March 06th, 2020
Author: Charles Golub – Market Development Manager – Medical Components – Life Sciences
Pump tubing is often one of the most critical components for a system that delivers fluid, and is often overlooked during the design phase. A number of key parameters go into pump tubing selection, such as:
1. How many rollers does your pump have?
This is an important initial thought to have because you want to know how many cycles the tube will withstand. For example, if you have a 4 roller pump, at 120 RPM’s, the tube sees 480 stress-relaxation cycles a minute. That is a lot of stress recovery on one product. This ties directly into the next question.
2. What is the percent occlusion of the pump design?
The percent occlusion of the pump is calculated by [(w*2)-h]/h*100 ; where w= wall thickness of the tube and h= the gap between the roller and housing. The result is the amount the tube is over compressed. This amount gives information about how much wear the tube will see as well as how much permanent set the tube will take (and how quickly). If the tube sets are loaded and shipped already assembled, the tube may take a large set and not dispense as accurately as tested to in an ideal state situation. These are important things to consider. While occlusion is important to ensure things like ability to hold prime and ease of pumping; two of the largest trade offs (specifically for long term pumping applications) are permanent set and spallation.
3. How accurate do you need the dispensing to be?
Most medical pumping requires very precise, short term dispensing. Not all, however, there are some longer term pumps that require pump life just as much as flow rate stability. For example, pumps that dose cleaning solutions or are used for long term applications can compromise a slight variation in flow rate to ensure the tube lasts the required timeframe before a scheduled change over. There are tubes that work best for each, some pump tubes offer very precise dosing for short term, however are very poor in longer term. Whereas some tubes that last a very long time can exhibit what’s known as a “break in” period, when the tube takes a slight set after the first day or two of continuous use. For this reason, it’s imperative to understand the true needs of the tubing product.
4. What drive is your pump using or how much torque does the tube see?
The type of drive, amount of torque and wear the tube sees, along with the percent occlusion will ultimately drive how much of a permanent set the tube will take as well as how much mechanical wear the tube may see. The permanent set is a measure of compressive response (usually tested to ASTM D395). Essentially material is compressed for a set time frame (22 hours is the standard for ASTM) and the amount the material rebounds is measured and compared to the original height. The result is percent permanent set and can be used as a leading indicator of how well a tube may perform in longer term pump performance.
5. How much does the tube wear or spallate?
Some products, specifically many of those designed for short term pumping, might start to spallate if improperly chosen for longer term continuous operation. The amount of spallation the tube exhibits is very much dependent on the prior questions, such as how much wear it sees and how much occlusion is designed into your pump. Tests such as tribology testing can give you some idea of the general properties of the wear characteristics of a material, but ultimately a combination of wear properties, compression properties and the design of the pump, all factor into the amount of spallation a tube might exhibit.
6. How is the product sterilized?
We all know various sterilization methods can play a role in the physical properties of materials, and pump performance is no different. Ensuring up front discussions about the sterilization and an understanding from the tubing manufacturer as to what those physical property changes are will ensure a seamless scale-up in your product validation.
A few other notes about choosing a peristaltic pump tubing; always tell your tubing manufacturer what chemicals the tube could be exposed to. For medical applications, it’s easy for a tube manufacturer to assume it would only be exposed to blood or saline; however, we know this might not be the case. Some pump tubes will perform dramatically differently with different chemistry of fluids.
Also, be sure to specify ID/wall for tolerances. This seems simple enough, but it may get overlooked in the tube selection process. General ID/OD tolerances are not sufficient for most medical pump applications. Ensuring concentricity of the lumen(s) for your pump tube is critical to ensuring consistent performance.
If you have further questions about pump tubing, please feel free to reach out to your local Saint-Gobain sales representative at: https://www.medical.saint-gobain.com/contact-us