In piping design, the corrosion allowance assigned specifically for small fittings such as couplings and unions should be considered __________ under normal service selection and thickness calculations.

Introduction / Context:
While sizing and specifying piping components, engineers add a corrosion allowance to compensate for expected wall loss over the service life. However, not all items in a line class are treated equally. Very small fittings like couplings and unions are typically purchased to standardized schedules and replaced as consumables. This question probes practical specification practice for such fittings.

Given Data / Assumptions:

• Standard carbon-steel or alloy steel piping in general service.
• Small screwed or socket-weld couplings and unions, not large fabricated nozzles.
• Normal plant corrosion rates; no severe corrosion service requiring special design.

Concept / Approach:
Corrosion allowance is primarily ensured on pressure parts whose wall thickness is directly sized by the designer (e.g., straight pipe, large nozzles). Commercial couplings and unions are catalog items with fixed wall thickness and short replacement cycles. Hence their life is not extended by adding “design allowance”; instead they are replaced on condition. Therefore, they are generally treated as having negligible design corrosion allowance.

Step-by-Step Solution:
Identify component category: small, standardized fittings (couplings, unions).Check specification practice: these are bought to standard wall schedules and replaced as needed.Conclusion: treat design corrosion allowance as negligible.

Verification / Alternative check:
Typical piping class notes specify corrosion allowance for pipe, flanges, and large branch connections; small couplings/unions are exempt and controlled by minimum schedule.

Why Other Options Are Wrong:
60 / 200 / 350: These have no unit context and do not represent a rational “allowance” entry; corrosion allowance is normally a thickness (e.g., 1.5 mm) not such figures.

Common Pitfalls:
Assuming the same allowance applies to every catalog fitting; ignoring that unions and couplings are inexpensive and routinely replaced.

Final Answer:
negligible