Negative Pressures

Hello Jose,

A negative pressure means that the hydraulic grade is below the physical elevation of the node. This could be due to an error in the configuration of the model somewhere else resulting in a lower HGL than expected, or it could be "real" based on assumed demands you have placed on the nodes. If you have closely examined the model configuration and user notifications and everything is set up as expected, with elevations representing where you want to measure pressure from, then the negative pressures could indeed be telling you that the system is not able to supply those demands. You can read more about this subject in the following article from our Wiki:

Troubleshooting negative pressures at pumps, junctions, & other node elements

Thank you for the reply Jesse,

I'm modeling an existing system which was very poorly designed and some zone do not get water when some valves are open at other zones. I know WaterCAD is demand driven so I still need to enter a demand to the nodes that in reality don't get water but was wondering if the negative pressures represent the lack of water in this zones which is what happens in reality. 

Hi Jose,

The following articles may also be helpful as they explain some other approaches to consider when modeling this type of situation:

Modeling Fixed Inflow from a Marginal Source

Modeling intermittent water supply

Hi Jesse,

I read the article for Modeling Inteermittent Water Supply and I'll go with the Pressure Dependent Demand approach but I have trouble understanding the reference pressure. How can I calculate this value?

Hello Jose,

The "Pressure (Reference)" in the PDD alternative represents the pressure at which 100% of the demand you entered on the node will be supplied. Meaning, if the pressure calculated in the model is equal to the reference pressure, the demand will be equal to the entered demand. If the pressure is higher than the reference pressure, the demand will be higher and if the pressure is lower than the reference pressure, the demand will be lower. A cutoff can also be specified.

Here is an article with more details on PDD, in case you have not seen it yet: Using Pressure Dependent Demands

The article gives more details along with examples to help you understand the required input and assumptions. As far as what to enter for the reference pressure, that is largely based on your engineering judgement for the particular system being modeled. In general you would probably want to enter a typical pressure that corresponds with the assumed demands. Meaning, when you enter a base demand on a node, what typical pressure would cause that outflow?

You can also choose to override the global reference pressure and enter per-node reference pressures, for cases where the typical pressure varies throughout the model. You can read more about that in the above article.

Hello Jose,

The "Pressure (Reference)" in the PDD alternative represents the pressure at which 100% of the demand you entered on the node will be supplied. Meaning, if the pressure calculated in the model is equal to the reference pressure, the demand will be equal to the entered demand. If the pressure is higher than the reference pressure, the demand will be higher and if the pressure is lower than the reference pressure, the demand will be lower. A cutoff can also be specified.

Here is an article with more details on PDD, in case you have not seen it yet: Using Pressure Dependent Demands

The article gives more details along with examples to help you understand the required input and assumptions. As far as what to enter for the reference pressure, that is largely based on your engineering judgement for the particular system being modeled. In general you would probably want to enter a typical pressure that corresponds with the assumed demands. Meaning, when you enter a base demand on a node, what typical pressure would cause that outflow?

You can also choose to override the global reference pressure and enter per-node reference pressures, for cases where the typical pressure varies throughout the model. You can read more about that in the above article.