What is Viscous Sampling and Why is it So Challenging?
Obtaining a representative sample from viscous fluids can be challenging due to their resistance to flow, sensitivity to temperature changes, and tendency to leave residue within sampling systems. Proper sampling practices are essential for achieving accurate analytical results while maintaining operator safety and process efficiency. In this video, you'll learn key considerations for sampling viscous fluids, including factors that can affect sample quality, system design strategies, and methods for improving sample transport and collection. Understanding these challenges can help reduce delays, improve consistency, and support more reliable analysis.
Video Transcript
KEVIN MCCLELLAND:
Welcome back to Ask Swagelok. My name is Kevin McClelland. I'm here with Matt Dixon, global technical lead. We are in the custom solutions building on Swagelok’s Solon campus today, and we're going to talk about viscous sampling and some design considerations.
KEVIN MCCLELLAND:
So Matt, can you describe viscous sampling and why it's so challenging?
MATT DIXON:
Sure. Viscous sampling, first of all, is known by a number of different names. It could be residual sampling or resin sampling, or more commonly, it's known as bottom sampling. And that comes from the refining industry where we're sampling the hot oil that's coming out of the distillation tower.
The oil is coming out at really high temperature—could be five, six, 700 degrees. And with that temperature come some really big safety concerns. We can't even let it cool down all that much because, if we do, it’s going to start to solidify and potentially plug up our system.
So, as a result, we need to keep it hot. We generally need to keep it flowing. But above all, we need to keep it safe. Those safety concerns are paramount, and that's compounded by the fact that a lot of times these viscous samples have gas trapped inside of them. As we're dispensing, the sample could be spitting and spittling a little bit, potentially splashing the operator. So again, some big safety concerns.
KEVIN MCCLELLAND:
What should we keep in mind when we are designing a sampler for these viscous liquids?
MATT DIXON:
The first consideration is safety. We want to make sure that the operator is safe when they're doing this. So we're going to look at things like insulating the hot surfaces to make sure they can't accidentally touch something that's hot. Protecting the operator from splatter—putting it inside an enclosure so they don't have the opportunity to get splashed.
We also want to make sure that we keep the sample hot and keep it moving. Normally, we're going to have some sort of fast loop arrangement where we take a sample from the process through a one‑inch pipe or tube to our sampling station and then return it to the process at some lower pressure point.
With that, we're able to keep the line nice and hot by steam tracing it, and we're able to keep the fluid moving because we don't want it to stop, stagnate, cool down, and potentially plug up.
Finally, we want to keep everything relatively simple in these sampling systems. With viscous sampling, keeping it simple with as few twists and turns as possible makes life easier. Fewer twists and turns mean fewer places where the sample could get hung up, cool down, solidify, and plug everything up. So we use nice swept tube bends instead of elbows.
KEVIN MCCLELLAND:
You mentioned valves. What are some of the important characteristics of a valve designed for viscous sampling?
MATT DIXON:
First, the valve has to meet the temperature profile. With the high temperatures of the sample, we need to meet those elevated temperatures. Beyond that, the valve needs relatively large orifices because of the thick nature of the liquid. Having orifice sizes at least a quarter inch in diameter is key to making sure we don't end up with plugs.
Whenever we have a fast loop arrangement, it's best to have an in‑line sampling valve. That way, the hot, viscous liquid can flow through the valve itself, which helps keep the valve warm, minimizes cool spots, and prevents buildup that could plug the valve.
It also minimizes the dead leg between the fast loop and the sample takeoff—so it's a double bonus. Beyond that, we want a valve we can purge so we can keep it clean. Using steam to purge the valve after sampling removes residue and prevents buildup that could cause plugging over time.
KEVIN MCCLELLAND:
Are there any special handling requirements for these viscous samples?
MATT DIXON:
Yes. Once the sample leaves the valve, we need to transport it into a sample container. Usually with a liquid, we use a glass or plastic bottle, but with this application that's not an option because of temperature. So we typically use a metallic sample container.
In the United States, the most common one I've seen is a metallic paint can from a hardware store—relatively inexpensive and widely available. Another common option is a stainless steel screw‑top bottle. Lots of options, but generally all metallic.
To dispense into the container, you typically have some sort of nozzle. It could be a simple tube stub from the final valve down to the container. But if it's just a simple tube stub, you risk plugging because it's not heated.
So sometimes we build a stinger assembly. With a stinger, we use the same tube stub but put a steam jacket around it. That keeps it warm to the tip—no cold spots, no places where it can cool down, solidify, and plug up.
Beyond that, purging the system after sampling is key. Purging removes residue, which helps with sample purity and reduces maintenance issues by preventing plugging.
KEVIN MCCLELLAND:
I want to say thank you once again to Matt for his wonderful technical expertise. This has been another episode of Ask Swagelok. We'll see you next time.
