Protecting Water Pumps Against the Elements

Nov. 1, 2015
The inherent losses in all pumping equipment can be divided into three main areas mechanical, leakage and hydraulic. This article looks at how the use of internal coating systems can improve pump efficient, including a case study from a water treatment plant in Portugal.

The inherent losses in all pumping equipment can be divided into three main areas mechanical, leakage and hydraulic. This article looks at how the use of internal coating systems can improve pump efficient, including a case study from a water treatment plant in Portugal.

By David Blackwell

Fluid handling equipment can suffer from multiple problems, dependent on the duty it is being asked to perform, such as mechanical damage, general or localised erosion and corrosion and chemical attack. These problems either singly or in concert result in decreased efficiency and poor performance which, if disregarded, in turn leads to increased operational costs.

Maximising and maintaining performance should therefore be an important consideration for both pump manufacturers and end users. Indeed, the losses inherent in even a new pump can account for relatively high efficiency losses (circa 16% for a mixed flow pump to 30% for radial flow).

Hydrophobic Coatings For Pump Efficiency Improvement

The inherent losses in all pumping equipment can be divided into three main areas, mechanical, leakage and hydraulic.

The application of an internal lining can do nothing to stem the mechanical losses which are generally produced by friction within the bearings and gland housings. Yet these losses can be minimised by using an appropriate and regular maintenance regime. Leakage and hydraulic losses, on the other hand can be influence for the better by a correctly specified internal lining. This can to minimise the erosive and corrosive effects of the fluid medium, thus reducing the increase in surface roughness of the wet areas of the equipment.

Graph 1: Surface smoothness comparison between polished stainless steel and Belzona 1341 (Supermetalglide)

Erosive forces such as entrainment, impingement, and cavitation together with corrosion (oxidation) and chemical attack cause the wet surface areas of the equipment to deteriorate and become increasingly rough over time.

This roughness in turn causes the flow patterns within the equipment to become more turbulent due to increased friction, which results in the driver absorbing more power to move liquid through the pump. Hence the predictable fall in system efficiency.

By introducing an internal lining which is impervious to corrosive elements and can resist the prevailing erosive forces for a longer period than the original material of manufacture then, the rate of internal deterioration can be managed and slowed, thus maintaining the operating efficiency at a higher level, than otherwise would be the case, for a longer period of time.

Furthermore introduction of hydrophobic properties to the internal lining coupled with the ability to cure to an ultra-smooth surface can actually boost the efficiency of the equipment to levels higher than those expected of the equipment in the “as new” condition. Ordinarily manufacturers strive to make their pump casings as smooth as possible in order to get the best possible efficiency measurements, since they are expected to guarantee specific efficiency ratings to their customers.

Unfortunately the process of smoothing the internal surface of a pump (fettling) is extremely time consuming and expensive.

Applying an ultra- smooth erosion-corrosion resistant efficiency coating to the wet areas of the pump in order to eliminate the casting irregularities is a much more effective and economical way of solving the problem. It has been shown to increase the efficiency of operation to levels unattainable by the traditional fettling method.

These polymeric coatings are specifically designed to improve efficiency of fluid handling systems, whilst protecting the metal surface against the effects of erosion-corrosion. The smooth surface condition which can be achieved by these hydrophobic epoxy coatings is demonstrated on Graph 1 where, for example, the surface profile of Belzona 1341 (Supermetalglide), a high performance coating was measured as fifteen times smoother than polished stainless steel.

As a result of the smoother surface and reduction in flow resistance (form drag) and friction, the hydraulic performance of the pump can be increased.

Graph 3 : Energy consumption comparison

Graph 2 shows the performance curves of a single stage, end suction centrifugal pump with 250 mm suction and discharge branches before and after being coated with Belzona 1341.

This test was performed by the British National Engineering Laboratories (N.E.L.) and the pump, in uncoated condition and running at 1,300 rpm, was originally found to deliver 875 m3/h at 26.5 meters head and with overall peak efficiency of 83.5% (overall efficiency defined as the ratio of water power output to mechanical power input at the shaft).

Testing of the Belzona 1341 coated pump gave a maximum of 6% increase in the peak efficiency and a reduction in power consumption of 5.1 kWh at duty point. Assuming a 5,000 hours operating cycle/annum, the power savings over this period would amount to 25,500 kWh.

Similar results have been measured by many pump manufacturers around the world, and feedback on industrial equipment protected with this coating show that it is possible to achieve a return in excess of the new pump condition.

Graph 4: Power consumption KW / m3 depending on the flow

It can be seen from graph 3 that the throttling effect of the application of a coating with too thick a cross section has the effect of moving the BEP to the left on the flow curve. The overall effect of this shift in BEP means that for duties to the left of the new BEP there is indeed an increase in pump efficiency.

However, for duties to the right of the new BEP the pumping efficiency falls rapidly and can actually exacerbate the damaging effects of the fluid flowing through the pump. Therefore in order to benefit financially from the application of this type of internal coating the operator must always ensure that the system demand (flow) does not exceed the new BEP.

Other products, which rely on a surface gel coat to give the efficiency increase, unfortunately begin to fall in efficiency when this gel coat becomes worn and begins to expose the underlying surface which is not able to maintain the effect.

Pump efficiency enhancement with four months return of investment

In 2013, a water elevation plant in Portugal was looking for a solution to improve its hydraulic efficiency. This plant was chosen because of its high energy consumption and costs, with an annual consumption of 1.7 GWh/year and a volume throughput of water of 1.2 Mm3/year recorded in 2012.

1: Pump impeller before application

The client was looking for a solution to restore the damaged pump while reducing internal friction to enhance hydraulic efficiency. The Belzona 1341 coating system was specified to maximise water flow and reduce energy consumption while reducing internal wear and minimise future maintenance requirements. The pump was disassembled and all internal surfaces were grit blasted to remove the previous coating, and create a substrate cleanliness of SA2,5 with an average a surface profile of at least 75μ. The surface was then cleaned and examined to ensure that it was free of dust and other particles.

2: Pump impeller repaired and coated

Prior to application of the coating the damaged areas of the pump volute were masked off and repaired with Belzona 1111 (Super Metal), an epoxy paste grade composite. Simply applied using an applicator, this material was used to reconstruct the areas damaged by corrosion and restore the original contours of the volute.

3: Pump before application

Within the two-hour over coating window, the first coat of Belzona 1341 (Supermetalglide) was applied by brush, followed by a second coat to obtain a total dry film thickness of 500μ maximum applied thickness ensuring that there is no detrimental hydraulic effect on the pump performance.

4: Belzona 1341 (Supermetalglide) applied

Measurements taken following reinstallation of the coated pump gave the following results:

  • Reduced energy consumption during non-peak hours 44.9% to 39.3% (-12.5%)
  • Reduced energy consumption by 14.9% (kW/m3), 147,247 KW/h recorded in eight months
  • 20.3% cost reduction
  • ROI: four months of operation.

The Belzona 1341 Supermetalglide system has been used and tested on all designs of pumps over the 25 years.

The efficiency gains do however vary dependent upon pump size, (and hence motor size) pump duty and pump design. In general the largest increases in efficiency are to be found in mixed flow pumps with smaller gains experienced in axial an radial designs. However, even the smaller gains obtained in the axial design range can be translated into large annual savings dependent on pump and motor size, as illustrated by the below graph.

5: The 1341 can reduce future maintenance requirements

In conclusion, coating technology can help to decreasing these losses, by maintaining maximum efficiency values over a longer period of time by providing a manmade barrier which is resistant to the forces that would normally create the losses thus increasing the pump’s overall performance and reducing operational costs.

David Blackwell is technical director of Belzona Polymerics.

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