Pressure Thrust – A Quick Tutorial
Pressure thrust is the single most important factor affecting the successful implementation of bellows in pipework. Proper consideration of this phenomena can literally spell the difference between success or catastrophic failure – so its vital for specifiers and end users to understand what pressure thrust is, and how to deal with it.
Imagine you are blowing up a balloon. Essentially what you are doing is increasing the pressure on the inside of the balloon, and the balloon then gets bigger. Now think about trying to do the same thing to a a more rigid container – say a glass bottle. If you try it, you will still be able to increase the pressure inside the bottle in the same way you could with the balloon, but the bottle won’t get bigger, as it is stronger. Pretty simple really.
Understanding Pressure thrust in terms of bellows in pipelines is exactly the same concept. In a normal pipeline, there is a certain pressure on the inside of the pipe line. If the pipeline is rigid enough, then the effect of the internal pressure goes un-noticed to the observer (much like the glass bottle). If the pipeline is flexible enough though, then the internal pressure will cause expansion (much like a balloon).
Given that bellows are designed to be flexible along their length, any internal pressure will tend to extend the bellows lengthwise, which is what we call pressure thrust. Pressure thrust is often not noticed with normal pipe, simply because the pipe isn’t flexible enough for the internal pressure to cause the pipe to expand.
The degree to which this is a problem, is simply determined by the flexibility of the bellows, and the pressure of the pipeline. For any given pipeline pressure and size, you can work out the maximum “pressure thrust” force which will be generated. (To work out pressure thrust, see our Pressure Thrust Calculator – try it!)
The formula for calculating pressure thrust is: Effective Area (mm) x Pressure (MPa) = Thrust force (N)
Its difficult to picture this in a normal piece of hard piping, but even between two fixed points in a hard pipe, there is a force trying to separate or break the pipe in the axial direction – however, since the pipe material is strong, the force results in no observable deflection – nonetheless, a stress is induced in the longitudinal direction in the pipe.
When you introduce a pipeline component which is flexible – like a metal bellows, a silicone coupler, etc, it is then more obvious that this thrust force exists. The flexible component will deflect as the thrust force is able to overcome the components stiffness and the stiffness of the connecting pipework. You can see an illustration of this in the diagrams above – when there is no pressure in the pipe, there is no thrust force generated – but when pressure is added, the thrust force acts to try to make the bellows longer. This can be a real problem if the thrust force is high, and if the pipe work is not designed to cope with these forces.
So there’s a force generated… what does that mean?
Good question! The answer is that it depends on just how large the force is.
For example, if you use some preset values from our calculator, you can see that for a 300nb bellows at a typical exhaust pressure, the thrust force is about 100kg of force. Ask yourself – can my pipeline handle 100kg of force acting on it from the bellows? Probably, yes – but it never hurts to check!
Lets ramp it up a bit though – lets consider the same bellows diameter, but now a steam application – not too high pressure, 700kPa (100psi in the old language) – and our pressure thrust is around 7,200kg.
Think carefully about that for a second. Can your pipework handle the weight of a bus acting along it, wherever you place an unrestrained bellows? That is one you definitely want to check, and its probably not an easy task to solve! Imagine, 7.2 tonnes of force pushing onto the side of the pump/boiler/valve/thrust block. Its a lot of force and its got to go somewhere. And that’s just a 300nb bellows, at a pressure very commonly encountered in pipework.
Typically, for practical engineering reasons, its at this kind of magnitude of force that unrestrained bellows become unusable in pipework – we need to use more clever solutions to try to introduce the flexibility in the pipework that is required – typically with Hinged, Gimballed or Pressure Balanced bellows. (See some of our product types here.)
The inline pressure balanced assembly pictured below is a special type of bellow which does not exert pressure thrust, but which can move axially. Typically, these types of bellows assemblies are larger and more complicated to implement, and offer less flexibility than an unrestrained bellows, but they will not destroy your pipeline either!
Pressure Balanced Assembly
An inline pressure balance bellows – this bellows is designed to move axially, but does not generate a thrust force
Hinged Bellows Assembly
A hinged bellows assembly – this bellows is designed for angular movements only and does not generate a thrust force
Pressure Thrust Failure
An unrestrained bellows that has failed due to pressure thrust.