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00:00:12 Lou Signorelli
Good day and welcome everyone to Power Bytes. I am your host, Lou Signorelli, and Power Bytes is your destination podcast for power generation conversations. As always, please know how much we appreciate you, our listeners. We hope that you find our topics both helpful and interesting.
00:00:27 Lou Signorelli
There are several ways you can get in touch with the show. You can send us an e-mail at powerbytes@cat.com. You can visit us at Cat Electric Power on Facebook or LinkedIn, but wherever you listen to your favorite podcast, please be sure to remember to subscribe, leave a comment, and like the show. It's very, very important to us. You know, we've done a number of successful episodes on technologies and engine attributes, and we wanted to share some insight on a challenge many users face called wet stacking. There are any number of names you might hear used about this; one’s called “slobber.” (Never thought I'd say that on the air, but slobber is a thing.) But “wet stacking” is the proper name for the condition. Joining me today for a second episode, we appreciate Roger Rossborough coming back to join us. Roger’s got 20 years of experience and a PhD in mechanical and manufacturing engineering from Queens University in Belfast, Northern Ireland. Roger, welcome back to the show!
00:01:24 Roger Rosborough
Thank you, Lou, and thanks everyone for listening in!
00:01:28 Lou Signorelli
Roger, let's start with the definition – or maybe a description – of wet stacking.
00:01:31 Roger Rosborough
Well, as you mentioned, Lou, the term “slobber,” is a common name for the condition wet stacking. And sometimes it's also referred to as “exhaust slobber” or “exhaust manifold slobber.” How do you know if your engine suffers from it? Well, you'll see a thick black liquid seeping from the joints of your exhaust system. That could be from the exhaust manifold itself or the turbocharger or the pipe work, and I would say the liquid itself looks like used lubricating oil that should have been changed a very long time ago: really thick, dirty oil. But that's not what it is! This thick black liquid is really composed of carbon particles, and that's combined with unburned fuel, and it can also contain some lubricating oil. But the carbon particles are really what gives it that black appearance.
00:02:24 Lou Signorelli
It sounds quite messy. What is the primary cause of wet stacking?
00:02:28 Roger Rosborough
The primary cause is due to operating your diesel generator set at low loads. And when we say “low loads,” we mean typically less than 30% of the rated load, and that's for an extended period of time. This light loading, it means that your engine is running below its designed operating temperature, and that's what can result in some of the fuel not getting completely burnt. For long periods of time this onboard fuel gets pushed into your exhaust system, which then wets the exhaust hardware. And that's where the term “wet stacking” comes from.
00:03:06 Lou Signorelli
That's quite interesting. How common is wet stacking?
00:03:09 Roger Rosborough
It is fairly common because standby gensets are typically sized for a worst-case scenario, which means that often when the genset is running, it may only be seeing a fraction of that worst-case loading scenario. As an example: if you think about the process of purchasing a standby genset for your home, you would add up all of your electrical loads in the house, including the cooker, the kettle, microwave oven, heating system, [other] heaters, air conditioning, fridge, freezer, dishwasher, lights, TVs, etc. And when you're doing this, you want to purchase a genset that has the capability of running everything or almost everything in your home at the same time. And that same approach is taking an industry, and it results in the possibility of your genset running at low loads for extended periods of time.
00:03:59 Lou Signorelli
That's right. When we size these things, we have to go for the worst-case scenario because Murphy's Law says it will be running during the worst-case scenario, many times. But is it bad for the engine?
00:04:12 Roger Rosborough
It certainly looks bad, having that thick black liquid dripping out of your exhaust connections! You would think there must be some big problems with the engine, but thankfully that's not necessarily the case. However, if you have long periods of this wet stacking, it can be bad for the engine and can result in reduced performance of the engine and premature wear.
00:04:37 Lou Signorelli
Let's take a look at that, Roger. How does it eventually lead to damaging the engine?
00:04:41 Roger Rosborough
The unburned fuel buildup and exhaust system can fill the fuel injectors that can cause buildup of carbon on the exhaust valves, the turbocharger, and exhaust. These carbon deposits can lead to gases bypassing the valve seatings, increasing the exhaust backpressure. And you can also get deposits on the turbo blades, reducing your turbo efficiency. All of these things will reduce the overall engine power output. Carbon deposits can damage engine surfaces over longer periods. And if the engine’s running below its design operating temperature for a long period, you can also get unburnt fuel leaking past the custom rings, diving into the oil pan, thus diluting your lubricating oil. The mechanism for that is: the piston rings don't expand enough, due to the lower temperatures, and so you don't get adequate sealing there. The dilution of that oil in the oil pan then reduces the engine lubrication, and that can lead to premature wear of components. That inadequate piston ring sealing can lead to some of the lubricating oil getting up past the piston rings into the cylinder and out the exhaust system.
00:05:55 Lou Signorelli
In understanding all of this: what are the best practices to keep wet stacking from prematurely damaging an engine?
00:06:01 Roger Rosborough
Best solution to avoid wet stacking is by running with sufficient loading to maintain that designed operating temperature. If it's not possible to do that, then there's NFPA 110, for example, which recommends exercising your genset at least once a month for 30 minutes, providing loading to it so that the minimum exhaust temperature’s maintained or, alternatively, at the design operating temperature and not less than 30% of the standby rating. If adequate loading can't be provided for this, then portable load banks can be used or even permanently installed load banks could be considered for that purpose.
00:06:39 Lou Signorelli
Well, there you have it, folks. I'd like to thank our guest, Roger Rosborough, for sharing his insights with us today and a special thanks to you, our listeners.
00:06:47 Lou Signorelli
If you'd like to read more about wet stacking, Roger has written a very good paper, and you can find that by going to cat.com/wetstacking. Please be sure to like, comment, and subscribe to Power Bytes wherever you listen to your favorite podcast. Again, you can follow us on Cat Electric Power on Facebook or LinkedIn. Until next time! This is Power Bytes. Thanks. Have a great rest of your day.
