Loss Coefficient for Fittings and Valves

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Loss coefficients for fittings and valves, often denoted as "K-values" or "Cv-values," are numerical factors used in fluid mechanics to quantify the pressure loss or resistance that occurs as a fluid flows through these components in a piping system.  These coefficients are used by engineers and designers to assess the performance of a system, particularly in applications involving the transport of liquids or gases.

Key Points about Loss Coefficients for Fittings and Valves

  • Loss Coefficient (K-value)  -  The loss coefficient, often denoted as K, represents the resistance to fluid flow through a particular fitting or valve in a piping system.  It is a dimensionless number that depends on the geometry of the component and the flow conditions.  Engineers use K-values to calculate pressure drops across fittings and valves.  The K-value is specific to each type of fitting or valve and is usually determined through experimentation or by referencing industry standards.
  • Flow Coefficient (Cv-value)  -  The flow coefficient, denoted as Cv, is another way to express the performance of a valve, particularly for control valves.  Like the K-value, it quantifies the capacity of a valve to pass a specific flow rate under given conditions.  The Cv-value is often used for valves that are designed to control flow, such as control valves used in process industries.  The Cv-value is also specific to the valve and is typically provided by the manufacturer.

Both K values and Cv-values are essential for engineers to understand the impact of fittings and valves on the overall system performance.  When designing a piping system, engineers can use these coefficients in pressure drop calculations and flow rate predictions.  Selecting the right fittings and valves with appropriate K or Cv-values is crucial to ensure that the system operates efficiently and meets its performance requirements.

It's important to note that these coefficients can vary for different types of fittings and valves, and they are typically provided by manufacturers or can be found in industry standards and reference materials.  Engineers must consider these coefficients when designing and analyzing fluid systems to ensure that the system functions as intended.


Loss Coefficient for Fittings and Valves 

Component Loss Coefficient K
90 Short Elbow, flanged  0.3
90 Short Elbow, threaded 1.5
90 Long Elbow, flanged 0.2
90 Long Elbow, threaded 0.7
45 Long Elbow, flanged 0.2
45 Short Elbow, threaded 0.4
180 Return  
180 Return, flanged 0.2
180 Return, threaded 1.5
Line flow, flanged 0.2
Line flow, threaded 0.9
Branch flow, flanged 1.0
Branch flow, threaded 2.0
Union, Threaded 0.08
Diaphragm, fully open 2.3
Diaphragm, 1/4 closed 21
Diaphragm, 1/2 closed 4.3
Globe, fully open 10
Angle, fully open 2
Gate, fully open 0.15
Gate, 1/4 closed 0.26
Gate, 1/2 closed 2.1
Gate, 3/4 closed 17
Swing Check, forward flow 2
Swing Check, backward flow  \(\infty\)
Ball, fully open 0.05
Ball, 1/3 closed 5.5
Ball, 2/3 closed 210
Water meter 7


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