Assumptions and limitations of tracking air or vapor pockets in HAMMER

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

Problem Description

What are the assumptions and limitations with tracking air or vapor (cavitation) pockets in HAMMER?

Resolution

HAMMER is able to track the volume of air or vapor in these situations:

- Air at Air Valve node elements upon subatmospheric pressure.
- Air at Discharge To Atmosphere node elements upon subatmospheric pressure.
- Vapor at any point along a pipeline, upon sub-vapor pressure.

The following assumptions/limitations apply:

Air at Air Valves and Discharge to Atmosphere

- The air pocket takes up the entire cross section of the pipe
- The air pocket is localized at the point of formation (the air valve node). So, the extent of the air pocket along the pipeline is unknown and the air-liquid interface is assumed to be at the node location. (by default - for air valves, some limited tracking can occur by way of the Extended CAV calculation option - see further below)
- The reduction in pressure-wave speed that can result from the presence of finely dispersed air or vapor bubbles in the fluid is accounted for by configuring the Wave Speed Reduction Factor in the calculation options.
- Air pockets entering an air valve can only exit the system through the same point. Basically it is assumed that the pocket cannot be swept downstream and expelled elsewhere.

In most modeling cases these assumptions are acceptable and should not result in significant error. In each case, the assumptions are made so that HAMMER's results provide conservative predictions of extreme transient pressures.

Vapor Pockets

Vapor pockets can form when pressure anywhere along the pipeline drops below the vapor pressure limit, as set in the transient calculation options. The following assumptions/limitations apply:

- HAMMER uses the Discrete Vapor Cavity Model (DVCM) method.
- The vapor pocket takes up the entire cross section of the pipe at the point of formation
- The air pocket is localized at the point of formation. So, the extent of the pocket along the pipeline is unknown and the vapor-liquid interface is assumed to be at the node location.
- The reduction in pressure-wave speed that can result from the presence of finely dispersed air or vapor bubbles in the fluid is accounted for by configuring the Wave Speed Reduction Factor in the calculation options.
- Vapor pockets that form can only collapse at the same point. Basically it is assumed that the pocket cannot be swept downstream.
- The "vaporization grade line" (HGL below which water turns to vapor) is based on the user-entered vapor pressure limit in the transient calculation options

Air and vapor formation at a Tee

If the location of the vapor/air collapse is directly on the profile, you will see the formation of the pocket in the transient profile and the subsequent pressure upsurge.

However, if the element is not along the profile, such as an air valve at a tee, you might see what appears to be an upsurge without a cause for it. But even if the air valve is at a tee, you would still see the impact of the air pocket collapse.

Air pocket tracking at air valves and using Extended CAV

Since the air pocket is reported at the air valve location, you will need to include the air valve in your profile in order to see air pockets forming in profile view. If your air valves are on a "Tee" from the main line, you will not see air volume reported in the profile, as the air valve element will not be directly included in the profile path.

If you need to track the location of the air-liquid interface of an air pocket entering the system (instead of assuming it's localized at the air valve node), you can use the Extended CAV method. To do this, select "True" for the "Run Extended CAV?" attribute, in the transient calculation options (Analysis > Calculation Options > Transient Solver).

When a sufficiently large volume of air enters a pipeline, the flow regime evolves from hydraulic transients to mass oscillations. Thus, at least in the vicinity of the air, the system may be represented by rigid-column theory in lieu of the elastic approach. Using the Extended CAV option activates this rigid (inelastic) approach. Besides improved computational efficiency, the rigid approach allows for the tracking of the air-liquid interface. When using extended CAV, the program will automatically switch between the regular (concentrated/elastic) and Extended (rigid) based on the percentage of the adjacent pipe volume that the air pocket occupies.

Limitations of using Extended CAV:

When using Extended CAV, the air valves must be set up as follows:

1) The air valve must have a higher elevation than the adjacent nodes.

2) The nodes on either side of the air valve must be junctions.

3) The junctions adjacent to the air valve cannot have a demand.

4) The air valve type must not be "slow closing" (which currently does not support Extended CAV)

If any of these qualifications are violated, the user notification above will be generated

Tracking the air/liquid interface:

There are two ways to observe the air/liquid interface tracking when using the Extended CAV option:

1. Open the Transient Analysis Output Log under Report > Transient Analysis Reports and scroll down to the section beginning with:

*** SNAPSHOT OF EVERY END POINT AT START OF TIME STEP 2 ***

Below this table, you will find information pertaining to element statuses, including Extended CAV air/liquid interface. For example:

At time step "4341" at CAV "Air Valve" with neighbor "J-3 ", the elevation, level and volume are: 137.000 135.361 0.966

1. Open the Transient results viewer and animate a report path including the air valve and adjacent pipes. As the pipeline fills with air, you can observe the change in HGL downstream of the air valve. This is the air/liquid interface:

In some cases, the extended CAV model may not be appropriate. For example, if you have a triple acting air valve with transition volume, it may not be appropriate since that is more of an elastic situation. The extended CAV option is typically used when relatively large volumes of air enter the system.

`Note: the Extended CAV option will only track air volume up to the extents of the adjacent pipe(s). In the event that the air expands greatly so that the interface moves down towards the neighbor node to the verge of draining, HAMMER issues a warning message, freezes the horizontal surface at the elevation of the neighbor node, and continues to track the volume (which could conceivably exceed the branch's volume).`

Modeling Reference - Air Valves

Modeling Reference - Discharge to Atmosphere

Does the Extended CAV option apply to vapor pockets in the system or only air?

Modeling large amounts of vapor or air volume

How does HAMMER calculate vapor pocket formation and collapse?

Slow Closing air valves and Extended CAV

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