Question 1

What are NPT threads and how do they differ from other thread types?

Accepted Answer

NPT (National Pipe Thread) is the US standard for pipe threads, with a tapered design so threads grip and seal as you tighten. The taper is 1 inch per 16 inches of length. NPTF (National Pipe Thread Fuel) is a variant with deeper roots for higher pressures. Metric pipe threads (M) and ISO taper threads (R, Rc) are standards outside North America. Never mix thread types: an NPT male on an NPTF female will leak under pressure. Always check the fitting specification. Sealing NPT requires thread tape or sealant compound; flared connections use a different mechanism (cone-to-cone contact).

Question 2

How do I know what pipe size to use for a water supply or gas line?

Accepted Answer

Pipe sizing depends on the flow rate (gallons per minute for water, cubic feet per hour for gas) and allowable pressure drop. A supply line to a single fixture (toilet, sink) is typically ½ inch; a main to a house is typically ¾ or 1 inch. Gas lines are sized by BTU load; a furnace and water heater might need ¾ inch copper or 1 inch black iron. Building codes specify minimum and maximum sizes, and pressure loss tables help you choose. Undersizing creates slow flow or pressure loss; oversizing wastes material. Consult the plumbing or gas code for your jurisdiction or a sizing chart from a supplier.

Question 3

What is the difference between hard and soft solder (or braze) for copper pipe?

Accepted Answer

Soft solder (lead-free tin-silver, melting point ~430°F) is used for water supply lines and low-pressure applications. Hard solder and brazing (copper-phosphorus or silver-based, melting point >1100°F) are for refrigeration, high-pressure steam, and where joint strength is critical. Soft solder is easier to learn (lower heat required) but weaker; hard solder is faster and stronger for experienced technicians. Always choose the joint strength appropriate for the application and code requirement.

Question 4

How do I thread a pipe by hand and avoid cross-threading?

Accepted Answer

Use a pipe die (circular cutting tool) fitted in a die stock (wrench-like handle). Start with the die centered on the pipe end, lightly butted against it. Apply cutting oil and rotate the die stock clockwise with moderate pressure. After each turn, reverse slightly (about ¼ turn) to break the chip and prevent jamming. Stop when the die is thread-deep (listen for the sound change or feel the resistance rise). For a machine threader, feed the pipe slowly and let the machine do the work. Cross-threading (threads misaligned) causes leaks and a stripped fitting; it happens if you start at an angle, so square the pipe to the die carefully.

Question 5

What is the pressure test procedure for a new piping system?

Accepted Answer

Pressure testing verifies that all joints hold under load and there are no leaks. For water, fill the system with water and use a hand pump or air pump to pressurize it to the code-required pressure (typically 1.5 times the working pressure, e.g., 75 psi for a 50 psi system). Leave it under pressure for 15 minutes and watch the gauge—if pressure drops, there is a leak. For gas, use nitrogen (never air or oxygen, which are explosive) at low pressure (10-20 psi) and listen for hisses or spray soapy water on suspected areas. Document the test (date, time, pressure held, pass/fail) and retain records for inspection.

Question 6

What thread sealant should I use: tape or dope, and how much?

Accepted Answer

Thread sealant tape (PTFE, white or yellow for gas) or pipe dope (liquid sealant) both work; many plumbers use both (tape plus dope for extra security). For tape: wrap 3-5 layers clockwise (the direction of tightening) around the male threads, overlapping slightly. Yellow tape is rated for gas and oils; white for water. For dope: apply a thin coat to the male threads before threading into the fitting. Don't overapply dope; excess can wash into the system and clog filters or valve seats. Gas fittings require either yellow tape and dope, or a product rated for gas; never use only tape on a gas connection (dope is required).

Question 7

How do I avoid leaks at pipe connections?

Accepted Answer

Common leak causes: (1) insufficient thread sealant, (2) stripped or damaged threads, (3) cross-threaded fitting, (4) not tightening enough (hand-tight plus ¾ turn with a wrench for NPT), (5) over-tightening, which can crack the fitting or distort threads. Use the correct wrench size (two wrenches: one to hold the fitting, one to turn the connector) to apply even force without stripping. Tighten incrementally and check for leaks. If a joint weeps after tightening, tighten a bit more; if it still leaks after two full turns, unthread, re-seal, and re-thread.

ภูมิภาค	เจ้าหน้าที่จูเนียร์	ระดับกลาง	เจ้าหน้าที่อาวุโส
USA	$35k	$60k	$85k
UK	£23k	£40k	£58k
EU	€26k	€45k	€65k
CANADA	C$42k	C$70k	C$100k

ภูมิภาค	เจ้าหน้าที่จูเนียร์	ระดับกลาง	เจ้าหน้าที่อาวุโส
USA	$35k	$60k	$85k
UK	£23k	£40k	£58k
EU	€26k	€45k	€65k
CANADA	C$42k	C$70k	C$100k

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