# 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.

Parents
• 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.

• 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.

Children
No Data