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