sound from -40¦C surge drum

[colonelbogart]

Hello guys...

In our meat processing company we operate ammonia refrigeration system. We have surge drums for 0°C, -5°, -15°, -27° system and -40°C system (used for blast freezing)...

We experienced (hearing) a very loud sound (like bang) on the surge drum. we're suspecting it as hammering. This sometimes occur when the level of the drum is low. What should be the reason of this?

thank you

 

[sterl]

Sure these are surge drums in the "gravity Flooded" sense? or possibly they are central "suction accumulators"? How is liquid fed to the evaporators? Is there a sustained Liquid level in the vessel, that is a make-up arrangement?

It could be a lot of things, but it should not be ignored...Even a few percentage points of water in the refrigerant will cause some knocking at the pressures you are dealing with to make -40 Deg. SST.

If it seems you do not have any identifiable coincident transitions occurring when shock is heard: such as rapid opening or closing of a suction isolation valve for defrost; or a transfer drum pushing accumulated liquid to a higher pressure containment...

From afar, my first guess would be: The low liquid level is calling in the makeup arrangement which tends to raise the pressure in the drum; may be marginal pressure rise as it applies to the vessel overall but locally it can cause liquid displacement and a condensate induced shock...Usually in an upstream section of piping.

Lots of data on the web and in the literature describes the mechanism as it applies to steam and especially long steam mains..not much on refrigerants. It is a 2-phase mechanism: requires defined containment.

With the very high latent heat combined with low pressures and the tolerance for minor levels of contamination, older ammonia plants are susceptible to such mechanisms, though it is not often that a vessel suffers from them. For the few proven incidents, most fractures or other inclemencies have occurred on piping, particularly horizontal runs at low pressures and low relative elevations, and on large pipes that have encountered a low flow condition for a period of time.

Every central refrigeration system is different, and if there is some energy associated with this noise, you should employ someone experienced and familiar with your industry operational practises to have a good look at piping, supports, operations and etc.

Pipe noises like this fall under the military's curious phrase "tolerance of deviation" which in effect says, if it repeats without exploding, it must be OK. Don't buy into it.