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[flash]

I have a compressed vapor ammonia line at about 150C and 17barg pressure that is needed to be heat traced against ammonia condensation during long shutdowns.
I understand I do not have to put on the steam during the normal operation because the conditions are far above the saturated conditions and there is no possibility for condensation, however it might happen when it is allowed to sit there for extended period of time.
I am interested in knowing if there is any standard for steam tracing for preventing ammonia condensation.

Thanks,

[Art Montemayor]

Flash:

There is no steam tracing “standard” for ensuring that you not condense ammonia.

You haven’t defined the conditions under which your pipe line will shutdown and this is important to know in order to define the “system” under which the blocked-in ammonia is working.

For example, if your pipe is initially filled with gaseous ammonia at 150 ^oC and 17 barg of pressure and then is “shut down” by blocking it in (shutting down valves at both ends of the pipe), then what you have is a constant volume system that is subjected to a differentially cooling mode. In other words, the hot ammonia gas will begin to cool down due to heat loss through the insulation. The initial temperature driving force is that of 150 ^oC minus the ambient air around the insulated pipe (I’m assuming the pipe is insulated). The hot ammonia gas will try to reach the ambient temperature, and will come close to it if given enough time – and bad insulation. As the gas temperature decreases, its pressure will also decrease according to the equation of state: P = ZnRT/V. The ammonia gas will probably not condense even if it achieves ambient temperature.

Another scenario is where the pipe line is shut down – but it is kept connected to an ammonia pressure source of 17 barg. Under this scenario, the system undergoes an isobaric mode: the pressure is kept constant. The pipe is blocked off so it represents an extension of that system that is static in flow – it is dead-ended. Consequently, if there are heat leaks through the pipe’s insulation and there are “low” points in the pipe, you could conceivably have ammonia gas condense in those low spots (at 45 ^oC) – particularly at the end of the static-flow pipe. However, you always have heat conduction through the pipe walls to keep the static flow at the end of the pipe heated to some degree. Whether you obtain any condensation under this scenario depends on your piping and system configuration and arrangement.

As you can see, without a detailed definition of your system, it is difficult to predict whether steam tracing is necessary or effective in your case. It won’ hurt, but whether it is necessary has to be evaluated under your detailed conditions of “shutdown”.