As part of its expert witness and break-investigation service, EHG has calibrated and validated HAMMER’s numerical simulations for different fluids and systems for clients in the civil (water and wastewater), mining (slurry), and hydropower sectors. Comparisons between computer models and validation data can be grouped into the following three categories:

• Cases for which closed-form analytical solutions exist given certain assumptions. If the model can directly reproduce the solution, is considered valid for this case. The example file (\\HAMR\Samples) hamsam01.hif is a validation case against the Joukowski equation.
• Laboratory experiments with flow and pressure data records. The model is calibrated using one set of data and, without changing parameter values, it is used to match a different set of results. If successful, it is considered valid for these cases.
• Field tests on actual systems with flow and pressure data records. These comparisons require threshold and span calibration of all sensor groups, multiple simultaneous datum and time base checks and careful test planning and interpretation. Sound calibrations match multiple sensor records and reproduce both peak timing and secondary signals—all measured every second or fraction of a second.

It is extremely difficult to develop a theoretical model that accurately simulates every physical phenomenon that can occur in a hydraulic system. Therefore, every hydraulic transient model involves some approximations and simplifications of the real problem. For designers trying to specify safe surge-control systems, conservative results are sufficient.

The differences between computer model results and actual system measurements are caused by several factors, including the following difficulties:

• Precise determination of the pressure-wave speed for the piping system is difficult, if not impossible. This is especially true for buried pipelines, whose wave speeds are influenced by bedding conditions and the compaction of the surrounding soil.
• Precise modeling of dynamic system elements (such as valves, pumps, and protection devices) is difficult because they are subject to deterioration with age and adjustments made during maintenance activities. Measurement equipment may also be inaccurate.
• Unsteady or transient friction coefficients and losses depend on fluid velocities and accelerations. These are difficult to predict and calibrate even in laboratory conditions.
• Prediction of the presence of free gases in the system liquid is sometimes impossible. These gases can significantly affect the pressure-wave speed. In addition, the exact timing of vapor-pocket formation and column separation are difficult to simulate.

Calibrating model parameters based on field data can minimize the first source of error listed above. Conversations with operators and a careful review of maintenance records can help obtain accurate operational characteristics of dynamic hydraulic elements. Unsteady or transient friction coefficients and the effects of free gases are more challenging to account for.

Fortunately, friction effects are usually minor in most water systems and vaporization can be avoided by specifying protection devices and/or stronger pipes and fittings able to withstand subatmospheric or vacuum conditions, which are usually short-lived.

For systems with free gas and the potential for water-column separation, the numerical simulation of hydraulic transients is more complex and the computed results are more uncertain. Small pressure spikes caused by the type of tiny vapor pockets that are difficult to simulate accurately seldom result in a significant change to the transient envelopes. Larger vapor-pocket collapse events resulting in significant upsurge pressures are simulated with enough accuracy to support definitive conclusions.

Consequently, HAMMER is a powerful and essential tool to design and operate hydraulic systems provided the results are interpreted carefully and scrutinized as follows:

• Perform what-if analyses to consider many more events and locations than can be tested, including events that would require destructive testing.
• Determine the sensitivity of the results to different operating times, system configurations, and operating- and protective-equipment combinations.
• Based on a calibrated or uncalibrated model, predict the effects of proposed system capacity and surge-protection upgrades by comparing them against each other.

These are facilitated if transient pressure or flow measurements are available for your system, but valid conclusions and recommendations can usually be obtained using HAMMER alone.