Pump efficiency: the road ahead to lower energy costs.

Efficiency is the relation between used and supplied energy in a system. In all technical processes or products there are losses that reduce efficiency. A small pump can achieve at most 50% efficiency, while a very large pump can reach as much as 93% efficiency.

Efficiency or--more to the point--your return from the power suppliers' bill, is crucial for almost all pump users. For instance, consider that for a typical industrial pump unit energy often represents 90%-95% of the total life cycle cost (LCC) after some years of duty. With this in mind, the consequences of just a few percent more or less on efficiency ratings are considerable.

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In this article we will glance at the subject of pump efficiency for rotodynamic pumps--centrifugal and propeller pumps--and focus on some important actors affecting it. As a whole, efficiency is an enormous field and, in many aspects, a very complex one.

WHAT IS PUMP EFFICIENCY AND HOW DO YOU MEASURE IT?

Pump efficiency is the ratio between the performed pump work and the energy input to the pump driver, including losses in any transmissions. The pump work is simply the physical work of lifting a given mass (volume x density) up to a given level. Pump work = P = H x Q x r x g/3600 kW (see definition 1).

The following can be seen as a standard checklist of factors affecting pump efficiency. Although important, these are just some of the factors to be considered in pump selection:

* Pump size and the combination of flow, head, and pump speed.

* Hydraulic design of the impeller.

* Hydraulic design of the pump housing.

* Mechanical losses in bearings and seals.

* Specific factors depending on the application (the right type of pump for the job).

* Dimensioning of the pump with respect to anticipated duty.

PUMP SIZE

The geometric design of the hydraulics (impeller + pump housing) is strongly connected to the type of work the pump will do. This design is often expressed as the specific speed (ns).

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The specific speed--which is not the pump's real speed--tells us what type of work the pump will perform most efficiently (see definition 2).

A typical radial (or centrifugal) pump will give a low ns value. Such a pump is best suited for applications where head is favored over capacity. An axial (or propeller) pump, on the other hand, will give a high ns value and is chosen when a large flow is to be pumped at a low head.

HYDRAULIC DESIGN

The impeller design is a typical conflict between high efficiency and specific application considerations. The design for ideal flow conditions assumes a clean medium with a viscosity close to that of water. Any deviations from that ideal fluid mean that some compromises must be made, with consequent negative effect on efficiency. If the fluid contains particles that may cause clogging, for instance, the gap between impeller and housing must be increased.

This will lead to a less ideal flow and a somewhat lowered efficiency. Liquids with high viscosity will also call for special impeller design. The solution will usually be semi-open impellers with a small gap between the vanes and the housing--a design that also affects efficiency.

Cavitation can occur--even for "ideal" fluids like water--in high-speed pumps. Cavitation is an efficiency lowering evaporation of the fluid at local areas on the impeller with low pressure.

Low pulsation pumps (needed in processes before the paper machine) require an increased number of vanes. One solution is to use an end suction double flow (ESDF) impeller. This can be described as two "superimposed" impellers that split the flow at the impeller inlet and will double the number of vane passages.

PUMP HOUSING DESIGN

The helix shape used for many rotodynamic pumps provides an even speed for the liquid around the circumference of the pump. Even velocity means fewer flow disturbances and higher efficiency. When reaching the outlet, the velocity of the medium often is very high. A diffuser is inserted in the flow to reduce velocity up to the connecting flange. The diffuser is a sort of spacer that transfers the flow and simultaneously reduces the velocity and converts it into static pressure at the out-let. The design of the diffuser is of crucial importance for pump efficiency.

Sometimes a pump is operated at varying flows and thus will often not run at the ideal flow. This will lead to an uneven distribution of forces in the housing, with consequent strong radial forces on the shaft and bearings. A way to deal with these forces--which can lead to increased risk of pump failure--is to give the housing a more cylindrical shape. This may, however, produce a some-what lower efficiency.

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MECHANICAL LOSSES

The relative importance of mechanical losses decreases with the increased size of the pump. However, seals (especially double seals) can cause substantial friction losses in smaller, high-pressure pumps. These losses will, of course, have a negative effect on pump efficiency.

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SPECIFIC FACTORS

Maximum efficiency is always desirable but several other demands can be of equal or greater importance:

* Reliability

* Overall length of service time

* Service conditions

* Environmental conditions

* Local climate conditions

DIMENSIONING OF THE PUMP

Pump size should, of course, be chosen based on the most efficient flow rate of the pump. It is best to have a wide range of pump sizes to choose from. If there will be variations in flow the chosen pump should have a maximum flow just above the most efficient flow of the pump. If there will be large variations in flow or head, one should perhaps consider a variable speed drive (VSD). The higher investment cost for a VSD operated pump can often be easily regained on energy savings and a lower LCC.

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SUMMING UP

Almost all factors can influence efficiency. As efficiency strongly affects energy consumption (often the bulk of LCC), LCC calculations can be a powerful tool for optimizing efficiency.

EDITOR'S NOTE:

This article originally appeared in the ABS Partners magazine.

RELATED ARTICLE: IN THIS ARTICLE YOU WILL LEARN:

* What is pump efficiency.

* How pump design affects efficiency.

* Factors for choosing the right pump.

ADDITIONAL RESOURCES:

* The three ISO standards for rotodynamic pumps are very useful tools for selecting pump type and optimizing efficiency for the specific application.

* EN-ISO 9905- Technical Specification for Centrifugal Pumps - Class I

* EN-ISO 5199- Technical Specification for Centrifugal Pumps - Class II

* EN-ISO 9908- Technical Specification for Centrifugal Pumps - Class III

RELATED ARTICLE

Definition 1

Pump efficiency = [h.sub.h]

[h.sub.h] = [Q x H x r x g]/[3600 x [P.sub.1] x [h.sub.m]]

where

Q = flow in [m.sup.3]/h

H = head in metres

r = density in kg/[m.sup.3]

g = acceleration due to gravity in m/[s.sup.2]

[P.sub.1] = input power in kW

[h.sub.h] = hydraulic efficiency in %

[h.sup.m] = motor efficiency in %

Definition 2

The specific speed = [n.sub.s]

[n.sub.s]=n x [[Q.sup.1/2]/[H.sup.3/4]]

where

Q = flow in [m.sup.3]/s

H = head in metres

[n.sub.s] = specific speed

n = speed in r.p.m.

KJELL ALFREDSSON, ABS

ABOUT THE AUTHOR

Kjell Alfredsson holds a Master of Science degree. He has been with ABS almost 40 years, and was named technical director of ABS and Cardo Pump AB in 1993. He retired from the company in 2001 but is still active as a senior consultant. Alfredsson has also been active within ISO and CEN standardization committees and within Europump, where he acted as secretary during the production of the LCC Guideline.