Binder Transfer in Paint and Coatings

The Challenge

Because of the unique types of binders that are handled, as well as the operators' requirements for clean, time-sensitive performance, a versatile pump needed to be incorporated-one that was sealless and provided low shear, clean-in-place capabilities and high volumetric efficiencies.

The Solution

The new pump selected for this type of application featured eccentric disc technology. The eccentric disc pump installed at the plant was sealless. Four years after installation, the pump is still working perfectly. At the end of the loading process, the pump totally drains the inlet hose, mobile tank and outlet pipe, and since the pump does not have a mechanical seal, there is no risk of product leakage. Eccentric disc pump in use at paint and coatings plant An eccentric disc pump provided the following design benefits:

• Sealless design without mechanical seals, magnets, rubber or PTFE diaphragms
• Low shear handling of products with low slip, lower internal velocities and ultra-low agitation
• Clean-in-place capability that allows the pump to be completely drained, flushed and cleaned without disassembly
• High volumetric efficiency capable of maintaining a constant flow rate at a given viscosity throughout its pressure range
• Good compression performance and the ability to run dry (up to 10 seconds) enable self-priming capabilities and complete line stripping of suction and discharge lines
• Self-adjusting operation that maintains delivery/pressure performance over time through the use of a self-adjusting disc/cylinder

Because of the latter characteristic, eccentric disc pumps can be used as dosing pumps. Since the pump is automatically self-adjusting, it maintains greater efficiency and repeatability over time than traditional lobe or gear pumps. All the pumps installed have a shear rate of rate of sec-1 = 0.9 rpm, which is lower than other types of pumps used in paint and coatings applications. This is due in part to the gentle, low velocity action of the disc and cylinder, and the extremely low slip rate of the pump. Unlike other technologies, eccentric disc pumps do not have required clearances that can cause slip, which is the portion of the pumped product forced back to the suction side of the pump due to pressure through the clearances. The discharge pressure exerts itself against the eccentric disc in a way that assists in maintaining axial contact with the cylinder, thus mitigating the usual effect that discharge pressure has on slip in pumps. It is this low slip between the disc and cylinder that gives the pump the ability to self-prime and line strip. The eccentric disc pump can handle viscosities of up to 10,000 Cp, working pressures up to 130 psi (9 bar), capacities of 4 to 158 gpm, operating temperatures up to 176 deg F and a particle-size range of 1 to 3 mm. The pump's particular clean-in-place (CIP) technology holds 3A Approval Certification and is designed per European Hygienic Equipment Design Group (EHEDG) specifications to be flushed and cleaned in place. Clean in place technology When installed for CIP operation, it experiences no loss of performance due to vertical drain porting, unlike a rotary lobe pump. When cleaning, pressure is introduced to the back of the eccentric disc through the pump chamber. When the flush pressure overcomes the spring, the disc moves away from the cylinder, allowing the cleaning solution to pass through the pumping chamber. This feature allows a relatively large volume of cleaning fluid to sweep through the pump, providing a thorough cleaning and often eliminating the need for bypass piping for the CIP mode. Where maintenance is concerned, eccentric disc pumps consist of few parts. The cylinder-disc assembly can be replaced without disturbing the suction piping or drive components. Cross section of eccentric disc pump

Eccentric Disc Technology versus Others

Because of all of these characteristics, eccentric disc pumps can supply benefits that some pumps traditionally used in the paint and coatings market sector cannot:

•  Air-Operated Diaphragm (AOD) Pumps-AODs have traditionally been the pump of choice in the P&C market because of their low initial purchase cost. However, they can be inefficient and require high maintenance, increasing their total cost of ownership.
• Gear Pumps-The second most popular pump choice behind AODs due to their capability of handling higher viscosity ranges. Gear pumps can have excessive seal leakage; inability to self prime; a flow rate that is jeopardized when wear begins; high internal velocities that affect fluid dynamics, resulting in shear; and clearances that result in slip as pressures increase and viscosities decrease.
• Centrifugal Pumps-Centrifugal pumps can have a high rate of slippage. Some centrifugal pumps have lower efficiencies than eccentric disc pumps.
• Lobe Pumps-Lobe-type pumps perform like gear pumps, meaning they can have many of the same drawbacks that gear pumps have. The need to seal two shafts doubles the expense of seals and the potential for leakage.

Conclusion

An eccentric disc pump is a multi-use piece of equipment, meaning that it can be used in many applications. In the paint and coatings industry, that could include the pumping of pigments, resins, solvents and additives. These capabilities help make eccentric disc technology a solution for the challenges of addressing pump seal, suction, product shear and volumetric efficiency concerns. Through the incorporation of leak-free operation and line-stripping capabilities, the eccentric disc principle makes the pump flexible, allowing the pumping of low-viscosity, high-viscosity and highly abrasive materials within a single process and with the same pump. The pump is not only a longer lasting, more efficient piece of equipment, but also a multi-tasking one. It might be the answer for many difficult pumping applications in the paint and coatings industry.   Pumps & Systems, November 2009