Safety - HAZOP of a Thermal Waste Treatment Plant – Rhetorical Questions

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.

Figure 1: Generic Storage Silos in a Waste to Energy Plant 

Screw conveyors

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

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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Hope you have enjoyed reading this

Chiraq