Hi everyone, I would like to share some notes and questions I made during a HAZOP (Hazard Operability) safety study for an Ammonia storage tank and pressurised cracker system.
Like my other posts, questions here can be asked for any generic process plants.
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| Photograph 1: Generic Ammonia Tank |
Storage Tank system:
1. First… the most basic question in process design…
(1) Has the system been modelled on a process simulation software? (e.g. Aspen HYSYS)
(2) Has back pressure calculation for the system been undertaken?
(3) Are the pressure relief valves (PSV's) sized for correct scenarios, e.g. thermal relief, fire case, 2 phase flow, range of flowrates, to get maximum orifice area size, etc.
2. Low temperature in pipework – the 'Joule Thomson' effect
(1) Ammonia usually self-vaporises when released – thus leads to rapid cooling
(2) Are the valves, gaskets and seals rated to handle cooling effect?
i. Different scenarios for liquid and gas ammonia
ii. Check rating of the PTFE seals for low temperature of ammonia, -33*C
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| Photograph 2: Generic Ammonia pipework |
3. Cold start
(1) Is there any pressure response from a cold start with ammonia trapped in the lines? e.g. slug of pressure or hot spots in furnace?
(2) Can this lead to cold flashing of ammonia?
4. Bund of storage tanks
(1) Is the transfer pump inside the bund?
(2) Is there a permit to entry for the bund?
(3) Is having a gas compressor better than a transfer pump for ammonia?
(4) Is it easy to break into the line? Given that liquid ammonia will boil, releasing toxic gas
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| Figure 1: Generic drawing of a ammonia tank |
5. Wrong – empty ammonia tank
(1) Is a reputable supplier used to supply ammonia? – can this be a safeguard
(2) Is there weighbridge at location? Is it public and calibrated/inspected at regular intervals?
6. Contamination of ammonia
(1) Organics + Ammonia may effect carbon seals and gaskets
(2) Some oil and seals may be soluble in ammonia
(3) Can grease get into the downstream process burner?
(4) Is there a potential to poison the catalyst, e.g. using silicon material?
(5) Ammonia does react with copper
7. Tank relief system
(1) Is the temperature relief valve (TRV) protected by a burst disc?
(2) Is there a tail for the bursting disc? Is the burst disc visually present for the operator?
(3) Could there be a sunshield (shade) leading lower temperatures?
(4) Relief valve usually work on differential pressure – use balance bellows
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| Photograph 3: Generic tank cleaning photo |
8. Reverse flow
(1) Does all backpressure from storage tank recirculation system equalise the pressure?
(2) Can transfer pump be installed incorrectly?
i. Can pump be wired incorrectly?
ii. Is phase inversion possible?
9. Tanks level
(1) Can dip pipe be above liquid level?
(2) Is splashing filling an issue? Electrostatic discharges possible?
(3) Can you put a tuning fork in the tube or flow switch in the suction line of transfer pump?
10. Flexible hoses
(1) Are they leak tested at regular intervals?
(2) Is contamination possible during storage and movement? E.g. while hanging in storage rack? Are caps needed?
(3) How tight fitting are the hose couplings?
11. Tanker filling - Icing
(1) Spraying of liquid into tank – cools vapour space, thus reduces vapour pressure
(2) Is the effect of icing of flanges accounted for in the tanker filling procedures?
(3) Are flange guards needed?
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| Figure 2: Generic release of fluids from flexible connections |
12. Purging system
(1) Are there multiple purge points? Is it in clean duty?
(2) Is there a local pressure gages around each purging point?
(3) Are purge/drain valves locked close when not in use? Can you remove the valve stem?
(4) Is it possible to use the transfer pump at the same while purging activities occur?
(5) Any incompatibility? E.g. methanol, hydrocyanide, or acid?
(6) Are materials of construction suitable for duty? E.g. seals, gaskets, Greece and oils, etc.
(7) Is there dedicated connection types for IBC outlets? E.g. for filling, mixing with water.
(8) Will there be any density change of nitrogen purging into ammonia liquid?
Reactor & cooling system:
13. Burner system
(1) Is the burner system made to standard? E.g. SIL 2 rated?
(2) Are thermocouples in furnace – single or double?
(3) Redundancy sensor for trip should be to standard, e.g. BS EN 746-2
(4) Is mercaptan (odorant) used in the propane gas fuel – an issue?
i. Will it react with the catalyst?
ii. Any potential for corrosion?
iii. Sulphur compounds or contaminants?
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| Photograph 4: Generic burner unit |
14. Catalyst
(1) Is there any issue for blocked catalyst, e.g. water flooding/logged?
(2) Is catalyst hydroscopic?
(3) Can catalyst be contaminated during transport?
(4) Is heat tracing needed for frost protection?
15. Adsorption column
(1) Will residence time in column effect hydrogen generation in cracked gas?
(2) Will excess undesired gases effect downstream gas cooling system?
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| Photograph 5: Generic adsorption column for gas cleaning |
Maintenance:
16. Cleaning of tank
(1) Is storage clean via a shot blasting process – will this lead to flammable dust atmospheres forming?
(2) Are leak and pressure testing undertaken as part of procedures?
17. For breaking to containment – are alignment of valves needed?
18.
Does vessel follow PSSR (more than 0.5bar) or
PED (less 0.5bar) regulations depending on operating pressure?
19. Do you need 3rd party for maintenance? Thus avoid access issues and risk assessment permits, etc.
20. Is noise levels an issue? If above 80dB
(1) Provide signage and PPE
(2) This usually depends on category A to C and frequency
(3) Usually in log scales that resonate
(4) Can you coat equipment in foam to reduce noise levels?
Thank you for reading, please comment, like and subscribe to this blog
Hope you have enjoyed reading this
Chiraq
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