Flow Rate vs. Flow Velocity- What’s the Difference?

At Triplex, when we size process pumps or suggest a line size, we often ask our customer’s for what their flow rate is. This is probably the most important process parameter. But when we’re sizing for CIP, that generally doesn’t come up. Why, might you ask? Because for CIP, we size for flow velocity and for process handling, we worry about rate. So what’s the difference? That will be the focus of the rest of today’s post.

Let’s start with flow rate. Flow rate, sometimes called Q is defined as the volume of fluid pass through an area during a given period of time. Put another way, flow rate is:

Q= V/t

Where V is volume and t is the time elapsed. In US applications, volume is typically in units of gallons and time in units of minutes. We also see volumes expressed as liters or even cubic meters (1000 Liters). Why are we using meters to measure volume? Well, remember that volume (Liters) is equal to length times width times height, or cubic meters.

So how is flow rate different than velocity? Well, while rate and velocity are related, there is one significant difference. Flow velocity depends significantly on the vessel or container the fluid is in, specifically, the cross-sectional area. Put another way:

Q=Av

Where A is the cross-sectional area of the container the fluid is floating in and v is the average velocity. This is why water coming out of a nozzle goes “fast”. The flow rate is unchanged, but because the cross-sectional area decreases, our velocity must increase.

So why is this so important for high purity process applications? One word- turbulence. When we size a pump for a process application, we want nice smooth laminar flow. When we size a pump for CIP, however, we want turbulent flow. Turbulent flow generally happens at flow rates of greater than 5 feet per second. In process, we want to avoid turbulent flow because it creates friction losses that require greater horsepower to overcome. For CIP applications, however, this is exactly what we want as it creates a “scrubbing” action within the tube that helps clear soil from the line.

So as you can see, knowing our flow rate and our line size will help us calculate velocity. And if we’re designing a skid and we don’t know what line size to select, we can use our 5 ft/second rule to help figure it out. Again, generally we will want to select a line size that is less than 5 ft/second for process flow rates. Because there aren’t that many sanitary tubing line sizes, we put this table together to help you pick your line size or check the velocity your pump is putting out:

In summary, flow rate is the amount of volume that passes through an area during a given period of time. Flow velocity, or speed, depends on the size on the container the fluid is moving through, so velocity varies with line size. This is important in helping us select the appropriate line, pump, and valve sizes. For any questions about your sanitary line size or what flow rate you should specify for your CIP supply pump, contact a Triplex Sales Engineer today!