HVAC INTERVIEW QUESTIONS FOR PIPING & FITTINGS

1. What is ERW & Seamless PIPING?

ERW Pipe:

In the ERW / EFW / HFW pipe process, the first plate is formed in a cylindrical shape and the longitudinal edges of the cylinder formed are welded by flash-welding, low-frequency resistance-welding, high-frequency induction welding, or high-frequency resistance welding.

Seamless Pipe:

Seamless Steel Pipe is made from a solid round steel ‘billet’ which is heated and pushed or pulled over a form until the steel is shaped into a hollow pipe. The hollow pipe is then extruded through a die and mandrel combination to reduce the outside diameter and to expand the inside diameter

2. Type of piping used according to the application?

ERW MS pipe for HVAC applications.

For Compressed air and corrosive applications use G.I Pipe or SS pipe.

3. What is the Schedule and type of class? Selection?

Pipe Schedule:

Pipe Schedule is the term used to describe the thickness of a PIPING. The outside diameter of a pipe is the same for all schedules in nominal pipe diameter. Standard pipe schedules or pipes sizes as given by ANSI / ASME B36.10M and API

Pipe Class:

Pipe class is developed considering Operating Pressure, temperature, and corrosive environment. Different material specifications are segregated in separate “Piping Class”. The pipe class is part of the Piping specification.

In ERW pipes and tubes there are three classes:

Class A
Class B
Class C

4. Advantages of GI Pipe over MS Pipe?

Lowest first cost: Galvanizing is lower in first cost than many other commonly specified protective coatings for steel. (The application cost of labor-intensive coatings such as painting has risen far more than the cost of factory operations such as galvanizing.)
Less maintenance/Lowest long-term cost. Even in cases where the initial cost of galvanizing is higher than alternative coatings, galvanizing is almost invariably cheapest in the long term (because it lasts longer and needs less maintenance). And, maintenance causes problems and adds to costs when structures are in remote areas, and when plant shutdown or disruption to production is involved.
Long life. The life expectancy of galvanized coatings on typical structural members is far in excess of 50 years in most rural environments, and 20 to 25 years plus, even in severe urban and coastal exposure.
Toughest coating. A galvanized coating has a unique metallurgical structure which gives outstanding resistance to mechanical damage in transport, erection and service.

5. Difference between SS-304 and SS-316.?

SS 304 contains 18% chromium and 8% nickel while SS 316 contains 16% chromium, 10% nickel and 2% molybdenum. The molybdenum is added to help resist corrosion to chlorides.

6. Difference between UPVC, CPVC, UPVC, and HDPE PIPING?

PVC (Polyvinyl chloride) – Polyvinyl chloride, more correctly but unusually poly (vinyl chloride), commonly abbreviated PVC, is the third-most widely produced synthetic plastic polymer, after polyethylene and polypropylene.
CPVC (Chlorinated polyvinyl chloride) – Chlorinated polyvinyl chloride (CPVC) is a thermoplastic produced by chlorination of polyvinyl chloride (PVC) resin which is significantly more flexible and can withstand higher temperatures than standard PVC.
Un-plasticized poly (vinyl chloride) (uPVC) (Polyvinyl chloride) – uPVC, also known as rigid PVC, is extensively used in the building industry as a low-maintenance material.
HDPE (High-density polyethylene)- High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a polyethylene thermoplastic made from petroleum.

7. What is FRP piping and basic application?

FRP Pipe / GRP Pipes are manufactured by the widely used helical filament winding method. FRP / GRP Pipes manufactured by filament winding method utilize continuous glass strand roving impregnated with polyester resin and helically wound on rotating mandrel at a specified winding angle. The winding process is continuous in bi-directional layers until the desired wall thickness is achieved. FRP / GRP pipes manufactured by filament winding offer better physical properties in comparison with another method so FRP / GRP Pipes manufactured with the helical filament winding method will have longer services life without affecting the cost of supply.

Features & Advantages of FRP Pipes:

Corrosion Resistance
Non-Toxicity
High Hydraulic Efficiency
Longer Services Life
High Resistance To Surge Pressure
Light Weight
Low Thermal And Electrical Conductivity

Applications:

Potable Water & Raw Water Supply
Effluent & Sewage Water Supply
Chemical & Fertilizer Plants
Desalination Plants
Refinery, Petrochemical & Petroleum Plant
Power Plant

8. Type of test used in HVAC PIPING?

Hydro test,
Radiography test,
Die penetration test.

9. Why do we provide High Point Vent (HPV) and Low Point Drain (LPD) in piping?

HPV – For removing Air during Hydro-test.
LPD – For draining water after conducting Hydro-test.

10. Type of joining method to connect PIPING?

Bronze / TIG /MIG Welding
Compression joints
Flange joints
Couplings.

11. Different types of Welding used in PIPING?

Arc Welding
Metal Inert Gas Welding
Tungsten Inert Gas
Oxy Acetylene Welding.

12. What is Pre-insulated pipe? Why it is used?

Pre-Insulated Pipe:

Construction – They consist of a steel pipe, an insulating layer, and an outer casing.
Uses & Applications – The main purpose of such pipes is to maintain the temperature of the fluid in the pipes. A common application is the hot water from district heating plants.
Types – Most commonly used are single insulated pipes, but more recently use two pipes insulated within the same casing is popularly used. By using insulated pipe supports, direct heat transfer between pipes and their supports are prevented.
Insulating MOC – The insulating material usually used is polyurethane foam or similar, with a coefficient of thermal conductivity “k=0.033-0.024” W/mK (thermal conductivity). Outer casing is usually high-density polyethylene (HDPE).
Feasibility – Insulated pipelines are usually assembled from pipes of 6 metres (20 ft), 12 metres (39 ft), or 16 metres (52 ft) in length, laid underground in depth 0.4–1.0 metre (1 ft 4 in–3 ft 3 in). Efficient working life of district heating pipelines networks is estimated at 25–30 years, after which they need to be replaced with new pipes

13. What are the different types of PIPING support?

The type of support for pipe is selected after analysis of the load that the pipe will be sustaining.

Types of fittings are as follows:

Spring Support
Snubbed or Shock Absorber
Rigid Support :-

Stanchion/Pipe Shoe
Rod Hanger
Rigid Strut

Chilled water PUF gutty or wooden block above that u clamp, threaded rod hangs to structure by a circular clamp. if piping is more Pipe rack is required.

14. What are the different PIPING Fittings?

The pipe fittings used are as follows:

Flanges
Nut bolts
Gasket
Short bend -Application
Long bend-
Tee
Reducer – Concentric, Eccentric
End connections
Barrel

15. Standard PIPING length available in the market?

Pipe length is mentioned in either meter or feet. During production, pipes are not manufactured in the same lengths, and during the construction of the process plant, you required various lengths of pipes. To address this issue standard has defined pipe lengths in single random and double random categories.

Single random pipes: These pipes come in 4.8m to 6.7m in lengths with 5% of lengths in between 3.7 to 4.8 m
Double random pipes: These pipes have a minimum average of 10.7 m and a minimum length of 4.8 m with 5 % of lengths in between 4.8 m to 10.7 m

16. What are the Standard painting used in chiller side, condenser side?

Chilled water -Blue Colour.
Cooling water-Green Colour.

17. What are the different Supports of Copper piping?

Supports of copper piping are as follows:

Slotted channels
Z angle
Supporting rod with bullet fastener
PVC sleeve (for protection of insulation)
Cable tie

18. What is the basis of using of short radius & long radius elbow?

Long radius elbows are used for small pressure drops whereas short radius elbow is used for high-pressure drops. For catalyst, flows very long radius elbows are used.

19. What should be the radius of the long radius elbow?

1.5D (Where “D” is the diameter of the pipe.)

20. What should be the radius of short radius elbow?

1D (Where “D” is the diameter of the pipe.)

21. From which size onwards NB of pipe is equal to OD of Pipe?

From the size 14” and onwards NB = OD of pipe.

22. What is the difference between Pipe and Tube?

The pipe is identified by NB and thickness is defined by Schedule whereas Tube is identified by OD & its thickness as BWG (Birmingham wire gauge or 1/100 inch)

23. Where the ERW & Seamless pipes are used?

Above 18” ERW pipes are used. Below 18” seamless pipes are used. Seamless pipes can sustain higher temperatures & pressure.

24. What is the main use of ASTM A53 & A106 Gr.B pipes?

ASTM A53 pipes are mainly used for utility services whereas A106 Gr. B pipes are used for high pressure & high-temperature services.

25. What is the minimum distance to be maintained between two welds in a pipe?

The thumb rule is that the minimum distance between adjacent butt welds is 1D. If not, it is never closer than 1-1/2″. This is supposed to prevent the overlap of HAZs. Minimum spacing of circumferential welds between centerlines shall not be less than 4 times the pipe wall thickness of 25 mm whichever is greater.

26. How the pipe fittings are classified based on end connections?

Pipe fittings are classified based on end connection as: –

Socket weld fittings.
Screwed end fittings.
Beveled end or Butt weld fittings.
Spigot socket fittings.
Buttress end fittings.

27. How the Gaskets are classified based on the type of construction?

Based on the type of construction, gaskets are classified as: –

Full face.
Spiral wound metallic.
Ring type.
Metal jacketed.
Inside bolt circle.

28. What is the most commonly used material for Gasket?

Compressed Asbestos Fiber.

29. Which type of gasket is recommended for high temperature & high-pressure application?

Spiral Wound Metallic Gasket

30. How can flanges be classified based on facing?

Flanges are classified based on facin

Flat face. (FF)
Raised face. (R/F)
Tongue and groove. (T/G)
Male and female. (M/F)
Ring type joint. (RTJ)

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