To add a component/valve with a Cv or Kv flow coefficient: 


  1. Add a component to a pipe as described in the previous section. 


  1. Type the name of the control valve in the Component Name field.


  1. Select the Cv Value or Kv Value Component Type.


  1. The symbol appropriate for the chosen Component Type is selected by default.  The symbol can be changed by clicking the Symbol Scroll Up or Scroll Down button to select the component pressure loss image you want displayed on the pipe system drawing.


  1. Enter the flow coefficient value for the component.


  1. Click OK to save and add the Cv/Kv flow coefficient data to the pipe.



Figure 65 Adding a Cv / Kv flow coefficient value

The component dialog includes ‘helpers’ to assist in calculating an appropriate Cv or Kv flow coefficient value for a particular flow rate and pressure drop (based on the current fluid). 

Click the calculator button to display the calculator.



Figure 66 Helper to calculate a Cv value

The user should be aware that the Cv or Kv flow coefficient specifies the flow rate of water for a particular pressure loss.


When the fluid density is greater or less than water, a different flow rate of the fluid will be required to produce a 1.00 psi or a 1 bar pressure loss through the valve.



CAUTIONS for Compressible Gas Flow: 


If the fluid is a gas, the flow rate entered into the calculator must represent the actual flow rate for the gas at the density that is defined in the fluid data for this fluid zone, i.e. the volumetric flow rate of the gas based on the gas density defined in the current fluid. The equivalent mass flow will then be calculated and this will be converted to an equivalent volume of water at normal conditions. The volume of water and the stated pressure drop will then be used to calculate a Cv value (which by definition is based on a volume of water flow for a certain pressure drop). Generally, for gas systems, it would be better to use a mass flow rate entry for the calculation instead of a volumetric flow rate.   


If the fluid is a gas and the pressure drop exceeds the critical pressure ratio when compared to the inlet pressure of the valve then the flow will become choked and it will not be possible to achieve the calculated flow rate. If this occurs, Pipe Flow Expert will warn of a problem in the Result Log (if the Component option to check for choked flow has been selected).



Gas Flow Calculation with the Compressible Flow Calculation Engine 


Gas systems should generally be solved using the Compressible flow calculation engine, which takes account of the pressure condition at the component and adjusts the density of the gas as appropriate when performing the component pressure loss calculation.


The fluid properties should be defined at the operating temperature for a particular section of the system (while the software adjusts fluid properties for changes of pressure as they are used in calculations, it assumes the temperature of the fluid remains the same and therefore the user must define the fluid properties for the appropriate temperature conditions within the pipe network). The software uses the viscosity of the gas as defined in the current fluid data and this is not adjusted for changes in pressure (pressure changes generally have a small impact on viscosity in comparison to temperature changes). 


See the section on Working with Compressible Fluids for more information.




Gas Flow Calculation with the Non-Compressible Flow Calculation Engine:


Prior to version 7, Pipe Flow Expert did not include a specialist Compressible Flow calculation engine, however it allowed calculation of gas systems using the non-compressible flow calculation engine that used the Darcy-Weisbach equation, which used a constant density and viscosity across the calculations. The user was required to define the fluid data to represent the density at the approximate average pressure condition in a section of the network. 


When using the Non-Compressible calculation engine with the Darcy-Weisbach equation to solve gas systems, Pipe Flow Expert will calculate the pressure drop through the component/valve for a particular flow rate at a given density as defined by the current fluid zone based on its Cv or Kv flow coefficient value.


If the fluid zone associated with the control valve does not represent the pressure condition at the outlet of the valve/component, it may be necessary to use an adjusted Cv (or Kv) value for valve selection to take in to account the effect of the gas expansion. The adjusted Cv (or Kv) value should be based on the Cv or Kv formula for sub critical gas pressure drop. 


We recommend you use the Compressible Flow calculation engine to solve gas systems.


See ‘Calculation Theory and Methods of Solution’ section for further details about Cv and Kv flow coefficient calculations for systems where the fluid is a gas.



Cv & Kv Control Valve Sizing and Open Position


The Cv (or Kv) flow coefficient of a component/valve is usually stated for the fully open flow condition. The Cv (or Kv) flow coefficient will be less when the valve is partly closed. In an actual system it is important to select a control valve which has an appropriate Cv or Kv flow coefficient for the actual valve position that will be used. 


A control valve that is too small or too large will not be able to provide the correct control characteristics in a pipe system.


Most control valve manufacturers recommend that you should select a valve where the required Cv (or Kv) value matches the given valve Cv (or Kv) value when operating within 20% - 80% of the valve’s range.


When selecting a valve to control a ‘top end’ pressure loss you should select a valve where the required Cv (or Kv) value is in the higher operating range of the valve, normally where the required Cv (or Kv) value matches the given valve Cv (or Kv) value when operating at about 70% of the valve’s range. 


Some control valve manufacturers recommend that an allowance of 30% should be added to the required Cv (or Kv) flow coefficient to obtain the minimum Cv (or Kv) flow coefficient rating that the selected valve should have.


Please check your control valve selection with the control valve manufacturer.