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HelpMeOmg's avatar

Why does pressure increase when you kink the hose?

Asked by HelpMeOmg (81points) May 7th, 2012

Everything I read on the topic of pressure, flow, and pipe size states that as pressure increases flow rate increases. A reduction in pipe size leads to a reduction in pressure and flow. Obstructions (resistance) leads to a drop in pressure and flow.

When you put your finger over the top of the hose (create an obstruction) pressure INCREASES and flow decreases. When you remove your finger flow increases and pressure decreases. This is contrary to the fact that they are directly proportional. Another example is a pressure washer forcing water out of that little opening at 3000 PSI. I assume if the opening was bigger and the hose was bigger the pressure would not be as high

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9 Answers

WestRiverrat's avatar

Flow does not decrease, the increased pressure is caused by the same amount of water flowing through a smaller opening.

HelpMeOmg's avatar

Flow doesn’t decrease? The same amount of water will come out of the hose in a given time if your finger is covering it? I don’t think so. Imagine opening the spicket ¼ turn instead of all the way which is the same concept as your finger covering a hose and you will see a lot less flow.

Just thinking about it, kinda seems like theirs an optimal point of max pressure\flow. If you open the hose sprayer a little you get a cone shape with a little bit of pressure\flow. If you open it all the way you get a nice pressure stream with more flow. If you take it off you get less pressure but more flow.

ragingloli's avatar

Yes, the flow does not decrease.
Think about it. The flow rate out of the tap stays the same, so the flow rate at the end of the hose also stays the same. If it did not, your hose would inflate like a balloon.

thorninmud's avatar

You can’t talk about “pressure” anymore once the water has left the hose. It will have velocity, but its pressure is then effectively zero because it is no longer contained.

You can talk about pressure differentials upstream and downstream from an obstruction somewhere along the hose’s length. So an obstruction will lead to a reduction in pressure downstream from the obstruction, but an increase in pressure upstream from the obstruction. The flow would drop both up- and downstream as a result of the obstruction.

CWOTUS's avatar

Welcome to Fluther.

You have to look at the preconditions that apply to your examples: I imagine that one of the conditions is “free flow”. Otherwise, closing the outlet valve and applying a heat source, for example, would increase pressure with zero flow.

The obstruction mentioned in your example means that “downstream pressure” is decreased (assuming the pipe / conduit diameter remains the same). Putting a kink in a hose won’t “increase the pressure” upstream of the kink, that only allows the pressure to equalize throughout the entire upstream system back to the source.

ETpro's avatar

The hose feels pressure to be normal, and making it kinky embarrasses it?

No, seriously. It’s true that an obstruction decreases pressure and flow BEYOND the obstruction, but it increases pressure upstream of the obstruction. Kinking a hose stops the flow. The pressure in the hose upstream of the kink then rises to the maximum pressure of the source supplying the water. Downstream of the kink, the pressure drops to atmospheric pressure.

HelpMeOmg's avatar

@ETpro – But it seems as if the pressure increases (or I guess velocity) after the kink. Spraying yourself with an open hose does not hurt however putting your finger over the top (obstructing) and spraying it hurts a little due to the increased pressure. Is this an increase in velocity or pressure?

ETpro's avatar

@HelpMeOmg Welcome to Fluther, BTW. You’re mixing metaphors now. Putting your finger over the nozzle increases the flow speed, so the spray stings. That’s an increase in velocity. Once out of the nozzle, the water pressure drops to atmospheric pressure. You asked about kinking the hose, which puts the obstruction well upstream of the nozzle. Kink the hose and the flow from the nozzle decreases in intensity. Kink it enough, and the flow stops altogether. The pressure upstream of the kink rises, and downstream it falls.

majorrich's avatar

Speaking of pressure, we had to have a regulator placed in our supply line. We were peaking over 100 psi! The water tower is at the top of the hill we live on. This applies in a similar manner to the subject at hand. The regulator is in simplest terms a partial obstruction of the line, dropping the pressure inside the house and keeping my icemaker hose from exploding…. again….XD We used to go through garden hoses quite a bit as well.

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