Understanding the pressure drop factor

By "Apollo" Valves
March 6, 2018

You can blow through one drinking straw, but what if you join two together? Three? Four? We've all tried it, and we learned that it's quite hard to move air through a long, thin tube.

What we're describing is pressure drop, and it's a critical issue in the design and installation of plumbing systems. Get it wrong and there's either too much flow or not enough. Here's a primer on the role valves play in flow rates and pressure drop.

Flow basics

Flow is usually specified in terms of gallons per minute (GPM). Flow through a pipe depends on the difference in pressure between the inlet and outlet. If that's very small, the fluid won't be in any great hurry to even out the difference. If the difference is bigger, the fluid will be more inclined to rush through the pipe.

Why we get a pressure drop

The difference in pressure between input and output ends of a pipe is known as “delta P.” It's determined by the diameter and length of the pipe. Fluid moving along the walls of the pipe is slowed down by friction. When the pipe diameter gets smaller, more of the fluid experiences this resistance. That's why fluid flows more easily through a larger pipe. In the same way, a longer pipe creates more drag.

The valve influence

A valve adds more resistance; how much depends on how the valve is designed. For example, consider a full port ball valve and a diaphragm valve. When fully open, the hole through the ball valve is the same diameter as the pipe. That means it adds very little extra resistance to the flow. In contrast, the fluid has to change direction as it flows through the diaphragm, and that creates more resistance.

Calculating flow (or pressure drop)

When you're selecting a valve, you want to know how much resistance it adds. That's determined by looking at the flow coefficient number, written as “Cv.” Cv is defined as the GPM through a valve with a pressure drop of 1 psi.

Cv provides a simple of way of comparing the flow through a valve. The formula relating flow and pressure drop is:

GPM = Cv x (square root of delta P)

Putting the knowledge to work

The full port ball valve in our example might have a Cv of 40. In contrast, that for the diaphragm valve would be closer to 15. So a higher Cv means more flow for a given pressure drop.

It may be easy to conclude that you should just look for valves with the highest Cv, but it's not that simple. While a low Cv is bad because it will throttle back the flow, a high Cv can also be a problem. A Cv too high for the system can lead to premature valve wear, poor flow control, and noise.

The key is to match the valve Cv to the rest of the system. If that looks like a special kind of math challenge, don't worry. A flow calculation program will handle it, or alternatively, valve specialists will be happy to help.

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