Overview

This technote explains how the Air Valve element works and its typical application in HAMMER V8i. It also provides an example model file for demonstration purposes. 

How it Works and When to Use it

The Air Valve element (sometimes referred to as a combination air valve or CAV) is typically placed at high points in the pipeline and other areas that are susceptible to subatmospheric pressure during a transient event. They allow air to enter into the system during periods when the head drops below the pipe elevation and expels air from the system when water columns begin to rejoin. After the air has been expelled and the pressure is positive, the valve becomes closed. 

If you are analyzing an existing system that has air valves, you should place them at the appropriate location(s). If analyzing a proposed system or system improvements, you will likely want to first compute the transient simulation without the air valves. In the transient results viewer, you would check the pressure envelope to identify critical points where air valves may help prevent vapor pockets. Then, you would place the air valve(s) along your pipeline and compute the transient simulation for a range of air valve types/configurations. Comparing the profile animation or pressure envelope for each trial of air valve configurations (orifice sizes/types, etc) will give you a good idea of the sensitivity/behavior of the air valve(s) in your system. 

There are essentially two ways in which an active air valve can behave:  

1. Pressure below atmospheric - air valve is open and acts to maintain pressure to 0 on the upstream end.
2. Pressure above atmospheric - air valve is closed and acts as any junction node.

The presence of air in the line limits subatmospheric pressures in the vicinity of the valve and for some distance to either side, as shown on HAMMER profile graphs. Air can also reduce high transient pressures if it is compressed enough to slow the water columns prior to impact.

Note: low or subatmospheric pressure can still occur further along the pipeline; the air valve element only provides local protection.

Typically, the air inlet orifice is large (1-3"), so as to allow free air intake and not throttle due to the sonic limit. If the air inflow orifice is too small, the model may show the hydraulic grade dipping below the physical elevation of the air valve (negative pressure) in an animation of the profile. Limiting air outflow using a small orifice will cause the air to compress inside the pipe and cushion the water column collapse.

Without an air valve, subatmospheric pressure (such as those caused by an emergency pump shutdown) can cause contaminants to be sucked into the system, thin-walled pipes can collapse and also vapor pockets can form (as the water boils at such low pressures) and subsequently collapse or damage pump impellers.

However, you must be careful when using the air valve, since extreme high pressure surges can be caused when the air pocket collapses. Meaning, if the air inside the air valve is expelled too quickly, the water columns in the adjacent pipes can collide at a high velocity and the force will cause a severe transient. This is similar to the surge that occurs when a water column slams against a closed valve, except in this case the momentum of two water columns are hitting each other, without the delay involved with valve closure. However, an air outlet orifice that is too small can also cause a problem, if the air cannot escape quickly enough. So, care must be taken to select an appropriate air valve type and size, so as not to cause worse transients than if no valve had been used. It is common to use a "triple-acting" air valve to help against this problem, as this type of air valve throttles the size of the outflow orifice (typically using a float.) 

For example, consider the below animations, which illustrate a pump shutdown event with an air valve at the high point. The first depicts a double acting air valve that releases air too quickly (outflow orifice size is likely too large.) Notice the high pressure transient that occurs when the water columns collide. The second animation depecits a triple acting air valve. Large to small outflow orifice transition is configured in such a way that it provides a cushion that helps with the water column collision, but also doesn't raise the pressure too much before that happens. You'll notice that the head starts to increase when the transition to the small outflow orifice occurs, but the flow is not restricted enough to cause this head increase to become too severe. 

Double-acting with large outflow orifice

Triple-Acting

Inactive Air Valves

In some cases, you may want to analyze the system without the air valve. For example, you may have a "no protection" scenario that describes the system without air valves, or a scenario where an alternative protection approach is taken. In these scenarios, you cannot simply delete the air valve, or even make them inactive by choosing "false" for "is active?". The reason is because every pipe must have a node at each endpoint. This situation should be approached by first using different active topology alternatives, then using one of two methods:

1. Place the air valves at a "tee" to the main pipeline. This way you can simply make the air valve and adjacent pipe inactive in the scenarios where the air valves are not present. This is the recommended method, as it is easier to manage and will account for headloss in the lateral pipe, which can sometimes be significant.

1. In the scenario(s) where the air valves are not present, make the air valve and both adjacent pipes inactive, then make a new pipe going around the air valve active. Do the opposite in the scenarios where the air valves are present - make the air valve and adjacent pipes active but the other, single pipe inactive. Be careful when using this approach, as friction headloss in the lateral pipe is omitted.

What if my air valve is open during the Initial Conditions?

If you are pumping over a high point with an air valve that is open under normal operating conditions, with some amount of part-full flow in the downstream pipe (which then resumes to pressure flow), there are some important considerations.

As seen in this technote, you can choose "false" for the "treat air valve as junction?", and the upstream pump will "see" the high point and know to add enough head to overcome it in the initial conditions (steady state).  

When an air valve is used in the initial conditions, it is internally treated as a PSV, in order to force an upstream pump to add enough head to keep positive pressure at the high point. Because of this, a head loss occurs through the air valve (PSV) in order to balance energy across the network. So, you may notice a large drop in hydraulic grade downstream of the air valve, without it being reported in the pipe's "head loss" field. In some cases, this may cause the pressure at downstream nodes to be negative. This situation should be interpreted as part-full flow when looking at the initial conditions. More on this is explained in this technote.

Because of the aforementioned behavior, you will have head losses and pressures that may not be realistic. The problem is that HAMMER requires the initial conditions to be very accurate. The equations behind HAMMER assume full-flow in pipes. Although the negative pressures seen in this case aren’t really negative, HAMMER doesn’t know this. So, if the negative pressure is below the vapor pressure limit set in your calculation options, HAMMER assumes that vapor would actually have formed at those locations. Even if the pressure does not drop to the vapor pressure limit in the part-full sections, you might encounter a friction loss error for the pipe.

Because of these complexities, the modeling approach must be modified in these situations in order to do a transient analysis. (assuming of course that part flow is really expected). Meaning, the system should be ended at the point where full flow transitions to part-full flow. It is recommended that this be done with a reservoir, demand or Discharge-to-Atmosphere. (see item 2 in this technote, under the section titled Common Applications of the D2A acting as an orifice). This approach is typically acceptable because the transient waves would not propogate past the air gap formed at the air valve.

Tracking of Air Pockets

HAMMER is able to track the volume of air entering the system at an air valve, but the following assumptions/limitations apply: 

- 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)
- 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. Note that 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. 

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 neighbour "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).

Air Flow Rate Calculation

To compute the flow rate of air through the air valve element when specify the openings as equivalent diameters, HAMMER uses the following equation:

 where Po is the density of air at 4°C and 1 atmosphere (=1.293 g/l), S=0.6A, with A being the cross-sectional area of the orifice. The throttling of air flow due to the "sonic velocity" is automatically calculated using the below formulation:

where Y is the exponent in the gas law, p is the absolute pressure, the subscript 0 denotes standard conditions, and p/py = constant. For air inflow, (1) is again applicable, except that the ratio within the square brackets is inverted to be p/p0 as p0>p in this instance. the exponent, Y, in the gas law is hard-coded as 1.4, which corresponds to adiabatic compression/expansion appropriate for the typically rapid processes which occur. Note that "Vmax" is not the same thing as the sonic limit. Vmax is the maximum velocity that would be achieved by a fluid when it is accelerated to absolute zero temperature in an imaginary adiabatic expansion process. It is a term used in the calculations for air flow rate, but the sonic limit is ~340m/s (1115 ft/s) at 60 degrees F.

Note: the above is used to calculate the "free air" flow rate, at atmospheric pressure. Currently, the air flow rate reported by HAMMER in the text reports is the flow rate at pipeline pressure, which will be different due to differences in air density.

Note: you can enter a rating curve of pressure versus air flow rate, instead of specifying an equivalent orifice. See further below.

Air Valve Types and Attributes

General

The following attributes are available in the air valve properties, regardless of the air valve type: 

"Treat Air Valve as junction" - This option specifies whether or not to treat the air valve as a junction element during the initial conditions (steady state or EPS). When set to "false", the valve may allow part full flow during the initial conditions, depending on the system conditions. This is mainly used for sewer forcemains and is typically not used during a transient analysis. This setting has no effect on the transient simulation itself. Meaning, the air valve will still function as an air valve during the transient simulation, even if this is set to "true". Further details on this feature are beyond the scope of this technote. 

"Elevation" - This field identifies the elevation of the air valve. The elevation is important because it determines the pressure at that node. It should be set to the elevation of the opening of the actual valve. When the hydraulic grade at the air valve location drops below the air valve's elevation, air intake starts to occur, since the pressure at that node would then be below zero. 

"Report Period" - entering a number in this field will allow HAMMER to report extended results for the air valve. For example, a report period of '10' would cause extended results to be reported every 10 timesteps. so, if the calculation timestep was 0.01 seconds, that means you will see these results at a 0.1second interval. To view these extended results after computing the transient simulation, go to Report > Transient Analysis Reports > Transient Analysis Detailed Report. Scroll down almost to the bottom, to the section beginning with " ** Air valve at node Air Valve**". Below this, you will see a table of time, air volume, head, air mass and air outflow rate. Note that the flow rate shown here is currently the "free air" flow rate, meaning the volumetric rate of air flow at atmospheric pressure conditions. The air flow inside the pipeline will be different due to differences in air density. 

"Air Volume (initial)" - This field is available when using either the Double Acting or Triple Acting air valve type. It is used when modeling an air valve that is initially open. Like the "Treat air valve as junction?" attribute, this is rarely used. Intuitively, the initial conditions pressure must be zero in this case (air valve is open), and the air present inside the air valve is entered in this field. This might occur at a high point that operates under partfull flow in normal conditions (when the pump is on.) 

"Air valve type" - This is where you specify which type of air valve will be used during the transient simulation. Details on how each type works and their corresponding input parameters are found below. 

"Air Flow Calculation Method" - This allows you to specify whether the air flow rate calculation is determined by a user-entered rating curve or calculated based on an equivalent orifice diameter.

Double Acting

With the double acting air valve, both inflow and outflow orifices are available. The diameters of these orifices don't change and there are two different actions:  

1. Air inflow through the inflow orifice diameter
2. Air outflow through the outflow orifice diameter

"Air Volume (Initial)" - The volume of air inside the air valve at the start of the simulation. If you need to enter a value here, then the pressure from the initial conditions must be zero (ie the air valve is open). This would only be used if you wanted to model an air valve that is open during the initial conditions, which is not typical. In most cases involving a pump, it is easier to begin the simulation with the pump on, then have the pump shut down and subsequently restart after an appropriate length of time, using the variable speed transient pump type.

"Diameter (Air Inflow Orifice)" - This is the diameter of the orifice for injection of air into the pipeline. This diameter should be large enough to allow the free entry of air into the pipeline. If set to zero, no inflow will occur.

"Diameter (Air Outflow Orifice)" - This is the diameter of the orifice that allows discharge of air out of the air valve, upon increase in pipeline pressure. It should be small enough to throttle the air flow and cushion the speed of the air pocket collapse. If set to zero, the air valve will act like a vacuum breaker type, in that no air can be released and the trapped air pocket will be compressed.

Triple Acting

This air valve type is used to model a triple acting air valve, which has an air inflow orifice at a fixed size and a variable-diameter air outflow orifice. Typically a float is used to decrease the orifice size, just before the air pocket is completely expelled. 

There are three different actions:  

1. Air Inflow
2. Air Outflow through the large orifice 
3. Air Outflow through the small orifice 

When the air valve opens, air inflow comes in through the inflow diameter. When pressure returns, air escapes out of the large diameter outflow orifice. Just before all of the air has escaped, the float is pushed up, which decreases the diameter of the outflow orifice down to the "small" value. This cushions the air pocket collapse and subsequent collision of the water columns.



"Diameter (Air Inflow Orifice)" - This is the diameter of the orifice for injection of air into the pipeline. This diameter should be large enough to allow the free entry of air into the pipeline. If set to zero, no inflow will occur.

"Diameter (Large Air Outflow Orifice)" - This is the diameter of the outflow orifice when the float is at the lowest position. It is the size of the orifice when the air volume inside the air valve is greater than or equal to the transition volume or when the air pressure is less than or equal to the transition pressure (depending on the method you selected to trigger the switch from large to small outflow orifice).

"Diameter (Small Air Outflow Orifice)" - This is the diameter of the outflow orifice when the float is at the highest position. It is the size of the orifice when the air volume inside the valve is less than the transition volume or when the air pressure is greater than the transition pressure. (depending on the selected method)

"Trigger to Switch Outflow Orifice Size" - You can choose to have the triple-acting air valve switch from the large to the small outflow orifice size based on a transition pressure or a transition volume. When selecting "Transition Volume", a "Transition Volume" input field is available and when selecting "Transition Pressure", a "Transition Pressure" field becomes available.

"Transition Volume" - If you're using the transition volume option, this is the Volume of air between the lowest and highest position of the float. (The amount it can change) Basically it is the volume of air left in the system when the water starts to raise the float to decrease the orifice size. It is usually approximated as the volume of the body of the valve.

"Transition Pressure" - If you're using the transition pressure option, this is the pressure at the air valve location (i.e. the HGL minus the elevation) above which the outflow orifice switches from the large to the small size.

Note: typically in real life it actually takes a small amount of time to transition from the large to the small orifice diameter, but it is generally pretty quick so we model it as instantaneous. Meaning, the diameter decreases to the "small" value, as soon as the volume of air is less than the "transition volume", or as soon as the pressure is greater than the "transition pressure" (depending on the method you selected.)

Not all triple-acting air valves trigger the outflow orifice transition based on a transition volume or pressure. For example, it may be based on velocity. In these cases, you will need to determine the air volume or system pressure at the air valve, at the time when your conditions is met. Start by setting the small outflow orifice diameter equal to the large, then enter a number in the "report period" field. After computing the transient simulation, open the transient analysis detailed report from the Report menu and scroll down to the bottom. You will see a table of air flow rate, air volume, pressure, etc over time, which you can use to determine this.

Vacuum Breaker

With the vacuum breaker air valve type, only the air inflow orifice diameter needs to be configured. This air valve type lets air into the system during subatmospheric pressure, but assumes the outflow diameter is very small (effectively zero) so it doesn't let air out. You will see the air volume change as the air pocket is compressed, but the mass of air in the pipe doesn't reduce. There is probably a very limited number of applications for this type valve. However, it could be used for a draining pipeline. 

Note: any air pocket left in the system due to a vacuum breaker valve is assumed to be expelled out of the system by some other means. HAMMER currently cannot track the behavior of these trapped air pockets (the underlying assumption is that the air must exit the system where it came in) 

"Diameter (Air Inflow Orifice)" - This is the diameter of the orifice for injection of air into the pipeline. This diameter should be large enough to allow the free entry of air into the pipeline.

Slow Closing

Although similar to the other air valve types, the slow-closing air valve only has a single orifice involved; for the expulsion of air and liquid. An air inflow orifice is not required because HAMMER assumes that air will be freely allowed into the system (no throttling) when the head drops below the air valve elevation. The valve starts to close linearly with respect to area only when air begins to exit from the pipeline (after the head begins to rise).  

It is possible for liquid to be discharged through this valve for a period after the air has been expelled, unlike the other air valve types, which closes when all the air has been evacuated from the pipeline. Typically you will want the valve to be fully closed after all air has been expelled, but before too much water has been expelled. 

"Diameter (Air Outflow Orifice)" - This is the diameter of the orifice that allows discharge of air out of the air valve, upon increase in pipeline pressure. It should be small enough to throttle the air flow and cushion the speed of the air pocket collapse. 

Note: there are many other advanced air valves that work differently than the types currently available in HAMMER (some work on flow rate), but they are not yet supported. A conservative approximation using one of the available types should normally suffice in this case. Future versions of HAMMER may allow the user to enter a custom pressure versus air flow rating table.  

Using a Custom Air Flow Curve

Traditionally, the openings for air flow into and out of an air valve are specified in terms of an equivalent diameter. As of V8i SELECTseries 2 (08.11.02.31) you can now specify a pressure vs. air flow rating curve for any of the openings, instead of an equivalent orifice.

• This is convenient in cases where the manufacturer provides a rating curve instead of orifice sizes
• Positive flows and pressures should be entered for outflow and negative flows and pressures should be entered for inflow.
• Air valve rating curves can be stored in a new air flow curve engineering library (some example data is included)
• Note that the flow rates entered here are the "free air" flow rates, at atmospheric pressure.

Example Model

Click to Download

Note: the above model is for example purposes only. It can be opened in version 08.11.00.30 and above and you can find additional information under File > Project Properties.

See Also

Product TechNotes and FAQs

Haestad Methods Product Tech Notes And FAQs

Protective Equipment FAQ

General HAMMER V8i FAQ

External Links

Water and Wastewater Forum

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

ARI Air Valves (contains many animations)