Protective Equipment [FAQ]

  Applies To 
  Product(s): Bentley HAMMER
  Version(s): V8i
  Area:  N/A
  Original Author: Jesse Dringoli, Bentley Technical Support Group

Hydropneumatic Tanks

How can I view the gas volume over time? 

First, enter a number for the "Report period" attribute of your hydropneumatic tank. This represents how often the results will be saved. For example, a report period of '10' means that extended results will be shown every 10 timesteps. Make sure text reports are enabled in the transient calculation options. Now, when you compute the transient simulation, extended results will be displayed, under Report > Transient Analysis Reports > Transient Analysis Detailed Report. Scroll down near the bottom, to the section starting with " ** Gas vessel at node X ** " and you will find a table of gas volume, tank hydraulic grade, pipeline hydraulic grade and tank inflow, over time. 

See also: How to view and graph extended transient results such as gas volume for hydropneumatic tanks, pump or turbine speed, air valve extended data, etc.

How can I determine the headloss through a hydropneumatic tank for a given timestep?

There is no direct way to report it, but it can be derived from the text report. First, enter a number in the "report period" field of the hydropneumatic tank, which represents the increment of timesteps at which some extended data will be reported. Then, re-compute the transient simulation. Go to reports > transient analysis reports > transient analysis detailed report. Scroll down (likely very close to the bottom) and you'll see a table starting with " ** Gas vessel at node X ** ", including "time", "volume", "head-gas", "head-pipe" and "inflow".

To determine the headloss, look at the difference between the values for "head-pipe" and "head-gas". When the value for "inflow" is positive, that means the tank is filling, so the head in the tank (head-gas) may be less than the head in the pipe (head-pipe) due to inflow headlosses. When the "inflow" is negative, that means the tank is draining, so the head-pipe may be less than the head-gas due to outflow headlosses. 

See also: Headloss through a hydropneumatic tank

How does the "ratio of losses" attribute work? 

The headloss through the orifice connecting the tank to the pipeline is determined by the "minor loss coefficient (outflow)" attribute. When the tank is draining (outflow), just this coefficient alone is used to determine the losses, based on the velocity through the orifice, using the standard headloss equation H = KV^2/(2g). When the tank is filling, the minor loss coefficient is multiplied by the value entered in the "ratio of losses" field to determine the inflow headloss. 

See Also: Ratio of losses field in hydropneumatic tank

Should I model the hydropneumatic tank on the main pipeline or should I include the short connecting pipe? 

Modeling the gas vessel as being located along the main line is the preferred way to simulate this device. The influence of the short piping between the main and the vessel can be represented by means of three parameters provided with the gas vessel: Diameter of Orifice or Throat, Head Loss Coefficient, and Ratio of Losses. Essentially, the idea is to match the head loss in these short pipes with the loss incurred across a differential orifice situated in the vessel's throat, given that the gas vessel is located along the main. 

Although explicitly entering the short connecting pipes to the vessel is not incorrect in principle, nevertheless it may lead to excessive adjustments in the wave speed which in turn may have an impact on the results -- but, it is impossible to know for certain whether the output is markedly affected by a large wave speed adjustment. 

See also: Modeling Reference - Hydropneumatic Tanks

How does the bladder option work for a hydropneumatic tank? 

If your hydropneumatic tank has its gas contained within a flexible bladder, you should select "true" for 'has bladder?" In this case, you must also enter the pre-charge pressure, representing the pressure inside the bladder before it is submitted to pipeline pressure. Since this means that the gas takes up the entire tank volume, the 'K' constant in the gas law (PV = K) is computed based on this preset pressure and the full tank volume, "Volume (Tank)". The transient simulation's initial gas volume is then computed based on the K constant and the initial conditions pressure. 

See also: Modeling Reference - Hydropneumatic Tanks

I've entered a full tank volume but the computed max gas volume exceeds this. Why? 

The "Volume (Tank)" field is for reference purposes during the transient simulation. If the volume of gas during the transient simulation exceeds the total tank volume that you entered, you'll encounter a user notification about it. However, HAMMER will still compute gas volumes above the total tank volume, based on the gas law. Not only will this indicate that there is something wrong, but it will also indicate by how much. Meaning, you will still be able to view the maximum gas volume required (in the text output log) with the current tank configuration, make the necessary adjustments, then re-run the simulation.  

Basically your hydropneumatic tank needs to be large enough so that it does not become empty during the transient simulation. HAMMER assumes that the water volume in the tank is enough so that this does not happen. In the Transient Analysis Output Log (Under Report > Transient Analysis Reports), you will see the maximum volume of gas that is needed during the transient analysis. You will then need to provide a hydropneumatic tank that will be able to accommodate that maximum volume of gas and still not become empty of water (assuming that you don't want it to become empty.) 

See also: Hydropneumatic tank user notification: Calculated volume of gas is more than volume of tank

Why do I not see gas volume changing in a time history of "volume" in the Transient Results Viewer? 

The "volume" reported in the transient results viewer is only air or vapor introduced into the pipeline. It does not show the volume of gas inside the hydropneumatic tank itself. To see the tank gas volume over time, enter a report period in the tank properties and look at the bottom of the transient analysis detailed report. 

See also: Hydropneumatic tank profile animation and time history shows zero gas volume

How does the "treat as junction?" option work for a hydropneumatic tank? 

In many cases a hydropneumatic tank may be implemented only for transient protection. During a steady state condition, the tank may simply operate under the corresponding normal / steady state head ('line pressure'). So, for simplification, it is sometimes preferable to select "true" for the "treat as junction" attribute in the tank properties. Doing this allows the initial conditions solver to compute a hydraulic grade at the tank location and the user simply assumes that the tank has already responded to the hydraulic grade and the air volume has expanded or contracted accordingly. In this case, the user only needs to enter the initial volume of air under the "transient" section of the tank properties, corresponding to that initial conditions hydraulic grade. The transient simulation will use that hydraulic grade and air volume as the initial conditions. The air will then expand and contract accordingly during the transient simulation, based on the gas law. 

If you already know the hydraulic grade that you'd like to use as the initial conditions, you would choose "false" for "treat as junction?" and enter it under the "physical" section of the tank properties. The initial conditions solver will then compute the flow/head in the rest of the system, with the hydropneumatic tank as the boundary condition. In this case, the tank will likely have either a net inflow or outflow, to balance energy across the system. So, your transient simulation may not begin at a true "steady" condition. 

See also: Modeling Reference - Hydropneumatic Tanks

Why does changing the atmospheric pressure property not seem to effect the transient simulation? 

The "Atmospheric Pressure" property of a hydropneumatic tank only applies to the initial conditions. Meaning in an EPS simulation, you can use a custom atmospheric pressure value based on your location and it will be used when the gas volume compresses and expands (the gas law works with absolute pressure.) However, during a transient simulation, the atmospheric pressure is assumed to be 1atm or 10.33m.  

See also: Hydropneumatic tank atmospheric pressure not changing results

Air Valves

What does each of the air valve types represent? 

The double acting type allows air inflow through one orifice and outflow through another one. The triple acting type allows the outflow orifice to be throttled (typically with a float) from a large to a small size (to cushion vapor pocket collapse) either based on transient pressure or volume. The slow closing type allows air inflow and outflow through the same orifice, with the orifice closes linearly, starting from the time that outflow first occurs. The vacuum breaker type only allows air into the system through one orifice; the trapped air pocket can become compressed. 

See also: Modeling Reference - Air Valves

Why does using an air valve sometimes result in higher maximum transient pressures than without it? 

This can occur sometimes, if the air pocket that enters the air valve is expelled too quickly. This can result in the adjacent water columns colliding at a high velocity, which causes a high pressure transient upsurge. 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. For example, a smaller outflow orifice may be necessary, to cushion air outflow. 

See also: Transient pressure worse with air valve added

What are the limitations/assumptions in HAMMER's tracking of air pockets? 

- 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.
- The reduction in pressure-wave speed that can result from the presence of finely dispersed air or vapor bubbles in the fluid is ignored in the current version. Future versions of HAMMER may handle this.
- 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.  

See also: Assumptions and limitations of tracking air or vapor pockets in HAMMER

Is there a suggested value for the Transition Volume of a triple-acting Air Valve? 

If the transition volume is not readily available, an estimate is often sufficient: take the valve's cylinder volume minus the volume of the float. You can also consider using the transition pressure option instead of transition volume. 

See also: Modeling Reference - Air Valves

What does the extended CAV calculation option do?  

Setting the "Run Extended CAV" calculation option to "true" changes the calculation approach for air valves. Sometimes it is more appropriate to use one versus the other. The extended CAV approach is an inelastic approach, so if you have a triple acting air valve with transition volume, it may not be appropriate. (since that is more of an elastic approach.) The extended CAV option is more suited for sufficiently large volume of air entering and the flow regime evolves from hydraulic transients to mass oscillations. Besides improved computational efficiency, the rigid approach allows for the tracing of the air-liquid interface (normally HAMMER only tracks the volume of air, not the location).  

See also: Modeling Reference - Air Valves

How does hammer calculate flow rate of air through the air valve element? 

To compute the flow rate of air through the air valve element, 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 above 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.  

See also: Modeling Reference - Air Valves

When using the Extended CAV calculation option in conjunction with a vacuum breaker air valve type, why is the air pocket fully expelled? 

This is due to a limitation with the current version of HAMMER. It is recommended that you do not use the extended CAV option when modeling a vacuum breaker air valve, which should not allow air to be expelled. 

What is the purpose of the "treat as junction?" attribute of an air valve? 

This option applies to the initial conditions calculation. When set to "true", the air valve node is treated as if it is a junction with no demand during the steady state/EPS initial conditions calculation. If set to "false", then the valve may allow part full flow, subject to the prevailing hydraulic conditions.  

By default, pumps only consider the boundary conditions (reservoirs and tank elevations) in your system. So, the pump will add enough head to lift the water to the downstream known hydraulic grade. It does not consider junction elevations (or air valve treated as junction) inbetween. By placing an air valve at the high point and choosing "false" for "treat as junction", the pump sees the air valve elevation as its downstream boundary condition for instances in which pressure would have otherwise been negative at the high point. A profile of the initial conditions can clearly illustrate this behavior. 

See also: Modeling Reference - Air Valves

How can I see extended results for my air valve, such as air volume, air flow rate, head and air mass, over time? 

First, enter a number in the "Report Period" field of the air valve properties. This represents how often the results will be reported. 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, either look at the "Extended node data" tab of the transient results viewer (V8i SELECTseries 5 and greater) or for earlier versions, 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. You import or copy/paste this data into an external program such as Microsoft Excel, to develop a graph if needed. 

See also: How to view and graph extended transient results such as gas volume for hydropneumatic tanks, pump or turbine speed, air valve extended data, etc.

Surge Valves

What does the surge valve do?  

The Surge Valve element encompasses both a Surge Anticipator Valve (SAV) and Surge Relief Valve (SRV). You can configure the valve to act as one of these types or both. When the pressure at an SAV valve falls below the threshold value, it opens up to the atmosphere, in anticipation of a subsequent upsurge (high pressure.) The SRV opens up to the atmosphere when the pressure at the SRV rises above the threshold pressure and closes immediately after pressure drops below this setting. 

See also: Modeling Reference - Surge Valves

How is the surge valve treated during the initial conditions? 

When computing initial conditions, a surge valve is treated as a junction with no demand (simulating the closed condition.) 

Does a surge valve open/close between the adjacent pipes? 

No - when this valve opens, it discharges to the atmosphere, not between the adjacent pipes.  

See also: Modeling a Surge valve that opens between two pipes

If subatmospheric pressure occurs at the surge valve location, will air be sucked in? 

No - if subatmospheric pressure occurs at the surge valve location, air inflow will not occur and you may need to consider other approaches. 

How can I see how much flow is leaving the surge valve during the transient simulation? 

If your surge valve is at a "Tee" (separate short pipe going from the main line to the surge valve at a dead end) then you can graph the flow over time by adding the pipe end as a report point. To do this, go to Analysis > Calculation Options, open your transient calculation options and make sure "report points" is set to "all points", or "selected points", with the surge valve added as a report point in the report point collection. Then, after computing the transient simulation, go to Analysis > Transient Results Viewer and plot a time history of "flow" for the pipe endpoint adjacent to the surge valve. 

See Also

General HAMMER FAQ  

Licensing TechNotes and FAQs

OpenFlows Product Tech Notes And FAQs

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