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Process Safety Moments

This page contains brief descriptions of situations that have occurred at process facilities which can teach important lessons. Questions are posed to help focus on the key points.

Field Tours by PHA Teams Are Beneficial

A PHA facilitator working with an experienced PHA team convinced the team that a field tour prior to beginning a study was good engineering practice. The team reluctantly agreed to the tour. The facilitator emphasized the need to look at the facility with “fresh” eyes as they made the tour. During the tour the team found several process safety concerns that could have had serious consequences. In one area, they found several broken and rusted grounding wires that connected instrument cable aluminum conduit to instruments. None of the grounding wires had been marked for repair or replacement even though it was obvious the wires had been separated for some time.

As the team toured one building, they came to a relief valve that discharged through the building wall to the outside area. During the tour outside the building, it was found that the relief valve discharge line was routed to the ground but was then buried in about 3” -4” of asphalt from the neighboring roadway. The roadway had been resurfaced about 4 months prior to the PHA and no one had detected the blocked relief valve discharge line prior to the PHA.


Do your PHA guidelines require the PHA team to participate in a field tour prior to conducting the PHA?

Investigation of Potentially Hazardous Situations by Facility Personnel

A company had a waste tank, normally receiving water with trace amounts of process material, which began to make unusual rumbling noises.  An operating supervisor, operator, and mechanic made a field trip to determine the problem.  After checking the tank and adjacent equipment, they started back to the operations building. The tank exploded violently when they were only about 50 feet away from the tank. The fire ball from the explosion killed the operator and the mechanic; the operating supervisor survived because he was shielded by the other two personnel from the direct effects. The accident investigation uncovered the fact that a process line tie-in connection was incorrectly piped into the tank and the isolation valve had been inadvertently left open following a shutdown.


Have you established guidelines for when and how it is appropriate for facility personnel to intervene and respond to operational problems?

Key Components Of Critical Equipment Should Be Included In Mechanical Integrity Programs

A company developed an extensive inspection program for pumps that handled a highly flammable material, including double mechanical seals with a monitored seal flush system and pump deadhead / cavitation protections to reduce the potential for loss of containment due to mechanical damage of the pump or seals. Over a weekend, a pump seal failure occurred leading to a large spill and a fire that took several hours to bring under control because the primary isolation valves were located in close proximity to the pump and could not be operated.

The pump seal failure was caused by the catastrophic failure of the pump motor bearings and the drop of the entire shaft assembly (motor and pump shaft) which caused major damage to the double seal assembly leading to the large release. Periodic monitoring of pump motor vibration was being done but failed to detect the bearing condition.


Does your mechanical integrity program take into account all the essential parts of critical equipment that could fail and cause a process safety incident?

Reliance on Pressure Safety Valves (PSVs) and the importance of a Preventive Maintenance (PM) program.

PSVs had been installed 5 years prior to a process hazard analysis study that was evaluating them. The study team questioned the number of demands that the PSVs faced from various multiple hazard scenarios. On consulting with the maintenance department, it was determined that the valves had yet to be tested and little was being done to track PSV failures. The facility realized that the PSVs could not be claimed as safeguards until such time as a PM program was in place for them.

Field Tour for an Existing Plant

PHA teams may try to avoid a field tour prior to starting the PHA, claiming that all the team members are familiar with the process and have years of operating experience. However, a field tour with either a new team member or with personnel who will provide "fresh" eyes can prove highly beneficial. In one particular case, an experienced PHA team was conducting a field tour for a PHA which included an enclosed building that housed part of the process and had several tanks with relief valves discharging to the ground outside the building. After the interior tour, the team checked the discharge locations for the relief valves and found that one of the relief discharge lines was now encased in almost 4" of asphalt road mix. The access road that ran beside the building had been resurfaced about 6 months earlier and no one had observed that the "overspill" from the resurfacing operation had traveled downhill and collected around the discharge line.


When you conduct field tours for your process PHA do you include newer members in the tour or encourage the team to conduct the tour as if it was their first time in the area and really challenge what they are observing?

Relying on Human Intervention as a Safeguard

A company wrote a procedure for an operator to go into the field and close a manual block valve in an elevated line to mitigate a loss of containment event involving a hazardous chemical. To reach the valve, the operator had to first don a SCBA (Self-Contained Breathing Apparatus) unit to protect against possible exposure to the hazardous chemical before climbing a vertical, caged ladder to reach the valve. A trial was conducted to determine how long it would take for the operator to take reach the valve and stop the leak. It revealed that the operator could not reach the valve because the SCBA unit would not allow the operator to get past the cage on the ladder.


Are field trials conducted for your critical safety procedures to assure that operating personnel actually are able to perform the required tasks and do so in a reasonable amount of time so that they are not put in jeopardy during the incident that they are trying to address?

Effects of Extreme Environmental Conditions on Process Safety Controls

A batch pilot plant operation involving a highly flammable solvent operates during daytime shifts, and is unmanned during "off hours". The unit is housed within a building that features a local Programmable Logic Control (PLC) system with mostly local alarms, with select alarming to the supporting primary process plant which is constantly operating and attended.

A serious near miss occurred one evening when ambient temperatures dropped and remained well below 0°F. The last operator to leave pushed the close button on a roll-up door, which happens to be very near and facing the PLC module. The operator left the site not noticing that the doorway failed to close, and fully re-opened due to an object in its pathway (much like our home garage door openers would operate).

From a review of the alarm history recorded within the PLC, the incident investigation concluded that spurious signals and false control inputs occurred due to extreme low-temperature effects on the PLC module circuitry. The most serious malfunction sent a "request signal" to deliver flammable solvent from a large remote storage vessel to the pilot plant receiving vessel, sending solvent for ten-minute increments (as programmed) and eventually delivering enough solvent to overfill the pilot plant vessel. Luckily, valve lineups were such that the overflow was diverted to connected vessels and the incident did not result in a release from the receiving tank's atmospheric vent to the building floor.

Corrective actions involved consultation with the PLC manufacturer, who explained that such malfunctions could occur given extreme low temperature exposure (the module was specified and purchased for indoor application). Also, alarming to the primary plant control room was rearranged to alert the constantly-attended location that flammable solvent was flowing to the pilot unit. The operating company also rearranged the roll-up door controls to require that the operator remain and hold the closure button until full door closure is verified.

Prior Process Hazards Analyses (PHAs) considered extreme weather conditions as "external events"; however, safeguards were documented as: "The process is located indoors with HVAC service". Safeguards were deemed adequate by the PHA Team.


  • As we perform our PHAs, are we certain that we fully understand the limits of control systems when exposed to extreme environments, or do we just assume that since it was specified for its intended service, its functionality is a given?
  • Do we adequately address failures of environmental controls (such as HVAC system failures or an inadvertent exposure such as an open door) in evaluating the hazards that can result from extreme exposures to the process?

Field Tours as part of Process Hazard Analysis (PHA)

PHA teams may question the need for conducting a field tour of an older existing facility when most or all of the team members have years of experience in the facility. During a field tour in preparation for a new baseline HAZOP study for an older facility that had not used HAZOP methodology for their earlier PHAs, the PHA team was walking through the operating area during the field tour and one of the team members noted that there was a small drain valve that was missing a plug. That observation led to a closer inspection of other drain valves on equipment and vessels and the discovery of a pattern of missing plugs or caps on drain valves, including several 2" quarter turn ball valves.


Are your field reviews done with a "fresh pair of eyes" or a "business as usual" attitude?

Does your PHA assume that caps, plugs or blinds are installed on the drains and vents without a field check?

When conducting field tours, do you take notes on the field tour findings and include them in your PHA documentation, especially any findings that warrant a PHA recommendation?

Flammable Atmospheres in Tanks

Several atmospheric storage tanks handled a product that emits a flammable vapor if it has not fully reacted. One of the tanks was taken out of service to patch some corroded areas. The product side of the tank was isolated. A hot work permit was issued and the atmosphere around the tank was checked for flammables and found to be in order.

Was it safe to proceed with welding? Absolutely not! While the atmosphere around the outside of the tank was checked, noone considered that there could be a flammable atmosphere inside the tank. Even though the process connections were locked out, the gas phase connection was left open to the other tanks. Noone recognized that flammables in the atmosphere of one tank could transport to the other tanks.

A 2010 Chemical Safety Board (CSB) study found that there were at least eleven instances where a severe injury or fatality resulted from welding on a tank containing a flammable atmosphere.


Is this issue covered by your procedures?

Contractor Awareness of Hazards

The fourth floor of the main processing building at a facility was being repaired. Cement was being pumped from the ground level. The cement truck was located next to the facility's tank farm. A crane was used to hold the cement hose above the tank farm which contained two large storage vessels of chlorine gas. When asked, the cement truck driver and crane operator did not know what was contained in the vessels or if the contents were hazardous.

Repairing floors and pumping concrete are not covered operations under OSHA's process safety management (PSM) standard. However, performing work within or in close proximity to a PSM-covered process requires proper awareness of the hazards present. In this particular situation, the cement truck driver and crane operator were subcontractors. The main contractor failed to share basic information about the process with those companies working for it.

The provisions of the PSM standard, and certainly the impacts of the hazards of an operation subject to PSM, apply not only to employees but also to visitors, contractors and subcontractors. Anyone present on-site needs to know the hazards present in areas where they will work and adjacent to them, and the provisions of the emergency action plan that pertain.


Is this true for your facility?