By Eric Rentsch, Product Marketing Manager, Fluid Handling
Optimize your pump tubing
Let’s say you have already selected a great chemical metering system by investing in a peristaltic pump and the pump works great. It’s dosing within error limits, no failures, easy maintenance—working as advertised. However, you notice that sometimes the flow rate doesn’t match what you pumped the other day: You didn’t change the RPM or the tubing, it’s the same fluid and, it’s the same room. What changed? It’s most likely not something you did, but the nature of your tubing.
How pump tubing ages
The peristaltic pump is named for the peristalsis action demonstrated in the human body by the constricting human muscles in the digestive tract as food or other material is “swallowed”. While your body is a living thing that can move and repair itself, thermoplastic elastomers aren’t very much alive as we know. The elastic memory of the tubing will decrease with use and at greater expediency, depending on the operational and environmental parameters. A few of the most significant factors in tube wear are system pressure, temperature and drive RPM. Many manufacturers will provide tubing life expectancy in operating hours, or the period of optimal tube performance under specific conditions such as room temperature water at atmospheric pressure and static RPM. This is a best-case scenario when it comes to impact on tube vitality, but it provides the appropriate parameters for tube life expectancy. Additionally, tubing used after the expiration date will no longer carry the guarantee of the manufacturer to behave as expected. This means there might be more variation in the 6accuracy when used for dosing applications but will otherwise still move fluid unless it is brittle from age (which takes years).
Working with “cold” tubing
On the other end of the tubing life cycle, “cold” tubing, or tubing that hasn’t been broken in, will result in similar inefficiencies. This period of time is known as the break-in period. When pump manufacturers test tubing accuracy, they do so after the break-in period, as the majority of the tubing’s operating life will be in this state. After a period of use, the tubing warms up and becomes more supple and elastic, as it reaches thermal equilibrium inside the pump head. This equilibrium varies with many factors such as fluid temperature, drive rpm, and tubing formulation (or material), but the thermal system will plateau. This is when you achieve peak accuracy and flow rates.
So, what should you do when first stepping into the lab or onto the process floor to get ready to start up your metering system and still need to break the tubing in? How do you optimize the accuracy of your first flow rates for the day and skip the break-in period? How do you get the most out of your tube’s operating lifetime? Here are four solutions from our experts to help you:
4 methods to optimize peristaltic tube performance
- Calibrate often – Work with the tube age and break-in period. Get to work early, run the pump, grab a coffee (or tea), and wait for a while to prime the tubing for use. Then start your dosing application. After about 30 minutes and again at about one hour, calibrate the tubing. Count the hours the tubing is in use, and start monitoring closely near the end of it’s life to avoid unexpected tube rupture. However, this still uses precious operating life for break-in periods and this method requires more mind or lab space to total the operating hours of the tubing in the pump. Fortunately, the technology is there. Cloud-based monitoring isn’t reserved for industrial process plants and a few manufacturers are offering services at a more commercial level such as MasterflexLive® cloud-based technology.
- Do not store the tubing in the pump head – This keeps the tubing in a deformed state, affecting its elastic memory and therefore its optimum performance lifetime.
- Make sure the tubing is loaded correctly – If the tube is not centered on the rotor it may wear faster or prematurely rupture due to “rotor strike”. This is when the tubing is too close to the disc rotor, and the disc grinds into the tube wall.
- Use a flow controller – Masterflex® now offers a near-perfect solution. By placing an ultrasonic flow meter downstream from your pump and using the Masterflex® ultrasonic flow controller, all you need to do is dial in the desired flow rate and the pump will compensate for cold tubing, worn tubing, back pressures, fluid friction, and almost anything else that would have an effect on your flow rate.
Peristaltic pumps dispense the desired volume by calculating the required drive RPM needed given the volume of the fluid packet in between each roller. This information is usually provided to the drive during calibration. This is essentially why they are less accurate when the tubing is not broken in because the tubing is not yet at its “normal” state of use. However, the flow controller will read the flow rate in real-time via the ultrasonic sensor and compensate by modifying the motor RPM. If the tubing is cold, the pump will increase RPM and gradually reduce as the tube warms. This even compensates for the gradual tubing wear over time as the ultrasonic technology is quite accurate.
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