Pipe Systems and Fluid Mechanics Flashcards

What is a 'spectacle blind' (also called a 'figure-8 blind'), and when is it used in piping systems?

Answer: A permanent flanged isolation device with a solid disc and ring (open) joined together, rotatable to switch between positive isolation and open-flow positions

A spectacle blind is a figure-8 shaped device installed between flanges with a solid disc (blind) and an open ring, bolted permanently into the flange. Rotating it 180 degrees switches between full isolation (blind in line) and full flow (ring in line) without removing the device.

What is the primary difference between a globe valve and a gate valve in terms of appropriate application?

Answer: Gate valves are designed for fully open or fully closed service with minimal pressure drop; globe valves provide throttling control but have higher pressure drop

Gate valves have a straight-through flow path and minimal pressure drop when fully open, making them ideal for isolation service (fully open or fully closed). Globe valves have a tortuous flow path and high pressure drop but excellent throttling characteristics.

When installing an orifice plate flow element in a horizontal pipeline, where must the differential pressure taps be positioned for liquid service versus gas service?

Answer: Liquid: taps at side or bottom (3 to 8 o'clock); Gas: taps at top or side (10 to 2 o'clock)

For liquid service, DP taps should be at the side or slightly below center to prevent gas pockets from entering impulse lines. For gas service, taps should be at the top or side to prevent condensate from accumulating in the impulse lines and causing measurement errors.

What is the function of a 'check valve' in a piping system, and what does 'cracking pressure' mean for a check valve?

Answer: Prevents backflow; cracking pressure is the minimum upstream pressure required to open the valve disc and allow forward flow

A check valve is a one-way valve that allows flow in one direction and automatically closes to prevent backflow. Cracking pressure is the minimum differential pressure across the valve needed to overcome the spring or gravity force and crack open the disc.

What is the difference between a '2-inch 150# flange' and a '2-inch 600# flange' in terms of their application?

Answer: The 600# flange has a higher pressure-temperature rating, larger bolt circle, more bolts, and thicker flange body than the 150# flange of the same nominal size

ASME B16.5 flange Class 150 and Class 600 have different pressure-temperature ratings, physical dimensions, bolt patterns, and bolt sizes. Class 600 flanges are physically larger, heavier, and use more bolts than Class 150 flanges of the same nominal pipe size.

What is the purpose of a 'hydrostatic leak test' at 1.5 times design pressure, and why is water used instead of compressed gas?

Answer: Water is non-compressible; in the event of a catastrophic failure during testing, minimal stored energy is released compared to a pressurized gas, greatly reducing the explosion hazard

Hydrostatic testing uses water because it is virtually incompressible. The stored energy in a pressurized water system is minimal compared to compressed gas. If a failure occurs during a water test, the pipe may crack or rupture locally, but there is no explosive decompression as with compressed air or process gas.