HAZOP - Ammonia Storage Tank & Cracker - Rhetorical Questions

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

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

 

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

 

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?

 

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?

 

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?

 

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 3^rd 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

 

Equipment Risk Assessment for ATEX Compliance - Some thoughts

Hi everyone, I would like to share some notes and questions on certifying non electrical equipment in hazardous areas using the methods outlined by the ISO standard - 80079-36 & 37.

This is used to self-certify legacy equipment as ‘safe’, and is significantly cheaper than buying expensive ATEX rated equipment! having saved 10s of thousands of £££s in the many project I’ve worked on.  

Figure 1: Generic mechanical equipment

Typically a risk assessment table is filled out, with example scenarios on how an ignition source can occur, its safety system in place, giving a resulting equipment protection level (EPL) (hazardous zone). Improvements to the safety guards like maintenance procedures, monitoring system, etc, may also be given as a result to achieve compliance.

Figure 2: Risk assesment table example from ISO 80079-36

 The following are some questions when undertaking this assessment. 

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Photograph 1: Generic pump opened up showing shaft and flanges

Pump system:

1.      Can there be frictional heating of impeller mechanical seals between rotating shaft in stationary outer part? - May not be likely as this part will be immersed and cooled with process fluid

 

2.      Can faulty bearings go undetected for some time – leading to the motor cooling fan blades hitting the cowling?  (applicable to other rotary equipment like conveyors and fans, etc.)

(1)   Can unchecked faulty bearings now be considered to be run in ‘normal operation’ ?

(2)   Are the bearings replaced at regular intervals, regardless of condition? See a typical bearing failure bath tub curve

 

Figure 3: Typical bath tube curve for bearings

3.      For the pump lubrication oil system:

(1)   Is there an alarm present if there is a leak on the liquid seals?

(2)   Are the seals monitored for flow out or pressure inside?

(3)   Is there any consequence for incorrect type of lubrication oil used for the pump?

(4)   Is there any issues for using contaminated or degraded oil? E.g. static discharges into a gas zone?

(5)   Is under and over lubrication an issue?

 

4.      Is there a limit on the amount of insulting surfaces inside the pump?

(1)   This is to prevent brush discharges, e.g. during cleaning operations, plastic parts can be rubbed to clean them, leading to brush discharges

(2)   There should be a limit of insulating surfaces depending on gas group, as per IEC 60079-32, e.g. 5,000 mm2 for IIA, 2,500 mm2 for IIB, etc.

 

5.      Typical Safeguards:

(1)   For the Planned Preventative Maintenance (PPM) routines:

                                          i.     Is there a check, lubrication and greasing of bearings periodically?

                                         ii.     Is there a visual check for faulty bearings?

                                        iii.     Are regular thermography undertaken on motor/equipment?

(2)   Are approved seals used & maintained in accordance with the manufacturers’ instructions?

(3)   Can you have the pump’s amperage sensor on the motor and link it to an alarm and trip? Is this an adequate safeguard for abnormal operation, overheating, etc?

(4)   For ‘Ex’ rated motors – are the flange gaps for ‘Ex d’ flameproof enclosures ensured that it is not painted over or otherwise blocked (as this may compromise sealing)?

 

Figure 4: Thermal imaging of pump motor

Screw conveyor:

1.      Can trapped powders within mechanical seals become heated to ignition temperature?

2.      Is the bending of screw or main shaft by powder weight credible?

3.      For tramp metal – is there filter/screening quality control process? Is rusting credible?

4.      Is charge accumulation on screw conveyor body by powder movement possible?

(1)   Can there already be charge on feed powders?

(2)   Infeed powder may generate static or inherently have static charges present (charge relaxation time)

(3)   Are the powders fed from earthed bodies/locations?

(4)   What is the powder resistivity? – typically high for organic powders like flour – difficult to charge.

 

Figure 5: Generic screw conveyor componants

5.      Safeguards (some similar to the pump system)

(1)   Is there a check for trapped powders around rotating seals?

(2)   Is the equipment run in short duration batches? This may allow a degree of cooling

(3)   Is the running rotational speed expected to be slow?

(4)   Is there checks for rusting?

(5)   Is there clearance inherent in the design of motor fan? Maybe this has changed over time due to rust or deposited/built up layers?

(6)   Is the shaft welded or screwed in? is the screw strength/torque checked as part of PPM?

Figure 6: Screw conveyor shaft failure

Reactor Agitator:

1.      Is rotating the shaft in the wrong direction an issue?

2.      Can a failed agitator or malfunction cause a chemical reaction runway potential? Is there any safeguards like process control or operating procedures of this?

3.      Drive system issues:

(1)   Is misalignment of agitator drive possible? This is usually the cause for increased load bearings, gears and seal damage

(2)   Has the agitator installation been verified using laser alignment and by a competent engineer?

(3)   Does the agitator drive belt need to be anti-static?

(4)   Is there a pressure testing system on the vessels prior to starting a new batch – to check for faulty/leaky bearings?

 

Figure 7: Generic reactor agitator

Thank you for reading, please comment, like and subscribe to this blog

Hope you have enjoyed reading this

Chiraq