( with aide from Cole-Palmer website)
Each pump system is divided into three parts: pump head, drive, and tubing. The pump head consists of the rotor and housing; the drive creates the force needed to move the rotor; and the tubing acts as the pump chamber. These three parts pump fluid by following these steps:
The tubing is placed into the tubing bed—between the rotor and housing—where it is occluded (squeezed).
The rollers on the rotor move across the tubing, pushing the fluid. The tubing behind the rollers recovers its shape, creates a vacuum, and draws fluid in behind it.
A "pillow" of fluid is formed between the rollers ( with two roller application a vaccuum is created but and a check valve is needed so no possible back siphon can occur). This is specific to the ID of the tubing and the geometry of the rotor. Flow rate is determined by multiplying speed by the size of the pillow. This pillow stays fairly constant except with very viscous fluids.
the tubing is the pump chamber. The elasticity of the tubing provides suction lift; its strength provides pressure handling ability; its flexiblity determines pumping life; its bore determines the flow rate, and its wall thickness determines pumping efficiency.
As i stated before if you use a two roller type pump (like the one below) you need to use a one-way check valve (always on the suction side) ensures positive progress of the fluid through the dosing line while eliminating back-siphoning
Tubing life is inversely proportional to pump speed. Minimum speed gives maximum life. You should always size your pump to the largest tubing size to the slowest RPM
Increasing Tubing Life
A tubing failure is defined as one of two conditions.
1. The tubing wears out and breaks. It generally splits along the longitudinal axis where the largest amount of stress is being placed on the tubing (see diagram). A piece of tubing has a finite # of times it can be compressed before breaking. This is a function of the tubing material and the wall thickness to inside diameter (I.D.) ratio.
2. A loss of 50% of flow rate The tubing fatigues or loses its ability to snap back to its original shape when it is released from the rollers. The flow rate decreases significantly because the vacuum created behind the roller as the tubing regains its shape decreases with fatigue. Therefore the fluid will not be pulled through the pump head as quickly. Figure A shows a cross sectional area of tubing. Figure B shows the same piece of tubing compressed by the rollers in the pump head and Figure C shows the fatigued tubing. Like #1 above, this is also a function of tubing material and wall thickness to I.D. ratio.
Keeping these points in mind, here are some ways to increase the life of your tubing:
1. Slow down the drive speed This decreases the frequency that the tubing is being compressed. To maintain the same flow rate, use a larger tubing size.
2. Choose a thick-walled tubing This will increase the number of compressions required to wear the tubing out, and it will increase its memory capacity. e.g. size 25 and size 15 tubing both have the same I.D. and provide the same flow rate, but size 15 has a thicker wall and will last longer.
3. Select the longest life tubing formulation that is compatible with your fluid Norprene® and Pharmed® have the longest lifetimes, C-flex® and silicone have the next longest lifetime, Tygon® formulations have shorter life, and Viton® is generally the shortest lived (less than 250 hours at 100 rpm).
4. Reduce system pressure Tubing lifetime decreases significantly at higher pressures, especially tubing formulations such as C-flex and silicone which have low pressure ratings.
5. Reduce the occlusion This decreases the stress applied to the tubing every time it is compressed.
6. Avoid sharp tubing bends, especially near the pump head. Run the tubing straight out of the pump head for at least 6-8". This will keep the tubing centered over the rollers.
7. Develop a preventative maintenance program Move the piece of tubing that is over the rollers 6-8" after a set number of hours before it reaches its failure point.
Later I make a photo blog on how to set up a Dosing system
Edited by scottyreef, 11 March 2009 - 03:36 PM.