RE: Definition of System Head Curve

Hello Sayed,

You will find information about system head curve in the help documentation of WaterCAD, please search by term “system head curve”.

Here are some support solutions on system head curve which you may find useful.

communities.bentley.com/.../15371.system-head-curves-with-no-downstream-storage-solution-500000056093.aspx

communities.bentley.com/.../12132.how-to-manual-generate-a-system-head-curve-solution-500000058706.aspx

Dear Sushma

Thanks a lot for useful links which you provided. I do not know which equation to represent the PDD equation and this equation should be fixed and used in all projects?. Actually if you can provided me with simple example with numbers (one reservoir, one pump and one junction), and the step to do the system head curve , it will be great help.

Regarding PDD - this is largely a matter of engineering judgment. You'll need to decide on the pressure/demand relationship that you are comfortable with assuming. If you have storage (tanks) downstream of the pumps that you want to plot the system head curve for, you may not necessarily need to set up PDD for your demands; PDD is useful for system head curves where you only have demands downstream, so that the flow is allowed to vary.

As for the steps for a system head curve, you simply right click on the pump of interest, choose "System Head curve", select the time(s) that you want to plot for (if your model is set as EPS), then click the compute button. You typically don't need to change the "maximum flow" and "number of intervals".  Keep in mind the assumptions documented in the help and in the articles that Sushma provided above. You can use one of the included sample model for illustrative purposes, if needed (located in the Samples folder within your WaterCAD/WaterGEMS installation folder). For instance, Example1.wtg is a simple network with a pump and downstream tank.

If you do need to set up PDD for your model, here is a section from an article on PDD that is linked to from one of the articles that Sushma mentioned, which talks about the power function option:

"...The Power Function option is used to define the exponential relationship between the nodal pressure and demand. The ratio of actual supplied demand to the reference demand (i.e., percentage of defined nodal demand designated as pressure-dependent) is defined as a power function of the ratio of actual pressure to reference pressure. (Defining of reference pressure and pressure-dependent demand percentage is done in the Alternative, as described in the next section.) Using a power equation for your Pressure Dependent Demands is like you are assuming that each 'demand' in your system acts like an orifice. The orifice equation can be written like this: Q = K*P0.5 Where Q is flow through the orifice, P is pressure upstream of the orifice, and K is some coefficient (which is a function of orifice area, coefficient of discharge, etc.) You can specify a desired Power Function Exponent. The default value provided is 0.5, which is the exponent used in the orifice equation. "

Hope this helps,

Regarding PDD - this is largely a matter of engineering judgment. You'll need to decide on the pressure/demand relationship that you are comfortable with assuming. If you have storage (tanks) downstream of the pumps that you want to plot the system head curve for, you may not necessarily need to set up PDD for your demands; PDD is useful for system head curves where you only have demands downstream, so that the flow is allowed to vary.

As for the steps for a system head curve, you simply right click on the pump of interest, choose "System Head curve", select the time(s) that you want to plot for (if your model is set as EPS), then click the compute button. You typically don't need to change the "maximum flow" and "number of intervals".  Keep in mind the assumptions documented in the help and in the articles that Sushma provided above. You can use one of the included sample model for illustrative purposes, if needed (located in the Samples folder within your WaterCAD/WaterGEMS installation folder). For instance, Example1.wtg is a simple network with a pump and downstream tank.

If you do need to set up PDD for your model, here is a section from an article on PDD that is linked to from one of the articles that Sushma mentioned, which talks about the power function option:

"...The Power Function option is used to define the exponential relationship between the nodal pressure and demand. The ratio of actual supplied demand to the reference demand (i.e., percentage of defined nodal demand designated as pressure-dependent) is defined as a power function of the ratio of actual pressure to reference pressure. (Defining of reference pressure and pressure-dependent demand percentage is done in the Alternative, as described in the next section.) Using a power equation for your Pressure Dependent Demands is like you are assuming that each 'demand' in your system acts like an orifice. The orifice equation can be written like this: Q = K*P0.5 Where Q is flow through the orifice, P is pressure upstream of the orifice, and K is some coefficient (which is a function of orifice area, coefficient of discharge, etc.) You can specify a desired Power Function Exponent. The default value provided is 0.5, which is the exponent used in the orifice equation. "

Hope this helps,