“By becoming interested in the cause, we are less likely to dislike the effect.” — Dale Carnegie
I love the English language, even though I make a hash of it on occasions. I love its complexity and irregularity. I especially love the words. There are so many words that mean almost the same thing that the search for exactly the right word is like a treasure hunt. And on occasion, a single word or phrase can mean so many things.
So, when I talk about common causes, I could be talking about a diverse group of people coming together to tackle a problem that they all share—a common cause. Or in the area of process risk reduction, I could be talking about a source of failure that impacts all redundant components—a common cause. Or I could be talking the reasons for a process deviation that occur most often in a PHA—common causes.
The Role of the Facilitator in a PHA
“You mean that as a PHA facilitator, all you do is ask questions?”
Yes, when things are going well in a Process Hazard Analysis, usually in the form of a Hazard and Operability Study—a HazOp—all a facilitator should be doing is asking questions. And pursuing those questions until there is an answer. And when the answer sounds bogus, then asking the next question, which should be, with one eyebrow raised, “Really?”
Sometimes, though, the team is stuck for an answer. For instance, what failure in the process could cause a deviation in the node being considered? By deviation, I mean something like “high pressure”, “low temperature”, “reverse flow”, or “wrong composition”.
It’s not the job of the facilitator to know the answers, although a knowledgeable facilitator will be better able to detect bogus answers. It’s the job of the facilitator to know the questions.
So, when the team is stuck for an answer, it is helpful if the facilitator can suggest possibilities for the team’s consideration.
What follows is a list of some of those possibilities. In other words, some common causes.
Universal
Control loop failure. This can happen anywhere there is a control loop, and in any direction. We generally plan on failures to the deenergized state having a frequency of 0.1/year, while we plan on failures to the energized state having a frequency of 0.01/year.
High Pressure
There are more comprehensive lists, but for our purposes, these are the common causes:
In vessels
- Blocked in and external fire
- Blocked in during pump in
- Gas regulator fails open
In heat exchangers
- Heat exchanger tube failure
- Cold side blocked in while the hot side continues to run
In lines
- Blocked pump discharge
- Blocked in cold with heat tracing on or solar gain
Low Pressure
In vessels
- Blocked in with condensing vapor (steam equipment is especially vulnerable to this)
- Blocked in during pump out
- Blocked in while draining
In lines
- Blocked pump suction
Surging Pressure
- Pumps in series
- Blowers in series
- Double-diaphragm pump
- Hydraulic shock
High Temperature
- Steam regulator fails open
- Tracing fails high
- Loss of cooling water supply
- Chiller failure
Low Temperature
- Freeze protection failure
- Insulation damage
- Tracing fails low or off
- Heating failure
High Flow
Flow is typically just a concern in lines, not in vessels.
- Inadvertently opened manual valve, or manual valve that is opened too much
- Block valve fails open
- Downstream line rupture-struck by equipment
- Downstream hose rupture-drive away/pull away
Low Flow
- Inadvertently closed manual valve, or manual valve that is closed too much
- Block valve fails closed
- Plugged line or orifice
- Plugged filter media
Reverse Flow
- Gravity or siphoning
- Pump failure while pumping to high pressure/high elevation destination
Misdirected Flow
- Inadvertently misaligning valves
- Sequence failure
- Block valve fails to wrong position
High Level
The inverse of the statement about flow, level is typically just a concern in vessels, not in lines. High level results from either putting too much into the vessel or not taking enough out.
- Adding to a vessel with an unrecognized heel
- Adding to vessel faster than it can be removed
- Pump out or drain failure
Low Level
Low level results from putting too little into the vessel or taking too much out.
- Loss of source material
- Feed pump failure.
Loss of Containment
This is probably the most frequently encountered deviation because eventually everything leaks.
- Component failure (gasket, seal, valve seat, etc.) – leak
- Drain, vent, or sample valve inadvertently left open – spill
- Spurious operation of PSV or rupture disk
- Rupture from being struck by equipment
High Composition
High composition refers to a component that is supposed to be or expected to be present, but is present in a concentration that is too high.
- Raw material is received out of spec
- Inadvertently charge too much
- Inadvertently set feed rate too high
Low Composition
Low composition refers to a component that is supposed to be or expected to be present, but is present in a concentration that is too low. Stabilizers are an example of a component that could be present in too low of a concentration.
- Raw material is received out of spec
- Inadvertently charge too little
- Inadvertently set feed rate too low
Contaminants
Contaminants refers to a component that is not supposed to be or expected to be present, but could be, and would cause a problem if it were.
- Inadvertently charging wrong material
- Spotting wrong drum/tote/truck/rail car
- Raw material quality issue
- Heat exchanger coil/tube leak
High Mixing
- Inadvertently setting agitator rate too high, causing splashing or frothing
- Turning on agitator when level is not high enough to stabilize agitator, causing damage
Low Mixing
- Agitator failure
- Eductor failure
- Static mixer failure
- Insufficient eductor flow
- Loss of recirculation
- Mixing for too short of time
Stratification
Stratification has many of the same causes as low mixing. The difference is that low mixing is the failure to combine two or more components into a single mixture, while stratification results when a single mixture separates into two or more components. Examples include an emulsion separating into separate phases, or the solids in a slurry settling out.
- Causes for low mixing
- Low flow rate or loss of flow in slurry line
Know the Questions
We won’t pretend that this is a comprehensive list, but it is a good list of the causes most commonly encountered. As facilitators, we don’t have to know the answers. We do, however, have to know the questions. I hope this helps new facilitators to be better prepared to fulfill that role.
