I’m at the start of my vacation, but I find it necessary to first make a point about the press release the city issued yesterday concerning the sewage plant (see previous post.)
The press release blames a back-up generator, and other media seem to be jumping on that explanation as the cause of the failure. They’re wrong.
The failure on the generators was merely the proximate cause of the failure, but doesn’t explain why we are without a sewer plant for over a year. See, the plant should have been idiot proof—that is, no matter what combination of stupid mistakes, bad luck and acts of a vengeful god conspired to bring down the plant, it should have been designed in such a way that it would be impossible to flood out the equipment.
As I explained in last week’s feature, that means a passive—meaning no moving parts, no machinery— a passive bypass should have been built at the top of the well. And there was no bypass in the plant. This is a fundamental, bone-headed, design error.
Think of the bypass as the holes just below the rim of your bathroom sink—you accidentally leave the stopper down in the drain, and the sink fills up, but rather than flow over the top of the sink and onto your floor, destroying your woodwork and flooding the rest of the house, the water instead goes through the holes, and down the drain safely.
Likewise, as sewage filled the well at the sewage plant, instead of flowing out over the top and drowning the equipment in the basement, it should have gone through a bypass, right to the tail pipe of the plant itself, out to the harbour. We’re talking about a 20-metre long segment of pipe, is all. Not rocket science.
Had there been a bypass, the plant still would have failed, but safely so, without destroying all the equipment in the basement. The problem could have then been identified, and the plant put back to working relatively quickly—not in a year and a half.
Until I see otherwise, I’m convinced that this design error is precisely why the city is not releasing the full forensic audit.
Now, on to vacation. I’ll follow-up on this as I find time.
Related Stories
How the sewage plant broke
This article appears in Jul 2-8, 2009.
It is difficult to imagine any design intentionally leaving water-vulnerable equipment in a sewage system under the hydraulic grade line of the collection system. When mechanical/electrical equipment is allowed to stay in a vulnerable location, it is designed to withstand inundation. Normally, there is a path of last resort – untreated sewage running down the street until the power comes back on is a better worst case scenario than that we are now enduring.
Perhaps the “difficult” and “complicated” dry weather diversion, now being constructed, was not specifically asked for in the Terms of Reference that the builder was obligated to satisfy? Perhaps it can serve to provide the common sense solution Mr. Bousquet advocates, to protect the plant from future similar events?
Water ALWAYS runs downhill, mechanical systems ALWAYS fail (eventually, and especially when very new or very old).
I am not familiar with the details of this system, but these questions would logically seem to need to be answered by the audit.
Tim, it’s a cover up. Both sides are to blame so they’ll probably share the cost. I understan fromthe release that design mods for Dartmouth & Herring Cove have not yet been made. Get a copy of the contract and see what is in there.
If the contractors have been briefed and have a copy I see no reason for the report not to be released.
Tim, I agree with the passive “last resort” bypass, but there is also the issue of this large iron valve that you mentioned last week. It’s unclear to me – I’ve never seen plant schematics – as to whether this valve is normally supposed to be fully closed. I’m guessing that it probably is. In which case by what kind of perverse design did it end up slightly ajar, thereby allowing all the sewage and water to flood out the dry well in the first place?
Brewnoser nailed it – mechanical systems always fail. However, laws of nature don’t. In particular, for a system of this sort gravity is your best backup – the passive bypass being one, and another being that mechanical valves are so designed that when shit happens (literally) that they assume a position best suited for minimizing damage.
I tend to agree with Joeblow that we’re looking at a coverup in the making. Neither side stands to gain from a legal battle; both stand to gain from downplaying (read hiding) mistakes they made and from emphasizing that it was an unfortunate act of God. I expect they’re feverishly trying to concoct a palatable explanation for why the valve was open.
When all is said and done the city’s lawyers aren’t *my* lawyers. I figure if every adult in HRM chipped in 5 bucks that we could hire some decent lawyers of our own to shake some trees. Never happen but it’s an appealing thought.
To Realist in Dartmouth – The 8 ft. diameter valve is at the inlet to the plant from the sewer line so it would vary from full open in normal times to close to closed whrn there is a heavy rain storm and the CSO’s are diverting flow to the harbour.
The valve is at the interface with the wet well where the five pumps are located. In Tim’s article he said it was the dry well but the section through the plant calls it the wet well.
I would agree that the plant should have included the gravity overflow feature as the CSO and likely we will see that added before this plant is put back in operation and added top the other palnts as well.
As Brewmaster indicates the level of water was very close to “Flowing down the street”. If it had overflowed it would have flooded the Dockyard property.
I have never seen an explanation of why the flood only went to the height it did and never overflowed. Was that just “good luck”?
http://decisions.courts.ns.ca/nssc/2003/20…
http://decisions.courts.ns.ca/nssc/2003/20…
http://decisions.courts.ns.ca/nssc/2003/20…
found this in my travels about Carl Yates