Transient implications of soil, pipe lining, material and thickness

Product(s): HAMMER
Version(s): V8i, CONNECT Edition
Area: Modeling

Problem

What impact and implications do the following have on the transient response?

1) Soil type/properties

2) Pipe Lining (compound pipe)

3) Pipe thickness

4) Pipe Material

Soil Type / Properties

HAMMER's wave speed calculator assumes that the pipe is not buried, so soil properties are not included in the calculations. This assumption is in place because a) it simplifies the calculations, and b) when buried pipes are expanding then contracting, they can push the soil away from the pipe so that there is little direct interaction between the two. However, if you want to include soil interactions, you would need to apply this to the wave speed calculation.

Regarding energy dissipation by the soil deformation, this is a possibility in a system, however HAMMER doesn't have a way to account for that. HAMMER strictly looks at energy dissipation through the friction caused by water flowing along the pipes, which is why a transient will continue forever in a frictionless pipe, or a pipe with zero flow. This is an assumption that HAMMER makes in order to simplify the calculations. This is common practice in transient software. Consensus across the industry is that energy dissipation through soil deformation is typically insignificant and can be ignored.

Pipe Lining (compound pipe)

The lining of a pipe, or weather it is a compound pipe, will impact the wave speed. This is a matter of engineering judgement. If the lining is fairly thin, most likely it won't have a noticeable impact on the wave speed. It might be best to ignore the lining and base the wave speed on the main material (such as steel) of the pipe. The Wave Speed Calculator tool in HAMMER can assist you with this.

Generally speaking, if in doubt, make a conservative estimate and/or consider a sensitivity analysis. For instance, you can derive values for pipe thickness and Young's modulus and see how this impacts the results. You could compute the model with the wave speed based on ignoring the lining. You could compute once more  with an estimate on the biggest possible impact (perhaps reduce the wave speed by a certain percentage). If there is no significant difference in the transient results, then you'll have more confidence that it's OK to ignore.

Related forum discussion: Calculate celerity of a compound pipe

Pipe Material

The material of a pipe (such as HDPE vs ductile iron) will impact:

1) The wave speed / celerity of the pipe (how fast a wave travels), by way of the thickness, rigidit and other factors. See more on wall thickness below.

2) Pressure tolerance. Certain pipe materials will be able to withstand higher pressures than others before bursting, cracking or leaking. Similarly, different materials will have different ability to withstand low or sub-atmospheric pressure before rupturing.

3) Joints. The joints used for different types of pipes may have different abilities to withstand transient pressures, or leak (or suck in contaminants) upon subatmospheric pressure conditions.

If in doubt, check with the pipe manufacturer to get more information on the properties that will need to be entered to determine the pressure tolerance and wave speed.

Pipe Thickness

Hydraulically, wall thickness only effects the wave speed (celerity) of a pipe. So, it is only directly modeled by way of the wave speed attribute entered in the pipe properties, or the global wave speed entered in the transient calculation options.

Wall thickness also effects the minimum and maximum pressure that the pipe can withstand during a transient condition (pressure rating). Contact the manufacturer for specifications and compare to the min and max calculated pressure during the transient simulation.

To calculate the wave speed of your pipes and account for the effect of the wall thickness, use the Wave Speed Calculator tool under the Tools menu. If you have pipes with different thicknesses, you would need to do this in batch. Meaning, select all the pipes of the same thickness in the drawing (for example by using a query on diameter or some other attribute), then open the tool and choose "Selection" (or pick the selection set, if you have made one) Then, enter the corresponding wall thickness in the calculator and click Ok to apply the wave speed to just those pipes. Now, repeat these steps for each set of pipes of a particular thickness.

If you'd like to see the wall thickness in the pipe properties or in table form (pipe flextable), you will need to add the field using the User Data Extensions tool. See more in the article below "Pipe wall thickness in flextables"

See Also

Equation used in Wave Speed Calculator tool

Pipe wall thickness in Flextables

The nature of polyethylene pipe failure (External)

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