Hi everyone, I would like to share some engineering questions
and notes, I made during a HAZOP (Hazard Operability) safety study for a
thermal waste treatment plant.
This plant consists of a series of screw conveyors (augers),
burner units, gas cleaning, condensing towers and waste oil tanks. Like my other posts, these
questions can be applied to any generic process plants and I hope it can be of
some benefit to the readers.
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| Figure 1: Generic Storage Silos in a Waste to Energy Plant |
1.
For the flexible bellow expansion joints present
on the conveyors, it can fail due to excess heating/cooling
(1)
Is the life expectancy of such joints/bellows
known?
(2)
Is there a suitable replacement/inspection
regime created? Ensure to hold essential stock for replacement.
(3)
Are the seals rated for the correct duty, design
temperature, and chemicals handled in the process?
2.
For the over-pressuring of a screw conveyor
(Auger) – A shear pin is usually present on the motor body, which indicates if
the screw is blocked
(1)
Is there any safeguard before shear pin is
activated?
(2)
Should there be alarms, differential pressure indicator,
shut off trip, etc?
(3)
It should be noted that the closure of discharge
valves (outlet of screw) is not recommended, as it is better to close the inlet
and leave outlet open
3.
Is it possible that one of screw (auger) will be
maintained when the other is in use? Can back flow of gas occur (from the
burner unit)? Is there adequate protection for persons working on the system?
Burner unit
4.
For the purging of a burner unit with inert gas
(Nitrogen):
(1)
Is there a written and verified procedure for
the purging process?
(2)
Is there 2 independent means of verifying that an
inert atmosphere is present within the burner chamber? – as purging of a burner
is not simply by pressing a button alone
(3)
Has a nitrogen demand balance been undertaken to
ensure there is sufficient nitrogen at peak flow?
(4)
For an increase in feedstock into the burner
(screw conveyor malfunction/overpressure) this would lead to an increase in
temperature - Will there be a delay in nitrogen purging in shutdown? Ideally this
should be immediate.
(5)
Backflow of gas into burner/screw – Can a Pressure
Safety Valve (PSV) on the purge nitrogen inlet line, with a low set pressure, stop
nitrogen back flowing into upstream screw conveyor?
5.
Is it possible for the upstream auger screw to
fail in such a way that fuel gas would bypass from inlet to outlet without
going along its desired path? (possible leakages)
6.
High temperature discharge from the burner unit
- has consideration been given for a jacketed water cooling system for the
outlet discharge conveyor?
7.
For the Combustion air fan
(1)
Is it a direct drive fan? Can it overspeed? If
there is a spare fan used, if so, could it be of a different specification?
(2)
Does the fan have a 3 phase fan motor, and could
the wires be installed incorrectly? i.e. two phases swapped, thus the fan would
run in reverse flow – therefore check this upon commissioning
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| Figure 2: Generic Air Cooler Fans at a Facility |
Heat Exchangers / Gas Quench Towers
8.
Is it possible for the circulation cooling water
to leak into the oil circulation system?
(1)
Are the tanks, equipment and towers capable
withstanding the extra pressure of cooling water into the system? It should be
noted that if a PSV is seeing higher pressure, that does not make a system safe
(oil/water mixture)
(2)
It should be noted that the Aims of a safety
system…
·
Know as soon as possible that a leak has
occurred
·
Ensure that if worst did happen – there is a
sufficient safety control to prevent failure of vessel (over pressurisation)
·
There should be alarm to show loss of control,
thus automated action, before safety valve is used
(3)
Reliability – should there be additional static
water supply available for the cooling system?
9.
Level monitoring of the quench towers:
(1)
Are there two independent layers for level
alarms and trips? i.e. One indicator should NOT have a ‘high’ and ‘high high’
alarm on it, as it should be independent (reliable).
(2)
Are switches, alarms and trips a different type
so that ‘common mode failure’ does not apply? I.e. Level alarm low (LAL), or
low low (LALL), etc.
(3)
Given that a sight glass shows liquid is flowing
out and doesn’t say how much is in the tank, if a sight glass is not inspected
– then can a high level in the quench tower/tank build up over time?
(4)
Could an overflow trap be fitted at the nozzle
into the quench tower? i.e. a dip leg / u shape pipe
General
10.
Are lines insulated with non-porous insulation
and or is there a procedure / permit for the removal of insulation?
11.
Draining system / waste oil tank
(1)
If oil is to be drained and system is under
negative pressure, will air bubble in though the oil as it comes out due to
gravity?
(2)
Is there positive isolation for drainage system?
i.e. typically a double valve or valve and blind setup, and this would be
needed for vents too
(3)
Is the whole site of the waste oil tank bunded for
fire water/deluge use? What about the catchment pit? Is the recovery of all the
deluge water important?
(4)
Blind flange – is there a safe system of removal
of the blind?
12.
Heat tracing - Is it in series? If so a failure
at any point would show as a failure of the system, BUT how is the location of
the failure diagnosed as it could be anywhere? It should be noted that pumps
are not usually heat traced, due to problem of removing them for maintenance
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this blog
Hope you have enjoyed reading this
Chiraq
