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
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.
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.