Piping Interview Questions and Answers - Part 05
What is preferred location for drain?
Answer: Must be located at the bottom section of drum.
What are the steps in selection of valve?
Answer: What to handle, liquid, gas or powder, fluid nature, function, construction material, disc type, stem type, how to operate, bonnet type, body ends, delivery time, cost, warranty.
What are functions of valves?
Answer: Isolation, regulation, non-return and special purposes.
What are isolating valves?
Answer: Gate, ball, plug, piston, diaphragm, butterfly, pinch.
What are regulation valves?
Answer: Globe, needle, butterfly, diaphragm, piston, pinch.
What are non-return valves?
Answer: check valve,
What are special valves?
Answer: multi-port, flush bottom, float, foot, pressure relief, breather.
What materials are used for construction of valves?
Answer: Cast iron, bronze, gun metal, carbon steel, stainless steel, alloy carbon steel, polypropylene and other plastics, special alloys.
What is trim?
Answer: Trim is composed of stem, seat surfaces, back seat bushing and other small internal parts that normally contact the surface fluid.
Which standard specifies trim numbers for valve?
Answer: API 600.
What are wetted parts of valve?
Answer: All parts that come in contact with surface fluid are called wetted parts.
What is wire drawing?
Answer: This term is used to indicate the premature erosion of the valve seat caused by excessive velocity between seat and seat disc, when valve is not closed tightly.
What is straight through valve?
Answer: Valve in which the closing operation of valve is achieved by 90 degrees turn of the closing element.
What pressure tests are carried out on valves?
Answer: Shell-hydrostatic, seat-hydrostatic, seat-pneumatic
What are available valve operators?
Answer: Hand-lever, hand-wheel, chain operator, gear operator etc.
What is the full form of ASME?
Answer: American Society for Mechanical Engineers.
Which Piping code is used for Power piping and which code is used for Process Piping design?
Answer:
Power Piping: ASME B 31.1
Process Piping: ASME B 31.3
What are the main differences between ASME B 31.1 and ASME B 31.3?
Answer: The main differences are listed below:
Material allowable stresses are different in both codes.
Stress increases due to occasional loads are different in each code.
B 31.3 neglects torsion while calculating sustained stresses, but B 31.1 includes it.
Sustained stress calculation is specific in B 31.1 but undefined for B 31.3.
B 31.1 intensifies torsion but B31.3 does not intensify it.
How to calculate the basic allowable stress for a material?
Answer: The basic allowable stress is defined in respective code. For example as per B 31.3 the basic allowable stress for a material is the minimum of the following:
- 1/3rd of tensile strength at design temperature.
- 2/3rd of yield strength at design temperature.
- 100% of average stress for a creep rate of 0.01% per 1000 hours.
- 67% of average stress for rupture at the end of 100000 hours.
- 80% of minimum stress for ruptures at the end of 100000 hours.
For austenitic stainless steel or nickel alloys the lower of yield strength and 90% of yield strength at temperature.
For structural grade materials 0.92 times of the lowest value of point (a) to (f)
What is the main difference between Constant and Variable Spring Hanger? When to use these hangers?
Answer: In Constant Spring hanger the load remains constant throughout its travel range. But In variable spring hanger the load varies with displacement.
Spring hangers are used when thermal displacements are upwards and piping system is lifted off from the support position. Variable spring hanger is preferable as this is less costly.
Constant springs are used:
- When thermal displacement exceeds 50 mm
- When variability exceeds 25%
- Sometimes when piping is connected to strain sensitive equipment like steam turbines, centrifugal compressors etc and it becomes very difficult to qualify nozzle loads by variable spring hangers, constant spring hangers can be used.
What do you mean by variability? What is the industry approved limit for variability?
Answer: Variability= (Hot Load-Cold load)/Hot load = (Spring Constant × displacement)/Hot load.
Limit for variability for variable spring hangers is 25%.
What are the major parameters you must address while making a Spring Data-sheet?
Answer: Major parameters are: Spring TAG, Cold load/Installed load, Vertical and horizontal movement, Piping design temperature, Piping Material, Insulation thickness, Hydro-test load, Line number etc.
How to calculate the height of a Variable Spring hanger?
- Answer: Select the height from vendor catalog based on spring size and stiffness class.
- For base mounted variable spring hanger the height is mentioned directly. It is the spring height.
- For top mounted variable spring hanger’s ass spring height with turnbuckle length, clamp/lug length and rod length.
Can you select a proper spring hanger if you do not make it program defined in your software?
What is the procedure?
Answer: In your system first decide the location where you want to install the spring. Then remove all nearby supports which are not taking load in thermal operating case. Now run the program and the sustained load on that support node is your hot load. The thermal movement in that location is your thermal movement for your spring. Now assume variability for your spring. So calculate
Spring constant = (Hot load × variability)/displacement. Now with spring constant and hot load enter any vendor catalog to select spring inside the travel range.
What is the software available for performing piping stress analysis?
Answer: Caesar II, AutoPipe etc.