This paper assesses two aspects of liquid flow in pipes. The first aspect is the relation between pipe critical flow depth and flow discharge. The critical flow depth in a pipe is generally expressed as an implicit function. The paper proposes two simple expressions that can be used to explicitly express pipe critical flow depth as a function of pipe diameter and discharge. The approximation errors associated with these proposed expressions were found to be within 1.0%. The second aspect is the flow transition from open pipe flow to full pipe flow in a long pipe. Theoretically, when a pipe is close to flowing full, a given discharge may correspond to multiple head differences. This paper presents a practical solution approach that yields a unique solution for the pipe flow depth for a given discharge in the transition zone.
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