Enter the valve's Cv rating found in the spec sheet.
Upstream pressure in PSI.
Downstream pressure in PSI.
Water = 1.0. Lower for lighter fluids, higher for heavier.
Calculated Flow Rate
0.00 GPM
Pressure Drop: 0 PSI
function calculateFlowRate() {
// Get input elements
var cvElem = document.getElementById('cvInput');
var p1Elem = document.getElementById('p1Input');
var p2Elem = document.getElementById('p2Input');
var sgElem = document.getElementById('sgInput');
var resultBox = document.getElementById('resultDisplay');
var errorBox = document.getElementById('errorDisplay');
var flowOutput = document.getElementById('flowResult');
var dpOutput = document.getElementById('dpResult');
// Parse values
var cv = parseFloat(cvElem.value);
var p1 = parseFloat(p1Elem.value);
var p2 = parseFloat(p2Elem.value);
var sg = parseFloat(sgElem.value);
// Reset display
errorBox.style.display = 'none';
resultBox.style.display = 'none';
// Validation logic
if (isNaN(cv) || isNaN(p1) || isNaN(p2) || isNaN(sg)) {
errorBox.innerText = "Please enter valid numbers for all fields.";
errorBox.style.display = 'block';
return;
}
if (cv < 0) {
errorBox.innerText = "Cv value cannot be negative.";
errorBox.style.display = 'block';
return;
}
if (sg <= 0) {
errorBox.innerText = "Specific Gravity must be greater than zero.";
errorBox.style.display = 'block';
return;
}
// Calculate Pressure Drop
var deltaP = p1 – p2;
if (deltaP < 0) {
errorBox.innerText = "Inlet Pressure (P1) must be greater than Outlet Pressure (P2) to generate flow.";
errorBox.style.display = 'block';
return;
}
if (deltaP === 0) {
resultBox.style.display = 'block';
flowOutput.innerText = "0.00 GPM";
dpOutput.innerText = "Pressure Drop: 0 PSI (No Flow)";
return;
}
// Calculation Formula: Q = Cv * sqrt(DeltaP / SG)
var flowRate = cv * Math.sqrt(deltaP / sg);
// Display results
resultBox.style.display = 'block';
flowOutput.innerText = flowRate.toFixed(2) + " GPM";
dpOutput.innerText = "Pressure Drop: " + deltaP.toFixed(2) + " PSI";
}
How to Calculate Flow Rate from Cv
The flow coefficient, denoted as Cv, is a crucial metric in fluid dynamics and valve sizing. It represents the flow capacity of a valve. Specifically, a Cv of 1 means that one US gallon of water at 60°F will flow through the valve in one minute with a pressure drop of 1 PSI.
Engineers and technicians use the Cv value to determine how much fluid will pass through a specific valve under a known pressure gradient. This calculation is essential for sizing control valves to ensure systems operate efficiently without excessive pressure loss or noise.
The Flow Rate Formula
To calculate the liquid flow rate ($Q$) in gallons per minute (GPM) given the Cv value, the pressure drop, and the specific gravity of the fluid, we use the following standard equation:
Q = Cv × √(ΔP / SG)
Where:
Q = Flow Rate in US Gallons per Minute (GPM).
Cv = Valve Flow Coefficient.
ΔP = Pressure Drop across the valve in PSI ($P_{inlet} – P_{outlet}$).
SG = Specific Gravity of the fluid (Water = 1.0).
Definition of Input Parameters
Valve Flow Coefficient (Cv)
This is a constant value provided by the valve manufacturer. It indicates the valve's efficiency at allowing fluid flow. A higher Cv indicates a larger opening or a streamlined path that allows more fluid to pass with less resistance.
Pressure Drop (ΔP)
This is the difference between the pressure entering the valve (Inlet Pressure, $P_1$) and the pressure leaving the valve (Outlet Pressure, $P_2$). Flow only occurs when $P_1$ is greater than $P_2$. The greater the pressure difference, the higher the flow rate.
Specific Gravity (SG)
Specific gravity compares the density of the fluid to the density of water.
Water has an SG of 1.0.
Oils often have an SG less than 1.0 (e.g., 0.85).
Heavier fluids like brine may have an SG greater than 1.0 (e.g., 1.2).
Heavier fluids (higher SG) flow more slowly than lighter fluids for the same pressure drop and valve size.
Example Calculation
Let's assume you have a control valve with a known Cv of 12. You are pumping water ($SG = 1.0$) through the system. Your gauge shows an upstream pressure of 60 PSI and a downstream pressure of 50 PSI.
Therefore, the flow rate through the valve is approximately 37.94 Gallons per Minute.
Why is this calculation important?
Correctly calculating flow rate from Cv prevents undersizing or oversizing valves:
Undersized Valves (Cv too low): Create a "choked" flow, causing high pressure drops, potential cavitation (formation of vapor bubbles), excessive noise, and mechanical damage.
Oversized Valves (Cv too high): Result in poor control resolution. The valve may operate constantly near the closed position, leading to "hunting" (cycling) and premature wear on the valve seat.