Flush-out processes
Flush-out processes can be controlled in a numberof ways.Settinga flush-out
timeis the least favoured solution, because of the dependenceon supply
pressure. Controllingflush-out by conductivity readingsis the optimum method
for transitionzonesbetween cleaningsolutionsand water, and controlling via
volumereadingsis a good solution for optimizingwaterflush-outphases.
It is importantwhendesigning flush-outprocesses to consider not onlythe
mediumbeingusedto do the flushingout, but also that whichis beingflushed
out. For instance, flushingout cleaning solution with water has different
characteristicsfromflushing out waterwithcleaningsolution.Thisis most
obvious if the sametimesare specified for carrying out bothprocesses.
Tankcleaning
Whentanks are cleaned,all elementsinstalled in them,such as measuring
instruments,safetyequipmentand agitators, mustalso be cleaned.Therefore it is
importantto avoidsprayshadows behind installedequipment,and considerall
aspectsof tankgeometry.
If spray ballsare usedfor tankcleaning, the recommendedflowof liquidis
30±50litresper minuteper metreof tankcircumference. So, for instance, a
heavilysoiledtank witha diameterof 4.7 m (circumferenceapproximately15 m)
needsa flow rate of cleaning solution of approximately 45 000 litres per hour.
The flowrate for rotatingjet cleanersis lower, at about50±35%of this amount.
Whendesigninga system to cleanlarge tankoutletpipes, it is importantto
remember that the flowratesof supply and returnpumps onlymatchin a few
cases,and that trappedair and low flowin the outlet pipecan lead to unreliable
cleaning. Largeoutletpipesare generallycleanedin a pulse±pause manner, with
rinsing pulses fromthe bottomof the tankto ensurereliableand complete
wettingof the tankoutletzone.It is alsoimportantto ensurethat thereis a
sufficient volumeof cleaning solution in the CIP-buffer tank.
Tankphases
For hot cleaning, in particular when alternatinghot and coldsprayingprocesses
in tanks,the effectsof temperatureon gas volumemust be takeninto account.
For instance, whenchangingfromcoldpre-rinse to hot detergentrinse,the gas
volumein the tankstrongly expands. Generally, the result of this expansionis
not that the pressureexceeds the permitted level, but that the cleaningsolution
violentlyburstsout when the safety and vacuumrelief valvesare open.
Whenchangingfromhot cleaning to a cold final rinse, the gas in the tank cools
downrapidly and its volumedecreases accordingly.Thisdropin gas volume
mustbe compensated for via a vacuumreliefvalve, otherwisethe pressurewill
fall below the permitted vacuumleveland the tankwill be destroyed.
Typicalproblemsduringcleaning system operations
∑ Sprayheadpartly clogged:leadsto formation of deposits at certain pointsdue
to sprayshadows, followedby pressure build-upand atomization of the
cleaning solution.
Improving cleaning-in-place (CIP) 437