Hello, Pix!
VLR purging and Implicit Detach are independent procedures in a VLR. They are used to avoid unwanted signalling load on network (paging and signalling between HLR and VLR) when subscriber is unreachable a long time. In our network Implicit Detach Timer may be changed from 6 min. to 30 hours. When Implicit Detach timer expires than subscriber is detached but isn’t cleared from VLR. When VLR purges subscriber, then it deletes his entry and sends Purge message to HLR.