Latest Chemical Interview Questions Part – 5
Are There Flow Velocity Restrictions To Avoid Static Charge Build Up In Pipelines?
There is an Australian standard “AS1020 (1984) – Control of undesirable Static Electricity” In it, there is a table for flammable hydrocarbons as follows:
Pipe Size (mm) Max Velocity (m/s)
10 8
25 4.9
50 3.5
100 2.5
200 1.8
400 1.3
600+ 1.0
This is based on pure hydrocarbons, and there is a correction, which can be applied for fluids of different conductivity. Methanol has a higher polarity than hydrocarbons and hence is more conductive.
The resistivity of diesel is 1013 ohm-m vs 108 for methanol. In addition to this, normal piping design guidelines should however be followed, such as appropriate earthing, and ensuring exit velocities into tanks of 1 m/s.
What Are Some Guidelines For Sizing A Psv For A Fire Scenario On A Vessel In A Refinery Service?
Sizing a PSV on your vessel is a matter of calculating how much heat is inputted from the fire. API-520 uses Q = FA0.82 where Q is BTU/hr, F is the insulation factor (commonly taken as 1.0 but can be less than 1.0 if your insulation will remain effective during the fire and not be dislodged by fire hoses) and finally, A is the external area in ft2.
The vapor load is then the total heat input from the fire divided by the liquid’s latent heat (BTU/lb).
As a fluid approaches its critical pressure, the latent heat as it boils decreases so the relieving flow rate increases. At the critical point, the latent heat goes to 0. Some companies simply use a minimum 50 BTU/lb latent heat others look at de-pressuring equipment, etc. One point is the protection, or potential lack of it, provided by a PSV during a fire.
The boiling liquid in the vessel from the fire helps keep the metal ‘cool’ so it retains its strength. Once the liquid is gone or the flame impinges on the wall not in contact with liquid, the metal can quickly reach a temperature where it has insufficient strength to withstand the internal pressure and you have a BLEVE. Not something, you want to be around.
As an added point to the information above, if 50 Btu/lb is not your company’s minimum standard for latent heat, here is an alternative to calculate the latent heat:
If you want to try and determine latent heat the most accurate thing would be to take a process simulation tool such as PROII, HYSYS, winsim or the like.
Set up a stream with the relevant composition and pressure and make a bubble point calculation (=the program calculates the temperature where the liquid starts to boil at the PSV set point P+allowable overpressure).
Then add a heater to the stream with a specified duty e.g. the resulting vapor stream and the duty that you specified can now be used to calculate the latent heat. Important: You have to specify a relatively low heat input because you want the latent heat for the first fraction that boils off
How Can One Determine If A Particular Solid Can Be Fluidized As In A Fluidized Bed?
Mr. Alex C. Hoffmann of the Stratingh Institute for Chemistry and Chemical Engineering states: “Whether a material can be fluidized at all is the question: if it is fine or sticky, the bed will be cohesive. It will then tend to form channels through which the aeration gas will escape rather than being dispersed through the interstices supporting the particles.
In the other extreme:
if the particles are too large and heavy the bed will not fluidized well either, but tend to be very turbulent and form a spout.” He goes on to present classification of fluidization by Geldart by use of the chart shown below. On this chart, the x-axis is the average particle diameter and the y-axis is the bulk density of the bed.
How Does A Tank-blanketing Valve Operate?
Tank Blanketing Valves provided an effective means of preventing and controlling fires in flammable liquid storage tanks. Vapors cannot be ignited in the absence of an adequate supply of oxygen. In most instances, this oxygen is provided by air drawn into the tank from the atmosphere during tank emptying operations.
Tank Blanketing Valves are installed with their inlet connected to a supply of pressurized inert gas (usually Nitrogen), and their outlet piped into the tanks vapor space. When the tank pressure drops below a predetermined level, the blanketing valve opens and allows a flow of inert gas into the vapor space. The blanketing valve reseals when pressure in the tank has returned to an acceptable level.
What Is A Common Failure Mechanism For Above Ground Atmospheric Storage Tanks?
Tanks constructed prior to the 1950’s are notorious for failing along the shell-to-bottom seam or on the side seam. The principle reason for this is that these tanks were constructed before there were established procedures and codes for such a tank (Ex/ API-650 “Welded Steel Tanks for Oil Storage”).
One of the key features of these codes and procedures was to make sure that tanks were designed to fail along the shell-to-seam such that the liquid remained largely contained.
Are There Any Methods Of Preventing Cracking Of Carbon Steel Welds In Refining Environments?
Where carbon steel is an appropriate material of construction, NACE (National Association of Corrosion Engineers) has issued the following standard: NACE RP0472, “Methods and controls to prevent in-service environmental cracking of carbon-steel weldments in corrosive petroleum refining environments”.
For welds where hardness testing is required, RP0472 give the following guidelines: A. Testing shall be taken with a portable Brinell hardness tester. Test technique guidelines are given in an appendix in the standard. B. Testing shall be done on the process side whenever possible.
C. For vessel or tank butt welds, one test per 10 feet of seam with a minimum of one location per seam is required. One test shall be done on each nozzle flange-to-neck and nozzle neck-to-shell (or neck-to-head) weld. D. A percentage of helping welds shall be tested (5 percent minimum is suggested).
E. Testing of fillet welds should be done when feasible (with the testing frequency similar to the butt welds). F. Each welding procedure used shall be tested. G. Welds that exceed 200 Brinell shall be heat treated or removed.
What Are Some Common Problems Associated With Bellow Expansion Joints?
Bellow expansion joints have gained a reputation for being “weak” points in piping. Usually they are used to remove piping stresses from equipment or to allow for minor piping moments. If they are used properly, expansion joints can save equipment and/or equipment welds from stresses generated from piping forces. The two most common complaints about bellows are
They tend to build up dirt
They are “weak” point in piping (as noted earlier).
To overcome these issues, manufacturers can began installing drains in the bellows to allow for the period purging of material.
Additionally, bellow manufacturers have placed much emphasis on installation advice and showing their customers how to protect the bellow from unnecessary damage. One such method is the use of tie rods between the end flanges to avoid pressure thrust movements (beyond the bellow’s design conditions) which are often the cause of bellow failures
What Is A Good Method Of Analyzing Powders For Composition?
A method known as Fourier transform-infrared (FT-IR) spectroscopy is often used for this purpose. FT-IR sends light beams of varying wavelength through the sample and the reflected light is analyzed by spectroscopy to find the absorption of each wavelength.
The measured wavelengths are compared with a reference laser and the sample composition can be calculated. Analect Instruments Inc. specializes in FT-IR measurement.
What Is A Common Source Of Error In Determining The Percent Spent Caustic In Refinery Applications?
In titrations, a common error made is that the technicians stop at the phenolphthalein endpoint (which is incorrect) rather than the methyl orange endpoint (which is correct). Stopping the titration too soon can cause the results to be grossly under-reported. Equation
2NaOH + H2S -> Na2S + 2H20 Equation
Na2S + H2S -> 2NaSH Overall Equation: NaOH + H2S -> NaSH + H2O
What Are Some Common Methods For Helium Leak Testing A Vacuum System?
It is common to have a location in the suction line of the pump to detect the helium. Then, the helium source is passed over the flanges and other possible sources of leakage. This is done while monitoring the detector at the pump suction for detectable amount of helium.
Alternatively, if your system can take pressure as well as vacuum you can try pressuring it up and looking for the leaks that way. As yet another alternative, you can install an IR unit to the suction of the pump and spray isopropyl alcohol on the flanges.
What Is A Good Device To Use For Obtaining Viscosity Data For A Non-newtonian Fluid?
Consider a rotational viscometer. It will measure the shear rate applied and the subsequent viscosity at the same time. You can also vary the temperature and time the stresses are applied for the truly “fun” non-Newtonian fluids. According to Cole-Parmer, “The rotational viscometer measures viscosity by determining the viscous resistance of the fluid.
This measurement is obtained by immersing a spindle into the test fluid. The viscometer measures the additional torque required for the spindle to overcome viscous resistance and regain constant speed. This value is then converted to centipoises and displayed on the instrument’s LCD readout.”
When testing a tomato sauce sample, the following results were observed: “A sample of tomato sauce was analyzed to determine the product’s viscosity profile. The test was conducted at a temperature of 25°C. An up/down speed ramp was performed from 10 to 100 RPM, giving a viscosity range of from 3,800 to 632.5 cP, over shear rates from 3.4 to 34.0 reciprocal seconds.
The test data obtained for tomato sauce shows that this product exhibits a marked shear thinning viscosity profile over the test conditions.
What Is Screen Analysis And What Are Its Applications In The Chemical Industry?
A screen analysis is the one passes solids through various sizes of screen mesh. This is done to get a particle size distribution. A group of solids is first passes through fine mesh and the amount that passes is noted, then a little larger mesh and the amount recorded and so on.
What Is A Quick Way To Calculate Frictional Pressure Drops In Carbon Steel Pipe?
The relationship shown below is valid for Reynolds numbers in the range of 2100 to 106. For smooth tubes, a constant of 23,000 should be used rather than 20,000.
How Can You Estimate A Gas Flow Based On Two Pressure Measurements?
You can use the Weymouth equation to estimate the gas flow. Below is the equation. The compressibility should be evaluated at Pavg shown below.
Nomenclature is as follows:
Q = flow rate, Million Cubic Feet per Day (MCFD) Tb = base Temperature, degrees Rankin Pb = base pressure, psia G = gas specific gravity (reference air=1) L = line length, miles T = gas temperature, degrees Rankin Z = gas compressibility factor D = pipe inside diameter, in.
E = Efficiency factor E=1 for new pipes with no bends E=0.95 for pipe less than a year old E=0.92 for average operating conditions E=0.85 for unfavorable operating conditions
What Are The Affinity Laws Associated With Dynamics Pumps?
Capacity varies directly with impeller diameter and speed.
Head varies directly with the square of impeller diameter and speed.
Horsepower varies directly with the cube of impeller diameter and speed.
How Can You Quickly Estimate The Horsepower Of A Pump?
Try this handy little equation:
Horsepower = (GPM)(Delivered Pressure) / 1715 (Efficiency) GPM = Gallon per minute of flow Delivered pressure = Discharge minus suction pressure, psi Efficiency = Fractional pump efficiency
How Can You Estimate The Efficiency Of A Pump?
The following method, developed by M.W. Kellogg, gives results within 3.5% of most manufacturers’ curves. Eff % = 80-0.2855H+3.78×10-4HF-2.23×10-7HF2+5.39×10-4H2-6.39×10-7H2F+4.0×10-10H2F2 H = Developed head, ft F = Flow in GPM (gallons per minute) Applicable for heads from 50 to 300 ft and flows from 100 to 1000 GPM
What Is The Significance Of The Minimum Flow Required By A Pump?
The minimum flow that a pump requires describes the flow below which the pump will experience what is called “shutoff”. At shutoff, most of the pump’s horsepower or work is converted to heat that can vaporize the fluid and cause cavitations that will severely damage the pump.
The minimum flow of a pump is particularly important in the design of boiler feed pumps where the fluid is near its boiling point.