Question

3. For piping assemblies, screwed fittings cannot be used for: (a) Galvanised piping as corrosion may develop around the threaded section (b) High flow applications as turbulence may be initiated at the threaded section (c) Sections that have to be dismantled for maintenance (d) Low pressure applications as the depth of the pipe thread reduces the pressure rating of the pipe.

Answer 1

Let's look at each option one by one.

(A) Screwed fittings can be used for galvanized pipes as long as it not large in diameter (<4") which is the fundamental restriction on its use. Variety of options for screwed pipes with GI as their MOC is available.

(B) Each type of fitting disturbs the flow by some amount and screwed fitting doesn't exacerbate it any further. Threaded faces doesn't come in contact with flowing fluid so statement is FALSE.

(C) This is a TRUE statement. Sections which require frequent dismantling for maintenance need to have flange joints which can be either screwed or welded on pipe. These flange joints provide an option for blinding of the downstream section in case of maintenance. It's a standard practice to provide a flanged fitting(flanged body valve etc) downstream of screwed joint, if used. Flanged joints are the easiest for assembling/disassembling for maintenance. Ofcourse, screwed joints are better than welded joints in terms for dismantling but NOT as flange joints.

(D) Although the last part of statement is correct that pressure rating of thread is lesser than that of pipe itself but for low pressure applications this fact doesn't hold importance because even though lower rating is available but it is usually satisfactory for lower pressure application. Infact, screwed fittings are best used for low pressure, moderate temperatures, small diameter (<4") fitting with less bending or thermal stress exposure. Hence, FALSE!