What are the current international standards of MTBF for kilns and mills?

There is a difference between mean time between failures and mean time between stops. Best practice for kiln MTBF in the cement industry is in excess of 750 hours, while MTBS is somewhat shorter. The problem with MTBF is that it is easier to massage the data and cheat because some stops are planned and therefore do not count. In that situation it is easy to bring forward a planned stop when you have an unexpected unplanned stop.

We carryout refractory work with screw jacks, maximum 10m lining at a time (ie 5m maximum simultaneously at two different locations in same kiln). As ours is a small plant of 1000tpd, with single kiln of 64m length, deploying brick lining machine and dismantling machines are not viable in terms of cost for us. What therefore is the maximum length of refractory work can be done at a time, in how many parts, simultaneously, from the various aspects such as safety, life of refractory etc.

I would recommend using the glueing method rather than screw jacks and then you can replace as long a section as you like without the concern of installing and removing the jacks. My understanding is that the glueing method will have no problems with a kiln of 3.95m diameter. However, you would be better to check this with suppliers of the refractory glue when placing your order.