check main motor & connect
it properly

V.S.T.C. becomes too hot
during start-up
Too high starting torque (high
motor current consumption)
Driven m/c is blocked
Check driven m/c for smooth
running, check fusible plugs
Driven m/c is jammed, other
Check oil circulation
Oil flow rate too low
Output Speed:

O/p speed hunting at constant
scoop tube position

Oil foaming (oil temp.
downstream of cooler too low,
thus air release property too
Warm up the oil in the tank >
45 oC
Check the oil level & the
filling pump
Check the system, vent &
stabilize if necessary
O/p speed hunting at
automatic scoop tube position
Controller doesn’t have the
correct constraint with regard=
to control system=
Tune the controller to the=
control system=(dampen the
controller, if necessary)=


O/p speed can’t be controlled
Scoop tube & scoop tube
actuator jammed
Check scoop tube for every
movt., remove obstruction
Scoop tube actuator defective Check the scoop tube actuator

Maxm o/p speed can’t be
Scoop tube not in 100%
Check the maxm scoop tube
Fusible plugs not responding Find ; eliminate the cause.
Insert new Fusible plug
Power requirement of the m/c
too high
Compare the power data with
the projected data.
Check driven m/c for smooth

Lube oil flow too low to give
release for start
Leak in oil cuts Check the oil level, check the
pipe work for leaks
Double oil filter clogged Change over double oil filter
; clean the filtering jug
Lube oil pressure too low
during normal operation
Double oil filter clogged Change over double oil filter
; clean the filtering jug

Filling pump pressure too low
Oil temp. in the tank
45 oC
Oil level too low Check oil level ; top-up to a
value between minm ; maxm
mark, check fusible plugs
Oil foaming (oil temp. too
low, oil contains water, poor
air release property, wrong oil
Check the oil for impurities.
Centrifuge or separate the oil.
Change the oil, if necessary.

V.S.T.C. becomes too hot
during operation
Cooling water flow rate too
Increase the cooling water
flow rate
Cooling water too warm.
Cooler is contaminated.
Check ; clean the cooling
Operation of V.S.T.C. outside
the permitted characteristic
Operate the V.S.T.C. within
the characteristic curve
Fusible plugs melted

Find ; eliminate the cause.
Insert new fusible plug


Bearing temperature high
Bearing damage Check quiet running
Check the bearing ; replace
Lube oil temp. too high Check the oil cooler, change
over double oil filter ; clean
the filtering jug.

Lube oil pressure too low
Check lube oil system
Change over double oil filter
; clean the filtering jug.
Check oil level
Increase the lube oil pressure
Quiet Running:

Uneven Running
Vibration Noise
Faulty Alignment Check ; correct the alignment
Insufficient shimming
between m/c ; foundation
Check alignment ; shimming,
and correct it.
The support is not uniform.
Machine is distorted
Provide proper packing
between m/c ; supports
Foundation bolts are loose.
The foundation is defective
Check the foundation ;
re-tighten the foundation bolts.
Wear / insufficient lubrication
of connecting coupling, sleeve
for curved tooth couplings
can’t be displaced axially
Check the connecting
Measure the vibration & make
frequent analysis of the whole
Check & replace the bearings.


Maintenance Measures & Intervals:

With the Unit Running:
Cause / Duration Action
When the differential pressure
on double filter rises
Change over & clean the double oil filter

In case of speed variations of
driven m/c
Check the air release property of operating oil
Check continuous venting of oil cooler (direct circulation)
In case of increased oil level Check the operating oil for water content

Check temperature, lube oil pressure, as well as well as the
differential pressure on the double oil filter
Every 100 operating hours Check & record all the temperature & pressure indications

Every 500 operating hours
Measure, record & compare quiet running & noise pattern
under the same operating conditions.
In case of changed values for smooth running, check &
correct the alignment of V.S.T.C.
Check foundation firing.
Analyse the operating oil for ageing & take appropriate
When the unit is stopped:

Check the oil level, top-up if the oil level is
too low
In case of too high oil level, check the
operating oil for water content.
Find out & remove the cause.
Separate/change the oil

After 100 operating hours
Clean the double oil filter
Clean the vent filter
Check smooth running of scoop tube,
Check the scoop tube stroke limitation.
Grease the joints & bright parts
Check the oil cooler & clean it, if necessary
according to manufacturer’s instruction.
Check piping ; greased oil lubricated
connecting coupling for leaks
Check the operating oil for impurities ; water
content. Separate / change the oil.


After 500 operating hours
Clean the double filter
Clean the vent filter
Check the piping ; connecting coupling for
After 1000 operating hours Check the maintenance work, as after 1000
operating hours
Every 1000 operating hours / when the double
oil filter is clogged
Check the double oil filter ; clean it, if

Every 6000 operating hours at least manually
Analyze the operating oil for ageing ; take
appropriate measures
Check ; service the connecting coupling
Check ; correct the alignment of V.S.T.C.
Check foundation fixing
Check general condition of V.S.T.C.
Open the inspection hole cover on the housing
cover ; check the correct pattern of the filling
pump drive, tooth flanges ; the condition of
fusible plug

Maintenance Measures When the Unit is at Standstill:
Every 8000 hours of operation / at least annually:
? Analyze operating oil for ageing and other specifications
? Inspect and maintain connecting couplings
? Check the actuator functioning
? Check alignment and foundation fixing of hydraulic coupling
? Visual inspection (Corrosion, general condition) of hydraulic coupling internally by
removing inspection cover on top housing
? Visual inspection of tooth contact pattern
? Inspection of fusible plug
? Check and maintain AOP motor

General Overhaul:
In the event of change in operating behaviors or after maximum 5 years operating time, V.S.T.C.
should be overhauled.


The following improvements are proposed regarding V.S.T.C.
? Gear Box:
Double helical gearing with large tooth width in order to reduce gear centre distances. Gears are
designed as single piece forging. The pinion is integral with shaft and the gear wheel shrink
? Couplings:
Runners made from high alloy steel for higher circumferential speeds.
? Scoop tube:
Scoop tube is operated by an electro hydraulic actuator (VEHS)
? Oil Circuit:
Separate oil circuit for working oil and lube oil closed working oil circuit with mechanically
operated flow control valve.
? Pumps:
Mechanically driven working oil and lube oil pump. Electrically driven AOD.
Improved Control System for V.S.T.C.:
Voith Electro Hydraulic Positioning Control System (VEHS)
An excellent regulation quality reached by:
? High sensitivity
? High positioning accuracy
? High reproducibility ; 0.05% (dissolution) with short floating times (Dynamics)

Electronics is implemented in analog technique o/p signal (4 to 20 mA): 0 to 100%

The VEHS is a compact unit of:
? Electromagnetic actuator
? Control magnet, 4/3 way valve
? Double acting hydraulic cylinder
? Electronic positioner.
Speed Control:
The scoop tube position operates as a function of VEHS control.

Advantages of V.S.T.C.:
? Higher technical flexibility
? Optimum adaptation a result of different possible combination between gearbox and
? Compact design
? Short delivery time due to standardize modules
? Compact actuator with integrated positioner for direct control (4 to 20 mA )
? Integrated rapid start up device
? Easier maintenance, as a result of modular design.

? Energy ; Cost Savings:
Speed control offers high efficiency compared with throttle control, especially during part load
operation. Driven m/c and the motor require less energy and this results in low operating costs.
These energy saving results in lower investment cost for components and reduce investment
costs for the driven m/c meaning payback times for 6 to 20 months.
? Reduced Emission:
Energy savings as a result of speed control result in lower consumption of primary energy and
hence protect the environment due to reduced emission.
? Increased Flexibility:
The application of variable speed drives allows considerably higher flexibility in operation of a
plant, since the driven m/c can be adjusted precisely to the actual conditions with highest

? Increased Service Life of Motor and Driven m/c:
The load requirements for the motor and the driving m/c are continuously tower as a result of
speed adoption and power take-up adjustments.
? Reduced Maintenance Requirements:
Simple and robust mechanical designs reduce maintenance requirements of a minimum.
? Extremely High Availability and Reliability:
Avg. value calculated on the basis of existing plants is over 99.9%.
? Lower Space Requirements:
Owing to the compact design a hydrodynamic drive require a minimum of space-compared with
alternative solutions and hence saves costs.

Figure 16: Comparison of Losses at Parabolic Torque Curves

1. NPTI Course Material- P.P.F. Vol-III.
2. NPTI Course Material- B.F.P. Design and Construction ; Operation.
3. Modern Power Station Practices, VoI-II, CEGB.
4. VOITH COUPLING Manual: 100 MW of Hirakud Power.
5. Power Plant Engineering- P. K. Nag

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