LBC II - Fahlke Control Systems KG

Fahlke Control Systems GmbH Germany
Rudolf-Diesel-Str. 3 • D-27356 Rotenburg
Tel.: (0 42 61) 91 43-0

THEORY OF OPERATION
The LBC II watches the preselected pressure within a pipeline and shut down a valve whenever a
pressure drop above a certain value is observed. This threshold depends on the operating conditions
of the line, and must be determined in a way that it does not interfere with normal operational
pressure drops.
The common mechanical line break systems are not independent of some physical parameters like
absolute pressure, dynamic viscosity, density and temperature of the linefill. By this reasons, the
threshold for closing the valve can not be maintained precisely, and the user is forced to select a
worst case limit. In some cases this must be a very rough estimation and makes the line break device
too sensitive for practical use.
The best way to detect a line break is to measure the pressure and to react on a certain pressure
gradient. This can only be achieved by an electronic device. The transport-lines can lead pressure
waves, which can simulate pressure drops, and this is why the selected gradient must be observed for
a certain period of time to decide whether the change in pressure is tolerable or not. A gradient of 0,6
bar/minute can also be a drop of 10 millibars per second, which is certainly a value which should be
ignored. This example shows, that not only the pressure gradient but also the time span is significant
for a system leak.
The combination of the gradient and the observing time results in an absolute value of linefill that will
be lost, in respect to the line length observed. These parameters must be discussed when fixing a
limit for the gradient and reaction speed of the protective device. It is important to be aware of the
fact, that the oil (gas) loss until the moment of reaction will be very small in comparison to the loss
that will occure after closing a valve anyway, and that on the other hand the safety for a proper
decision will increase with the observing time for the controller.
The basic function of the processor is to measure the pressure and to shift the measured values into a
buffer. Then it calculates the average of the two halves of the buffer and compares them, which gives
the average of a pressure gradient within the measuring period. The selection of the gradient is done
by means of a thumbwheel switch on the front panel of the processor. The gradient can be selected
within the limit of 0,2 until 1,8 bar/minute.

MAIN COMPONENTS OF THE LBC
The LBC has the dimension of 105 x 90 x
195 mm and will be delivered installed into
a stainless steel box IP54 of 380 x 210 x
600 mm which includes battery back up
with charger unit, junction box, relay box
and select switch.
The supply for the processor can be
adapted to practically all voltages. For AC
supplies, a transformer will convert the
input voltage to the internal supply of 12 V
DC. When DC supply is given, a DC/DC
converter is installed on the supply board
of the processor. The power used to feed
the processor is less than 200 milliwatt, that
can also easily be done by a solar cell,
which can be delivered by Fahlke as an
option.
In all cases the processor will be backed up
by a battery that makes sure operation
continues for a minimum of ten days after
the power supply fails.

THE MICROPROCESSOR
has its program stored in proms. This allows to match its characteristics in line with the customers demands.
Even if the basic parameters required change, a reprogramming can be achieved by simply changing the proms
on site, because they are not soldered into the printed circuit board, but only plugged into sockets.
The design is based on long practical experience with various leak detection systems, therefore it is very likely,
that hard- and software configuration will cover all practical applications for detection of leaks in oil- and gas-
lines.
The hardware has an indication panel where serveral
parameters can be seen, like:
a) gradient positiv,
b) gradient negative,
c) if the order for closing the actuator (valve) has
been given and the actuator is still running into
closed position,
d) or if actuator and valve are already closed,
e) indication if reopening the actuator (valve) is
working,
f) battery low alarm,
g) malfunction indication
h) and processor function indication.
All of them are shown by LED signals on the display.
The actual measured line pressure and the pressure
gradient are both alternating shown on an extra LCD
display.
The lower part of the front also consists of a two
decade thumbwheel switch for the selection of the
pressure gradient.
LEITUNGSBRUCH-DETEKTIONS-SYSTEM
LINEBREAK-DETECTION-SYSTEM
Anzeige
indication
LBC II
Fahlke
Control Systems
Gradient Positiv
Gradient Negative
Läuft zu
traveling closed
Befehl zu
acting closed
Freigabe auf
reopening
Batterie Alarm
Störung
malfunction
Computer Funktion
Druck / pressure
Funktionsschalter
function
switch
Gradient
Gradient

INDICATION PANEL OF THE LBC
LEITUNGSBRUCH-DETEKTIONS-SYSTEM
LINEBREAK-DETECTION-SYSTEM
Anzeige
indication
LBC II
Fahlke
Control Systems
Gradient Positiv
Gradient Negative
Läuft zu
traveling closed
Befehl zu
acting closed
Freigabe auf
reopening
Batterie Alarm
Störung
malfunction
Computer Funktion
Druck / pressure
Funktionsschalter
function
switch
Gradient
Gradient
An analog output represents the line pressure,
this output can be switched to give either 0 to 10
V DC or 0 to 20 milliamps, a life zero output is
optional.
With a modem the complete status of the
processor can be transmitted to a control center,
which can be very useful to monitor the line
pressure and open/close status of the valve.
Commands can also be transmitted to the
compressor, thus using the line break device as
a simple telecommunication plant to telecontrol
a valve.
0 = P 70 bar
1 = GRAD 70 bar
2 = P/G 70 bar
3 = P 40 bar
4 = GRAD 40 bar
5 = P/G 40 bar
6 = LBC sends data to modem
7 = failure signalisation output 20 mA
8 = signalisation voltage battery
9 = signalisation A/D transducer coefficient

FUNCTION OF THE CONTROLLER
Switched on, the processor will release the open command, thus allowing the valve to go
to the open position until the limit switch "open" is reached. A LED "open" will be lit for
this period, to show, that the output transistor is activated and the valve should be
running.
The computer measures the pressure in the line in adequate points and shifts the values
into a buffer. At every shift the oldest value is pushed out of the buffer. Whenever a new
value is sampled, the averages of the two halves of the buffer are calculated and also the
difference between both averages.
The LCD display blinks and gives for a period of seven seconds the value of absolute
pressure, then for three seconds the pressure gradient. Two of the LEDs are used to
indicate whether the sign of gradient is positive or negative.
When a pressure drop reaches the predefined threshold, the valve will be closed. During
the closing period a LED also indicates the fact that a control output is energized.
The closing function will also be executed, when the line pressure goes below a certain
pressure (ppmax).

ILLUSTRATION NO. 1
The thumbwheel switch on the front panel of the processor serves for setting the gradient that will
close the valve when a leak is detected in the line. A gradient of this magnitude will close the valve
after 60 seconds. When the leak is greater, the time for detection becomes shorter.
The relation between size of the leak and the time used for detection is a non linear function, that is
shown in illustration no. 1.
P
K: GR > S
A V 2
G R / 2
A V 1
30 30
G
t

1,2
1,1
1,0
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
S (bar / min.)
10,0 5,0 3,0 2,0
10 20 30 40 50 60
gradient (bar/minute)
1,2
1,0
G
0,7
0,4
0,2
t (s)
ILLUSTRATION NO. 2
The following table gives the appoximate
values for different leaks
and selected thresholds:
threshold selected (bar/minute)
0,2 0,4 0,7 1,0 1,2 2,0 3,0
0,2 60 - - - - - -
0,3 32 - - - - - -
0,4 29 60 - - - - -
0,5 26 33 - - - - -
0,6 24 36 - - - - -
0,7 22 32 60 - - - -
0,8 20 29 39 - - - -
1,0 18 26 36 60 - - -
1,2 18 24 32 42 60 - -
3,0 10 15 20 24 26 30 60
response time (seconds)

PRESSURE MEASUREMENT
For on-line pressure measurement the system is
fitted with a piezoresistiv transducer (Kistler 4043
A50). The transducer is feeded by 3 milliampere
constantly, the measure signal will be taken as an
voltage (0...500 MV) at the wheatstone bridge and
will be prepared by a 12 bit analog digital
converter.
The complete error of the transducer is smaller than 0,5 % by the whole temperature range from -20 up to
+80° Celsius. The longterm change is less than 0,2 % a year. That indicates the transducer works excellent
for absolute pressure measuring. It is mounted in a way to be isolation proof for serveral kilovolts.
The connection to the processor is a four wire connection: two wires to feed the bridge ( 3 milliamps),
and another two to give back the measured voltage (0,375 volts). The function of the transducer is watched
by the processor. If the transducer fails or a wire is interrupted, the processor will give an alarm via a light
emitting diode and will stop operation immediately in order to avoid a wrong operation.

DIMENSIONS OF THE LBC COMPONENTS
( 3 )
D = 9
( 4 )
( 1 )
250
D = 6,8
( 7 )
( 6 )
2 x PG 21
1 x PG 13,5
380
( 2 )
( 5 )
LBC II
( 6 )
1 x PG 21
2 x PG 13,5
1 x PG 13,5 / blue
250
600
210
1 = battery
2 = terminal box
3 = on-off switch
4 = battery charger
5 = microprocessor LBC
6 = cable glands
7 = relay box
The electronic line break system
can also be mounted in an explosion
proof cabinet if necessary.

acoustic
alarm
CPU
BLOCK DIAGRAMM OF THE LBC
alarm
LEDs
RAM
2 K
LCD-pressure
control
UV-EPROM
2.........6 K
interface-controller
autotest
modem
command open
command close
limit switch
open actuator
limit switch
close actuator
measurement UB
telefon
connection
watchdog output
analog-output
0.........20 mA
piezo
pressure
transmitter