INDICATING ALARM METERS

1. INTRODUCTION INDICATING ALARM METERS

INDICATING ALARM METERS -Indicating  alarm  meters  are  mounted  in  control  room  as  wells  as  in  various  local  operator panels to indicate the parameters like flow, temperature, pressure etc.  And to generate alarm when these parameters exceed set value. The alarm contacts can be used to generate alarm or to initiate safety actions like tripping of the plant. Field sensing devices  measure  various parameters  of  a  plant  which  require  indication  in  control  room  for  operator  information.

Since  it  may  not  be  possible  to  monitor  the  large  number  of  indicators  simultaneously an alarm  may  be  generated  to  draw  the  attention  of  the  operator  to  take  corrective  actions immediately. The measure signal from field sensor is converted in to a proportional current signal in the range of 4-20 m A. This is always necessary because the indicating meters are calibrated for 4-20 mA current input. Thus to measure temperature, temperature sensing devices like R.T.D.  will  be  connected  to  an  interposing  device  called  resistance  to  current converter  which  after  suitable  calibration  will    provide  current  signal  proportional  to temperature in the range of 4-20 m A.

2.  LEKTROLAB IAM

The lektrolab make IAM consists of a permanent magnet and a moving coil, which moves in response to the field created by the magnet. The 4-20 ma current signal is given to the coil and  a  pointer  is  attached  along  with  this  coil  which  will  deflect  proportionate  to  the magnitude of the current signal. An adjustable index also called setter(alarm set point),  may be positioned by convenient external means, at any point analog the scale at which control action is desired. Thus position of setter decides the set value. Lektrolab IAMs are available with low as well as high alarm set points.

A focused light source is fixed on this movable setter which also carries a photo diode. As long  as  the  light  bream  strikes  the  photo  diode  head,  its  resistance,  remains  low.  The controller  senses  this  low  resistance  in  its  input  circuit  and  acts  so  as  to  keep  the  relay energized. Thus falling of light on photo diode acts as the “keep the relay energized “signal to the controller /indicator. The indicator pointer  carries a small light vane  at its other end and this can pass through between  the  light  source  and  photo  diode  to  obstruct  the  light  beam,  thus  increasing  the resistance of photo diode.

The controller with this higher resistance in its input circuit acts in a way so as to “ de-energize the relay “ signal for the controller. This vane, shaped in form of an arc starts moving between the light source and photo diode only when the pointer reaches the set  point.  The  vane  is  designed  in  such  a  way  so  that  for  any  position  of  the  pointer beyond the set point its certain portion (which depends on the position of the pointer) always remains in between the light source and photo diode. Thus even when the pointer exceeds the set point (which may happen due to certain failure in the control loop or due to some other reason), the light in photo diode remains obstructed and the relay remains de-energies which is a position in which controller takes corrective action.

3. Fail Safe Feature:

If the power supply of the instrument or LED fails the relay de-energizes which is a  safer side because this is the position relay assumes whenever the process signal strength exceeds the set value.  If input signal fails Pointer goes below 0% on minimum value. If indication lamp is glowing, Process variable measured has reached the set point and Relay will be de- energized.

4. Application: –

Alarm indicator is used to indicate the instantaneous value of the process parameters. It also generates the contact input when process parameter exceeds form the set point which can be used for generating alarm, operation of the logic condition and to operate safety devices.

5. Description of parts: –

1.  Pointer: – Knife edge type pointer mounting on the scale over the parallax mirror and scale.

2.  Parallax mirror: – A mirror is provided on the full scale to take the reading without parallax error. While  taking reading pointer & pointer image should be in the same line.

3.  Scale: – Scale is mounted in actual unit with magnitude or percentage. Set point adjustment: – With the help of the screwdriver the set point position on the scale can be changed. Some instruments have double set point, high & low, high & very high, low & very low etc.

4.  Set point indicator: – Red marked pointer is provided on the scale, which indicates the set point value.

5.  Lamp indication: – Top of the front a lamp is provided, normally if will be off, it became on when pointer exceed from set point.

6. Procedure for taking  reading: –

1.  Set your self in front of alarm indicator.

2.  Match the pointer and its image on same line.

3.  Look the reading on the scale.

4.  Write magnitude by unit

IAM FRONT LOOK

7. MASIBUS ALARM INDICATOR (DIGITAL CONTROLLER)

Masibus  digital  controller  is  a  versatile  microprocessor  based  ON/OFF  controller  designed for  universal  input  type  and  up  to  two  set  points  with  individual  relays.  The  programming, calibration and  operation    of  the  instrument  is  by  four  simple  keys  with  two  independent displays for set point and process variable. The operation of the equipment is menu driven with user understandable prompts. Various options are protected by back plate DIP switch / jumpers to avoid unwanted access. The product is made to accept 4-20 mA D.C. and 10-50 mA D.C. inputs. Two set points with individual hysteresis are provided with separate relays. The set points can be programmed as control or alarm type, relay on below or above set point and as deviation from main set point, relay normally ON or OFF. Provision for zero and span setting is made to restrict range of usage and programming.

FRONT PANAL LAYOUT

There are four modes of operation for the unit.

•  Run mode

•  Verify mode

•  Program mode

•  Configuration mode

The run mode is the normal mode of display. Verify mode is used for checking set points and hysteresis settings for all set points without altering the data. The program mode is used for programming the set points. The Configuration mode is used to configure the controller. The input type, Zero & Span setting, Number of set points etc. can be configured in this mode.

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