Release Notes for EPANET 2.2 (Draft)

This document describes the changes and updates that have been made to version 2.2 of EPANET.

Thread-Safe API Functions

A duplicate set of the version 2.1 API functions has been provided that allow multiple EPANET projects to be analyzed concurrently in a thread-safe manner. These functions maintain the same name as the original but use a EN_ prefix instead of EN. In addition, the first argument to each of these functions is a pointer to an EN_Project structure that encapsulates the network data for the particular project being analyzed. For example, instead of writing:

ENgetnodevalue(nodeIndex, EN_ELEVATION, &elev)

one would use:

EN_getnodevalue(pr, nodeIndex, EN_ELEVATION, &elev)

where pr is the pointer to an EN_Project.

Two new functions have been added to the API to manage the creation and deletion of project pointers. EN_createproject creates a new project along with a pointer to it, while EN_deleteproject deletes a project. An example of using the thread-safe version of the API is shown below:

#include "epanet2.h"
int runEpanet(char *finp, char *frpt)
{
    EN_Project *pr = NULL;
    int err;
    err = EN_createproject(&pr);
    if (err) return err;
    err = EN_open(pr, finp, frpt, "");
    if (!err) err = EN_solveH(pr);
    if (!err) err = EN_report(pr);
    EN_close(pr);
    EN_deleteproject(pr);
    return err;
}

Additional Convergence Parameters

Two new analysis options have been added to provide more rigorous convergence criteria for EPANET's hydraulic solver. In the API they are named EN_HEADERROR and EN_FLOWCHANGE while in the [OPTIONS] section of an EPANET input file they are named HEADERROR and FLOWCHANGE, respectively.

EN_HEADERROR is the maximum head loss error that any network link can have for hydraulic convergence to occur. A link's head loss error is the difference between the head loss found as a function of computed flow in the link (such as by the Hazen-Williams equation for a pipe) and the difference in computed heads for the link's end nodes. The units of this parameter are feet (or meters for SI units). The default value of 0 indicates that no head error limit applies.

EN_FLOWCHANGE is the largest change in flow that any network element (link, emitter, or pressure-dependent demand) can have for hydraulic convergence to occur. It is specified in whatever flow units the project is using. The default value of 0 indicates that no flow change limit applies.

These new parameters augment the current EN_ACCURACY option which always remains in effect. In addition, both EN_HEADERROR and EN_FLOWCHANGE can be used as parameters in the ENgetstatistic (or EN_getstatistic) function to retrieve their computed values (even when their option values are 0) after a hydraulic solution has been completed.

Improved Linear Solver Routine

EPANET's hydraulic solver requires solving a system of linear equations over a series of iterations until a set of convergence criteria are met. The coefficient matrix of this linear system is square and symmetric. It has a row for each network node and a non-zero off-diagonal coefficient for each link. The numerical effort needed to solve the linear system can be reduced if the nodes are re-ordered so that the non-zero coefficients cluster more tightly around the diagonal.

EPANET's original node re-ordering scheme has been replaced by the more powerful Multiple Minimum Degree (MMD) algorithm. On a series of eight networks ranging in size from 7,700 to 100,000 nodes MMD reduced the solution time for a single period (steady state) hydraulic analysis by an average of 58%.

Pressure Dependent Demands

EPANET has always employed a Demand Driven Analysis (DDA) when modeling network hydraulics. Under this approach nodal demands at a given point in time are fixed values that must be delivered no matter what nodal heads and link flows are produced by a hydraulic solution. This can result in situations where required demands are satisfied at nodes that have negative pressures - a physical impossibility.

To address this issue EPANET has been extended to use a Pressure Driven Analysis (PDA) if so desired. Under PDA, the demand D delivered at a node depends on the node's available pressure P according to:

D = D_f [ (P - P_min) / (P_req - P_min) ]^P_exp

where D_f is the full demand required, P_min is the pressure below which demand is zero, P_req is the pressure required to deliver the full required demand and P_exp is an exponent. When P < P_min demand is 0 and when P > P_req demand equals D_f.

To implement pressure driven analysis four new parameters have been added to the [OPTIONS] section of the EPANET input file:

Parameter	Description	Default
DEMAND MODEL	either DDA or PDA	DDA
MINIMUM PRESSURE	value for P_min	0
REQUIRED PRESSURE	value for P_req	0
PRESSURE EXPONENT	value for P_exp	0.5

These parameters can also be set and retrieved in code using the following API functions

int ENsetdemandmodel(int modelType, double pMin, double pReq, double pExp);
int ENgetdemandmodel(int *modelType, double *pMin, double *pReq, double *pExp);

for the legacy API and

int EN_setdemandmodel(EN_Project *pr, int modelType, double pMin, double pReq, double pExp);
int EN_getdemandmodel(EN_Project *pr, int *modelType, double *pMin, double *pReq, double *pExp);

for the thread-safe API. Some additional points regarding the new PDA option are:

If no DEMAND MODEL and its parameters are specified then the analysis defaults to being demand driven (DDA).
This implementation of PDA assumes that the same parameters apply to all nodes in the network. Extending the framework to allow different parameters for specific nodes is straightforward to do but is left as a future feature to implement.
P_min is allowed to equal to P_req. This condition can be used to find a solution that results in the smallest amount of demand reductions needed to insure that no node delivers positive demand at a pressure below P_min.

Code Changes

The header file vars.h containing global variables has been eliminated. Instead a number of new structures incorporating these variables has been added to types.h. These structures have been incorporated into the new EN_Project structure, also defined in types.h, which gets passed into each of the thread-safe API functions as a pointer.
Each of the legacy API functions now simply calls its thread-safe counterpart passing in a pointer to a default globalEN_Project variable that is declared in types.h.
Throughout all code modules, global variables that were previously accessed through vars.h are now accessed using the EN_Project pointer that is passed into the functions where the variables appear.
The exceedingly long hydraul.c file has been split into four separate files:
- hydraul.c now contains just the code needed to initialize a hydraulic analysis, set demands and control actions at each time step, and determine the length of the next time step to take.
- hydsolver.c implements EPANET's hydraulic solver at a single point in time.
- hydcoeffs.c computes values of the matrix coefficients (derived from link head losses and their gradients) used by the hydraulic solver.
- hydstatus.c checks for status changes in valves and pumps as requested by the hydraulic solver.
The Multiple Minimum Degree re-ordering algorithm appears in a new file named genmmd.c. This is 1990's legacy code that is readily available on the web and can be found in several linear equation solver libraries.

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