One risk is that a tube fails and piping or water hits the worker. Typically the risk of rupturing a 1/2″ thick tube during the pigging process is low. However, for those who have concerns, a simple hydrotest can be performed by the pigging contractor prior to pigging.
Another potential hazard is falling refractory and/or tube hangers. During one pig decoke, a 4’x6′ section of refractory fell off of the end wall from 10′-15′ above the floor. To a lesser extent (but with potentially more severe consequences), we have broken hangers, which also have fallen to the floor. Therefore, consideration should be given to both as to their impact on safety, prior to allowing entry during pigging.
Some sites requires the following entry procedures:
– After steaming coil to remove hydrocarbon, blind and hydrotest the coil with well/fire water prior to pigging.
– Have the pump safety valve set appropriately for the tubes being pigged.
– Alert all persons entering the box that the tubes are pressured and no work can be done to the tubes during the pigging operation (such as grinding).
– Pressure in the tubes to be monitored and recorded.
Typical water pressure at inlet to furnace during mechanical pigging can reach maximum in the range of 400-600 psig. This too is the maximum pressure the pump can deliver. Typical furnace tube wall thickness can withstand this pressure – thus, technically it is assessed ‘safe’ to work inside the furnace while pigging. However, there are other factors to consider – like noise, vibration of the tubes (and scaffolding, if tied to tubes/supports), etc.
Personal experience – “I have seen and worked inside a furnace while the furnace was being pigged. Though, refinery typically did not allow such simultaneous work (just to avoid any potential incident) in the other furnace (Visb) using pig decoke, but this was done as a ‘measured risk’ after a thorough Safe Operations Committee review. Furnace work was on critical path and offered enough incentive and low risk to execute. Precautions and contingencies were exercised carefully. The work was done quite well, and in time. I have been in the furnace – there was mild vibration, and you could hear a moving sound when the pig moved in the tube.
I have seen such simultaneous work in other refineries, inside furnace while furnace being pigged. I had similar observation when I was inside furnace. I am not aware what procedures are followed/reviewed there to allow furnace entry during pigging.
Specific site may have their own opinion or preference on this. However, there must be a few issues considered…what comes to my mind right now…
=>confirm tube remaining thickness is suitable for pump discharge pressure
=> Inspection to evaluate furnace inside is okay for ‘small amount’ of vibration (tubes, supports, loose castables, etc) – tubes vibrates a little when the pig runs. Form my observation so far, vibration is really small
confirm vibration inside furnace (dring pigging) is acceptable – may take out all personnel from inside, start pigging, assess situation and allow them in again if okay
=> scaffolding should be built considering vibration
=> COMMUNICATE clearly to all concerned personnel (worker, pig-operator, technicians, etc) about procedure. Last thing wanted is somebody falling from scaffolding being shocked by sudden sound!
In Singapore, a lower pressure for water at furnace inlet was set as an alarm – If this pressure is reached, pigging folks to stop procedure, inform Operator and evacuate personnel from inside the firebox. Resume pigging and allow work again if situation judged okay. If I remember correctly, about 250psig was set as an alarm (pump discharge was about 600psig), but never reached at that level. Typical pressure at inlet was about 150psig.
It may be prudent to install a temporary barrier when pigging is taking place.
Some sites will allow work in a firing zone while pigging is occurring in another firing zone. In that case the risk is failure of a convection section tube, which most assess to be low probability and low risk should it occur (it is much less likely that shattered pieces, or much water, will hit a worker.
Regards – Mitch Moloney (ExxonMobil)