Q & A > Question Details
Our catalytic reforming unit is running at 500°C . A short length flame and smoke was seen at the heater outlet flange; we have a steam ring which we have used to extinguish the fire. Now my query about hot bolting: is it safe to tighten the bolt at 500°C to stop the leak? If not then what should be the maximum temperature and safe procedure for hot bolting?
20/07/2020 A: RAUL FLORES, PDVSA INTEVEP, fraulf@hotmail.com
Be careful!! I'm not a materials specialist, only a chemist, but it is a matter of "common sense" that the any material properties change with the conditions, most of all temperature. Please review the design book. A bolt is a serious piece regarding any structure in a refinery. Please, do not touch it without reviewing the design conditions.
20/07/2020 A: keith bowers, B and B Consulting, kebowers47@gmail.com
'Further tightening of Bolting of a plain flange leak is always dangerous for several reasons: 1) Bolts are not strong enough at the hot temp. 2) Bolts have deteriorated and are yielding and tightening may cause complete failure 3) Gasket and sealing surfaces may be eroded and further tightening will not work 4) Leak may increase suddenly and risk injury if leak has 'coked up.' 5) Metallurgical/mechanical calculations of residual strength at max temperature should be conducted, and prudent 'discount' for deterioration must be included. IF torsional strength is not well above predicted twisting torque, NO GO. IF condition of bolting cannot be estimated with confidence--no go. IF calculated strength IS sufficiently high, consider replacing the bolts one at a time. Reduce feed rate and flange temperature and pressure by reducing heater firing. Consider using a leak fix firm to inject proven sealant into custom built wrapper to seal the flange. Carefully evaluate estimated financial impact of a short feed-rate reduction or total 'hot shutdown' for re-bolting. The potential increase of profits from 'hot bolting' MUST be high to justify employee risk and major fire risk . Consider continuous 15# steam ring cooling of flange system to 'cool off' the bolting and flanges until a shutdown can be conducted to re-bolt. Cooling of bolts may reduce/stop the leak and bolt replacement is then possible. Increasing torque on a hot bolt operating at/above normal safe temp is inherently dangerous and complete technical evaluation must be performed before attempted.
20/07/2020 A: Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
It is important that you investigate the root cause of the flange fire. It could be all the usual suspects; bolt relaxation, flange face damage, flange misalignment, uneven gasket loading due to improper bolt torquing, etc, etc. However, another possibility is gasket damage due to graphite oxidation. If graphite oxidation is the cause, simply replacing and retorquing bolts may not eliminate the problem. I would recommend you inspect the gasket in this flange at the next opportunity. If oxidation damage is revealed, replace all other gaskets in the reaction section flanged joints which operate at high temperature (say > 400 deg. C) with an Alloy 600 carrier ring, a stabilised graphite inner filler and an oxidation-resistant outer filler such as vermiculite or mica. You should carry out a risk assessment before attempting to re-torque the bolts with the plant in operation. You could consider using a multi-head hydraulic bolt tensioning with a specialist maintenance contractor to reduce the risk of injury to maintenance crews during bolt loosening (remote operation) and reduce the risk of uneven gasket loading during re-torquing of bolts.