3 replies to this topic

[Art Montemayor]

Steve:

I consider this type of application a very serious one and with a high possibility of hazardous accidents – if not designed, instrumented, and operated properly 100% of the time.

I also was faced with a similar, manually-operated installation. Mine was dealing with liquid Phenol – a substance not as immediately hazardous as your HCL, but one with toxic and serious medical consequences. Please refer to the attached Excel workbook where I sketch a possible safety oriented installation.

You have not described your total installation, and this is important to note. I believe you are unloading 35% (wt) HCL from tanker trucks directly into a storage tank (20,000 gallons capacity) with an acid scrubber on its vent pipe utilizing what the old timers (me included) call an “acid egg” type of transfer. This is the way sulfuric and hydrochloric acids were transferred all the time long before there were centrifugal pumps, stainless steel, instruments, etc. Note that the storage tank that I sketched shows what I would describe as the MINIMUM in safety pressure relief devices required for such an operation,. I have not shown your vent scrubber because I don’t believe the vent scrubber is the real important hazardous issue here. Storage Tank Over Pressure is the main hazard to be avoided at all costs – especially in your case where you can’t count on the higher tensile strength of steel. You must ensure that any air pressure regulator failure upstream does not cause any over pressure.

You must also always consider that there is a possibility that a HIGH HCL level reading read falsely as a LOW level can cause a deadly and serious disaster in overfilling the storage tank with liquid and causing a total tank rupture if the vents cannot safely relieve the hydraulic pressure built up. The tank must have redundant high-high level alarms and shutdowns, in my opinion.

Note that I installed a flow switch device that senses the potential break-through of liquid followed by air and that this signal immediately closes a block valve in the storage tank inlet. In the event you don’t have enough travel time in the unloading line distance indicated, then you can resort to an intermediate hold tank (installation #2) that gives you sufficient residence time to actuate the block valve. All this required time must be accurately calculated and allowed for. Please excuse my bad sketches. I consider this such an important subject and hazard that I have done the sketches very quickly to answer this post as soon as I could.

Your vents should be sized to handle safely the total, maximum air quantity that can flow through the air regulator upstream of the tanker trucks. I would insert a restriction orifice downstream of the regulator(s). I would also use a 2-stage air regulator with a lock-out on the set pressure to ensure that it complies with calculated values for the installation. I would tag all these instruments as “critical”.

You have a formidable job ahead of you and I recommend that you seriously spend more than enough time to contemplate, identify, and prepare yourself to mitigate all credible and possible hazards that can occur in this operation. You cannot afford a rupture and/or a subsequent spill out into a human environment.

I encourage you to take this project as seriously as you can, considering that your supervisors have picked you for this very, very important assignment that can’t be taken lightly. Don't let them down. They must think highly of you and your capabilities for them to have picked a rookie for this application. Use every resource and experienced engineer/operator at your disposal. Research the operation’s history and seek out Lessons Learned in order not to repeat past mistakes and correct for past errors/oversights. Do not give in to installing a “cheap” operation with a minimum of instrumentation. If you can seriously point to, and defend, a safety required instrument, do so – and do it with all your ability to stress safety and proper engineering design and operations. Defend your position on safety and proper operation; your supervisors can only admire and respect you if they are worth their salt – and I suspect they are.

Good Luck with good, safe engineering.
 HCL_Acid_Egg_Transfer.xls   47.5KB   405 downloads

[Art Montemayor]

Steve:

In my hasty attempt to quickly furnish you with something, I omitted two important additional recommendations:

1) Employ the MOC (Management of Change) methodology and all of its documentation - especially calculations & drawings - requirements;

2) I would apply at least two (2) Hazops to this project - one preliminary Hazop immediately after the generation of Prelim P&IDs and another after the issue of P&IDs for approval. I would also hold a PSSR (Pre-Startup Safety Review) with all operators and supervisors involved in this operation and part of your plant, prior to the initial startup. I would not attempt a startup without a PSSR.

I hope this helps.