By: Frank Caprio | On: February 20, 2018
Understanding pressure drop is critical to optimizing the performance of a fluid system. In order for liquid or gaseous media (“fluid”) to flow through a piping system, the fluid must have sufficient potential energy, which we measure as system pressure. Remember that energy can neither be created nor destroyed, but it can be converted into other energy. Some of the ways potential energy can be converted include:
- Friction between the fluid and the wall of the pipe
- Friction between adjacent layers of the fluid itself
- Friction loss as the fluid passes through any pipe fittings, bends, valves, or components
- Pressure loss due to a change in elevation of the fluid (if the pipe is not horizontal)
- Pressure gain due to any fluid head that is added by a pump
Proper function of a piping system requires the designer to identify any change in the system pressure between two points. This change is referred to as pressure drop. For pressure drop that results from the friction-related variables listed above, there are various reference charts and online tools that can assist the designer in determining and understanding pressure drop through a piping system. However, when working with flexible hose and expansion joints, the matter becomes much more complicated.
Pressure drop can be readily estimated through rigid piping components, because the interior surfaces of these products are fairly well-defined and consistent. The pressure drop can then be calculated as long as the following variables are known: the flow rate, the pipe diameter and length, and the media properties, such as the initial pressure and the density/viscosity.
However, when dealing with flexible components such as corrugated metal hose and expansion joints, many other variables (in addition to those shown above) can affect the flow characteristics significantly. Some of the variables specific to metal hose and expansion joints:
- The geometry of the corrugations (height, width, pitch)
- Corrugation size relative to the hose diameter
- Annular vs. helical corrugation profile
- The rigidity of the corrugations (single- vs. multiple-ply bellows)
- Any bends in the component
- Any changes in diameter (such as hose-to-fitting transitions, etc.)
- Vibration, pulsation, or pressure spikes in the system
There have been numerous attempts to produce charts or formulas that can assist designers in estimating pressure drop, but even the best formulas cannot take all of these nuances into account. Here is the pressure drop chart Hose Master publishes for compressed air lines.
Let’s take a quick look at how to use this chart. If a customer wants to calculate the pressure drop for compressed air running through a 2” I.D. corrugated hose that is 5 feet long, we must first obtain the inlet air pressure, temperature, and the flow velocity in cubic feet per minute (CFM). Why is the temperature required when it is not included in our variables listed above? Because it helps us determine the density of the air. For our example, let’s assume the customer is operating at 100 psi and 600 CFM at 60 degrees F. First, we locate the given CFM on the “X” (horizontal) axis. We then follow this line vertically until it intersects the slanted line indicating 2” I.D. hose. Once we locate the intersection of these two lines, we then follow that point over to locate the pressure drop per foot on the “Y” (vertical) axis. So, for our example, the pressure drop per foot would be 0.06 PSIG/foot, multiplied by the length of the hose (5 feet), or 0.30 PSIG. Easy, right? Well…
Here’s where all those other variables come into play.
Q: What if the air is at some pressure other than 100 PSIG? A: There’s another formula for that (see our Corrugated Metal Hose and Assemblies Catalog)
Q: What if water is being transferred instead of air? A: There’s a different table for that (see our catalog)
Q: What if the media temperature is other than 60 degrees F.? A: Then we have to determine the change in density at actual operating temperature and then recalculate the pressure drop.
Q: How do different corrugation geometries affect the pressure drop? A: The effect of different corrugations geometries on pressure drop is interdependent on a number of other factors and does not fit neatly into a mathematical formula.
Q: What if there is a bend (or several) in the hose? A: That changes things, too. Complex tools, an output of which is shown in the above video, are used to calculate the effect of the hose bends on the pressure and velocity of the media.
These and other factors will skew the accuracy of the tables, which is why everyone states the values are approximate.
For expansion joints, the relatively short bellows lengths generally don’t produce significant amounts of pressure drop. However, increased flow velocity resulting from pressure drop through the piping system upstream of the expansion joint could require the addition of a flow liner, so these variables are still important. Additionally, using multiple-plied bellows adds another layer of complexity.
Because each of these factors can affect the pressure drop for a given application, the best way to determine the pressure drop is through testing under actual operating conditions. The pressure drop is determined by measuring the pressure at the start of the piping (or hose) run, then measuring the pressure again at the end of the run.
If this is not a viable option, then a better, more reliable method to determine pressure drop is required. Hose Master is actually working on some pretty cool stuff in this area, so stay tuned for future updates. Until then, contact us or call us at 800-221-2319 for any assistance we can provide regarding pressure drop or any other technical issue.
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