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Segei
06-10-2015, 12:38 AM
Hello, guys.
What do you think about this video? http://www.csb.gov/assets/1/16/thumbs/Pipe_09_01_10_012.Crop.jpgCSB Releases New Safety Video Entitled Shock to the System Offering Key Lessons for Preventing Hydraulic Shock in Ammonia Refrigeration System (http://www.csb.gov/-csb-releases-new-safety-video-entitled-shock-to-the-system-offering-key-lessons-for-preventing-hydraulic-shock-in-ammonia-refrigeration-systems/)

RANGER1
06-10-2015, 05:02 AM
For me need more information than they are giving.
Would be interesting to see real pipe work lay out.
Have heard of it once before in Melbourne, but don't know details.
Also was pipe suitable for low temperature application.
The scenario doesn't seem that abnormal in the real world,
so would definitely like to understand it.
Video appears incorrect as how can cold liquid enter evaporators if it has "hot gas"
in it?
Hot gas is not hot by the time it gets there, it's probably 50C if that's hot.
There has to be a better explanation than this, this video is for people not in refrgeration.

Magoo
07-10-2015, 02:15 AM
Hi Ranger.
I am inclined to agree we are not seeing the whole picture, the blown out blank end looks more like material failure that happens with trapped sub cooled liquid line that is allowed to rise in temperature as with a power failure.

RANGER1
07-10-2015, 07:36 PM
This explains in more detail
http://www.csb.gov/assets/1/19/final_CSB_CaseStudy_Millard_0114_0543PM.pdf

Segei
10-10-2015, 11:38 PM
I don't agree with some conclusions of this investigation.
1. They believe that initial rinture of the evaporator coil happen when cold ammonia have entered evaporator full of hot gas. First, 7 hours is more than enough for that gas to condense. Second, pressure inside defrosting coil is at least 70 psig, pressure of the cold liquid before it enter coil is 15-20 psig. How pressure 20 psig can overcome pressure 70 psig?
2. They believe that condensation induced shock happen when suction valve open and hot gas was release into suction line. I don't think so.
Most likely what happen. When defrost was interrupted by power failure, suction valves of the defrosted coils have opened. During 7 hours of power off pressure in suction line has increased and ammonia vapour start to condensate(through suction valves) into the cold evaporators. When power was back on, defrost cycle was started but from the middle of this cycle. It means hot gas main valves opened. Hot gas at high speed entered coils full of liquid ammonia and vapor propelled liquid sludge happened. Coil was ruptured. After that suction valves were opened and large quantity of mixture liquid and gas rushed into suction line. This time vapor propelled liquid sludge was created in the suction line and it was ruptured at dead end.
Similar issue happen at one Nestle plant and this accident was presented at IIAR conference several years ago(reference 3 in this report). At this plant, blast freezer coil ruptured when it was not pumped out before defrosting. This happen because of electrical malfunction. In this case conclusion was that PLC should be programmed to prevent opening main hot gas unless pressure in the coil increase to 20 psig during pilot hot gas valve operation.
I think major reason of this two accidents is that too much hot gas has entered the coils when main hot gas valves were opened. Unfortunately, we have misconception in the industry that for good fast defrosting hot gas should be supplied as much as possible. However, hot gas defrosting is triple process. Hot gas supply, ammonia condensation and ammonia liquid draining. For good safe efficient defrosting these 3 parts should be balanced. Typically, we put a lot of hot gas into the coil but we forget about liquid ammonia condensate draining. For bottom feed overfed coils liquid ammonia should be drained through small orifices at the entrance of each circuit. Oversupplied hot gas will defrost the top of the coil but botton of the coil will not defrost until all liquid ammonia has been drained out. It means that moderate supply of the hot gas will give us similar defrost time compare to oversupplied hot gas but it will be much safer and more efficient.

RANGER1
11-10-2015, 02:55 AM
There is more questions than answers on this one.
Because a number of air coolers were with 1 set of controls valves throws a curve ball.
Programme definately wrong for this set up.
You would need to install a pressure switch or transducer to prevent main suction valve opening if pressure not reduced enough, linked to a fault or alarm, as what if pressure bleed off solenoid burns out, or someone leaves a valve closed in maintenance!
What happens if air coolers in general cannot pump down before defrost, maybe frosted over in some cases?
Not all units have soft defrost & pressure bleed off, but of course nice.

The plant had been off for a long time so Evap pressure would be equivalent to room temp.

Would be nice to see pipe work layout & where damaged header pipe work was in relation, as you would have thought suction return line on roof would slope towards plant room or accumulator.

Maybe hot gas line full of liquid as well, we don't know layout.

Was hot gas top or bottom feed, as gravity would help with bottom feed if hot gas enters in top.

This had to be a bit special, as why isn't it more common.

Danfoss have a good valve for this purpose of gentle pressure equalisation PMLX or a motorised version.
It would be interesting if hot gas defrost returns to interstage or low temp suction.