Where’s that vapor going?


Where’s that vapor going? Sooner or later that question must be answered when some type of work is required on an ammonia refrigeration system.

To accomplish the work, the system may have to be opened, and later that portion of the system pressure tested.

The pressure testing process will typically involve some amount of ammonia used in the final test, after which the pressure is removed again before the portion of the system tested is placed back into operation.

Even when a small amount of vapor is vented to atmosphere, several seemingly innocuous details can add up to a big problem if they aren’t carefully considered.

In this edition of “Lesson Learned,” we’ll look at a specific case where those details were overlooked.

It was a routine day and a routine procedure that took the contractor in this case to the facility roof.

In this instance, pressure was applied to the piping associated with a newly installed automatic purger. After installation of the new purger, solenoids at several condenser connection points, and all of the associated piping, a pressure test was done.

The majority of the new valve assemblies and piping was located up near the evaporative condensers, which were mounted above the machine room.

The only new equipment and piping in the machine room was for the purger. The distance from the ground level outside of the machine room to the condensers located directly above the machine room was approximately 35 to 40 feet.

The contractor doing this work knew he was well above ground level and above the roof level.

He also knew that there was not much ammonia vapor to release due to the volume of small size piping and only one circuit of one condenser.

The contractor estimated that the location and the small vapor volume of the ammonia in question would make the decision to vent the ammonia pressure to atmosphere a sound choice.

With the pressure test successfully completed, it only took a few minutes to release the ammonia pressure.

Up to this point things had been uneventful, however, one important detail had not been considered. Where is that ammonia vapor going?

Many times, even normal wind flow can be affected by building structures and other natural barriers such as trees.

As the wind hits these objects, it can create eddies or other temporary changes of wind direction, pushing vapor in unexpected directions.

In this particular case, not only were there building structures, but the prevailing wind was not coming from the normal direction. As the ammonia vapor released, it was quickly taken over the roof of the building where the condensers were mounted, across an alley between buildings, and then, due to an eddy effect, pushed to ground level beyond the next building.

As the vapor went to ground level, it also spread out and traveled through a group of construction workers in the nearby area.

Even though the amount of vapor released was well below the federal reporting quantity of 100 pounds in 24 hours, it was sufficient to cause the workers in the impact area to feel very uncomfortable.

The smell of ammonia was quickly reported to the facility personnel, even as the vapor rapidly dissipated due to the small amount released.

So what lessons were learned from this event?

Probably the first and most obvious one is to make sure you know what direction the wind is blowing before blowing off the vapor. As in this case, it would be wise to not only check the wind direction at the release location but also downstream where there may be eddy effects.

Another important consideration that should have been made before the contractor got to this point in his work is the facility emergency plan. A contractor must understand what to do if there is an emergency, and he should also know what to do and who to call if he causes the emergency.

Along with this, prior to releasing ammonia, the contractor and facility personnel should coordinate as best they can to determine any potential negative impacts to people and the environment.

This coordination should involve making sure there are no downwind concerns, such as people, in this case, that could be affected. With this analysis, the means and method of release would be adjusted to eliminate potential unwanted impacts.

In thinking about releasing pressure, a determination should be made on whether to release the pressure to atmosphere, or purge into water, or use some other means to neutralize the released vapor.

These methods all have their place and can be very effective if the focus is not only on where the ammonia is coming out, but also on people and the environment in the immediate area, as well as further downwind.

If the decision is made to release to the atmosphere, knowing that ammonia is a natural part of our environment and will usually dissipate quickly, the release of the pressure should be done in a controlled fashion.

In this particular case, the purging valve used to release the pressure was opened quickly to rapidly dump the pressure.

A better approach would have been to partially open the valve and let the pressure slowly bleed down. A slow release would have allowed the vapor to thoroughly mix with the air and would have significantly reduced the downwind distance where a noticeable level of ammonia smell could have gone.

In this case, the lessons weren’t over with the actual event. Staff at the facility itself also has a responsibility when an ammonia event occurs.

The question was asked in this example: “Was this a near miss since no one was injured or even affected that much?”

The answer is: No! It was a direct hit! And an incident investigation must be done in a case like this to not only find out what happened, but also to reduce or eliminate the possibility of it happening again.

In addition, just because this was a release that was well under the federal reporting quantity, it does not mean other agencies or groups don’t need to be notified.

In the particular location, both the state emergency management division, and the local emergency planning committee were notified. The fire department may also need to be notified.

This event could easily have been a non-event if a few of the above mentioned thoughts had been considered.

Luckily, no one was injured and the lessons learned by both the contractor and the facility were valuable. Hopefully we all can learn from this example, and ask the important question –where’s that vapor going? – sooner, rather than too late.