WORLD PUMPS ® All Ground Down - How Getting the Grinding Right Makes For Reliability Page 3 of 5

ALL GROUND DOWN-HOW GETTING THE

GRINDING RIGHT MAKES FOR RELIABILITY cont...

Table 1: Inner Layer Materials Available For Watson-Marlow Bredel Hoses And Their Areas Of Application
	Natural Rubber (NR)	Acrylonitrile-butadiene rubber (NBR)	Ethylene propylene diene terpolymer (EPDM)	Chlorosulphonyl Polyethylene (HypalonÒ/CSM)
General Properties	Highly resilient; excellent abrasion resistance and mechanical strength	Highly abrasion and wear resistant	Good chemical resistance to a wide range of fluids	Excellent chemical resistance; extending the range of applications
Applications	Diluted acids and alcohols	Oils, fats. Alkalis and detergents	Concentrated acids and alkalis	Concentrated acids and alkalis
Examples	-	Food handling: FDA and 3A approved	Weakly polar solvents	Strong oxidizing products
Temperature Range	-20 to 80 ºC	-10 to 80 ºC	-10 to 90 ºC	-10 to 80 ºC

The right balance in compression

For the working of a hose pump in general and its positive displacement characteristic, the hose needs to be compressed to full closure. One can imagine that the force needed to squeeze the hose flat steadily increases during this movement.

Figure 6. the layered construction of a typical hose

However, the force increases dramatically when the hose is compressed beyond the point of full closure. This is shown in the force-displacement curve (Figure 7) made by compressing a SP(X)40 hose using a hydraulic press. In the vicinity of complete closure, around 26 mm in this example, a millimeter extra compression leads to an increase of a few kilo-Newtons in compression force. Due to the incompressibility characteristic of rubber, high stresses are formed inside the hose.

With increasing compression internal frictions also increase, resulting in a higher hose temperature and a shorter fatigue lifetime. The pressing shoes and the rotor deliver the counterforce needed to compress the hose. Too much compression can give an unacceptable load on the bearings of the rotor. Also the pump casing and the rotor shaft can be damaged.

On the other hand, incomplete closing of the hose causes (high velocity) backflow. Especially with abrasive media, severe wear of the inner layer can occur. From experience it is known that a minimum over-compression is less harmful for the hose than under-compression. Under the influence of internal (system) pressure, rotor speed and temperature, different displacements are necessary to completely close the hose. For this shims can be put underneath the pressing shoes to increase the total diameter of the rotor, just to the point where there is no more backflow. Shimming curves per pump type are available for the right selection. Since, after grinding, twice the tolerance on the wall thickness of the Watson-Marlow Bredel hoses is smaller than the thickness of one shim (0.5 mm), the shimming does not need to be adapted after hose exchange.

Figure 7. force-displacement curve of a 40 mm Watson-Marlow Bredel hose with a wall thickness of 13.2mm, measured with a hydraulic press.