Special Considerations for High-Pressure Applications
In Part 2 of our blog on precision tubing in liquid dispensing systems, we mentioned that the ID of the tubing will determine how evenly the liquid samples are dispensed. Imperfections in the surface finish ID can result in conditions (e.g., swirling, eddying, or buffeting) that prevent the liquid from flowing out of the tube smoothly and, in turn, make it more difficult to get good test results. Swirling, eddying, or buffeting can also occur when the tubing in liquid dispensing systems must perform under high pressure — as when the testing system is for or related to chromatography, where components of a mixture are separated to prepare for a more advanced use and/or to measure the relative proportions of the different components, as in a chromatogram.
What is important to know about precision tubing under high pressure?
High-performance liquid chromatography (HPLC) is a powerful and versatile tool in analytical chemistry. Providing the ability to separate, identify, and quantify the components present in any sample that can be dissolved in a liquid, today’s HPLC technology can detect trace concentrations as low as parts per trillion (ppt) and is widely used in pharmaceuticals, food processing, cosmetics, environmental sciences, forensics, the chemical industry, Defense and Homeland Security, and more.
In HPLC and other high-pressure liquid dispensing systems that utilize pumps (rather than gravity), seamless tubing is highly preferred to tubing that is welded and redrawn. That is because any weakness at the weld line of the tubing can pose a risk of leaks, cracks, or even explosions when pressure is applied. As chromatography has moved to higher bars of pressure (from 6,000 to 9,000 psi), it has also led to changes in the materials used for the tubing. For example, a nickel-cobalt alloy such as Carpenter MP-35N™ allows for tubing that withstands higher pressure than, but with the same wall thickness as, stainless steel.
In addition, while the rule of thumb is that the smaller the ID, the better the results, it is important to remember that as the ID decreases, the pressure created by the tubing can increase significantly. This in turn has the potential to create higher backpressure than the system can withstand.
How do you choose a tubing material for HPLC and liquid dispensing systems?
When deciding whether a particular tubing material can and should be used in HPLC or other liquid dispensing systems, there are additional factors to consider besides the tube’s ability to withstand pressure. For example, the tubing material must be chemically compatible with the sample and the testing system itself. Some biological samples will “stick” to the surface of stainless steel tubing, causing issues such as loss of sample, low yield, and carryover. So, in this case a commonly used tubing material such as SS 316, a molybdenum-alloyed stainless steel, might be replaced by titanium or any number of other metals that won’t have this problematic “sticking” issue.
And of course, it is critical to be sure the tubing material chosen can be cut, drawn, and finished to provide the right length and the smooth, burr-free ID, OD, and end cut you need. For instance, plastic might seem to be a good, cost-effective option for the task; however, it cannot be cut as exactly as metal can be — an important consideration where dispensing accuracy is vital — and would additionally have the issue of withstanding high pressure.
So, then, when evaluating a tubing material for liquid dispensing systems, ask these questions:
- Will the material interact biologically or chemically with the sample? With other system parts?
- How short can the tubing be cut?
- Can the material be drawn to the size needed? Just how small can the tube ID and OD be?
- How smooth can the ID surface finish be made? Using what method?
- How hard is it to cut the tubing without collapsing, chipping, or other deformations?
- Is the end of the tubing burr-free?
- Is the tubing’s ID centered?
The choice of tubing material is a critically important step in getting the best results in chromatography and other testing that utilizes liquid dispensing systems. Fortunately, your metal tubing provider and its engineering team can help you decide on cutting method, ID surface finishing, and other features that will meet your needs and be cost-effective for your particular application.
In the meantime, to explore different precision cutting options for your metal tubing or other metal parts needs, please download our free guide, Choose with Confidence: Comparing 2-Axis Precision Cutting Methods.