Summary

CvC_v and KvK_v are singles values in units of flowrate that may be used to characterise the relationship between flowrate and pressure loss for fittings and equipment. This article demonstrates how to calculate the CvC_v or KvK_v values, and how to use these values to determine the pressure loss for a given flowrate.

Definitions

CvC_{v}:Flow coefficient determined on a US customary basis
KvK_{v}:Flow coefficient determined on a metric basis
QQ:Flowrate of fluid
SGSG:Specific gravity of the fluid
ΔP\Delta P:Pressure change through the equipment
ρ\rho:Density of the fluid

Introduction

There are many ways of describing the pressure loss through pipes, fittings and equipment. The CvC_v and KvK_v methods use a single number to characterise the flowrate through the equipment at a known pressure loss, this number may then be used to estimate the pressure loss characteristics of the equipment at other flowrates.

Control Valve

While the CvC_v and KvK_v methods may be used to describe many types of fitting and equipment, the most common use is for describing the flow characteristics of control valves. The CvC_v value is in units of US gallons per minute, while the KvK_v value is in units of m3/hrm^3/hr . The CvC_v values, using US customary units are far more widely reported than KvK_v values, using Metric units.

Calculation of Pressure Loss using the CvC_v Method

A CvC_v value is defined as the rate of flow of water in US gallons per Minute at 60°F at a pressure drop of 1 psi across the equipment. CvC_v relates to pressure drop and flowrate via the following expression (imperial units):

Cv=QSGΔP C_v = Q \sqrt{\frac{SG}{\Delta P}}

Where Q is in USGPM and ΔP is in psi.

Thus for a given CvC_v , SG and Q the pressure drop across an item of equipment is calculated as:

ΔP=SG(QCv)2 \Delta P = SG \left( \frac{Q}{C_v} \right)^2

CvC_v relates to pressure drop and flow rate via the following expression (Metric Units):

Cv=0.0694QρΔP×999 C_v = 0.0694 Q \sqrt{ \frac{\rho}{\Delta P \times 999}}

Where Q is in L/min, ρ\rho is in kg/m3 and ΔP is in bar.

Thus for a given CvC_v , ρ\rho and Q the pressure drop across an item of equipment is calculated as:

ΔP=ρ999(0.0694QCv)2 \Delta P = \frac{\rho}{999} \left( \frac{0.0694Q}{C_v} \right)^2

Calculation of Pressure Loss using the KvK_v Method

A KvK_v value is defined as the rate of flow of water in m3/hrm^3/hr and 4°C at a pressure drop of 1 bar across the equipment. KvK_v relates to pressure drop and flow rate via the following expression:

Kv=QSGΔP K_v = Q \sqrt{ \frac{SG}{ \Delta P}}

Where Q is in m3/hrm^3/hr and ΔP is in bar.

Thus for a given KvK_v , ρ\rho and Q the pressure drop across an item of equipment is calculated as:

ΔP=SG(QKv)2 \Delta P = SG \left( \frac{Q}{K_v} \right)^2

Conversion Between CvC_v and KvK_v Values

Values of CvC_v and KvK_v can be converted using the formulae below:

Cv=1.157×Kv C_{v} = 1.157 \times K_{v}

Kv=0.8646×Cv K_{v} = 0.8646 \times C_{v}