User Notification: "Initial pressure less than vapor pressure. At the pipe end(s), the elevation(s) or head(s) are incorrect"

  Applies To 
  Product(s):  HAMMER
  Version(s): CONNECT Edition, V8i
  Area:  Modeling
  Original Author: Nancy Mahmoud, Bentley Technical Support Group

Problem

The following User notification is seen when attempting to compute a transient simulation:

"Initial pressure less than vapor pressure. At the pipe end(s), the elevation(s) or head(s) are incorrect".

Solution

Because of HAMMER's assumption of pressurized pipes, the occurrence of sub-vapor pressure in the initial conditions means that you would actually have a vapor pocket at that location, before the transient event even starts. Basically it means the model is not in a true steady state and the model needs to be fixed so that the pressures are correct.

The solution to this is different for every model, as there are a number of factors that effect the pressure at a node. Trace the HGL from that node to the upstream boundary condition to get a better idea of why the pressure is that low.

To resolve this issue, you will need to adjust your initial conditions so that pressure is above the vapor pressure limit at all locations. Review the pipe connections, and make sure the the elevation and head values are correct. For instance, you should review the elevation data at junctions, pump, and reservoirs/tanks. For the reservoir make sure that the value for "Elevation (Inlet/Outlet Invert)" is set correctly and makes sense in relation to the "Elevation". See more tips here: Troubleshooting negative pressures at pumps, junctions, & other node elements

Also, ensure that you have configured HAMMER to use the computed initial conditions and not user-defined initial conditions. In the properties of the Transient Calculation Options, check the "Specify Initial Conditions?" option at the bottom. This should be set to "False" in almost all cases. If it is set to "True", then additional user-defined initial conditions fields will be visible in the properties of elements like pipes and pumps, and HAMMER will use what you enter there for the initial conditions. This can cause confusion if you do not realize that it is doing this instead of using the calculated initial conditions, and can cause the notification in question if for example the user defined pipe start/stop HGL is set to the default of zero.

Local High points

In some cases, these negative pressures could occur at a high point where an air valve would actually exist. In such a case, you can place an air valve at the high points to help with this issue. More information on this can be found in the Modeling air valves at high points TechNote and background information can also be found here. You would need to set the air valve property "Treat as junction?" to False and the program will know to have the upstream pump add enough head to overcome the high point. You may still end up with negative pressure downstream of the high point though, which means that the pipe will not be flowing full past the local high point at that flow rate. (It may flow full at other flow rates.) If the pipe is flowing partially full then you may need to reevaluate your downstream boundary condition.

The equations in HAMMER are based on full pipes. Once you get to partly full pipes, you need to change the approach. The downstream end of the HAMMER model should be the last pipe that is flowing full (crest of the last high point). So, if an air valve would be open during normal operating conditions (in the initial conditions) with an air gap from part-full downstream flow, you could end the system at a reservoir, demand or discharge to atmosphere element.

Pump startup

If you are simulating a pump startup event and you have high points that would normally experience part-full flow when the pumps are off, you could encounter this situation. The problem in this case is that the initial conditions solver is only aware of the downstream boundary condition - the reservoir elevation. So, if you look at a profile of the system, you will see a flat HGL.

In the real system, assuming there is a check valve at the pump, water will probably remain in the pipes even after the pump turns off (with some air), and the weight of the water would result in an initial pressure above vapor pressure, but this is a challenging condition to simulate in a hydraulic model (initial conditions). The program is built upon the assumption that pipes are flowing full and energy is balanced based on boundary conditions.

Even by placing the air valves on the main line and setting "treat as junction?" to "false", it can be difficult for the model to solve the "correct" initial conditions with zero flow.

Here are three possible approaches to consider:

1) Set the transient calculation option "specify initial conditions?" to "true", then manually enter what you believe to be the correct initial HGL at each pipe. This will force the initial conditions for your pump startup run. You may need to specify an initial air volume at the air valve locations where part-full flow is expected.

Related article: Purpose of the specify initial conditions calculation option

2) Start the initial conditions with the pumps on, use the "variable speed/torque" transient pump type with a pattern that gradually turns the pumps off (ramps down to zero RPM), waits for a while, then performs the desired start up. Basically allow enough time for any transient effects of the pumps turning off to settle down and establish a more accurate initial conditions, then have the pump start up. With this approach, you may find that air pockets continue to increase in size as long as the pump is off. This is due to limitations of air volume tracking, whereby (by default), the transient solver does not know the extent along the pipeline that the air pocket would travel and instead assumes that they are concentrated at the air valve location. You can consider using the Extended CAV transient calculation option to help with this, but it can only track the air/liquid interface to the extend of the adjacent pipe(s).

Related article: Assumptions and limitations of tracking air or vapor pockets in HAMMER

3) End the system at the first high point that experiences part-full flow when the pump is off. You could use a discharge to atmosphere element to represent the opening at the top that essentially discharges to the part-full downstream pipe. This may be an acceptable approach if you only expect severe transients to occur in the segment of pipe between the pump and the the high point, since transient waves won't propagate across an air gap.

You may want to assess possible transient effects of the release of air from the other air valves after the pumps start back up.

Also related to a pump startup, if your initial conditions calculation includes a demand with no other source aside from the one connected to the pump that is starting up, this can result in disconnected demand nodes and invalid results. If this includes pressure results that are below vapor pressure, this error message can occur. In such a case, you can try starting with the pump initially on and using the operating rule to shut down the pump, then turn it back on. More information on this workflow can be found here: Modeling a pump startup and shutdown transient event in the same simulation.

See Also

Modeling a case where an empty pipe is filling using Bentley HAMMER

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

Recommended
Related