Suitable must have conditions to get correct/Ground reality results through Demand Dependent Analysis

I have come to know that \

1. there are two types of Analysis possible one is DDA and another is PDA.

2. In DDA, Demand is always met.

3. It is Suitable for Pressure sufficient systems. That means that we will get almost results on ground as the Model has predicted. 

I need help with below Questions. 

1. Are all the three Statements above correct?

2. How can we know exactly the system is pressure sufficient?  

  • Hello Chandan,

    WaterGEMS and WaterCAD are demand driven programs, so the demands should be satisfied. You can model demands that change based on pressure. See this link for more information on this: Options for modeling an outflow that varies with pressure.

    The link has further information on the difference between these: Demand-driven vs. Pressure-driven vs. Gravity-driven analysis in WaterGEMS and WaterCAD.

    For pressure results, it will depend on what you are modeling. If you need to see the pressure results for a typical hydraulic analysis, you can review the pressure results in the junction flextables. If you need to make sure the pressure is sufficient during a fire flow analysis, you can compute an automated fire flow analysis. We would need more information if this doesn't help.

    Regards,

    Scott

  • Is it correct that DDA will give correct results if System is Pressure sufficient. How can we establish whether a system is pressure sufficient or not? Which  of the below case(s) is pressure sufficient system?

     

  • WaterGEMS and WaterCAD can perform both fixed demand and pressure dependent analysis. The default method is fixed demand which is based on the assumption that pressure is adequate. If you suspect that demands in your system are pressure dependent, switch to PDD and see how different the demands are. We provide plenty of help on how to switch to PDD.

    See the Help provided in WaterGEMS. For additional help see

    communities.bentley.com/.../using-pressure-dependent-demands

    www.youtube.com/watch

  • Sir, I don't wish to use PDA instead I wish to use DDA which is default for EPANET and WaterGEMs. However, As I have come to understood that DDA will provide good results if System is Pressure Sufficient, I need to Understand When can I call a System to be pressure sufficent. Can you please some ceiteria to evalute when can we call a system pressure sufficent. For Example, if minimum pressure in network is 1.2 times atm pressure or 1.5 times atm pressure something like to easily understand whether system is pressure sufficient or not?

  • Hi Chandan,

    Whether or not a system is "Pressure Sufficient" is highly dependent on the type of system.   A domestic plumbing tap has different characteristics to a shower head, which has different characteristics to an irrigation system head etc. etc.

    For a plain fixed orifice like say a dust suppression system spray head, then for any given "Demand" Q, this will be "satisfied" by Pressure P like this.  In these "open" systems then there is no single Minimum P value,  the Demand is Pressure Dependent, and vice-versa, the Pressure required is Demand Dependant.   In modelling these type of high flow, PDD systems the modeller will tend to use the option of turning on Pressure Dependent Demands for these Demand Nodes

    However, in a house/unit, taps and showers have a completely different P vs Q curve.   In these "closed" systems, then it looks more like below.

    Generally this is where we say the model results are "good enough" to not bother with simulating PDD, because the user of the system has valves/taps that they use to throttle to near fixed flow  (In reality it is not quite fixed.........there is still always a little bit more system demand in these systems the higher the pressure)

    ....and then water efficient shower heads are different again.  These have internal pressure sensitive diaphragm that closes up as pressure increases to make sure the shower head discharges no more than a certain maximum flow rate.  So they look like this

    Industrial systems with a risk of backflow contamination to the main are yet different again.  They may have a spring loaded backflow prevention valve like an RPZD that makes their curves look like this

    So, there is no single value for "Pressure Sufficiency" to go to a model that simulates Fixed Demands.   Broadly the engineer looks at the range of water pressures likely to be in the network, and then looks at what plumbing, fixtures and appliances are connected to that network.

    If generally the pressure is on the parts of the above curves where Demand is highly insensitive to Main pressure = Assume Fixed Model Demands.

    If not, and the Demand is sensitive to the Pressure AND it makes a significant difference to the engineering solution (don't bother using PDD if it doesn't change the design outcomes!), then use Pressure Dependent Demand nodes, like Emitters.



    Answer Verified By: Chandan Kumar