An Open or closed low flash point close to operating temperatures is always a risky business. When it comes to thermal and heat transfer oils, temperatures can get very hot indeed, meaning the flash point needs to be extra high to accommodate this. However, what happens when your flash point is lower than it once was and close to or even below operating temperatures. This could be very costly and dangerous indeed. A typical oil change on an installed system could cost £50,000 depending on the oil brand and volume of the system. This is only covering the cost of the oil itself and not the cost and labour to shut down, change the oil and dispose of the old oil, which could easily double that price easily. Hence, to simply getting rid of the oil may seem like a financial pill you cannot swallow. Equally, you have the safety concerns of a low flash point oil in your working environment, which can often have legal implications for your country’s health and safety laws. So you need to do something, but the question is, what? In this article, I will discuss:
- The problem of a low flash point in thermal oils.
- What is a flash point, and how is it tested.
- What do the terms closed, open, PMCC (Pensky Martin Closed Cup) and COC (Cleveland Open Cup) and fire point all mean?
- Why might a thermal oil flash point drop over time?
- What are light ends?
- Removal methods of light ends including Light End Removal Kits (LERKs).
So what is a flash point?
It may surprise you to find out that liquids don’t actually burn, but it is the flammable vapours just above the liquid that actually ignite.
So a flash point is considered the temperature the liquid is at when the vapours above the liquid ignite but not to a sufficient degree to generate enough heat to set fire to the whole liquid. Hence the process is a flash of fire which quickly goes out.
What is the difference between an open and closed flash point?
There are a few different methods for measuring flash points, including Seta flash and Abel flash points for specific applications. However, by far the most popular two methods are PMCC and COC or Pensky Martin Closed Cup and Cleveland Open Cup.
The difference between these two methods, often abbreviated to closed and open flash, is related to the cup the test is performed in and whether it has a lid on it or not.
The difference a lid makes?
The lid acts to trap the build-up of gases and simulates an enclosed space where vapours cannot escape. It is typically is the worst-case scenario and used for e.g. rules of transportation etc. The open flash point by comparison is performed without a lid allowing vapours to escape and simulates a well vented space. The closed flash point is normally lower than the open one because the vapours have the chance to build up to a point there are enough to cause a flash and do not escape to the atmosphere.
So what is a fire point?
A fire point is when you take the open flash point past the flash point and keep going until not only the oil flashes repeatedly with increasing temperature but fully ignites to create a self-sustaining fire.
PMCC, COC Flashpoints and Fire Point types in 10 seconds of animation
That’s a lot of info and descriptions. It may help to show you some animations of the tests so you know what is happening.
Which flash point is better?
It depends on the scenario. For certain specifications and safety data the closed value is often used. For new oil product data, oil companies will quote the open flash point, mainly because it’s higher and hence looks better on marketing material, which is not an uncommon practice – see box below.
You will be surprised how many methods or even choices of metohds have been invented purely for marketing purposes. For instance the classic Base Number measurement of engine oils using perchloric acid was invented because it gives a higher and hence better number. This was then adopted by all the competitor oil companies and is now the standard method for performing the test.
Getting beyond the hype of product data sheet claims.
On thermal oils, as discussed today it is important to measure both open and closed flash points and compare the difference between the two as a large difference suggests a build-up of light ends.
What are light ends?
Light ends are lower molecular weight parts of the thermal oil. They are generated by the high temperatures of the oil physically breaking long heavy end chains into smaller light end chains. The lighter the chain the more easily distilled and the more flammable. So a large number of light ends means the flash point will be depressed, particularly on the closed cup measurement. This can lead to a separation between the closed and open cup measurements with the further apart the difference the worse the light ends problem.
So you have a light end issue. What now?
For safety reasons if the light end level has reached a point the closed flash point is coming close to operating or hotspot temperatures within or around the system conditions then this will need addressing. You have a few choices:
- A complete oil change – this is only really practical on very low volume systems owing to the costs involved.
- Improving the ventilation – this can involve active purging of the air space above the system to remove the likely flash point to be closer to the open flash point than the closed one. This assumes the system design allows this.
- Killing the fire triangle – you need oxygen to allow a fire to occur so some systems by design use purge gases such as nitrogen to remove the oxygen from the system and reduce the risk of fire in this respect.
- A partial change / sweetening of the oil – this works on the principle you will be diluting number of light ends and hence increasing the temperature point they form a flammable atmosphere above the liquid.
- A LERK (Light end removal kit) system to physically remove the light ends from the oil. I will discuss this in the next section
So what exactly is a LERK (Light End Removal Kit) and how does it work?
A LERK is essentially a distillation vessel that heats the oil up and depending on the exact design you buy or rent it has a spray chamber to aid in vapourising the light ends at elevated temperatures. These are then passed to a condenser that cools the vapours so you can collect and remove the light ends. These often have the oxygen removed from the systems to avoid oxidising the oil as a result of the process. If you have ever seen a cartoon of a chemistry lab, with things bubbling away in one container and collecting in another (if you look at the title picture of this article at the top of the page, you will see in the bottom left of the image I have chosen one of these cartoons too). This is essentially what is happening just in a slicker looking and more industrial bit of equipment.
Test, test and test again
If you ever listen to a politician speaking during an election campaign, it is quite popular to hear the same word iterated in triplicate to indicate its importance. If I were pitching for the vote of your thermal oil system, I would be going with “test, test and test again”. Why am I saying this? Well, yes I own a lab so if you send samples of oil that increases my sales figures. The point still stands whichever lab you use, though, even if it’s not mine. Testing is the only way to know how safe your thermal oil system is. It is the only way to tell if you have a light end issue, and it’s the only way to identify if your corrective solution for a light end issue has worked. Hence, I reiterate; test, test and test again your thermal oil. In some countries, this is a legal requirement to do so, but lets for a second ignore this and pretend it’s not. Even if it’s not a legal requirement, you should still do it as you can save a lot of money from a failed and inefficiently operating system, avoid unnecessary oil changes, test the effectiveness of your major maintenance actions and most of all, potentially save someone’s life by doing so in the process. A flash point problem can potentially lead to explosions and fires and, sadly, can lead to injury and loss of life. Testing makes sense for you, your company, your staff and your machinery. I reiterate for the third and final time, it is important to test, test and test again.
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