COMPONENT FOCUS
Josh Cosford • Contributing Editor
How do you determine pressure drop through fittings? In a recent fluid power webinar, I was presenting on Selecting & Sizing the Proper Fitting and I was asked a viewer question I had no good answer to. Near the latter portion of the webinar, I had been discussing pressure drop through fittings. Ensuring a healthy, unimpeded path for hydraulic fluid to flow saves critical energy for useful work, rather than just pushing molecules. The excessive use of tees, elbows and adapters creates pressure drop, so such fittings should be avoided, if possible. I had explained the factors for pressure drop through a fitting; diameter, flow rate, radius of bend, surface finish and Reynolds Number. Of course, most of you know there is a direct relationship between conduit diameter, flow rate and pressure drop. The higher the flow rate or smaller the fitting, the higher the pressure drop. It should be noted that pressure drop increases exponentially as diameter decreases … that’s how much fitting size affects energy loss. Less considered, bend radius is how tightly fluid is forced to change direction in a tube, hose or fitting. In the image, two 90° elbows are compared. One fitting’s internals smoothly transition its radius with little drama, ensuring flow is closer to laminar, which prevents excessive backpressure. The other forged and machined elbow fitting more severely alters the fluid path, which creates backpressure as oil loses energy changing direction. 70
FLUID POWER WORLD
Component Focus_4-19_FPW_V3.indd 70
4 • 2019
Picture, if you will, the extreme waterslide at your local amusement park … you know, the one with the trap door and fifty-foot plummet? The slide door opens and drops you straight down for ten feet before your posterior contacts the slide to start changing your direction from vertical to horizontal, and with surprising ease and speed, your plummet is managed safely. Now imagine the trap door opens, and you fall twenty feet and land with a thud. In front of you is a dark tunnel. However, you’ve lost all forward momentum, and not until more bodies are dropped to force you down the tunnel, your energy is lost. My example is extreme, but important nonetheless. It takes energy to turn a 90° corner, and the concept isn’t lost in fluid power. The combination of the bend radius and the smoothness of the walls of the conduit contribute to the Reynolds Number, which is just a dimensionless description dependant on other things like viscosity. The Reynolds Number is defined using the following, which I don’t expect you to calculate or memorize, but have a look at the factors:
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4/16/19 3:51 PM
