Every profession has its
inside lingo, expressions that mean something only to practitioners. In the disciplines of electrical and
mechanical engineering, one such expression is to “go open
loop.” The loop referred to is a
feedback loop, a concept that control systems of all kinds use to regulate quantities
such as speed, flow rate, and pressure.
A well-designed feedback loop works as a clever automatic hand on a
control valve to maintain constant pressure in a gas supply line, for instance,
in the face of variations in demand for gas or delivery pressure from the high-pressure
gas main. But in order to work, the
feedback loop must be closed, or complete, all the way from the regulator that
controls the pressure, say, through supply pipes to the sensor that tells the
system what the pressure is, back to the regulator. If the information flow is interrupted
anywhere in the loop, the regulator usually goes to an extreme and jams, which
can lead to dire consequences. So if you
ever hear an engineer talking about a person who lost control of his temper as “going
open loop,” that’s where the expression comes from.
On Sept. 13 of this year,
dozens of people were injured, one was killed, and over a hundred structures
were damaged in and around Lawrence, Massachusetts, when a natural-gas supply
line’s pressure soared and turned pilot lights into blowtorches and stove gas
burners into towering infernos. The
National Transportation Safety Board has released its preliminary report on the
cause of the disaster, and it looks very much like it was a classic case of
That afternoon, work crews
were in the final stages of performing a tie-in to connect a new set of plastic
gas-distribution pipes to the system, and to decommission a set of old cast-iron
pipes that dated back to the early 1900s.
While many modern gas distribution systems place individual regulators
at each customer’s location, the older systems such as the one in Lawrence used
low-pressure gas (about 0.5 pounds per square inch gauge, or psig) in the distribution
pipes. To regulate this pressure,
control-system sensors were placed on the pipes and fed their data back to
regulators at the junctions between the high-pressure transmission pipes and
the low-pressure distribution pipes.
The instructions to the work
crews did not say anything about what to do with the sensor signals coming from the old cast-iron pipes when
the gas was switched over from the old system to the new plastic one. So when the crews shut off the manual valve
that fed the old pipes, the sensors in them were still connected to the
regulator that was now feeding the new plastic pipes, which were connected to
most of the homes in the affected area.
It’s easy to see that cutting
off the valve broke the feedback loop.
The control system saw pressure falling in the old pipes, so it opened
up the regulator. But the higher
pressure wasn’t being sensed, because the crews hadn’t been told to switch the
sensor signals at the same time they switched the pipes.
Pressures are monitored
constantly on the system, but only at Columbia Gas headquarters back in
Columbus, Ohio. Overpressure alarms went
off shortly after 4 PM at the monitoring center, but the dispatchers there had
no way of shutting off the system. All
they could do was to try getting in touch with the technicians in Lawrence to
tell them to shut off the gas. Even
though that was done about a minute after the alarms went off, it wasn’t until
4:30 that the regulator supplying the overpressure gas was shut off. By then most of the damage had been
An article in the Boston Globe last week describes the
anguish of Lawrence residents who are still waiting for Columbia Gas to restore
gas service. Many homes in the area are
unlivable without gas heat, and as cold weather intensifies the situation is
getting intolerable. The company’s
self-imposed deadline of mid-November for having all customers back on line
recently slipped to Dec. 16, and there are many complaints about poor
communication regarding repair-work schedules and shoddy workmanship once the
crews arrive. Columbia Gas is going to
be paying for this mistake for a very long time, as are the affected residents
The behavior of Columbia Gas
approaches a poster-child status of how not to run a utility. You can bet that when the gas company was
locally owned and operated, the monitoring center in Massachusetts probably also
had the ability to shut off the gas. But
when consolidator Columbia Gas purchased the system, the move of the monitoring
operation to Ohio was probably done to save money, money which was not spent on
a corresponding control system to allow Ohio monitors to shut off regulators
and valves remotely.
Work-crew training and
planning also came up short in this disaster.
It’s sometimes hard to say exactly how much knowledge technicians and
their immediate supervisors should have about the technology they are working
on. There’s no need for every lowly tech
to know enough to design the entire system, for example. That’s what engineers
are for. But on the other hand, you
would hope that the person in charge of the work crew would know enough to
realize that if the sensor signals were not swapped at the same time as the
pipe systems they were connected to, there could be trouble. Presumably the company has done this sort of
thing before successfully, so it may be a case of an isolated incompetent
worker rather than a systemic issue. But
clearly, more explicit instructions and better training are needed.
Finally, Columbia Gas hasn’t
exactly covered themselves with glory in the public-relations department. Gas utilities are by nature local monopolies,
and it is easy for them to act like what they are, namely the only gas company
in town. Unfortunately, in misleading
their customers as to when gas would be restored, they have created a lot of
ill will that more careful and cautious scheduling could have avoided.
Let’s hope for Lawrence’s sake
that it’s a mild winter in Massachusetts, and that everybody gets their gas
lines working again before it’s too much colder. And for all you other gas utilities out
there: don’t act like Columbia Gas.