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I have a question regarding valves: Let's assume a district heating system where you want to control the mass flow in a sub-station with a valve.

  1. Is (in normal cases) the diameter of the valve (fully open) smaller than the diameter of the pipe that is going to the house (substation)?
  2. Is this the reason, why a valve has a nominal pressure drop at full opening? Because when the diameter of the pipe and the valve would be equal, then the valve would have no pressure-drop when it is fully open right?
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  1. While there is some variation on valve type (gate, ball, needle etc.), the answer is yes, the valve body and internals constrict the aperture for flow even when the valve is in the full open position. This is visually evident when looking at a cross section of a typical valve body (a great example of which is shown here: http://news.cision.com/the-nanosteel-company/i/valve-body-cross-section-is-one-example-of-many-large-scale-industrial-part-designs-which-can-be-for,c1227018). In addition, most valves (all but ball valves really) force the fluid to significantly shift its path as it travels through the valve, which causes significant pressure drop as well.

  2. The answer to this question is "mostly". As a matter of clarification, recall that there is a pressure drop across a length of pipe. In the case of an open valve, there is some pressure drop associated with the frictional losses of the fluid across the surface. However, the vast majority of the pressure losses are due to the constriction of the flow (and sometimes the introduction of drag centers into the fluid) as it flows through the valve body. This can best be seen by looking at the values presented for valve recovery coefficients (higher is more "pipelike"), a small example of which are at the bottom of this page: http://www.mycheme.com/calculating-liquid-pressure-drops-across-control-valves/

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