Introduction

This article explains how hydropneumatic tanks and surge tanks are modeled in WaterGEMS and WaterCAD. 

Background

A hydropneumatic tank uses compressed air or gas to provide a relatively small amount of water storage volume while keeping the hydraulic grade much higher than the physical top of the tank.

Hydropneumatic tanks can be set to either follow the traditional Gas Law equation or use the Constant Area Approximation method to calculate a change in pressure/HGL and volume. The constant area approximation uses a linear relationship; the user must specify minimum/maximum HGL and the corresponding volume between. The Gas Law method is non-linear and follows the Gas Law--as gas is compressed, it becomes harder to compress it more. 

Unless you have a specific need to use the constant area method, it is recommended that you use the Gas Law. The Constant Area Approximation method is primarily left in place for legacy purposes, to provide a way for users to achieve equivalent results compared to the method that was used before this element type was available in the product. Meaning, before the hydropneumatic tank element, it was common to model a hydropneumatic tank using a normal tank node element configured to be very tall with the appropriate constant diameter. With the hydropneumatic tank node element, an older model (or one from another product such as EPANET) could have a hydropneumatic tank inserted instead of the regular tank and configured with the Constant Area Approximation method to achieve comparable results.

How it works

The two method of modeling a hydropneumatic tank are described in more detail below:

• Constant Area Approximation: This method approximates a hydropneumatic tank by using a tall, thin tank whose water surface elevation approximates the HGL in a hydropneumatic tank. The "HGL on" and "HGL off" fields represent the maximum and minimum hydraulic grade lines within the hydropneumatic tank (i.e., when an associated booster pump would turn on or off). An approximate diameter is computed based on the effective volume of the hydropneumatic tank so that the tank cross sectional area multiplied by the distance between HGL on and HGL off gives the same volume as the hydropneumatic tank.

• Gas Law: This method uses the ideal gas law, PV=nRT. Given the initial level/hydraulic grade and liquid volume, the gas law model keeps the "nrt" part of the equation constant (called K) and is thus able to compute hydraulic grade for a given volume, or vice versa, as the conditions in the model change over the course of an EPS simulation. One should still use controls to turn the pump on or off based on the minimum/maximum level or hydraulic grade in the hydropneumatic tank. The initial liquid volume is subtracted from the total tank volume to find the gas volume. The physical "elevation" is subtracted from the initial HGL to find the gauge pressure. The atmospheric pressure is added to the gauge pressure to get absolute pressure, which is used in the ideal gas law equation.

*Both methods typically yield similar results within the "effective" control range, but the gas law is technically more accurate. 

"Treat as Junction?" Field

In many cases a hydropneumatic tank (or surge tank) may be implemented only for transient protection (which can be analyzed in Bentley HAMMER). During a steady state condition, the tank may simply operate under the corresponding normal / steady state head ("line pressure"). To simplify this during a model simulation there is field called  "Treat as junction" that can be set to "True". Doing this allows the WaterGEMS/WaterCAD solver to compute a hydraulic grade at the tank location and the user simply assumes that the tank has already responded to the hydraulic grade and the air volume has expanded or contracted accordingly. When using this option the user only needs to enter the initial volume of gas under the "Transient" section of the tank properties that corresponds to the hydraulic grade (unless using a bladder instead of gas law setting). 

Troubleshooting: Why does the HGL or pressure become large and exceed the pump control range?

Hydropneumatic tanks have a very short cycle time compared with large tanks. Therefore, when hydropneumatic tanks are used in a model, a very short hydraulic time step may be needed or the tank may overshoot its on and off levels. If this occurs, a reduction in the timestep may be necessary. You will want to make sure the time is small enough, so that if intermediate time steps are necessary to include for the controls they don't skip over the controls. Intermediate time steps are calculation time steps that are inserted between regular interval time steps to account for control changes to element statuses. These time steps are inserted at 1/10th of the normal hydraulic time step. You will know if you need a smaller hydraulic time step because the HGL in the tank will shoot past the control elevation in 1/10 of a calculation hydraulic time step. This helps to ensure that the controls do not end up overshooting the fast level changes that can occur.

Surge Tanks

Surge tanks are handled just as regular tanks are in WaterCAD and WaterGEMS. They are available in the software because these products share a common file format with Bentley HAMMER. This enables you to lay out the location of surge tanks while working in WaterCAD or WaterGEMS, so that the model is ready for a transient analysis in HAMMER (the model can be opened directly, assuming the same version or later of HAMMER is used)

Transient Simulation (HAMMER)

For more information on how hydropneumatic tanks work in Bentley HAMMER, see the following article: Modeling Reference - Hydropneumatic tanks

Example Model

Click the below link to download a small example model with a Reservoir > Pump > Hydropneumatic tank > Piping > Demand. The Gas Law is used with pump controls based on the tank hydraulic grade, with a suitably small time step.

Hydropneumatic Tank Example EPS model.zip

See Also

Why are there so many extra elements listed for WaterGEMS and WaterCAD such as surge tanks, rupture disks, or periodic head-flow elements?