It Pays to Keep Your Pipes Clean
Keep your pipes clean and your ammonia refrigeration system will work like a charm. I can't tell you how many times I have heard that in all the years I have been working in this industry. But, I still find that it bears repeating.
The IIAR Ammonia Refrigeration Piping Handbook spells it out pretty clearly:
Check all pipe for rust and foreign material when the pipe is purchased. Return any pipe that is not clean. Store pipe under a roof or protect it from rusting and from blowing debris.
All pipes shall be capped on both ends with a color-coded solid yellow cap to prevent dust and dirt from entering the pipe. All pipes should be tilted and hammered (to loosen any scale) and swabbed out before installation. Avoid allowing dirt, sand, water or other items in the pipe during field construction.
When possible, purchase only clean pipe with closed (capped) ends.
When making welded joints, backup rings are permitted, though not necessarily recommended. Backup rings keep weld material out of the pipe during welding, but they may also act as a small dam in the line. In those lines where this is undesirable, use consumable rings (designer's discretion).
I should say at this point that I am not a fan of backup rings and I don't recommend them. But that's a subject for another column. For now, we will simply agree that new systems and older systems alike do a better job if the pipes are clean. Let's look at a couple of examples.
Lubricant ChangeI know of one system that's about 30 years old. It's had a number of additions and changes over the years. The original plant had reciprocating and rotary compressors lubricated with standard naphenic lubricants. It's safe to say that the lubricant had migrated pretty much throughout the system. A few years back, the plant converted the reciprocating compressors to screw compressors. Each screw package now has a special 10 micron dual-lube filter. In all probability, the original compressor's filters did not perform because of dirt in the system.
About five years ago, the plant made some other changes, including a swap to a PAO lubricant. A couple of years later after the oil conversion was complete, the seats and discs in the stop and check valves began to wear rapidly. The company called me in and I took a closer look, and it seemed fairly clear that the damage appeared to be erosion.
The plant had several places where velocities in the valves and piping exceeded 20 ft/sec. Dirt and rust were evident in an ammonia pump. But, what baffled us was: Why was this occurring now, about five years after the conversion? If there was going to be a mixing of oils, why didn't it happen after the conversion?
PAO lube will slowly dissolve wax, and some was probably left behind by the naphenic lubricants. Undoubtedly, it was hung up in various traps, possibly backup rings, and improperly installed globe valves. PAO also loosens rust, which was likely in the pipes from the time they were first installed. The dirt and rust then moved via the wet return to the recirculators. Each of the additions and changes to the system apparently added to the problem.
Contamination in Lube CoolerIn this instance, several screw compressors were converted from injection cooling to thermosyphon cooling. The thermosyphon vessel was located about 40' above the lubricant coolers. Soon after startup following the conversion, the tubes in the lube cooler started to leak.
When we took a look at it, we found that some of the tubes had been damaged, apparently by erosion. Why? Well, even though the system was not very many years old, it had been contaminated with rust and dirt. We were able to correct the problem by installing filters ahead of each lubricant cooler.
These are all expensive lessons that the result in unnecessary maintenance and lots of strainer and filter cleaning.
For existing systems, one solution operators can try is to install dual, low micron (maximum 15) filters in the main liquid line at the high-pressure receiver. Another idea is to install an ammonia still at each recirculator, making sure that the inlet to the still is taken from a connection at the bottom of the recirculator. There also are companies that provide a cleanup service for ammonia systems.
But, the best solution is to head it off before it becomes a problem. Some folks may not want to admit this, but sometimes the crew is just not as careful as it ought to be when system piping is being installed. Those contractors I've talked with who absolutely insist on clean pipe from their suppliers, and proper care from their installers, have found that it's a practice that pays off. They tell me that call-back costs have dropped from about 2 percent of the project cost to almost zero.