Mechanical engineer interviews questions

Anaerobic  Process Principle:  Anaerobic treatment is efficient method for treating industrial effluent water. It uses anaerobic bacteria to convert organic pollutants or chemical oxygen demand into biogas. i.e. it is a biological digestion process through which bacteria breakdown organic matter in the absence of oxygen in closed vessel. This process consists of two stages : An Acidification phase - anaerobes breakdown complex organic compounds into simpler, short chain volatile organic acids.  Methane production phase - anaerobes synthesize organic acids to form acetate, CO2, H2 and anaerobic microorganism's act upon newly formed molecules to form CH4 and CO2.    Microbial reactions take place in the absence of molecular/ free oxygen, Reactions products are carbon dioxide, methane and excess biomass. It reduces BOD, COD and TSS as well as produce biogas as byproduct. Applications:  Net Sludge Yield: Relatively low Waste water with medium to high o...

Aerobic Process Principle:  Microbial reactions take place in the presence of molecular/ free oxygen, Reactions products are carbon dioxide, water and excess biomass. Applications:  Net Sludge Yield: Relatively high Waste water with low to medium organic impurities (COD < 1000 ppm) and for waste water that are difficult to biodegrade e.g. industry waste water, municipal sewage, refinery waste water etc. Post Treatment : Typically direct discharge or filtration Example Technologies: Activated Sludge e.g. Extended Aeration, MBR, Fixed Film Processes e.g. Trickling Filter/Bio tower

As per ASME code, for pressure vessels and piping, if the thickness of the parts being welded exceeds a specified value, PWHT cannot be avoided. Charpy test properties of the material and the minimum service temperature required should determine this. In general, US codes limit the thickness of C / C-Mn Steels to 32 mm, beyond which PWHT cannot be avoided whether it be pressure vessel, piping or storage tank. As per ASME code VIII Div.1 UCS-79 as it stated that the required material will need heat treatment when the resulting fiber elongation is more than 5% from the as rolled condition. When comes to the low alloy steels, ASME B31.3 has set a limit thickness of 19 mm, above which PWHT cannot be avoided. Refer ASME Section VIII - Div. 1, UCS-56 for Post Weld Heat Treatment requirements detailed. Also refer UCS-68 where it is mentioned that PWHT cannot be avoided when minimum design metal temperature is colder than -12 degree C.

Tube is specified by its outside diameter. Tubing is normally "cold drawn" seamless with the O.D. the same as the size designation (i.e.: 1" (25mm) inch tubing has an O. D. of 1 inch (25mm)). Tube is OD x wall unless DOM (Drawn over Mandrel) which is technically OD & ID, DOM in SSID (Special smooth ID) is ID and Wall.  Pipe is specified by its nominal inside diameter and Pipe has a wider variance in wall thickness. Pipe is to the pipe charts as per SB's comments NPS and schedule. Pipe can be made off the "pipe charts" but the reality is the more generous tolerances vs tube and uses the NPS for the first designation. It gets a few wrinkles in welded vs seamless for tolerances. 1-1/2" SCH40 Pipe, the 1-1/2" is the inside dimension but the actual inside dimension might be 1-5/8" instead. Pipe is made seamless or rolled and welded (sometimes referred to as ERW pipe) . The pipe sizes up to 12' has a nominal size which is close bu...

Yes, it is true and yes this material could be welded. It can be welding as per following standard. ASTM 216/216M -"Specification for steel castings, carbon, Suitable for fusion welding for High temperature service" Bad reputation of castings are due to, 1. Most of the casting standards are lenient in composition, heat treatment and inspection. 2. The control exercised by the foundries, due to lack of knowledge or economics, in both composition and heat treatments. Due to this the quality factor on general castings (derating factor) is specified in the ASME/VIII which with the proper NDE and testing could be again made 100%. However since these additional NDE requirements are not mandatory (supplementary requirements) would be enforced only if specified in specific purchase orders. These (Radiography, LPI, MPI etc) ensures the structural rigidity. It is possible for welding but definitely not recommended for strength concerns. Cast Iron/steel are britt...

Diesel engine has higher compression ratio, hence it is subjected to very high cylinder pressures which may cause higher vibrations. In order to counter the vibrations the diesel engines have heavier flywheel than petrol engines. Diesel engine is bigger causing it absorbing vibration, a lot of compression going on in this cycle. The principle of a flywheel is to reduce variations in the rotational speed of the shaft to which they are attached. As such they dampen an impulsive drive or load.  All internal combustion engines have speed variations and offer an impulsive drive due to their cyclic processes of induction, compression, combustion and exhaust. The extent of the speed variations is dependent on the number of cylinders and the engine configuration. A single cylinder engine exhibits the most significant difference. The momentum (stored energy) of the flywheel reduces these variations by accepting and discharging energy as required.  Diesel engines due to th...

Nozzle: Reduction in pressure and increase in velocity of fluid  Diffuser: Reduction in velocity and Increase in pressure of fluid A nozzle accelerates the flow of a gas or fluid with a resulting decrease in the pressure while a diffuser does pretty much the opposite; it slows down the flow of the gas or fluid and has a resulting increase in the pressure. The mass flow, for a system that has both nozzle and diffuser, remains the same but there is a decrease (nozzle) or increase (diffuser) in the cross sectional area of the boundary of the system. In Incomprehensible Flow.  In Nozzle Static Pressure is decrease and Velocity is increased.  In Diffuser Static Pressure is increased and velocity is decreased.  Nozzle in compressible supersonic Nozzle  Nozzle has generally three sections. 1. Convergent..2. Divergent. 3. Throat.  In subsonic flow Mach No<1 : Static Pressure is decreased velocity increase  In throat Mach ...

The full form of DTSI is "Digital Twin Spark Ignition." Two sparks, offset by a time interval, is used to ignite the fuel air mixture. The initial spark burns most of the mixture while a second spark is used to burn the rest. This increases fuel economy as less unburnt fuel escapes through the exhaust port. It also provides increased power.  A knock occurs when the fuel is ignited while the piston is undergoing the compression stroke not when it is going back. When the second spark fires, the amount of charge in the cylinder is considerably less and therefore the pressure spike created by it does not produce any jerking effect. DTSI system has been the driving technology in the new generation motor cycles to be more fuel economical & to meet the emission standards. We are all aware of Internal Combustion engine with single spark-plug system. DTSI engines use two spark plug one at the top of the combustion chamber & the other somewhere in-between the TDC ...

Design – It is conceptualization of an idea based on theory and actual practice or experience.  Drafting – It is putting the conceptualized idea in paper which would be the basis or reference in its implementation.  Design - It is the complete process of calculations and drawing. Drafting - It means drawing. It is the part of design. Design - It is knowledge base concept  Drafting – It is art. Design - It is a raw idea conceptualized through gathering of facts, ideas and research; a product of an initial study.  Drafting – It is when putting such design into its detailed drawing/plan, complete with dimensions ready for implementation. A designer is someone with subject matter knowledge and is experienced enough to make decisions influencing a design.  A draftsman delineates a design without influencing that design. Drafting is the visual representation of a Design. Difference can be explained by just a simple example.. A riv...

For the FD, calculate your boiler´s total air requirements. The total air flow is the sum of theoretical air plus excess air. Once you have the air in mass units ( kg/s , lb/h ) convert it to volume units ( multiply by the specific volume of air at a standard temperature, for example 20 deg C and correct for the ambient temperature and altitude of your locality ).  For the fan discharge pressure, you will need to add all of the pressure drops of your boiler system ( air pressure drop in air preheater and, in the air ductwork, in the burner wind box and register. Flue gas pressure drops in the radiant superheater panels, in the reheater, in the convective superheater panels, in the boiler bank, in the economizer banks, in the gas side of your air preheater and, in the scrubbers, cyclone dust collectors and bag or electrostatic collectors)  If your boiler system is of the balanced draught type, the FD handles air pressure drops and the ID fan flue gas pressure drops a...

The ASME Code Section VIII, Division 1, does not have mandatory rules for the calculations of vessel component thickness for hydrostatic test condition. The requirement for such calculations comes from the customer in its vessel specification. As the test is for short duration at ambient temperature, the customer specification often requires that no components of the vessel to be subjected to above the 90% of the material yield strength during the test.  The pressure due to liquid head is always added to the design internal pressure and the total pressure is used to calculate the thickness in the UG-27 (shell) and UG-32 (heads). The Code allowable stress value at the design temperature is used in the formula. The operating liquid may not be always full of the vessel. For example, consider a tall distillation tower, where liquid level of sp. gr. greater than water is at the maximum just below the lowest down comer, which could be only one-sixth or less of the tower height....

There are a lot of tables, but you can use formula of nominal stud diameter x 1.5 + 1/8 inch in most cases, for example a 1-1/2" stud uses a 2-3/8" wrench size.

ASME B16.5 or ASME B16.47 contain definitions of class, pressure rating,, and pressure-temperature rating. "Pressure Rating Designation Class, followed by a dimensionless number, is the designation for pressure-temperature ratings as follows: Class 150, Class 300, Class 600, Class 900 etc.. "Pressure-Temperature Ratings  Pressure-temperature ratings are the maximum allowable working gage pressure...at the temperature shown in Tables...for the applicable material and class designation..." Rating is the qualifying parameter.  Class is a designation or name for rating. The pressure classes were originally designated as “pound”, that is, Class 150 = 150 pound flange. The relationship between pressure class and rating for flanges is a complicated one and is defined in ASME B16.5. For most carbon steels, the rating of a Class 150 flange is 285 psi at 100 F. Similarly, the rating of the Class 300 flange is 740 psi. For threaded and socket welding fitt...

The flow rate Q, can be calculated using the tank volume,  Q=dV/dt Where, V- tank volume,  B- the bulk modulus of water, dP/dt -the known pressure surge rate,  Q=dV/dt=(dP/dt)(V/B)  or  dV= (V/B)(dP)  Since water is virtually incompressible, only a small flow of water needs to be released at a rate on par with the pressure surge rate to prevent rupturing the tank.

Laboratory chemical wet analysis test. XRF portable machine. X-ray Gun. The most popular standard for this type of analysis in metallic materials is ASTM-E415. This is considered the most accurate method.  ASTM E-415- Standard Test Method for Analysis of Carbon and Low-Alloy Steel by Spark Atomic Emission Spectrometry.

Condition Monitoring, is all about proactive/predictive maintenance. How we use gathered data such as maintenance history, visual inspection, oil sample history, etc, and generate backlog activity to prevent further damage Condition monitoring is the process of determining the condition of machinery while in operation. The key to a successful condition monitoring program includes:  Knowing what to listen for  How to interpret it  When to put this knowledge to use  successfully using a program enables the repair of problem components prior to failure. Condition monitoring not only helps plant personnel reduce the possibility of catastrophic failure, but also allows them to order parts in advance, schedule manpower, and plan other repairs during the downtime.  On-line monitoring is practiced by the installed supervisory instrumentation of the major plant equipment. This includes instrumentation such as vibration, pressure, differential pressure,...

Before preparing preventive maintenance for your equipment, forget it is an equipment, think about it as a living creature, best to consider it your child, then ask yourself, how can I keep my child healthy all the time, think what are different diseases he can catch, and put a plan to vaccinate him, and measuring points that will tell you how is he performing, then in case he got ill, how you can treat him to return to his shape! All these together will create the preventive plan. The best approach if accurate information and technical knowledge is readily available and time allows is to build your PM plan based upon the actual failure modes experienced or likely to experience. This means performing an FMEA. Mark suggested purchasing a template of ready made PM's if the equipment is within the data base. I like this approach, however running it through a validation filter with members of your team is an important step. You are likely to find and make subtle changes, but the ...

Planning = facilitates preparation and readiness  Scheduling = facilitates execution and completion Planning is the "What" and the "How";  Scheduling is the "Who" and the "When".  Planning simply means to plan the event.  Scheduling means to set a time frame to accomplish the event or actually do the work. Planning is General.  Scheduling is specific / detailed.  For Ex. Planning: Part ABC to be made 5000 Nos. in first shift as per marketing requirement.  Scheduling contains: Men, Machine, Material, etc.,  Planning is thought Process including Collection of data and listing down all activities that must be done to achieve perfect end result.(500 MT ton of desired output is--- what is Plan)  Scheduling is distribution of activity plan to respective locations that must be adhered to provide Planned Output.  Well Known Saying -- Plan Your Work and Work your plan. Planning is all about describing t...

Company role is to allow the maintenance department to perform the needed maintenance when required thru breakdown & preventive measures. This should also include training for the maintenance department concerning the equipment used in the facility. In a lot of cases, the company must take a hold of the purse strings to control equipment maintenance & have management for maintenance justify the sometimes overwhelming cost that can be often times seen. The most important role they can play is to include maintenance as part of their overall strategy of doing business. By elevating maintenance to a strategic function you open the door to interdepartmental cooperation and begin building a partnership focused on reliability and, in the end, a more productive company. The true function of Maintenance is to Maintain Equipment and Production Systems in a continuously improving state of reliable operation that meets its functional specifications and requirements of the Organiz...

TPM was invented by JIPM to increase the overall productivity of the process or machine by enhancing maintenance, reducing unscheduled downtime, increasing production capacity, and quality (example the OEE module).  RCM is a boost performance process that focuses on Improving the reliability and life time of the product or machine. This can be done through improving the maintenance, the manufacturing process, and the design process by applying a total quality management system.  TPM views itself as supporting Production by being productive in its tasks i.e. Total Productive Maintenance.  Whereas RCM is centered on Reliability and supporting the entire business with the understanding that Asset Reliability supports all other functions, such as Production Trough-put, Quality, Safety, Environmental Compliance, Asset Management, Sales, Investor Confidence and continued Business Growth.  TPM promotes the use of planned maintenance, e.g. Preventive mainte...

REACTIVE means - wait to something happen to act. PROACTIVE means- Anticipate the actions before something happen. Predictive and Preventive maintenance are strategies to anticipate the consequences. Predictive should have "Run to Failure" data as the baseline data for life expectancy. You can predict when a part will fail. Corrective Maintenance or repair failures are Reactive mode.  RCM propose several strategies in maintenance according with the Operative Context and consequences in the asset and organization, in some cases is better works in the case "Run to Fail". Proactive maintenance is some improvement for solving problem or increase production and reduce cost, including quality. Proactive maintenance is based on records and facts. We can for sure minimize down time by planning ahead based on records and observations. Some improvement you take from RCA (root cause analysis) and KAIZEN (continuous improvement). As I see it, Proactive Ma...

Predictive maintenance can be Vibration Analysis, Ultrasonic Analysis, MCA and Thermography.  Good Practice would be Acoustic Bearing Lubrication (greasing), Proper shaft alignment for shaft coupled installations, proper pulley alignment and belt tension for belt coupled installations.  Trimming keyway keys to the proper length and checking balance after install. Tropical environments, may recommend heaters if they are shut off for any length of time. Also can specify epoxy sealed windings for tropical environments.  Considerations in storing larger spare motors 10 hp plus should keep them in a dry, clean, vibration free place and rotate the motor shaft a few revolutions once every 30 days to prevent false brinelling in the bearings and cover the output shaft with something to prevent shaft corrosion. You can also use MCA to checking winding health in storage every 3 months. If no good dry place to store motors heaters can be used in storage to keep windings...

One of the most dangerous things about H2S is that it deadens your sense of smell at low levels. If you begin to smell H2S, you should immediately vacate the area. The toxicity comes from the affinity to bind to iron and prevent cellular respiration in such places like in the mitochondria of any cells or on hemoglobin on red blood cells. In short its toxic because it prevents our cells from getting oxygen properly. As the H2S levels increase, an untrained individual may take this as a sign that they are now out of harm’s way, but unfortunately they may be exposed to lethal levels of the chemical. If you are in a plant that processes H2S, ask for a H2S badge or monitor. At least 1 in 5 people working in an area should have a monitor on them. It also have wide explosive limit and spark is enough to initiate ignition

Predictive maintenance tools are given below: V ibration Analysis -It is useful to check health check of rotating equipment's by  checking vibration of bearing. Vibration checking shall be carried out for horizontal, vertical and axial vibration. Thermography - Thermography is carried out for checking invisible thermal signatures of impeding damage before the damage occur in electrical circuit. When current flows through an electrical circuit electrical energy converted in to heat energy. Sometimes due to abnormality high heat generated and it is wasteful and damaging electrical circuit. Ultrasonic - It uses high frequency sound energy to carry out examination and measurements. It is also useful to find out dimensional measurement and detection of flow in pipes. In non destructive material testing method it is useful for find out discontinuity in material. Infrared temperature - It is useful to check temperature of gearbox, rotating equipment's bearing te...

Determine the Capacity Required (Unit- M3/Hr) Determine the Viscosity of the Liquid (Unit-Centistokes) Determine the Differential Pressure of the System (Unit-Meter) Pumps with the Best efficiency point at the design operating point should be selected.  Best efficiency point is the operating point at which a pump most efficiently converts shaft power to flow. Higher energy efficiencies are achieved by selecting the pump to operate as close as possible to the BEP. Do not oversize pumps. This leads to uneconomical operation and generally narrows the safe operation range of capacities. An oversized pump, while providing sufficient flow, can cause other negative consequences, such as: Higher purchase costs for the pump and motor assembly. Larger electrical switchgear and cables. Higher energy costs. Higher maintenance requirements; because as pumps operate further from their BEP they experience greater stress. Do not try to select pumps with excessively lo...