How to Calculate Pressure Drop Across a Valve?

Joseph Hebert
Owner at - HVAC Buster

Joseph is an HVAC technician and a hobbyist blogger. He’s been working as an HVAC technician for almost 13 years, and he started blogging just...Read more

When fluid flows through a valve there is always some pressure drop across the valve. The amount of pressure drop depends on many factors including the type of valve, the size of the valve, the fluid properties, and the flow rate. In this article we will discuss how to calculate pressure drop across a globe valve.

We will also provide some practical tips to help minimize pressure drop in your process.

• Determine the desired flow rate through the valve
• Select a valve with a Cv (flow coefficient) that will provide the desired flow rate at the required differential pressure
• Calculate the pressure drop across the valve using the given equation: ΔP = (Q x μ x γ)/(Cv x √ΔP) 4
• Repeat steps 2 and 3 for different valves until an acceptable valve is found

How Do I Calculate the Pressure Drop Across a Valve

When calculating the pressure drop across a valve, one must first determine the specific gravity of the fluid, the absolute viscosity of the fluid, the rate of flow through the valve in gallons per minute (gpm), and the size of the valve opening. With this information, one can use either the Darcy-Weisbach equation or the Moody friction factor equation to calculate pressure loss. The equations are as follows:

P = f * L * (V^2) / (2 * g * d) or P = 0.54 * f * L * (V^2) / ((D^5/4))

where P is pressure drop in psi/ft, f is friction factor from Moody diagram or Colebrook-White equation, L is length of pipe in ft, V is velocity in ft/s, g is acceleration due to gravity (=32.17 ft/s^2), and d is inside diameter of pipe in inches. D is also inside diameter but expressed in feet (d=D/12).

Conclusion

If you want to calculate the pressure drop across a valve, there are a few things you need to know. First, you’ll need to know the valve’s orifice diameter. Second, you’ll need to know the fluid’s density.

Third, you’ll need to know the fluid’s velocity. Fourth, you’ll need to know the fluid’s viscosity.

Joseph is an HVAC technician and a hobbyist blogger. He’s been working as an HVAC technician for almost 13 years, and he started blogging just a couple of years ago. Joseph loves to talk about HVAC devices, their uses, maintenance, installation, fixing, and different problems people face with their HVAC devices. He created Hvacbuster to share his knowledge and decade of experiences with people who don’t have any prior knowledge about these devices.