17.4 Product recovery
To minimiseproductlossesduring cleaning and or change-overit shouldbe
possible to purgethe system. The simplest wayis by purgingthe systemby
wateror product.Thisbecomesless effectivefor verylongpiperunsas the
mixing zoneincreases. Alsofor viscous products purging witha low-viscosity
fluidsuchas wateris not effective, as the waterwill pusha holethroughthe
centreof the pipeand the majorityof the viscousproduct will stay behind.The
most attractive way of reducingthe environmental implicationsfor closed
systems is product recoveryby pigging'.The so-calledpig' is sent,by means
of compressedair or wateras the drivingforce,throughthe pipeline,and expels
mostof the product fromthe system.The pig is often designedfroman elastic
material suchas siliconrubber witha closedsurface. Sponge-like pigsare not
recommended,as theyare difficult to clean. The earlypiggingsystemswere
operated manually. Today's piggingsystems are fully automatic and are
cleanable-in-place (CIP), avoidingany chance of contaminationby the operator.
Also,unlessthe manualsystemis carefullydesignedto preventthe pig from
escapingfromthe line underpressure,it can present a potentialdangerto the
operator.
The drawbacks of these systemsare the requirementstheyput on the design
and constructionof the pipingsystem, suchas:
∑ all pipes should havethe samediameter;
∑ piperoundness is important, especiallyin bends;
∑ no valves or sensorsprotrudinginto the pipeline;
∑ no weldsprotrudinginto the pipeline;
∑ branchesshould be constructed in sucha way that it is impossible for the pig
to go in the wrongdirection.
Oftenthese requirements are difficultto meet especially in slightly older
existingplant.
For some of the issues mentioned abovethere are solutions. To avoid
protrudingsensors somesuppliersdid developretractablesystemsfor temperature
sensors,pH sensors.Alsofull-boreball-valveshavebeendesigned whichare
claimedto be cleanablein place.As standardball-valvesare not `cleanable-in-
place'since the areabetweenthe sealscannotbe cleaned,this has nowbeen
overcomein an inventivewayby the use of inflatablesealswhichare deflated
during CIP, allowing the CIP fluidto flow around the ball. The design
incorporatestwo inflatablesealsthat grip the ball wheninflatedand releasedfrom
the ball when deflated.Thesevalvesare also availablein a three-way versionand
formthe basisfor a piggingsystemas providedby HygienicPiggingSystemsLtd.
The alternativeapproach is to makethe pig more tolerantfor geometrical
changes. Thishas for example beendonein a UK-LINK schemeled by the
University of Bristolwhere the pig was made fromslushice. The universityhas
patented this development. It should also be possible to makethe pig fromfood-
gradebiopolymer.
Improving the hygienicdesignof pipes 275