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

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

Hope you have enjoyed reading this

Chiraq

Safety - HAZOP Study Toxic Storage Tank - Rhetorical Questions...

Hi everyone, I would like to share some questions/notes, I made during a HAZOP (Hazard Operability) safety study for a 25 tonne storage tank and its distribution system, handling toxic materials at a facility.

Generic Chemical Storage Tank

Questions:

General System:

1.       Is a direct liquid feeding system safer than one with multiple smaller day tanks?

·       Safer for direct feeding as there is less instruments, leak paths & point of failures

·       BUT not good for production as day tanks provide buffer if a transfer pump shuts down

·       Direct feeds may have higher flowrates (unsafer if it leaks) – thus day tanks are preferred

2.       Is there any use of different material of construction, valves, seats or seals out of specifications in the distribution system? - This can be a source of leakages and fugitive emissions 

3.       Is there a common vent line shared between the multiple day storage tanks? Are these tanks sealed for overflow of liquid into the vapour line?

·       Vent lines equalises pressures & allow for expansion of vapours

·       A failure of overfilling liquid in one tank may trigger level alarms in all the day storage tanks - when it could be instrument fault or human error in one tank, Thus difficult to troubleshoot

4.       Are there any safeguards for improper connections of vapour return line onto tanker? i.e. CCTV, 2^nd signature checklist, labelling, etc.

Equipment:

5.       Is there an issue for pumping air into the tank? Will this effect quality of the chemical storage & used in the facility?

6.       Can the transfer pump can run dry? As magnets can burn out, overheat and be damaged. Wil the operator have to break into line? Can a seal-less pump be used instead? As it is self-priming, thus prevents restarts

7.       Can a spill back line (return) for the pump be used to mix with potentially frozen fluids?

8.       What is the procedure for changing the filters of the pump? Is there decontamination process? Is splashing a possibility? How is releases to drain or ground water prevented?

9.       For the gas scrubber/extractor – Is the activated carbon the right grade/type? Can a sign be put up to inform operators? Scrubbers usually catch fire with ketones – thus confirm suitability with toxic material

10.   Is there a possibility for obstruction in the exhaust outlet of an extraction fan? i.e. a birds nest.

Safety Systems

11.   Can the tanks relief valves/Burst discs be installed incorrectly backwards?

12.   Is foaming or bubbling a possibility in the burst disc? Check ullage/head space of tank for foaming allowance

13.   Does temperature effect the operation of the tank relief valve and does the room need temperature control?

·       Higher temperatures, thus vapour generation & more vapour flow through relief valve

·       Low temperatures, more crystallisation & solidifying may take place

·       Consider using adhesive surface temperature indicator, which changes colour (typically used in a fish tank)

14.   What is the sizing basis for the vacuum relief valves? Is there any potential for chemical reactions that remove gases from head space of the tank?

·       Due to corrosion of interior of a steel vessel

·       Newly cleaned steel with chemicals can remove oxygen

·       E.g. Ammonia reacting with chlorine to form ammonia chloride

15.   Is there potential for blockages in the tank and distribution system?

·       Material can polymerise, crystallise, condense or solidify

·       Monomers can escape a tank free of inhibitors

·       Waxes & viscous high boiling point materials can solidify

·       Dust accumulation

·       Water vapours can condense to form liquid seals in low points or freeze in winter!

·       Is there any

16.   How are leaks controlled in flanges of transfer pipework? Can you put portable sniffers into flange guards to detect leaks, rather than detecting in the local air/atmosphere

·       Can you use shrink wrap around flanges too, BUT may not be compatible – may melt if leaks

·       Still needs to remove containment toxic materials from flange guard (secondary) as also may lead to release

·       Can you plot a table to justify hiring one person for inspections/plant walk throughs – account for the following variables…

                                                               i.      Size of pipework (diameter in inches NB)

                                                             ii.      Proximity of release area of people

                                                           iii.      Activity in area – where 0 = heavy work and 1 = no work, i.e.. 0.2 = walkthrough

                                                           iv.      Ventilation – where 0 = none, 1 – full, i.e. 0.2 for doors, etc.

                                                             v.      Frequency to exposure

                                                           vi.      Maybe hire someone for 2 months of the year for regular emissions testing

 

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

Hope you have enjoyed reading this

Chiraq

Process Design - Control Valves - Questions about their origin...

 

Hello everyone, I would like to share some notes I made, when I had to design, purchase and install control valves for the plant I was working in.

Just simple questions we should ask ourselves when looking at control valves - A quick read, nothing too taxing as this is the general theme of this blog. 

Figure 1: Parallel control valves

Questions on Sizing of a control valve:

1.       Should the valve be sized for trapped volume, i.e. the volume length between Level Alarm High (LAH) to Level Alarm Low (LAL) set points of a vessel?

2.       How long is the control valve being used for? Is it for 15 seconds to 30 mins?

3.       Is it sized only for a ranges, i.e. usually 10%-90% open and does it account for a range of operating conditions? Typically control valves are sized at 65% open

4.       Is there an extra margin?

5.       Does it allow for “fluid expansion”?

Usually control valves are one pipe size down from source piping and sometimes have to fit into existing piping. Modifications can be made and information about the trim and body should be ascertained form the vendor.

It should be noted, vendors can sell novel products & exaggerate their benefits, whereas clients (plant operators) may seek economical & proven solutions.

 

What is the Basis of the control valve and how is it introduced into a system?

1.       Where does the set point of a control valve come from?

2.       How is introduced... Is it from the trends on measurement devices?

3.       Does equipment efficiency affect this value?

4.       Is it from the Operator or the Distributed Control System (DCS)?

 

Example of a cascade control system of a shell and tube heat exchanger:

1.       The control system must take account of temperature range

2.       Is temperature control critical?

3.       What are the criteria for override control?

a.       If the flow control of heating medium is linked to temperature control of process fluid, then is there a flow override value? Is there a region where the operation must be in to be feasible?

b.       It should be noted override control is used to regulate a process input to maintain one process output at target without violating a constraint on another output

Thank you for reading

Have a great day

Mr C

Process Design – Process Intensification (PI) plan for improving an existing manufacturing plant

 Process Intensification (PI) is a topic receiving a lot of attention recently. While working in a pilot plant, I came across the philosophy of Process Intensification (PI). This is to improve a facility either by introducing new technology or optimising existing equipment to output more products and profit, (essentially). At least I like to tell myself that, we did the best we could with the tools we have, while balancing safety, production and cost.

Generic birds eye view of a ‘plant’ - but with the right set of eyes, there is always potential for to improve this ‘plant’ 

Plant improvement can be taken under 2 approaches:

1.      Equipment driven approach

1)     Usually in testing new/novel application

2)     Investment in new

2.      Process intensification

1)     Time scales, flow patterns, & temperatures

2)     Inherent properties of material reaction & physical properties

3)     Build on process knowledge – small improvements with big performance changes

Questions to ask yourself:

Here we examine reaction kinetics, heat of reaction & physical properties of a process. The business drivers are of course, production, cost, project management, efficiency. To push the boundaries of process conditions. Some questions you should ask yourself….

1.      Why is the temperature limited?

2.      What happens when it is exceeded?

3.      Can you add reagents in different order?

4.      Aim to remove many rate limiting steps?

5.      And remove unwanted steps?

6.      Can you operate continuously rather than batch?

7.      Can you increase impeller speed of stir tank OR use extra reagents?

8.      Can you use impinging jets – i.e. change to reacting in a pipe rather than reactor? (more turbulence)

9.      Can you use inline mixer in pipe, to save energy usage downstream (of course you would need too elevated pressures & temperatures)

10.   Can you speed up a reaction?

1)     Use a new catalyst

2)     Use Jet mixed tank

3)     Is heat transfer & mixing efficiency important

Generic pipes with gate valves and flanges - Can this be optimised? heat loss, leakages are just some thoughts

Process Intensification is a very interesting high level. I urge all people working in the industry to read upon it. This philosophy should be deeply ingrained into the lives and minds of all who work in industrial processes.

 

Regards,

Mr C