Q. a) You are working onboard the engine room crane. What are the hazards associated with working at heights? (5 Marks) JUNE 2018
b) What risk mitigation measures will you take to mitigate the hazards identified by you while working at heights? (6 Marks) JUNE 2018
c) Which all matrix, checklists and permits will you fill up before commencing the work and who will authorize the work permits? (5 Marks) JUNE 2018
Ans- (a) The following hazard takes place while working on electric machinery at heights:
1) Slip and trip from heights while climbing.
2) Falling of tools from heights while working.
3) Electrical shock.
4) Limited space at heights.
5) Sudden change in Weather conditions
6) Rolling and pitching of vessel.
7) Falling from height while working
Ans (b) Existing Controls (mitigations)
1) Proper PPE to be donned, safety harness to be worn.
2) Work aloft permit to be complied with. Adequate illumination to be provided.
3) Communication with the working person to be tested and continue communication is to be established between work place and officer of the watch.
4) For aloft careful consideration is given for sea and weather condition prior taking up the job.
5) When working in cold climates. Adequate winter gear to be worn.
6) Material and strength of planks to be suitable for the job and personnel
7) Only experienced staff to work aloft, and the rigging of stage should also be supervised by a competent person.
8) Work Aloft Permit to be complied. Warning signs posted
9) Electric work permits to be complied. Electric machinery to be disconnected and isolated.
10) Warning signs to be posted.
Ans (c) The following matrix, checklist and permits are required to fill up and authorized by
I. Safe Job Analysis – A proper risk assessment is to be carried out and a matrix is to made for determining the risk and taking counter measure to mitigate the risk involved. In cases where the Determined Risk is:
High: Work shall not start. Appropriate additional controls must be taken to reduce residual risk level to Moderate or Low.
Moderate: Efforts should be made to reduce the risk, with heightened monitoring of the additional controls that are implemented
Low: No additional controls required, monitoring required to ensure existing controls are maintained.
This Job risk assessment to be approved by Chief Engineer.
II. Working Afloat Permit – all the checklist in the section 1 is to be completed and the work authorization is to be given by Master for deck / Chief Engineer for engine room.
III. Electric Work Permit – All the relevant precautionary measures are to compiled and checked by ETO. This work permit is to be authorized by Chief Engineer and it is to be approved by Master.
IV. Tool box talk – All the attendees are to do meeting and discuss the scope of work is to be carried out, safety measures, contingency plan, communications etc. This is to be signed by all attendees and is to be authorized by person in charge.
Q – What are the safety procedures if any work has to be carried out in live MSB bus bar? (8 Marks) JUNE 2018
Ans:- Following is the safety procedure to work on live MSB bus bar should be carried out only in extreme situations: –
1) The risk assessment should be conducted by a competent person with the appropriate knowledge and experience to assess all risks arising from the work. The person should have good understanding of the work as well as good knowledge of the safe practices and safety measures required. After safe Job analysis, this report is to be rendered to office to obtain permission.
2) A permit-to-work should be issued by authorized person and is to be approved by competent authority. Typical situation includes the nature of work is complicated and the work is carried out on live MSB bus bar. Where danger cannot be avoided for work on the live circuit, the circuit should be isolated, and the live work should be prohibited.
3) Only competent person who have received proper training and have the relevant knowledge, experience and understand the safety rules and procedures should undertake such work.
4) The work is to be discussed in tool box meeting by sketching a simple line diagram, and all persons of team is to be agreed. Any doubt is to be cleared. The emergency and correct action is to be discussed well.
5) Proper PPE is needed to accomplish such job safely such as insulated gloves, insulated mats, insulated screens, insulated clothing etc., of the suitable type and grade for the work. Use flash protection clothing, if the job requires operating, racking, circuit breakers with the doors open, or, working within reaching distances of exposed energized parts. Working on live MSB rated at 440 volts or greater must use rubber gloves, hard hats, safety boots, and other approved protective equipment or hot-line tools.
6) Work on live MSB should be done only in the presence of the responsible person, who has authorized such work. At least one person should be well versed with rendering first aid in case of electric shock. Safety rescue hook is to be used to remove live conductor of person.
7) Suitable safety equipment such as insulated tools, insulation mats, insulation stands etc., should be available to work in such unavoidable situations.
8) Sufficient working space should be allowed. Furthermore, access ways should be properly maintained and kept free from obstructions. It is important to prevent unauthorized personnel from encroaching on the area of work.
9) Suitable and adequate safety precautions is to be taken to prevent inadvertent contact with the live parts by keeping a safe clear distance from the live parts, erecting physical barriers around the live parts and posting of warning notices fixing of warning notices for repair in appropriate places.
10) There should be effective communication amongst the team and clear order of command at work to prevent accidents.
11) The duration and extension of the live work to be minimized. Once the unavoidable live work is finished, the involved electric circuit should be isolated and verified dead and with the power supply locked out before proceeding with the remaining work.
Note:-This is not a part of answer
The hazards in live MSB bus bar, if any work can be carried out, broadly grouped into the following major categories:
a) Electrical shock hazards – The persons are vulnerable to electric shock hazard whilst working on the conductive parts of the live MSB which has not been properly isolated from the power source. Electric shock hazard also arises from the improper application of high voltage test on the electrical switchgear. Electric shock can cause serious injury or even death to the person due to cardiac arrest, respiratory arrest or body burn.
b) Fire and explosion hazards – Fire and explosion in live MSB work can be caused by accidental short-circuited of live parts and earthed parts by, say, metal hand tools, substandard testing equipment, loosen parts detached or foreign objects left inside the switchgear work. Fire and explosion can cause serious injury or even death to the persons due to severe body burn caused by the hot gas mass or molten metal, inhalation of smoke caused by the fire or eye injury caused by the intensive light and ultra violet radiation generated by electric arcing.
c) Other related hazards – there are other hazards that relate to live MSB work. These Include, but not limited to, the following:
i) Tripping hazards due to tangling of electric wires of portable tools or loose hand tools placed on the floor; and
ii) Musculo skeletal problems resulting from manual handling of heavy objects such as air circuit breakers or prolonged periods of awkward postures.
Risk assessment should be specified to the required task. Factors to be considered in assessing risks associated with electrical switchgear maintenance work include, but not limited to, the following:
a) The location of MSB to work on;
b) The type of work to be carried out;
c) Whether the MSB to work on is isolated, energized or partially energized;
d) The working environment, including
i) Whether access to and egress from the workplace are safe, suitable and adequate;
ii) Whether lighting & ventilation of the workplace are sufficient & suitable;
iii) Whether the workplace is congested or restrictive;
iv) Whether working at height is necessary;
v) Whether there are combustible/flammable materials nearby;
vi) Whether the workplace is hot, damp and dusty or the environment is corrosive, etc;
e) The strength of the working team & the competence of team members.
Q – Methods of determining the voltage regulation of 3 phase alternator
Ans – Voltage Regulation-
1. Synchronous Impedance Method or EMF method.
2. Ampere-turn method or MMF method of Voltage Regulation.
3. Zero Power Factor method or Potier Method
Synchronous Impedance Method
The Synchronous Impedance Method or Emf Method is relied on the concept of replacing the effect of armature reaction by an imaginary reactance. For calculating the regulation, the synchronous method needs the following data; they are the armature resistance per phase & the open circuit characteristic. The open circuit characteristic is the graph of the circuit voltage & the field current. This method also needs short circuit characteristic which is the graph of the short circuit & the field current.
MMF Method of Voltage Regulation
MMF Method is also called as the ampere turn method. The synchronous impedance method is relied on the concept of replacing the effect of armature reaction by an imaginary reactance the Magneto motive force (MMF). The MMF method replaces the effect of the armature leakage reactance by an equivalent additional armature reaction MMF so that this MMF may be combined with the armature reaction MMF.
To calculate the voltage regulation by the MMF Method, the following information is required. They are as follows:
The resistance of the stator winding per phase
Open circuit characteristics at the synchronous speed.
Short circuit characteristic
Potier Triangle or Zero Power Factor Method
The Potier triangle determines the voltage regulation of the machines. This method rely on the separation of the leakage reactance of armature and their effects.
Q – Induction Motor Starting Methods
Most of the large induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of the voltage on the supply lines due to large starting current surges. To limit the starting current surge, large induction motors are started at the reduced voltage & then have full supply voltage reconnected when they run up to near rotated speed.
Two methods of the reduced voltage starting are star delta starting & autotransformer starting. Contactors carry out the switching action in the starter to connect & disconnect the power supply to the motor. If the current is above the rated current for the motor, the contactor will be tripped automatically to disconnect the motor from the supply.
A 3 phase supply is given to the stator of the 3 phase induction motor, & this in turn produces a magnetic field which revolves in space around the stator. As if the magnetic poles are being rotated, the speed of the rotating magnetic field is given by the formula N = 120 f /P
The high starting current will produce severe a voltage drop & will affect the operation of other equipment. It is not required to start large motors direct on line (giving full voltage to the stator). Generally with motors beyond 5 HP, starters are provided. For the reduction in the starting current, a lower voltage is applied to the stator, specially for the squirrel cage induction motors. Full voltage is only given when the motor picks up the speed.
Starting methods of the Induction motor include:
1. Direct –On– line (DOL) starters for less than(<) 10 Kw motors.
2. Star–Delta starters for the large motors. The stator winding is initially connected in a star configuration & later on changed over to a Delta connection, when the motor reaches rated speed.
3. Auto transformer.
Direct On Line Starter
1. It is simple & cheap starter for a 3-phase induction motor.
2. The contacts close against spring action.
3. This method is generally limited to smaller cage induction motors, because starting current can be as high as 8 times the full load current of motor. Use of a double cage rotor need lower staring current(approximately 4 times) & use of quick acting A.V.R enables motors of 75 Kw & above to be started direct on line.
4. An isolator is needed to isolate the starter from the supply for the maintenance.
5. Protection must be given for the motor. Some of the safety features are over-current protection, under-voltage protection, short circuit protection, etc. Control circuit voltage is sometimes stepped down with an autotransformer.
Star Delta Starter
A 3 phase motor will give 3 times the power output when the stator windings are connected in delta than if connected in the star, but will take 1/3 of the current from the supply when connected in star than when connected in delta. The starting torque generated in the star is ½ that when starting in delta.
1. A 2-position switch (manual or automatic) is provided through a timing relay.
2. Starting in star decreases the starting current.
3. When the motor has accelerated up to speed & the current is decreased to its normal value, the starter is moved to run position with the windings now connected in delta.
4. More complicated than the DOL starter, a motor with a star-delta starter may not generate sufficient torque to start against full load, so output is decreased in the start position. The motors are thus generally started under a light load condition.
5. Switching causes a transient current which may have peak values in excess of those with the DOL.
Auto Transformer Starter
1. Operated by a 2 position switch i.e. manually or automatically using a timer to change over from the start to run position.
2. In starting position supply is connected to the stator windings with an auto-transformer which decreases applied voltage to 50, 60, & 70% of normal value depending on the tapping used.
3. Reduced voltage decreases current in the motor windings with 50% tapping used motor current is halved & supply current will be half of the motor current. Thus starting current taken from the supply will only be 25% of the taken by the DOL starter.
4. For an induction motor, torque T is formed by V2, thus on 50% tapping, torque at starting is only (0.5V)2 of the obtained by the DOL starting. Hence 25% torque is generated.
5. Starters used in lager industries, it is larger in size & expensive.
6. Switching from start to run positions causing transient current, which can be greater in value than those obtained by the DOL starting.
Rotor Resistance Starter
1. This starter is used with the wound rotor induction motor. It uses an external resistance or phase in the rotor circuit so that rotor will generate a high value of torque.
2. High torque is generated at low speeds, when the external resistance is at its higher value.
3. At start, supply power is connected to the stator through a 3 pole contactor &, at a same time, an external rotor resistance is added.
4. The high resistance limits the staring current & enables the motor to start safely against high load.
5. Resistors are generally of the wire-wound type, connected through brushes & slip rings to each rotor phase. They are tapped with points brought out to fixed contactors.
6. As the motor starts, the external rotor resistance is gradually cut out of circuit; the handle or starter is turned & moves the 3 contacts simultaneously from 1 fixed contact to the next.
7. The 3 moving contacts are interconnected to form a start point for resistors.
8. To assure that the motor cannot be started till all rotor resistance is in circuit, an interlock is installed which averts the contactors from being closed till this condition is fulfilled.
Q – Explain the meaning of following terms with reference to Automatic Control System a) Control Loop b) Transmitter c) Controller d) Desired Value
Ans- What is an automatic control system?
Automation is the technology by which a process is completed without human assistance. Automation covers applications ranging from a household thermostat controlling the boiler, to a large industrial control system with tens of thousands of input measurements & output control signals.
a) Open Loop and Closed Loop Control Systems
Control Systems can be classified as open loop control systems & closed loop control systems depends on the feedback path.
In open loop control systems, output is not feedback to the input. So, the control action is independent of the desired output.
The following fig. shows the block diagram of the open loop control system.
Here, an input is applied to a controller & it produces an actuating signal or controlling signal. This signal is given as an input to the plant or process which is to be controlled. So, the plant produces an output, which is controlled. The traffic lights control system which we discussed earlier is an example of an open loop control system.
In closed loop control systems, output is feedback to the input. So, the control action is dependent on the desired output.
The following fig. shows the block diagram of negative feedback closed loop control system.
The error detector produces an error signal, which is the difference between the input & the feedback signal. This feedback signal is obtained from the block (feedback elements) by considering the output of the overall system as an input to this block. Instead of the direct input, the error signal is applied as an input to the controller.
So, the controller produces an actuating signal which controls the plant. In this combination, the output of the control system is adjusted automatically till we get the required response. Hence, the closed loop control systems are also called the automatic control systems. Traffic lights control system having sensor at the input is an example of the closed loop control system.
The differences between the open loop & the closed loop control systems are mentioned in the following table.
|Open Loop Control Systems||Closed Loop Control Systems|
|Control action is independent of the desired output.||Control action is dependent of the desired output.|
|Feedback path is not present.||Feedback path is present.|
|These are also called as non-feedback control systems.||These are also called as feedback control systems.|
|Easy to design.||Difficult to design.|
|These are economical||These are costlier.|
b) Sensors & Transmitters
A sensor element measures a process variable: flow rate, temperature, pressure, level, pH, density, composition, etc. Much of the time, the measurement is assumed from the second variable: flow & level are often computed from pressure measurements, composition from the temperature measurements.
A transducer is a device that receives a signal & retransmits it in a different form. For example, I/P transducers that convert a current signal to pneumatic form. Most industrial sensors act to detect process variables in the form of a position or the voltage change, & hence most sensors also function as the transducers. For example, a thermocouple shows a temperature change as a voltage change, while a displacer shows a level change as a change in position of a rotating element.
If the sensor element does not produce a signal suitable for the transmission through the plant, an additional transducer element is required. This combined sensor/transducer device is typically called a transmitter, at least in industrial settings. Laboratory equipment manufacturers are likely to refer to the combined device as the transducer.
Signals produced by the sensor measurement & transducer transmission are processed by the control system. They may be displayed on the control panels as indicators, stored by recorders, or used by alarms or switches. Standard symbols for these devices consist of two letter groups — the first indicates the process variable, the second the control function. So:
- PI – pressure indicator
- TC – temperature controller
- LAH – level alarm high
- FS – flow switch
- PRC – pressure recorder/controller
- TIC – temperature indicator/controller
A sensor device which provides local readout only is generally referred to as a gage. Local pressure gages and level gages (sight glasses) are very common.
When acquiring a transmitter, certain properties are important. These include:
- Accuracy — the difference between the measured value obtained by the sensor and the true value
- Repeatability — the difference between the sensed values obtained for the same true value
- Rangeability — the ratio of large to small readings that maintains accuracy
- Sensor Dynamics — the time constant of the sensor; how long it takes to detect and transmit a changed value
For an automatic control system, repeatability is the most critical of these; more so than accuracy.
The users of a transmitter must periodically calibrate the device. This is done by using the sensor to measure some fixed standard & adjusting its settings to assure accuracy & repeatability.
Users of a sensor/transmitter typically specify three values:
- the zero is the measurement value corresponding to minimum signal (20 C set to produce 4 mA)
- The range specifies the boundaries of an operating region. This term is used loosely and so it is important to distinguish between
a) the instrument range which is characteristic of the device and set by tolerances, materials of construction, etc. (0 to 500 C can be seen without mechanical failure)
b) the operating range or calibrated range which the device is set to detect (for example, 20 C to 200 C)
- The span is the size of the calibrated operating region (180 C)
Most transmitters have set screws or other means of adjusting the zero and span; this is done during the calibration process to obtain a desired operating range.
EXAMPLE: Consider a tank whose level is being measured and transmitted. The level is expected to fall between 0 and 24 inches.
The zero of the transmitter will be set to 0 inches. The span of the transmitter will be set to 24 inches. Thus, the “calibrated” or operating range of the transmitter will be 0-24 inches.
Next, consider the case where the level is expected to fall between 4 and 30 inches.
The zero will be set to 4 inches. The span will be set to 26 inches. The calibrated range will be 4-30 inches; the instrument range will remain 0-40 inches.
c) Controller- Types
In earlier days all the process controllers (P, PI, PID) are pneumatic type where there is no so much advancement or progress in the electronic components. The advantage of pneumatic controllers is its ruggedness, & major limitation is the slow response. This controllers are designed using mechanical components which works according to the air/liquid pressure. Due to this, mechanical components these controllers are strong & insensitive to the temperatures in plant. But, the response of mechanical components are slow compared to the electronic components so these controller slower in response. These pneumatic controllers will acts on the difference between the air/liquid pressure of measured signal & set point signal. The best example of pneumatic/hydraulic controller is the Hydraulic Governor which is placed to control the steam turbine parameters.
The simple scheme of the implementation of Proportional controller is shown in figure. Here the set point & measured values are converted as the hydraulic pressure signals & applied to one end of the force beam. The force beam moves up or down according to the differential pressure of measured & set values. If the set point is high then the left end of the force beam moves down & the right end of the force beam moves up. the movement ratio of left & right end of beam can be controlled by the adjustable weight which can be called as the proportional gain. Due to this movement of right end of the beam the flapper which is connected to it also moves upward towards fixed flapper. this reduces the nozzle gap between the fixed & moving flappers & hence reduces the oil flow through the nozzle. This in-turn increases the output control oil pressure & flow which is used drive the control valve actuator.
ii-Electronic or Hardwired controller:
The later advancements in the electronic components prompted the use of electronic components in the control systems. The development of this electronic control systems improved the performance of the control much faster compared to the mechanical systems. Due to the small size of the electronic components the overall size of the controller became very small. But, the system response is faster in this control system but it also has a disadvantage that it is very sensitive to temperature or any internal faults in the components. There will be some internal & external noise because of electronic components & care has to be taken to filter the noise which otherwise affects overall response of the controller.
Nowadays the electronic controllers with advanced technology which are offering with very less noise with low sensitivity for the internal faults. A simple schematic of electronic PID controller is shown in below figure.
The PID controller is constructed using basic electronic components like resistor, capacitor & op-amp comparator. The PID controller is the combination of Proportional gain, Integral component (Low-pass filter) & differential component (High Pass filter).
Output = Proportional gain * error + Low pass filter (error) + high pass filter (error).
The input comparator calculates the error between input set point & actual measured value. This error is parallelly applied to the gain amplifier, low pass filter & high pass filter. The parallel output is combined at the output node & given to the actuator to drive the actuator. The biggest disadvantage in hardwired controller is that it is not flexible for changing the logic of the controller. That is if the PI controller is constructed to control the liquid level in the tank, suppose with same controller should be used to control the temperature with PD design then the whole circuit should be redesigned for this operation. Hence it is not suitable in design & testing environment.
In the earlier days the controllers using digital systems are used in the power plant controls because of their flexibility. The complex control logic can be replaced by programming instructions & the digital computer executes these instructions & provide an output to the controller. Nowadays the advanced controllers are embedded with the Human Machine Interface (HMI) based control screens for the operator to change the controller parameters dynamically & to test the controller before putting it to the actual place. The control logic’s pertaining to the similar components like circuit breakers can be clubbed & embedded into digital system so that usage of excess hardware for each component can be reduced. This creates a lot of difference in the view of the plant cost.
The above figure shows the simple schematic digital controller for the liquid level of tank. The measured level is connected to the digital computer controller through I/O devices which converters the level value into corresponding digital signal. The digital computer scans this level signal at the specific intervals & compared with the set-point provided by the operator from the HMI control panel. Hence the error signal will be processed by the control instructions stored in controller memory & provides an output control signal to the final actuating device through the I/O device. The I/O device converts the digital output signal into analog output signal which is suitable for the actuation of final element. In the same controller we can embed the pump starting logic if the level is low by increasing I/O cards & by writing some extra programming instructions.
Block diagram of a negative feedback system used to maintain a set point in the face of a disturbance using error-controlled regulation. Positive error means feedback is too small (controller calls for an increase), & negative error means feedback is too large (controller calls for a decrease).
In control theory, a setpoint (also set point or set-point) is the desired or target value for an essential variable, or process value of the system. Departure of such a variable from its set point is one basis for error-controlled regulation using negative feedback for automatic control. The set point is generally abbreviated to SP, & the process value is usually abbreviated to PV.