Application is tracing. I need to install 200 traps. There is a significant price difference between the balanced pressure steam trap and the bimetallic (more expensive). Can the thermostatic balanced pressure do the job, internals are all stainless steel. Steam pressure is 3 barg. If not, why choose for the bimetallic. I'm not considering thermodynamic ones.

Check out GEM steam traps, maintenance free and 10 year warranty.

This takes me back many years(Screw pumps)!

Here are my two pfennig's worth:

Both bimetallic and  thermostatic steam traps result in sub-cooling. This means that they're not really steam traps at all but rather "hot condensate" traps; the steam/condensate interface is well upstream. Bimetallic traps suffer from hysterisis. They "remember their previous settings and become loathe to compensate for changing conditions. BP thermostatic traps respond much faster. Modern versions of the latter incorporate very robust, sealed thermostatic capsules thus overcoming the somewhat fragile characteristics of the antique bellows design.

Thermodynamic traps should not be used on low pressure tracing systems because they'll blow much too frequently; TD traps require higher operating pressures to behave properly. But, if one requires higher tracing temperatures and needs to avoid condensate build-up in the lines, increase the pressure (if the system allows it, of course) and use TDs.

I love traps.

I also like thermostatic traps for tracing and most drip trap applications, as long as the subcooling of condensate can be done in the piping and not in the steam line for drip traps.

Since bimetallic traps ARE thermostatic traps, I assume in your question that you are comparing them to liquid filled type thermostatic traps.

The bimetallic traps are more robust and will hold up better if there is water hammer present.  They also do not follow the steam curve as closely as the liquid filled traps.  So for constant pressure applications like drips and tracers - which also have very small condensate loads - they generally work just fine.

For liquid filled traps, if they are true bellows types, these should also not have much problems at 3 barg.  There is the capsule type that is an improvement on the bellow as there is less liquid fill and they are therefor more responsive, but the cost of these is usually a step up from the bellows, but probably still a bit less than the bimetallic.

So, without knowing more about the trap applications or your  exact system, in general I would say at 3 barg without water hammer go with a quality bellows or capsule for drip and tracers, with the stipulation that the drip trap installation must take into account the space for condensate subcooling.

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