Top Chemical Interview Questions Part – 6
How Can You Determine The Largest Impeller That A Pump Can Handle?
The motor amperage should be measured in the field with the pump discharge valve wide open. Subtract about 10% from the pumps maximum rated amperage. Than the maximum impeller size can be determined from A2 = A1 (d2/d1)3 A2 = Maximum amperage minus 10% A1 = Current operating amperage d2 = Maximum impeller diameter d1 = Current impeller diameter
What Is A Good Estimate For The Absolute Roughness For Epoxy Lined Carbon Steel Pipe?
The specific roughness for welded, seamless steel is .0002 ft. PVC has a specific roughness of 0.000005 ft. You may also want to consider using the Hazen-Williams formula, which lists a coefficient of 130-140 for cement-lined cast iron piping. You need to decide which is more conservative for your application.
What Types Of Valves Are Recommended For Slurry Services?
Typically straight-through diaphragm, clamp or pinch, and full-port ball valves with cavity fillers are the preferred type of slurry valves. In general, gate, needle, and globe valves are NOT recommended for slurry services.
What Is The Best Way To Configure A Bypass Line In Slurry Services?
Bypass lines should be placed ABOVE the control valve so that the slurry cannot settle out and build up in the line during bypass.
Should Slurry Pipes Be Sloped During Horizontal Runs?
If possible, slurry lines should indeed be sloped. Generally, to slope the pipes 1/2 inches for every 10 feet of pipe is recommended.
What Type Of Flow Measurement Devices Is Best For Slurries?
Any device that restricts the flow to perform measurements is not recommended for slurries. These devices include orifices and dampeners. These devices can lead to liquid/solid separation and they can lead to excessive erosion. Instead, measuring devices that do not restrict the flow should be used. One example of such a device is the magnetic flow meter.
What Type Of Pump May Be Appropriate For A Liquid Near Saturation, A Low Flow Rate, And Very Limited Npsha (net Positive Suction Head Available)?
This application is nearly perfect for a turbine regenerative type of pump. Factors that immediately identify your application and pump type are the small flow rate, low NPSHa, and high temperature. The regenerative turbine was specifically developed for these conditions and one more: high discharge pressures.
The high discharge pressure may not be necessary, but the regenerative turbine can give you an NPSHr of 0.5 feet with ease. They are particularly suited to saturated boiler feed water and your application is similar, albeit not in pressure. You can visit the site below to learn more about these types of pumps.
How Can One Estimate How The Friction Factor Changes In Heat Exchanger Tubes With A Change In Temperature?
Sieder and Tate recommended the following for determine friction factors inside heat exchanger tubes with varying temperatures: First, determine the average, bulk mean temperature in the processing line. For example if the fluid enters the line at 300 °C and leaves at 280 °C, use 290 °C to determine the physical properties and friction factors.
As for corrections: Laminar Flow If the liquid is cooling, the friction factor obtained from the mean temperature and bulk properties is divided by (bulk viscosity/wall viscosity)0.23 and for heating, it’s divided by (bulk viscosity/wall viscosity)0.38. Here, the bulk and wall viscosity are determined at the mean temperature over the length of the line.
Turbulent Flow If the liquid is cooling, the friction factor obtained from the mean temperature and bulk properties is divided by (bulk viscosity/wall viscosity)0.11 and for heating, it’s divided by (bulk viscosity/wall viscosity)0.17.
What Are The Advantages And Disadvantages Of Using Gear Pumps?
Gear pumps are a type of positive displacement pump that are appropriate for pumping relatively high pressures and low capacities. Advantages include the ability to handle a wide range of viscosities, less sensitivity to cavitation (than centrifugal style pumps), relatively simple to maintain and rebuild.
Disadvantages can include a limited array of materials of construction due to tight tolerances required, high shear placed on the liquid, and the fluid must be free of abrasives. Also, note that gear pumps must be controlled via the motor speed. Throttling the discharge is not an acceptable means of control.
Under What Circumstances Are Vortex Flowmeters The Most Accurate?
The accuracy of vortex flowmeters can be within 1% so long as they’re being operating within their recommended flow range, have a steady stream, and you have 10 pipe diameters of straight pipe behind the in front of the flowmeters. Outside of these parameters, these flowmeters are not accurate.
What Are Some Factors To Consider When Trying Choosing Between A Dry Screw Compressor And An Oil-flooded Screw Compressor?
Screw compressors utilize a pair of “meshing” helical screws to compress gases. These types of compressors a generally appropriate for a flow range of 85-170 m3/h (3000-6000 acfm) and discharge pressures in the range of 2070-2760 kPa (300-400 psig).
As the name implies, dry screw compressor run dry while oil-flooded compressors use oil for bearing lubrication as well as to seal the compression chamber. The oil also carries the heat from the compression away from the compressor. This heat is typically rejected to an external heat exchanger. Some factors to consider when choosing between the two types of screw compressors include
Is the process gas compatible with the oil? If the answer is no, use dry type Does the process gas have to be oil free? If the answer is yes, use dry type is efficiency the top priority. If the answer is yes, use oil-flooded type Are you looking to minimize shaft-seal leakage.
If the answer is yes, use oil-flooded type Are there any liquids in the incoming gas. If the answer is yes, use oil-flooded type Does the gas contain small particulate matter? If the answer is yes, use dry type these and other guidelines can help in choosing between the two types of screw compressors.
Is There A Handy Way To Determine If A Horizontal Pipe Is Running Full If The Flow Rate Is Known?
If Q/d2.5 is greater than or equal to about 10.2, then the pipe is said to full. In this case, Q is in GPM (U.S. Imperial gallons per minute) and d is in inches. Reference: Pocket Guide to Chemical Engineering, ISBN: 0884153118
How Is Plate Heat Exchangers Used In An Ammonia Refrigeration System?
Plate heat exchangers are widely used in ammonia refrigeration systems, and they can be much smaller than the equivalent tubular exchanger can. They work best flooded. A flooded exchanger system needs a way to separate the liquid from the vapor. A typical system has a vessel, which acts as knockout drum, accumulator, and header tank in one, along with the heat exchanger.
Liquid ammonia flows from the vessel to the exchanger, and liquid/vapor is returned to the middle of the drum. Vapor is removed from the top of the drum. The liquid/vapor mixture from the exchanger has a lower density than the liquid entering the exchanger, so gravity provides the driving force to circulate the refrigerant.
Is There A Difference In Mtd (mean Temperature Difference) Between “e” And “j” (divided Flow) Type Shell And Tube Heat Exchangers?
Divided flow (shell type J) does not have the same correction as the usual flow pattern (shell type E). Thermal design program make this correction factor mistake. True, there is very little difference at correction factors above 0.90. However, there is a difference at lower values.
For example, Equal outlet temperatures Shell type “E” correction Fn = 0.805 Shell type “J” correction Fn = 0.775 Cold outlet 5F higher than hot outlet Shell type “E” correction Fn = 0.765 Shell type “J” correction Fn = 0.65 Contact us if you do not have MTD correction factor charts for divided flow.
TEMA has one chart for a single shell but it gives high values for the above examples and it is hard to read in this range. Source: Gulley Computer Associates
What Is A Good Method Of Minimizing Shell Side Pressure Drop In A Shell And Tube Exchanger?
When shell pressure drop is critical and impingement protection is required, use rods or tube protectors in top rows instead of a plate. These create less pressure drop and better distribution than an impingement plate. An impingement plate causes an abrupt 90-degree turn of the shell stream, which causes extra pressure drop. Source: Gulley Computer Associates
Are There Any Alternatives To Scraping A Shell And Tube If A Capacity Increase Will Make The Pressure Drop Across The Exchanger Too Large?
When an increase in capacity will cause excessive pressure drop, you may not have to junk the heat exchangers. A relatively inexpensive alteration is to reduce the number of tube passes. Other possibilities are arranging the exchangers in parallel or using lowfins or other special tubing. Source: Gulley Computer Associates
Are Some Heat Transfer Services More Prone To Tube Vibration That Others For A Shell And Tube Exchanger?
Bundle vibration can cause leaks due to tubes being cut at the baffle holes or tubes being loosened at the tubesheet joint. There are services that are more likely to cause bundle vibration than others are.
The most likely service to cause vibration is a single-phase gas operating at a pressure of 100 to 300 PSI. This is especially true if the baffle spacing is greater than 18 inches and single segmental.
Source:
Gulley Computer Associates