Control Systems Interview Questions Answers
What is Resonant peak? What is Resonant Frequency? Answer: The frequency at which resonant peak occurs is called the resonant frequency. Resonance frequency explains about the speed of the transient response.youtube.com What is Cut-off rate? Answer: The slope of the log-magnitude curve near the cut-off frequency is called the cut-off rate. What is Phase cross over frequency? Answer: The frequency at which the phase of the open loop transfer function is 180o is called the phase cross over frequency. What is Gain Margin? Answer: The Gain Margin is defined as the reciprocal of the magnitude of open loop transfer function at phase cross over frequency. Answer: The phase margin is the amount of additional phase lag at the gain cross over frequency required to bring the system to the verge of instability.
We at Saudi Aramco developed a slightly modified categorization to simplify segregation and separation. The IEEE NSL levels settled into three levels based on our practical experiences. Level 2: Low susceptibility with switching signals greater than 30 V, analog signals greater than 50 V, and 120-240 AC feeders less than 20 amps. Signal segregation In instrumentation cabling, it is a good practice to segregate various signals from each other. For optimum segregation, each type of signal (within each NSL) shall transmit on dedicated cables and rout to dedicated junction boxes. For example, 4-20 mA signals shall rout on separate cables from all other signals under NSL-1. The same applies on all other signal types. From the junction boxes to the control room, the cables for each NSL level can share the same cable tray or trench.
They can also share the same marshalling cabinet provided the cables get enough air and adequate terminal strip identifications are in place.youtube.com In addition, all emergency shutdown signals should have their own cables, junction boxes, and marshalling cabinets. They also have to be segregated based on signal type as discussed above. Separation between different NSL: The recommended separation distances are from IEEE-518 and PIP standard PCCEL001. It is important to note the zero separation distances between signals of the same NSL do not mean different signals within the same NSL can use the same cable. Separate cables must carry and serve different signals even if they are of the same NSL. Common return wire for multiple signals: Utilizing a common wire for multiple signals is a common bad wiring practice that results in many covert noise problems.
This wiring practice is common in wiring multiple solenoids associated with equipment, MOV wiring, relays, and in some cases in DCS or PLC loops. The temptation to use such wiring finds supporters especially when designing or executing projects or when there are in-house projects that would utilize spare wiring. It would be justified based on cost savings but always has a negative impact on the integrity of the associated loops. To protect against common impedance coupling, each signal should have its own return wire extending from the source to the destination. Avoid using one or two return wires for multiple signals.
In automation systems, proper grounding plays a significant role in the overall health and integrity of process signals. It protects the automation systems from potential damages due to surges, voltage fluctuations, lightning, and short circuits. In addition, proper grounding hierarchy helps mitigate signal noises and interferences by providing a low resistance path for these unwanted voltages and currents that could result in safety hazards or degradation to process control signals. When attending to field problems associated with signal noise, erratic spikes, or interference problems, we found the majority of these problems stemmed from poor grounding. To ensure proper grounding of instrumentation systems, one must follow a clear grounding scheme. One should carefully evaluate the overall grounding system when diagnosing a problem or when designing for new plants.
These areas are grounding in the field, interconnection wiring, and grounding within central control or process interface buildings. Grounding in the field: In the field, the enclosures of all instrument devices have to connect to ground, typically the plant overall grounding grid, or bond to an electrically conductive structure that is connected to the grid. Raceways such as conduits and trays have to ground at both ends.youtube.com Handling of shield drain wires: One should properly cut and tape the shield and its drain wire in the field, near the instrument. From the field instruments all the way to the marshalling cabinets, the shield drain wires should be treated and terminated similar to the signal wires.
Exposed parts of the drain wires within junction boxes or marshalling cabinets should be inside insertion jackets to protect against the possibility of multiple drain wires touching each other. Once the loop reaches the marshalling cabinet, the shield drain wires have to consolidate and terminate at the DC and shield grounding bus bar. In addition, all spare pairs or triads extending between the field junction boxes and marshalling cabinets should terminate at both ends. In some cases, it would be useful to ground the spare wiring in the marshalling cabinets to minimize potential noise pick up. All shield drain wires and DC common wires must merge and connect to the isolated bus bars. It is vital to ensure shield drain wires ground at one end, typically in the marshalling cabinets.
Grounding them at both ends may result in ground loops, which happen to be one of the main causes of signal noise. The isolated DC and shield grounding bus bars within all cabinets should then be consolidate into a master instrument grounding bus bar within that building. Similarly, all AC bus bars within these cabinets should come together at a master safety-ground bus bar within the building. The two master ground bus bars should then be connected to the plant overall grid.endress.com Saeed M. AL-Abeediah (saeed.abeediah@aramco.com) is a senior engineering consultant in the process and control systems department and PID/instrumentation unit at Saudi Aramco.
The sample drawing presented here represents a typical arrangement generally used to represent control valves on P&ID. Depending on the projects legend sheets, control valves may be represented by globe or gate valves. Here a globe valve symbol is used. First of all a proper valve symbol should be selected to represent the control valve as per the project standards. Generally, the control valve size is smaller than the corresponding line size. This change in diameter should be clearly indicated in the P&ID with reducer and expander. Block valves should be provided upstream and downstream of the control valves in case of shutdown and maintenance. A drain valve is normally provided between the control valve and upstream block valve.
If the control valve is of ‘Fail Open’ type, this drain valve is sufficient to drain the piping segment. If the control valve is of ‘Fail Close’ or ‘Fail in Position’ type, then additional drain valve is required between the control valve and downstream block valve as shown in the sample drawing. Normally, either a bypass or a handwheel is provided for control valves which are under continuous service. If two or more control valves are installed in parallel, bypass or handwheel is not required. The choice between providing either a bypass or a handwheel for the control valve is made based on the size of the control valve. For control valves bigger than a certain size, provision of handwheel is preferred.
For control valves smaller than certain size, provision of bypass with block valves is preferred. For control valves on certain critical services, a spare control valve may be installed on the bypass of main control valve. This limiting control valve size between handwheel and bypass is specific for a project and may vary from one project to another. If the control valve is equipped with a handwheel, then only the drain between control valve and upstream block valve is sufficient for draining by opening the control valve using handwheel. Normally globe valve is selected as the bypass valve on the control valve as it allows better control with opening. P&ID for control valves, as per the project standards. All the guidelines given here are very general and may be modified as per specific requirements of any particular project.
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