Flexible hose pressure staightening forces 1

[richsad]

How can I determine the forces and moments applied to a flexible curved hose, steel braided type, subjected to pressure, 4000psi approx 1-1.5 inch diameter? The forces and moments are from restraining supports at each end of the hose and/or intermediate hose supports.

 

[umamagesh]

There is no Pressure thrust force transmitted onto the support by hoses as the braid takes all the forces.
Is this your querry.

 

[richsad]

I agree no pressure thrust but for a curved hose the braid angle will be different from the inside to the outside of the curved portion of the hose. So I would expect a straightening moment to generate restraining support forces as a result of the pressure in the hose.

 

[EdDanzer]

You will probably have to field test. The curve, the type of hose, and the lenght will effect this value. The speed the pressure pulse is applied will also have an effect.

 

[hacksaw]

As long as you comply with the minimum bending radius established by the manufacturer, it is my understanding that the pressure stresses (circumferential) govern.

The problem you have is that most hose shortens under pressure and if not properly installed it will develop tensile stresses.

 

[AeroE]

richsad

Please email me if you have found a method for calculating the end loads - I found your thread in a search for the same method.

These other guys are all wet - there are defintely external shear loads on the end of a flexible hydraulic hose, all you need to do to prove it to yourself is observe a hose as it pressurises from zero pressure, or is subjected to surge pressures.

My concern is for hoses that terminate in a metal line that is not clamped at the end of the hose, i.e., the metal line will experience a bending moment. I've looked all over for a method; our hydraulic epxerts at work don't even have one.

 

[EdDanzer]

AeroE
Your attitude is not appreciated. You may be the one who ends up all wet when you do plumbing.
I have plumbed hundreds of different machines, and attachments, no aerospace, and I don’t believe there is way to accurately determine what a hose will do when pressurized. Different sections of the same hose will have minor differences in reaction forces, the way the hose is installed will have an effect. Most hose is manufactured and coiled, this puts a set in the hose so it hangs differently when rotated about the center axis. This feature will cause variation during assembly that has a major effect on the forces at the connection point when pressurized. If you do testing, I think you will find each hose manufactures hose will be different in the reaction force, so you may need to test several makes of hose.

 

[patrickjmp]

The stainless steel braid is there to keep the hose in tension, therefore it shouldn't exert any thrust force on the pipe or other equipment. The braid keep the hose from expanding.

 

[aviat]

The amount of elogation allowable are given in some specs., for instance some SAE hose specs. give it. It is usually around +4% to -6% of the hose length at rated pressure. It is the reason why there should be some stack built into line when installing.

 

[athomas236]

If I was posed with this problem I would talk to the hose supplier and seek his advice.

There was a case in the UK some years ago at a place called Flixborough were I believe some maintenance guys changed the design of an unsupported flexible pipe joint. When returned to service the joint "sqirmed" and failed causing deaths and devastation from the resulting explosion. I understand that the fact that the maintenance guys did not understand how flexible joints behave under pressure was a significant factor in this incident.

Best regards,

athomas236

 

[StressGuy]

That was with expansion joints, not braided hoses and because they removed a reactor that was an anchor point for the system.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

All opinions expressed here are my own and not my company's.

 

[lilliput1]

St = E a dT
where:
St = thermal stress (psi)
E = Modulus of elasticity (psi)
a = coef of thermal expansion (in/(in °F))
dT = Tmax - Tinstall (°F)

Sp = p(D-t)/2t
where:
Sp = stress due to internasl pressure (psi)
D = pipe OD (in)
t = wall thickness (in)
p = system pressure (psi)

Solve for combined stress Sc (psi)

Sc = (St^2 + Sp^2)^0.5

Solve for F:

F = Sc A
Where F= end load (lbs)
Sc = combined stress psi
A = cross sectional area of pipe wall (in^2)

Use minimum 2.2 factor of safety or flex hose manuf recommendation whichever is maximum.

Draw force diagram of support & hose. Assume resultant force acts on centroid. Solve for moments.

 

[athomas236]

StressGuy

So whats your answer to the original question.

athomas236

 

[bvi]

The Flixborough failure was of a temperorary piping system connecting two reactors (the piping system took the place of the removed reactor). The piping had two expansion joints and a dogleg pipe section. It failed by squirm and subsequent jacknifing of the dogleg section.

With respect to braided hose, if the braid is taught, the braid takes the pressure thrust forces. However, if the hose is installed relatively straight between pieces of equipment, the equipment rather than the braid can end up taking the pressure thrust load. I have seen a nozzle fail due to this type of pressure thrust condition.

 

[athomas236]

so cb4 whats your answer to the original question.

Regards,

athomas236

 

[Whe]

Some interesting ideas in this post. I think the result may depend on the specific hoses used.

I agree with CB4 that there can be axial thrust loads on end points if the straight flexible hose is not taught - and in my experience hoses are not necessarily taught under all operating conditions.

As a sanity check though, take a hose in your back garden, lay it out in a bend, close the end nozzle so no water is flowing, and holding the nozzle, turn on the tap. What happens? Does the middle of the pipe move? What if your restraints do not allow this movement? Is there a load on your hand and the nozzle end? If you are using a trigger nozzle, press the trigger. Does the hose jump? Remember that this is small diameter, very flexible, and relatively low pressure. A couple of thoughts:

1) the difference in surface area of the wall of the pipe on the inside of the bend versus the outside causes the middle of the pipe to kick out which will load supports, especially intermediate ones (the static load can easily be calculated, but this may not be the whole story)

2) Once liquid is flowing, there will be lateral forces on the pipe due to change in momentum considerations (this can be estimated statically, too).

3) plus dynamic loads.

I suspect that the devil is in the detail of your specific installation. I think you are right in giving this careful consideration. Hope these comments help.