backflow: one construct, many systems

In plumbing and water supply, backflow is the unwanted reversal of flow in a distribution system, which can draw contaminated water back through a cross-connection into the potable mains, by back-siphonage when upstream pressure drops, or by back-pressure when downstream pressure rises. Bhardwaj and Reed's 2004 account of cross-connection and backflow prevention describes the hazard and the check valves and air gaps that guard against it. The referent is reverse flow in a piping network, the same physical event that, in other systems, shows up as valvular regurgitation or pump recirculation.[1]

Unit

Volume of flow reversed past an intended one-way point

Recovery means

Prevented and corrected by backflow preventers: check valves, air gaps, and reduced-pressure assemblies

Measured by

Detected by backflow-preventer testing and by pressure conditions that permit back-siphonage or back-pressure

In cardiology, backflow is regurgitation: blood that should move forward instead leaks backward through a heart valve that fails to close fully, so part of each stroke returns the way it came and loads the heart. Enriquez-Sarano and colleagues' 2005 study quantified the regurgitant backflow of mitral regurgitation and tied its volume to patient outcome. The referent is reverse blood flow across a valve, the same reverse-flow construct as in a water main or a pump, realised in the circulatory system.[2]

Unit

Regurgitant volume and fraction returning across the valve per beat

Recovery means

Addressed by valve repair or replacement; the leak itself does not resolve on its own

Measured by

Quantified by regurgitant volume, regurgitant fraction, and effective regurgitant orifice area on Doppler echocardiography

In turbomachinery, backflow is the reverse recirculation that forms at a centrifugal pump's inlet when it runs well below its design flow: fluid spills back out of the impeller eye and recirculates, degrading efficiency and destabilising the head–flow curve. Mengdehl and colleagues' 2015 study examined influencing this part-load recirculation to avoid an unstable performance curve. The referent is reverse flow within a pump, the same reverse-flow construct seen in a water main or a leaking heart valve.[3]

Unit

Reverse-recirculating flow at the impeller inlet at part load

Recovery means

Not applicable

Measured by

Characterised by the part-load flow at which recirculation onsets and its effect on the head–flow curve