Hi everyone, I would like to share some engineering questions
and notes, I made during a HAZOP (Hazard Operability) safety study for a vacuum
furnace process producing fine metal powder.
The melting
and spray drying of metal blocks into fine powder (extremely explosible!) is
undertaken in a vacuum furnace vessel. The process includes furnace tipping of
melted metal blocks, spray drying with a gas, solid separation via a cyclone
and dust filter/gas cleaning before it releases to the atmosphere. A new metal
feedstock and impure inert gas was considered in the study.
Like my other posts, questions here can be asked for any
generic process plants.
Please comment, like and subscribe.
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| Photograph 1: Generic Metal Production Plant |
Vacuum Furnace
Operation
1.
Given water is usually circulated in coiled pipe on the furnace body –
if a leak were to occur, is there a possibility of steam explosion occurring?
(1)
Condensation – possibly due to a leak in the vessel roof
(2)
Water droplets in presence of rust/coal/soot and molten metal, can
lead to an explosion
(3)
Caused by violent boiling or flashing of water into steam
2.
If a wrong feedstock (metal block) is used, Is there any possible
consequence of superheating?
(1)
Thermal expansion – may lead to possible rupture and leaks
(2)
Low density material can lead to overfilling – as higher volume for
same mass
(3)
Different vapour pressure between boiled vapour of new material –
boiling point reduced in mixture – thus over boiling?
3.
For the dust vent filter box –
is it possible that it could be incorrectly installed?
(1)
Could the vent panels or dust collection bin on the filter, not be secured?
(2) Could you consider installing a dust discharge monitor in the
vent stack?
(3)
Do explosion panels need to account for travel
back – therefore do they have an explosion isolation system? like a Slam shut
valves, deluge/suppression, trouser legs (to push explosion up - I
know it’s a funny word actually)
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| Photograph 2: Generic pipework |
Oxygen sampling
4.
The spray drying of flammable metal powders, is inerted, based on its
limited oxygen concentration (LOC) - to prevent explosive atmosphere. There is
an oxygen monitor and alarm with a set point to trip operations - Has this been
adjusted for operating temperature and pressure of the furnace?
(1)
Has a safety margin been applied to the set point of the oxygen O2
alarm?
(2)
Typically, the LOC is reduced by 17% reduced per 100°C up to 200°C,
then 12% reduced per 100°C afterwards
(3)
Include an extra 2% points extra margin for delay in reading! This is
concentration NOT percentage of an LOC (very important!!!)
5.
Has the complexity of temperature been accounted for the monitoring of
oxygen level in the furnace? Has a profile been developed as the furnace vessel
is being cooled?:
(1)
Heated inert gas could be heavier than air, but at temperature could
be at point of buoyancy
(2)
It is recommended to record an oxygen monitoring profile for
shutdown/cooling
(3)
Can you use a thermal temperature monitor to see inert gas profile?
(4)
BUT convection pattern of hot gas can be inert mixture – may be cooled
thus difficult to tell if air or inert gas present.
6.
How do you check if sample line for an oxygen analyser is blocked? Is there
a cycling of multiple channel sample points?
(1)
Could it be drawing from one channel and others are blocked?
(2)
Calibration not enough to fix sample line problems!
(3)
Can you use secondary handheld meters (2nd layer)
(4)
Can you reverse the flow of sample line to unblock line (purge)?
(5)
Is there a redundancy present (spare?)
(6)
Is there motive force (pressure) or a pump needed?
(7)
Is there a place to return the sample? E.g. vent stack or flare, if
hazardous
Inert Gas Supply
/ Storage
7.
Are there any cold surfaces in supply line for gas to expand? – may
cause injury to person by freeze burning
8.
Is the inert gas dry? – thus potential for condensation &
blockages in supply line to vessel
9.
Is there potential for air ingress into the gas supply line or in the vapour
space of the storage tank - when supply level is low?
(1)
Is there a monitor of oxygen gas to check quality of inert gas
quality.
(2)
Has a walk the supply pipework been undertaken as per its P&ID?
(3)
Safeguard may be that air is expected to be above the liquid CO2
surface (as CO2 is heavier)
(4)
Get vendor to do a hard change over & written confirmation of the
inert gas sample
(5)
Partial pressures – if CO2 or O2 which will boil off first? –
therefore is there a limit switch when the supply storage tank empties?
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| Photograph 3: Generic Factory Floor |
General / Maintenance:
10.
What if there is a power failure of a hydraulic crane unit used in
tipping the furnace into the spray dryer?
(1)
Does it lock in position upon failure? (continuous strain)
(2)
Does it return to default position in slow mode (or nearest point?)
(3)
Hydraulic system will need something to relief pressure for it to stop
dead
(4)
What is the fail-safe position of the hydraulic/electrically powered
crane?
11. Hydraulic oil hoses used in the mechanical systems, what are the quality checks, change policy and service life?
(1)
Are replacements like for like or better hose service duty
(2)
Can additives be used to prolong life of such hoses?
(3)
Is there a potential for fire in area and injury to person?
(4)
If left unchecked too long – may crack and degrade
12.
Are solvents used to clean the vacuum furnace vessel?
(1)
What respirator kits are used? Disposable? Tyvek (paper suit)?
(2)
Can water be used for cleaning? Deionised v distilled water (pure water)
(3)
Mixed bed resin is used for deionised (pass through)
(4)
Ensure ions have been removed – thus relatively fast process
(5)
But needs regeneration of resins that may degrade over time
(6)
Distillation – removal of contaminants but longer process
(7)
Consider waste water collection in bund area & disposal (HSE)
(8)
BUT water too cohesive – thus effects vacuuming process
Thank you for reading, please comment, like and subscribe to
this blog
Hope you have enjoyed reading this
Chiraq
#Hazop #Safety #IChemE #PHA
