Reacciones adversas Resumen del perfil de seguridad

Stage 5: Largest Liquid Release with a Bund Bund

The two inch liquid release that you have just modeled used conservative assump-tions in that it had no bund, so that the pool could spread to the largest possible size, giving the maximum evaporation rate from its surface.

In this stage, you will model the same release with a bund, to investigate the effect on the evaporation rate. The hydrogen cyanide tank has a bund 15 feet long by 10 feet wide, which will give a pool about four inches deep with the entire inventory released into the bund.

Create the Model and Set the

Create the Model and Set the Input ValuesInput Values

Copy the 2 inch Liquid model and name the copy 2 inch Liquid - bunded.

Set the values as shown for the bund data in the Location tab section:

Concrete is the default Bund Surface TypeBund Surface Type.

Run the Calculations and View the Results Run the Calculations and View the Results

You will see that the results are identical to those for the unbunded release. The Pool Vap Report states that the bund is not hit, and shows a maximum pool radius of more than 38 ft, which means a pool area of over 4500 ft²—much larger than the 150 ft² area that you specified in the input data. This is occurring because the rainout position, at 12.77 ft, is outside the bund. The program models the bund as a circle with the area given in the input data, and this equivalent circle has a radius of 6.9 ft, so the rainout position is well beyond the bund.

This behavior may puzzle you the first time you see it in the results, since the Report does not emphasize the fact that the rainout takes place outside the bund, and you may think that the program is ignoring the bund completely. This aspect of the modeling is realistic in that rainout is not likely to occur exactly at the release point, and may well be delayed until after the cloud has passed over the bund wall, in which case the bund will not be effective. However, the approach is also simplified in that it treats rainout as occurring at a single point, whereas in reality it would occur at a range of locations, and some of the rainout might be inside the bund, and some outside.

Chapter 4: Tutorial 2

There are several input variables that affect this aspect of bund modeling, and that you can use for control and investigation. The use of each of these variables is described below.

Variables that Affect Rainout: Release Direction and Elevation Variables that Affect Rainout: Release Direction and Elevation

You set the release direction in the Indoor/Outdoor tab section, and it is currently set as Horizontal, which was the value that you set for the worst case ammonia release at the beginning of Tutorial 1, and have copied to subsequent toxic Models.

If you change this setting to Down - impinging on the ground, as shown in the illustration, the program will ignore the Elevation that is set in the Location tab section, and will model the release as starting at ground level, and with reduced momentum.

In this situation, the liquid droplets do not have a “fall time” or “fall trajectory” as they do when released from a height, so they do not have the opportunity to evaporate before reaching the ground, or to travel any distance from the release location—and the program models this by assuming that all of the liquid rains out immediately, and at the release location.

This is a realistic setting to make for a liquid release, since many releases will be from the bottom of the equipment, and will therefore be directed towards the ground. If you make this setting for the 2 inch liquid - bunded Model, then you will ensure that the rainout does occur inside the 150 ft² bund. The mass that rains out will be larger than for the unbunded Model because the droplets do not evaporate before raining out, and that will also change the vaporization results.

With this setting, the program represents the pool vaporization with a single Average Rate, as shown in the illustration of the Pool Vap Report for the Model:

Changing the Release Direction

Getting Started with PHAST

Compare this with the Average Rates for the unbunded release, where the smallest rate was 3,377 lb/hr, and the rate with the longest duration was 5,324 lb/hr. This shows that a bund can have a significant effect on the results for a liquid release.

You can achieve a similar effect by setting the Release ElevationRelease Elevation to zero in the Location tab section. In this case, the release will be modeled with no reduction in momentum, because it does not impinge on the ground.

Variables that Affect Rainout: Bund Area Variables that Affect Rainout: Bund Area

Not all liquid releases will be directed downwards, and sometimes you will want to model a release as horizontal (or at some other angle). In this case, if you want to see the effect of the bund, you will have to set the bund area such that the radius of the equivalent circular bund is greater than the distance to rainout; for the2 inch liquid Model, this requires a bund area of 512 ft² or greater, giving a radius greater than 12.7 ft.

With this larger bund area, the program models the evaporation using two Average Rates, as shown in the illustration of the Pool Vap Report.

Average Rate 1 represents the stage during which the pool is spreading, and Average Rate 2 represents the stage after the pool has reached the edge of the bund.

Even with the larger pool size, the evaporation rates are still much lower than those for the unbunded Model.

Variables that Affect Rainout: Dispersion Parameter Variables that Affect Rainout: Dispersion Parameter Note—Applies to Instantaneous Releases only Note—Applies to Instantaneous Releases only

The third method for controlling the modeling of rainout cannot currently be used with the 2 inch liquid release, because the method is only applicable to instantane-ous releases. However, the method is discussed here because you may find it useful when you encounter an instantaneous release that is raining out beyond the bund.

The method involves the parameter called Cloud rainout at sourceCloud rainout at source, which is located on the Liquid tab section of the input dialog for Dispersion Parameters, as shown in the illustration on the next page.

Pool Vaporization Results

Chapter 4: Tutorial 2

To open this dialog in order to change this parameter, move to the Parameters section of the Study Tree and double-click on theDispersion icon in the Global Parameters folder.

The parameter has the default value Do not set rainout position, which means that the program will model the rainout at the location predicted by the droplet modeling (e.g. at 12.8 ft for the 2 inch liquid release).

If you change the parameter to Do reset rainout position, then the program will perform the droplet modeling in order to calculate the evaporation before rainout, but will ignore the calculated rainout position and will place the rainout at the release location instead.

This parameter is provided principally to give you the ability to control this aspect The Parameter for Controlling the Rainout Position

Getting Started with PHAST

If the rainout is close to the edge of the bund, then it is valid to model both situations, since both are likely to occur in reality; if the rainout is far beyond the edge of the bund, then it is less valid—and also non-conservative—to reset the rainout position to the release location.

In principle, this approach could also be applied to continuous releases. The parameter is currently only available for instantaneous releases because of differences in the way that the program devises release segments for the two types of release; it is easy to reset the position for an instantaneous release, but for a continuous release it would introduce problems in the time-sequencing of seg-ments. However, you should not find this a serious limitation, since you can achieve a similar effect for a continuous release by setting theDirectionDirection to Down or the Elevation

Elevation to zero, as described earlier.