Troubleshooting Screw Compressors
One thing I have come to the conclusion of about screw compressors is if you have a problem, it's not always readily recognizable. In a previous column, I suggested that one of the best ways to maintain a screw compressor was to have vibration analysis run on a regular basis. This type of preventive maintenance will, when performed by an experienced operator, determine the status of the internal components as well as the alignment and the motor status (that is, whether they have been lubricated recently) and determine if the motor has a soft foot. A soft foot normally indicates poor alignment.
Most analyses can determine the rate of decline of a screw compressor part, provided the analysis is performed on a periodic basis. Screw compressors often are more forgiving than other types of compressors. That means a bearing failure is something that usually happens gradually. I've known some vibration analysis operators who could predict the failure of a thrust bearing within the time frame of just a few weeks.
Screw compressors often will continue to operate well even with minor problems that might not be so obvious. Lubricant carryover is just one of those kinds of problems. When a screw compressor looses 3 or 4 parts per million of lubricant by weight, there may be a problem with the coalescers. For example, if you have a 500 TR compressor, it should not lose more than a gallon of oil in a week's time -- even if it operates nonstop, 24 hr/day (168 hr/week) at full load.
If the coalescers have been rated properly by the packager, the obvious problem is with the coalescer itself or the method by which it was installed. If the refrigeration system is dirty, then there's a pretty good chance that the coalescer is clogged with dirt. A fairly simple method of determining if that's the problem is to install a gauge on the oil-charging connection and another one on the purge valve on the coalescer portion of the lubricant separator. While the compressor is operating at full load, take a couple of simple measurements. If the differential between the gauges is greater than 3 psi, the coalescer probably needs to be replaced. If the pressure drop is less than 3 psi, there may be other problems with the coalescers.
There are other reasons for excessive lubricant loss. The O-ring that seals the coalescer against its support in the separator may be damaged or defective. Inspection requires the removal of the manway. If the O-ring is damaged, the coalescer support will have a streak of lubricant that can be seen with the aid of a flashlight. Just the slightest imperfection or scratch on the O-ring can permit a large amount of lubricant to escape. Although most of the lubricant will find its way back into the compressor, some of the oil will accompany the discharged ammonia.
Before investigating the coalescers, it's probably a good idea to check the vapor pressure of the lubricant. If you use synthetic lubricant in the compressor, go directly to the coalescers. But, if you are using a standard lubricant, I suggest you take a hard look at the lubricants that have been developed specifically for ammonia refrigeration, which have vapor pressures that are about one-tenth the standard oils. Thus, you lose about one-tenth the amount of lubricant that you might otherwise be losing!
There's another problem that is fairly common with screw compressors: worn out slide valves. This usually occurs because of improper adjustments to the level controls on the recirculators. The ideal way to maintain the proper liquid ammonia level in a recirculator is to employ a modulating feed system. This maintains a constant pressure in the vessel -- or at least close to constant pressure. However, many recirculators are fed with a solenoid and a hand expansion valve actuated by float switch. This type of feed must be adjusted properly or the result will be large upward pressure swings in the recirculator. This, in turn, will cause the screw compressor control to increase the capacity, thus moving the slide valve unnecessarily. The slide valve returns to its original position as soon as the compressor has reduced the pressure to the original setting. There is usually a way to slow down the slide valve operation. The lube lines to the four-way capacity control valve often have an adjustable orifice. That orifice should be adjusted so that the slide valve takes several minutes to move from one point to another.