How to input pressure sustaining valve (PSV) in watergems

Hi, 

I have a model that has a negative pressure at J-35, where in ground elevation is at 310m and the resulting hydraulic grade is at 302m only.

I tried replacing J-35 with a Pressure Sustaining Valve (PSV), set the pressure to 20psi,   with the aim to have the hydraulic grade at that junction to be equal to the ground elevation which is 310m. 

But after doing so, this is the new graph that I am getting, there is now a to and from value of hydraulic grade, from 324m to 302m. 

Hope you can enlighten me on this one, maybe I am using PSV incorrectly.

Note that my main aim is to have the hydraulic grade at that point to be equal to the ground elevation which is at 310m.

Thanks in advance. 

  • Hello Saide,

    PSVs are used to maintain the user-defined pressure or HGL on upstream side, it will throttle the flow to control upstream hydraulics. With PSVs pressure/HGL on upstream side will increase and it will decrease on the downstream side. PSVs are used as pressure relief valves. Here is detailed information on modeling PSVs in WaterGEMS and how they work. 

    How does the Pressure Sustaining Valve (PSV) work in WaterGEMS and WaterCAD? 

    I request you to please go through the wiki article above.

    If that doesn't help, we may need to see a copy of the model.

  • Saide,

    To add to Ankit's response - a PSV will throttle in order to increase the upstream hydraulic grade, but downstream boundary conditions may still control the downstream HGL. For example you may have a nearby downstream reservoir or tank whose elevation at around 302 m, so with relatively low friction losses, it isn't physically possible for the HGL downstream of the PSV to be above 310 m. The following article explains more about this: Why do I see such a large headloss through my FCV, PSV or PRV?

    Can you explain a bit more about this system and what you need to accomplish? For example what is upstream and downstream of the location in question? A pump upstream and a tank downstream? Only demands downstream?

    If this high point represents an air valve that is expected to remain open, then the negative pressure downstream of the PSV could be interpreted as partially-full flow. The air valve node element can be used as well, which internally is treated as a PSV whose HGL setting is equal to the air valve elevation. The following article has some illustrations to explain the interpretation of the part-full conditions: Modeling Air Valves At High Points in WaterCAD or WaterGEMS

    If this is not an air valve and you need to use an actual pressure sustaining valve, you may need to look closely at the downstream conditions (try creating a profile all the way down to the downstream boundary) and consider placing the PSV further downstream.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

  • Saide to come back to the engineering side:

    1. Negative 8 m gauge pressure in the model is not in itself a problem.  Water can and does flow through pipes at negative gauge pressures.
    2. Inserting Dummy PSVs at high points does not simulate what really happens in the hydraulics.  If the objective is to model what the pipeline hydraulics are in real-life then don't insert a PSV

    So on Point 1, the pressure in the fluid is usually higher than the WaterCAD Pressure Result.  The reason for this is that to get the true fluid pressure, the model needs to use the Pipe Invert Elevation...........not the Surface Level.   If the pipeline itself is for example 6 metres underground then the actual fluid pressure is -2 m , not -8 m.

    Also, water flows through pressure pipes normally at negative pressures so long as the fluid stays above its vapor pressure (the point at which the fluid will flash over to steam/vapor voids).  For water at 25 degrees Celsius, it will stay as a fluid up to an absolute pressure (not gauge pressure) minimum pressure of around 3 kPa.   In gauge pressure, with a standard 1 atmosphere of ambient air pressure, this means that water will stay as a fluid up to ~  negative 98 kPa, or about -9.6 metres fluid pressure.   WaterCAD will warn you when the model has reached pressures below standard minimum water vapor pressure but in this case the model has stayed above vapor pressure and water will continue to flow through the pipe with a negative gauge pressure and the HGL below Surface Level:  This is normal hydraulics and what happens in real life.

    So in your modelling case, if it is water, and the system is near sea level, the water will flow over the high point as pressurised flow at an absolute pressure  >20 kPa , and a gauge pressure of > -80 kPa.

    There is one downside to physical systems that do develop negative pressures, and that is a risk of contamination of the water through sucking particles through the pipe joint seals.  Normally this can be engineered out by say using welded pipe joints in the negative pressure area, or using a standpipe or header tank on the hill to create a break-of-head.