Physics of Taps
Most water and gas taps have adjustable flow. Turning the knob or working the lever sets the flow rate by adjusting the size of an opening in the valve assembly, giving rise to choked flow through the narrow opening in the valve. The choked flow rate is independent of the viscosity or temperature of the fluid or gas in the pipe, and depends only weakly on the supply pressure, so that flow rate is stable at a given setting. At intermediate flow settings the pressure at the valve restriction drops nearly to zero from the Venturi effect; in water taps, this causes the water to boil momentarily at room temperature as it passes through the restriction. Bubbles of cool water vapor form and collapse at the restriction, causing the familiar hissing sound. At very low flow settings, the viscosity of the water becomes important and the pressure drop (and hissing noise) vanish; at full flow settings, parasitic drag in the pipes becomes important and the water again becomes quiet.
One reason that most beer taps are not designed for adjustable flow is that the beer itself is damaged by the pressure drop in a choked-flow valve: holding a beer tap partially open causes the beer to foam vigorously, ruining the pour.
Read more about this topic: Tap (valve)
Famous quotes containing the words physics and/or taps:
“But this invites the occult mind,
Cancels our physics with a sneer,
And spatters all we knew of denouement
Across the expedient and wicked stones.”
—Karl Shapiro (b. 1913)
“She had thought the studio would keep itself;
no dust upon the furniture of love.
Half heresy, to wish the taps less vocal,
the panes relieved of grime.”
—Adrienne Rich (b. 1929)