Testing High Pressure Cutouts

The issue of how to test high pressure cutouts in a refrigeration system can be a contentious one. And although there is no simple solution, there are many guidelines that offer direction.

The Mechanical Code requires that every system have a cutout, but ASHRAE 15, considered the industry standard, forbids a shutoff valve between the source of the pressure and a shutoff switch. Meanwhile, IIAR-5 standards provide a clear definition for how to test a shutoff switch.

“IIAR-5 says that you close the discharge valve and you let it trip,” said Joe Pillis of Johnson Controls. The IIAR-5 standard reads as follows: A high pressure cutout should be tested and calibrated as required, plus discharge pressure must be increased slowly until the trained start-up technician can confirm that the cutout operates at the required setting. If the high pressure exceeds the cutout set point at which it is intended to operate, the compressor shall be manually stopped. It is unacceptable to adjust the high pressure cutout set point for testing purposes.”

“The margin between the high pressure cutout and the safety relief valve setting is important,” Pillis said. “The IIAR has taken a bold step in saying this is the way to do this.”

Whether the IIAR-5 standard is good or bad depends on your point of view, added Pillis. “The good thing is that you know it works,” he said. “The bad thing is that you must do it very carefully. There is an issue of how much margin you have between your high pressure cutout setting and the point where your relief valves may start to leak. If you’re playing around with the discharge stop valve and the pressure is rising, you need to have another pressure measurement checking that your transducer is reading accurately and that your switch is set accurately.”

“So you’re watching the gauge and the readout, bringing up the pressure, and if it doesn’t trip you don’t have long to get that stop valve back open before you have an ammonia release,” he said. “It’s a bit delicate because you may not have a lot of margin between your high pressure cutout level and your safety relief valve setting.”

There are alternative methods for testing a switch, but each has a downside, according to Pillis.

One alternative is to remove the transducer, depressurize the package and use a nitrogen bottle with a pressure measurement device. A bench calibration can then be performed by slowly increasing the pressure with nitrogen and watching the safety trip electronically on the panel.

“This gets around ASHRAE 15, but not IIAR-5,” Pillis says.

The downside is the complexity of the method. The refrigerant must be removed from the package, isolated and pumped out. The transducer must also be removed, yet left wired to the panel. After applying nitrogen pressure and watching it trip, the transducer must be hooked back up, and the package evacuated and recharged with refrigerant.

“There are a lot of moving parts,” Pillis says. “You’ve got to have enough flexibility in your wire so you’re able to keep the transducer wired while using the nitrogen bottle.”

A second alternative is to install an additional high pressure cutout with a shutoff valve below it. The second cutout can be locked and tagged out. It can then be tested with a nitrogen bottle to make sure it trips and then locked open.

“The problem is it doesn’t meet ASHRAE safety requirements,” Pillis says. “Plus, although you might be comfortable that it will trip by testing the parallel cutout, you haven’t tested the actual one.”

The questions surrounding the methods of testing high pressure cutouts can become hot topics, because if the procedure is not done correctly, it can result in an ammonia release. “It has happened that a high pressure cutout was not set correctly and there has been a reportable release. Nobody wants that,” Pillis said. “So that’s where the concern comes in. How do we make sure the cutout is set correctly and in compliance with codes?”

Pillis suggested an acceptable alternative that would require modification of the ASHRAE 15 standard.

“A safer way would be to allow the shutoff valve to be between the pressure source and the high pressure cutout,” he said. “But it would have to be locked out and tagged out with administrative controls that guarantee that it’s operating correctly when the compressor is in operation.”