Leak Detection on Large Systems

Imagine, if you will, a 20-year-old kid. He has no formal training in the HVAC/R field, only ride along “training” and book learning as he reads along with his journeyman father with whom he's riding. One stormy summer evening, a service call comes in for an ice cream freezer down. After calling and talking to the site manager, the old-timer decides that this call is within the abilities of this fresh-faced kid, so he throws him the keys and says, “Tag!  You’re IT!” and wishes the kid well.

Flash to that same kid pulling into the parking lot and meeting an anxious manager at the door. “They’re all down. Everything in frozen foods is down.”

Oh boy!!! Here we go!

The heart-dropping sensation those words brought to me is beyond description, but it's never far from my memory. That night, I learned a lot. I learned that I was capable of much more than I gave myself credit because I managed to diagnose the problem (for the most part), and I learned that leak checking is a difficult job. The problem, by the way, was that the relief valve on the receiver had opened, dumping over 500 pounds of refrigerant into the atmosphere.

That is an example of a (mostly) non-preventable leak, although I do think that those sorts of leaks are somewhat manageable. We’re going to explore the various means of finding, repairing, and, possibly most importantly, preventing leaks on larger refrigeration equipment.

Let’s start by saying that leak detection on large refrigeration, like I work on every day, is very much a full-time job. With dozens of accounts, each one with multiple large pieces of equipment on-site, leak detection is an important part of a maintenance program and, sometimes, a 24/7 job.

For starters, what are we talking about when I say “large refrigeration equipment?” I’m mostly talking about supermarket rack refrigeration systems. These pieces of equipment can hold anywhere between a couple hundred pounds up to well over a ton of refrigerant. They’re made up of a central pumping station where several compressors work together to maintain a set suction pressure, a condenser unit that's typically mounted on the roof, the various fixtures where the product is stored, and, importantly, the interconnecting piping system.

A solid leak prevention system starts before a drop of refrigerant is ever added. It starts at installation with good piping practices. Piping should be installed so that it can move naturally with temperature changes without rubbing on supports. On top of that, valve openings and closings shouldn’t cause liquid hammer and result in either line movement or breakage (ideally, we'd work to eliminate that type of condition altogether). Any control lines also need to be properly and safely secured to prevent them from vibrating and breaking. If your system is poorly installed, you will have more leaks than if the system is properly installed.

For the most part, though, that is beyond our control. We’re service guys. We’re called when that installation breaks, and we can’t change how the equipment was installed. We just have to deal with it the best that we can. Sometimes we can make small changes such as securing capillary control lines with silicone to prevent them from vibrating against themselves and other things, switching to ‘armored’ cap tubes when we service a part of the machine, etc. However, these are generally very small changes in the grand scheme of things.

Let’s start with a new customer, and we can talk about a multi-step process to find, pinpoint, and potentially prevent leaks for that customer.

My first step is to do a customer interview and a quick, gross inspection of the equipment. I’ll talk to the customer about refrigerant leaks. Do they have to add a lot of refrigerant? Are leaks a problem for them? I usually keep nothing more than a flashlight in hand. I’m looking at refrigerant levels, looking for a pile of empty drums in the mechanical room corner, or a nice stack of fresh, full ones. I’m looking for puddles of oil or piles of cat litter under the equipment to indicate where a large leak may have occurred and hasn't been cleaned up. I'm looking for oil spots, smears, and trails on the piping and valves. I’m also looking for those practices I mentioned earlier where piping is visible.

Now, I’ve got an idea of what I’m in for. I’ve got 2” of dirty, oily cat litter under the unit, a dozen empties in one corner, and a fresh stack of full cans in the other. The customer is complaining that the ‘other guys’ can’t find any leaks but have to add gas once every week to 10 days and have started making excuses. One tech actually tried to tell the owner, “Well, you see, these new gases, the molecules are smaller than the old gases. That means that the molecules can just leak out of the pipes, so you just have to add gas sometimes.”

Shaking my head, I’ll hike out to the truck and fire up the leak detector.

When it comes to leak detectors, I’ve got some opinions based on years of field experience. I use the H-10PM or the Tif ZX1. While both have some faults, both have proven to be very reliable and consistent detectors.

So, for step 2, I'm going to turn that detector on at the truck and let it warm up. I'll go through its calibration out there so that I can be reasonably sure it's calibrating in a clean environment. The startup and warmup process of a leak detector is also a calibration process. With large units and large amounts of refrigerant, the entire building can be contaminated with refrigerant, so starting the detector up in that contaminated environment can be problematic.

If you get an alert as soon as you step through the doors, you have a leak. This is where you have to place a degree of trust in your instrument. It's telling you there is a leak in that building. It's up to you to find it.

Start a methodical search of the building, notebook in hand. If the leak detector alerts in a certain cooler or even a certain area of the store, walk away and revisit that spot, then make a note of the area and move on. We're just surveying right now; making notes allows us to come back later and work to pinpoint and repair the leaks. Also, while you're searching, keep a constant lookout for oil spots. You'd be surprised how many leaks I have found by the oil spot rather than the detector.

After all of the fixtures are searched and appropriate notes made, move on to the mechanical room and repeat the search. Then, move on to the remote condenser. Because you're usually outdoors in the wind when you inspect that condenser, your eyes are quite often a bigger asset than any leak detector. Look for oily spots, particularly places where dirt and dust have accumulated and are soaked with oil.

With the survey done, it's time to zoom in on each leak and make the repairs. Let's look at some methods to make that process more simple.

Finding a leak is part skill, part art, and all determination. As an art and a skill, it's something that you develop over time and with practice. However, a few pointers can help hone those skills.

First, and I know I've said this, but it bears repeating, trust your instruments. In this instance, your instrument knows better than you do. If it says there is a leak, you need to believe it and work to find that leak.  Another thing is that you're looking for a leak, so LOOK. Sometimes you'll see an oily patch or a weird bubble where one shouldn't be. It is those little details that reveal a leak.

If I'm prioritizing places to look for leaks, I'm starting with flare fittings. Between poor factory flares, vibration, and temperature-related expansion and contraction of those fittings, that's a high-likelihood spot to look. Follow that up with just about any mechanical or O-ring seal on a valve. Those are kind of high-percentage spots to look at, too.

Now, remember that survey we did? Dig that notebook back out and pick a spot. We’ll get to all of them. Maybe you noted that one was a large area, or the alert on the detector was particularly intense. That’s as good a spot as any to start. Head back to that spot and have a look around. Remember, we’re looking for oil spots and staining first. Nothing? Okay, now we have two directions to go in. If it is in a closed box like a walk-in, your leak detector usually isn’t going to be of too much use inside. The sealed nature of the box tends to concentrate refrigerant, making leaks appear larger than they actually are. Start by removing the covers for the ends of the evaporator coil. Again, look for leaks, then use soap bubbles to try to find them.

In open-type cases, it's a little easier to narrow down the leak. Pass over the leaking area back and forth, resetting the calibration on your detector until you've got a narrow band. That will narrow down the leak area, and then you can start using soap bubbles to pinpoint the leak.

Some people complain about finding little tiny leaks. Very often, the small ones aren’t the tough ones to find. Sometimes, you get a large leak that is blowing refrigerant far away from the source, and you’ll get alerts several feet away from the actual source of the leak. While this can happen almost anywhere, it seems more common in mechanical rooms because of the control hoses and the high vibration. First thing I’ll do if I suspect a leak like this is shut down everything in the mechanical room. Shut off the compressors, shut off any exhaust fans and start looking. First, without the machinery running, you may well be able to hear the leak. Without all of the air movement, it may be easier to locate the leak. This trick can be extended to the entire building. Shutting off all HVAC equipment can allow refrigerant that is being circulated to settle near the leak, allowing you to narrow in on it.

Repeat this procedure on each leak until every last one is found and repaired, then start all over again and repeat until no leaks are found on the storewide assessment.

Once we've got a leak-free system, take the time to clean up the mess in the mechanical room. Clean up that nasty cat litter, clean up the spilled and leaked oil, dispose of the empty drums, and stack any full ones neatly and secure them per local and national codes. Take a clean rag and wipe the oily dirt away from other repair sites. Clean equipment means that you can spot a leaky spot more easily because an oil spot is the exception rather than the rule.

Leak detection is a full time job.

You really have to think about the amount of gas we’re dealing with here. Think about how much room 30, 30-pound refrigerant cylinders take up? Want to jam those in your truck? Think about the cost of that refrigerant. Now, remember back to when you took your EPA certification. Remember the 35% EPA leak rate trigger? We need to keep leak rates down on larger equipment.

So, how do you treat a store that you regularly service?

Really, you use a similar process. You just don't normally have to spend a lot of time tracking down old leaks. Each month, on the preventive maintenance visit, the tech doing the PM does a leak check similar to the survey check outlined before. If the PM tech finds any indication of a leak, the tech takes corrective action. If the PM tech has the skills and the tools, he will repair the leak. If not, a more skilled tech should be dispatched to the site to correct the leak and to guide and educate the PM tech. You don’t want to let leaks go unrepaired, so any leak indication really needs to be followed up.

Where things get tough is that a PM visit is only a monthly thing while this machine is running 24/7/365 with all of the things that can cause leaks happening on a constant and ongoing basis. It’s possible for a tech to either miss a leak on a PM or for a leak to form soon after the PM was completed. Then, that leak just sits unaddressed for the next 28 days or so.

There are a couple of backstops to watch for that.

One method is refrigerant level monitoring. Most of these large machines have sensors in the receiver of various types to monitor the amount of refrigerant in that tank. From a simple float switch to give a digital signal when the level drops below a certain point to analog signals indicating what percentage of the tank is filled, we’re watching this and monitoring it with a digital control system. When the level falls below a predetermined level, an alarm is generated. That can prevent a serious problem resulting in a loss of product by catching the loss of refrigerant before so much refrigerant is lost that it causes problems at the fixture. The drawback of this system is that we can still lose a significant amount of refrigerant before a technician gets involved in correcting the problem.

Enter continuous refrigerant monitoring…

The newest buildings are being equipped with machinery to continuously sample the air in the building and alert when even small quantities of refrigerant are detected. How this works is that sampling tubes are run throughout the building to areas where refrigeration equipment is. You’ll see sampling tubes under fixtures, sampling tubes located in piping chases, in walk-in boxes, and around the machinery itself. A detector pumps air samples through that tubing and checks it for refrigerant, which it reports in parts per million. When that level exceeds a limit, again, an alarm is generated, and a technician is dispatched to investigate and correct.

These machines need service and calibration as well, so that can mean more work for the technician.

 

Cracking the tough ones

So, let’s say that we’ve fixed all of the leaks on our initial survey and got the units charged back up to normal levels, but we still see a slow drop in the amount of refrigerant in the rack. We KNOW there’s a leak somewhere. We bring our detector in the building from a clean calibration out by the truck, and it alerts at the door.   Yeah, we’ve got a leak in this building. A check of the mechanical room, the fixtures, and the condenser, however, show nothing. Maybe we’re getting an alert in one part of the building, so let’s focus our efforts there. The first step will typically be shutting down the air circulation. Kill HVAC equipment, exhaust fans, and anything else that’s going to circulate air. Pay attention to smaller fans because sometimes it doesn’t take much air to move that refrigerant around. What we’re looking to do is allow that refrigerant to settle into one area. That’s going to help us to narrow down the location of that leak.

Work methodically, checking every bit of exposed piping, under insulation, and in chases both overhead and underground. Follow every single alert your detector gives. There are no shortcuts to this job. It is tedious, difficult, and very often frustrating work. Be patient and methodical in your search, and bring lots of soap and flashlight batteries.

Another common trick used to pinpoint leaks, and this one is usually employed once you’ve got the leak localized to a fairly small area, is to raise the system pressure. Turn that system off or close an isolation valve; allowing pressure to build will make a leak form bubbles better. We’ve all tried this sort of trick, even by adding nitrogen to raise the pressure within the unit. No surprise there, right? Let me throw a curveball at you now…

Sometimes, the leak is so large that the system pressure creates such a large leak that bubbles won’t even start to form. That's because the refrigerant coming out of the leak simply blows the soap away from the leak.

Depending on the location and angle of the leak, you can’t always hear it—or at least my old, defective ears can’t. In cases like this, I’ll often lower system pressure closer and closer to atmospheric without going below that. That allows a large leak to be located more easily. Also, back to basic observation techniques, I’ll run my fingers around and over the pipes. Even though I can’t hear the leaks, I can feel the escaping refrigerant gas.

Another problem with very large leaks and electronic leak detectors is that, once a large leak is repaired, the refrigerant in the ambient air doesn't just ‘magically’ go away. Sometimes we need to walk away after repairing a large leak and allow the building's HVAC system to do its job of circulating air in the building, bringing in some fresh air, and diluting the refrigerant in the atmosphere, so it doesn't affect our search for more leaks.

In the beginning, I said that a leak like the one on that service call was ultimately preventable. Again, this is where a comprehensive maintenance program helps a lot. The ultimate CAUSE of that relief valve opening traced back to a defective high-pressure cutout control. A single-phase power outage caused the condenser to stop, which resulted in higher than normal head pressure. Had the safety controls been operating normally, the safeties would have shut the compressors down and prevented that problem.  It would have still resulted in a service call, but it wouldn’t have been a 500-pound refrigerant loss. Detailed and comprehensive service and maintenance procedures include more than just leak checking.

—Jeremy Smith

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