80% of analyzer problems occur in the sample system.
I can personnally vouche for that. Whether it's a plugged filter or a heater not working, in some way we are not getting a good representative sample to the analyzer. We must have a REPRESENTATIVE sample.
What does that mean????? Deliver a process sample to the analyzer without changing the integrity of the sample.
Extractive Sample System and In-situ Sample System
The extractive system will pull the sample from the process and prepare it for the analyzer. Depending on the sample depends on how we will condition it. We might have to drop the pressure or dry it or remove particulates and water.
The In-situ measuring means we will be in the process measuring directly. So the analyzer is in the stack taking a direct measurement.
We will discuss the advantages of each system and their drawbacks.
So what would be a disadvantage of an extractive system? If we have a long sample line from our sample tap we will have lag time. Each piece of a sample system cost money and adds up quickly. There is more maintenance to be performed so more man hours in maintenance. The advantages ----- the analyzers are not exsposed to harsh conditions. With proper maintenance they will run like a clock and produce consistent results. Prevents our people from being being exposed to the process as much.
The in-situ is in the process so when it has issues, guess where the tech has to be to fix it? Thats right, in it too. But they are a direct measurement with fast results. Most of the time they can come in cheaper than an extractive system.
There is always that engineer that actually wants to come in under budget and get praised and hands the technician a nightmare.
So let's say that we need to design a sample system. As the engineer we can choose who we buy our valves, coolers, pumps, analyzers and so on from. So who do we go with?
If we are actually doing this right then we would consider these factors.
1. $$$$$$$ Price
2. Your plants engineering guidelines
3. How available are the parts? You can't have a 6 week deliver time....
And mostly just copy what has worked before.
Let's take a look at how we would use an extractive sample system for a flare. In this exercise we will draw out the components in the box.