Very High Fluid Velocity in Hoses

[etaylor]

I have been searching for ages to find a good reason not to ignore the guidelines from hydraulic hose suppliers which state that you should not exceed the velocity limit in pressure lines of 6 or 10 m/s(depending on supplier).
We have peak velocities of 43m/s in a 2" hose! Theses peaks occur very infrequently and only last for less than a second. The average speed is 4.3 m/s. I have calculated the pressure loss and the heat generation and dissipation, and as this hose is underwater, overheating is not an issue. The fluid is water glycol and highly filtered so erosion due to fast moving solid particles will not be an issue. The fluid pressure ranges from 20 Bar to 140 Bar so this is well above the vapour pressure so cavitation will not occur.

My question to you clever people is:
Apart from pressure loss and heat generation and erosion from solid particles and cavitation which I deem all to be acceptable or non existent, is there anything else that I should be concerned about?

 

[LittleInch]

Try surge pressures. Past about 5-6 m/sec, pressure surge from sudden cessation of flow can reach very high pressures.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

YES the 140 bar pressure in the hose!
You need to mitigate that surge pressure before you use it again.

Independent events are seldomly independent.

 

[etaylor]

Surge pressure are the pressures created when the flow starts or stops quickly like when you rapidly close a valve. The system that I am talking about has pressure ranges between 20 and 140 bar which is due to the fluctuating flow rates and has be carefully modeled in simulink. The hose will have a working pressure of 140 bar or greater.

To summarize:

Pressure loss: acceptable
Heating: acceptable
Cavitation: not a problem
Surge pressure: already accounted for

Is there anything else that I should be concerned about when exceeding the max recommended flow rate for the hose?

 

[BigInch]

Really?

Somehow or other I don't think a 140 barg transient pressure is the same thing as a 140 barg constant pressure load, especially in a hose, where you can have significant bending and other whip effect loads moving up and down the hose associated with those transients. When you have transient pressures as high as 140/20 where the fluid hits the front of a closed valve, you have the potential to generate negative pressures on the downstream side of 20-(140-20), that's -100, yes that's impossible, so we would limit that to the fluid's vapor pressure, ie... cavitation.

Plus ... Erosion can take place from high velocity contact with a clean fluid. You are only considering "hard particles" grit blast effect. Have you not ever directed a high velocity spray of clean water on anything. A dirty fluid may have an erosion limit of 15 fps, while the same fluid in a clean state may begin to cause erosion at 2 x that or about 30 fps.

Independent events are seldomly independent.

 

[LittleInch]

To a certain extent it depends on the length and orientation of your hose and nature of your flow. A fairly straight hose you might get away with higher velocities, but the vendor doesn't know where his hose is going to be used and hence has to assume that you will use the flexibility of the hose to its full extent, even 180 degree bend or 270. At the min radius of the hose, there are both erosional forces and some element of axial force from momemtum change and the varying loads caused by pressure trying to striaghten out your bent hose. These forces increase with velocity.

Your responses are a little uneven as you quote 43m/sec for less than a second. That sounds like a healthy surge pressure to me, then your reply above states 140 to 20 bar due to fluctuaing flows - Is this Steady State pressure or transient pressure. Please be clear.

At the end of this a good reason not to ignore the guidelines / limits from the suppliers is that if you design to exceed this and the hose fails, the supplier will not be liable or responsible - you will be - and as big Inch notes above, erosion is more abrasive with hard particles, but that does not eleminate erosion. Many of these effects end up proportional to some power of the velocity, so what is OK at 6 m/sec, isn't at 9m/sec.

You really need to do this on a case by case basis and get buy in from your hose vendor giving him all the info including the amounts of time the hose will operate at different velocities.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[etaylor]

Hi Inches

Thanks for your help. To be clear the flow is not steady state it is transient state. The flow and pressure vary continuously and are never constant, I would like to show you a graph of it but haven't worked out how to attach a file to this post. There are not any valves that open and close quickly causing large pressure spikes or drops. These flows and pressures are generated by ocean waves and not a pump.
I would like to stay within the suppliers recommended flow rates and understand the liability issues but am out of options, if you know any professional consultancies that specialize in this area it would be appreciated. All the hose suppliers offer very little technical expertise and I have not found any that can offer a workable solution to this.

 

[MikeHalloran]

The hose will also be in the habitat of some animals and microbes who may find it quite tasty.

They may also react in odd ways to the hoise associated with the flow peaks.





Mike Halloran
Pembroke Pines, FL, USA

 

[BigInch]

If you abide by their (and our) recommendations, not a very high level of tech expertise is necessary.

We told you what we thought, but we can't make you do anything, so good luck.

Independent events are seldomly independent.

 

[LittleInch]

To upload a file go to the bottom of the post writing area and the easiest is to click on the "or upload your file to engineering.com" link which uploads a file from any of your drives / folders..


If you can give us a decent description of your real issue / problem then you may get some better help. Sounds like you've got a wave energy device of some sort, but details are always good.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way