This Month’s Puzzler
A welder at our site died from asphyxiation. He came in early to install new pipe brackets in the corner of our gas plant. The gas chromatograph is under the new rack. When I arrived to write the initial report after his body was removed, I noticed that the welding screen blocked in the corner. A door is nearby but always is closed as per our safety standards. The third shift operator didn’t notice the welder at the end of his shift, although the unit logbook shows the welder signing in at 6:58 am.
The first shift operator found the body during his walkthrough after the morning meeting about 8:45 am. The operator called for help and waited for the rescue team to arrive in self-contained breathing apparatus gear. The welder was pronounced dead on the scene.
My initial findings are a little unsettling: 1) recently, the N2 supply solenoid valve that purges the chromatograph was changed; 2) some PLC programming was altered to add an extra sampling point — the PLC operates the sampling pump and valves and an N2 purge used to flush the sample line; 3) the sampling line vent was recently shortened because it broke off — N2 and gas could be purging inside the building; and 4) training records show that only the operators were provided safety training — recent budget cuts eliminated training for mechanics, engineers, managers and welders.
I discussed my findings with the safety manager and the head of process engineering. The safety manager says I nailed it but the process manager scoffs, dismissively declaring that nobody could die from a tiny purge line. What do you think? If N2 is the problem, what do you reckon is causing the issue? What can we do in the short term to make the pump room safe?
Check The HVAC Air Exchanges
An oxygen analyzer — whether a permanent installation or procedural use of a temporary/portable device prior to entering the room — seems like a good idea in an enclosed room where asphyxiates are present.
As far as the process manager’s comments go, I am sure the O2 concentration could be readily calculated based on the number of air exchanges provided by the HVAC system for the room, the room dimensions, and the maximum flow rate of the N2 from the line. N2 and air have essentially the same density, so it’s not likely the N2 will concentrate in low or high spaces in the room unless there is zero air movement. If this were the case, the calculation mentioned previously would clearly indicate there’s a potential for a low oxygen environment.
David S. Tascarella, research scientist
Dow Corning Corp., Midland, Mich.
Get Corporate Help
Remember, it’s not what you believe or even what you think, it’s what you can prove. I don’t know what the politics are like in your plant but normally the process manager has more clout than the safety manager. You must prove your point. Even then, expect some pushback.
Many years ago, at a company that’s changed hands more often than a 1963 dollar bill, I ran into a similar circumstance. We’d been burning through O2 coils. I had the science. I had the lab data. I had the trend data. I had everything. I even predicted a failure. It took three years to convince managers to consider changes. The opposing superintendent attacked my conclusions and shortened my career at that company. That’s what you’re up against. Hopefully, you’ve got some support from corporate when you publish your report.
First, you must calculate the concentration of N2 in the enclosed area. A simple orifice calculation, assuming critical flow, will give you the supply rate based on a starting pressure (P1). Flow is critical, i.e., choked, because the end pressure (P2) is atmospheric, P2< 0.51P1 means choked flow except for thin-wall openings (refer to http://goo.gl/pLMXLq). Next, you must estimate the diffusion rate to surroundings. Granted this is all theoretical but math is our bread and butter in engineering so the proof is in the numbers.
This calculation also can be used to evaluate options for reducing risk in the pump room. In this situation, calculations aren’t enough. You must use O2 analyzers for the room.
For the sake of better safety, I recommend adding O2 monitors during normal welding operations because welders use inerting gas. In addition, why wasn’t a job safety analysis conducted prior to doing the work? This is standard practice at many job sites. Lastly, the operator in an area and that person’s supervisor always are responsible for new people in that area. The welder shouldn’t have been allowed entry to any job site until an operator trained to spot hazards there has inspected the area and given the okay. This accident suggests that safety training doesn’t include managers and support staff like the welder.
As for the root cause, first look at the programming of the PLC. If someone switched the contact from normally closed to normally open, then it’s possible the N2 supply valve remains open all the time. I’ve seen air lines in a packaging room left open by design; the plant used over 750 scfm of air even when the room wasn’t packing a thing.
Making the room safe from asphyxiation may be a problem considering it’s in a gas plant. Issues involving electrical area classification (API-500) demand careful assessment. Propping a door open means that flammable gas vapors could more easily enter the room.
Dirk Willard, consultant
Wooster, Ohio
October’s Puzzler
The pipeline company I work for replaced some centrifugal pumps (CPs) that performed well for over 20 years with progressive cavity (PC) pumps at an old oilfield gathering station (Figure 1). After two months’ operation, the PC pumps suddenly failed. Because the station is down, the company decided to do some much needed maintenance. While sniffing for leaks before construction could begin, we discovered several, in particular around the new pumps. In addition, it looks like regulators will ding us for over-using our flare and for complaints about hydrogen sulfide emissions because the flare failed to ignite properly.
My first problem is finding out why the PC pumps failed and what to do about the flare. We had relied on a world-renowned engineering company for the previous work; it had highly recommended a particular make of PC pumps. However, my company opted for a different pump vendor because maintenance insisted on the brand for “consistency” in the equipment inventory. The engineering company also installed a new flare ignition system about the time of the pump replacement.
Why do you think the CPs ran so well for so long and the PC pumps failed? Is the flare problem somehow connected with the pump failure? What other thoughts do you have?
