Technician safety: Learn from the mistakes of others
This week we will look at specific jobs that were made dangerous.
Job history no. 1The place: Wilmington, Delaware; the year: 1960. We were starting a 150-hp compressor. All valves were checked for proper position for start-up. It was an ammonia system.
I told all personnel to stand clear of the compressor in the event of seizure, or worse.
It was worse. The three compressor heads exploded, throwing shrapnel out like a hand grenade. It was later found that the discharge check valve had been installed backwards.
Yes, the high-pressure safety had opened, but the compressor flywheel, weighing 200 lb, continued to turn and the internal compressor relief valve had too small of an orifice to relieve the tremendous pressure that had been created.
The only thing left on the compressor heads were the hold-down bolts. Fortunately, no one was hurt.
Job history no. 2Many years ago, I was called to a job at 4 Penn Center in Philadelphia. A compressor had been changed out and two of the three compressors were in operation.
This was a computer center, and the owners requested that the two compressors on-line not be shut down. Since the main power to all three compressors was common, there was power to the cartridge fuses to be installed to allow operation of the replacement compressor. This was a 460-V, three-phase, 60-Hz system.
The controls were in the “off” position for this compressor and, using proper procedure, I used fuse pullers to insert the fuses. The first of three fuses was installed — no problem.
Very carefully, I installed the second fuse. There was no contact between the first and second fuses. I wore safety glasses as required.
Suddenly the electrical panel on the unit exploded, blowing off my tie and knocking me back 5 ft. The building superintendent lost his hearing in one ear for two weeks. We also blew out the 600-amp breaker for the entire floor. I was rushed to the infirmary for shock, burns of the hands, and flash burns of the eyes.
This accident was caused by the lack of insulators between the phases. As the second fuse was installed, the metal fuse holders were less than 3/8 in. apart, which is the critical distance for 460 V without insulation between phases. The higher the voltage, the larger the gap requirement becomes.
The lesson learned is to always shut off all power, for your safety and others’. Most electrical codes specify that a disconnect switch be provided within sight of a unit. This is not always done.
If you are required to work on a system without positive electrical lockout, you either padlock the nearest disconnect switch or remove the load leads for safety. It may also be noted some controls are fed from a separate source. Check all leads before beginning work.
Job history no. 3In 1962, at the Amoco Building in New York City, I was supervising startup of a 1,000-ton chilled-water system.
Upon arriving one morning, I saw that the NYFD had surrounded the building. Someone had called the fire department because “smoke” was coming from the roof. It was actually steam from the water tower on the roof of the 20-story building.
My day wasn’t over. The discharge check valve on one of the compressors was chattering due to low gas flow when the compressor was in an unloaded position. All check valves should be sized at least one pipe diameter smaller than the discharge line to prevent this occurrence.
In any event, work was begun to remove the check valve for replacement. The compressor discharge valve was shut off, along with the shut-off valve between the condenser and the check valve. The bolts were removed from the top of the check valve and the round plate was taken off.
The 7-lb cylinder for the check valve was still in position, but not for long. Pressure was still in the discharge line between the condenser shut-off valve and the check valve. Suddenly, the check valve cylinder blew out and stuck in the ceiling. No one was injured.
The message is, be certain no pressure exists on either side of an object to be removed. This same rule applies when soldering any pipe, anywhere. If necessary, I have drilled holes in the pipe to ensure that zero pressure exists.
Job history no. 4It was in New York City, 1964: All belt-driven compressors are supposed to have guards covering the belts and pulleys.
Not so on this job. A mechanic climbing through a trap door reached inside the mechanical room and had all the fingers on his right hand cut off because the unit was mounted directly next to the opening, without a safety guard around the belts and pulleys.
This happened two weeks prior to my arrival at the jobsite. The guards for the belts and pulleys were in the corner of the room and still not in place.
Job history no. 5Change the scene to Vineland, N.J., 1958: I was assigned the startup of a low-temperature locker storage system.
This installation was provided with a field-mounted oil separator on the discharge line. It was properly mounted on a horizontal run about 10 ft from the compressor. It was cylindrical and approximately 20 in. wide by 30 in. high.
After system startup, all pressures and temperatures were perfect and pull-down was proceeding as expected. After two hours, I decided to take a coffee break. One step out of the room, I heard an explosion. It was the oil separator; one side of it was missing, and pieces of steel were all over the room.
Any manufactured, pressurized cylinder must have a certain wall thickness, which is determined by the cylinder diameter, operating pressure, and temperature. It was found, upon examination of the fragments, that the wall thickness was below standards on the one side of the cylinder.
I have no explanation why I left the room. Perhaps someone wanted me to live long enough to write this article.
Next week: Safety specifics in various hvac scenarios (or, how not to ruin equipment or freeze your toes off).