Concept reference

Firm Capacity and Redundancy

C.1Plain-language definition

Installed capacity adds everything that exists. Firm capacity asks what the station can deliver after a specified credible outage, commonly the largest duty unit unavailable. It is an availability claim, so pumps alone are insufficient if every unit shares one blocked header, transformer or control panel.

C.2Governing relation or decision logic

A simple screening expression is Qfirm = Qavailable combination with the designated unit out, not generally the arithmetic sum of rated pump flows. Parallel capacity must be found from the combined pump curve intersecting the system curve. The outage definition and required design flow come from governing criteria and risk basis.

C.3The failure to name

Calling three 50 L/s pumps “100 L/s firm” assumes two pumps actually deliver 50 L/s together against the higher system loss. Redundant pumps behind one non-isolatable valve or one power feed are not independent against that common failure.

C.4Field review checklist

• State the required flow and precise unavailable-unit/event definition.
• Calculate remaining pump combinations on the governing system curves.
• Trace common electrical, control, suction, discharge, valve and access dependencies.
• Document storage, standby power, bypass and repair-time assumptions where they support resilience.

C.5Where it fits in the course

§1.4 calculates firm output; §8.6 expands the check beyond pumps.