TechArchive

Original article date: May 1998

Ingersoll-Dresser Pumps has developed a new impeller with double row vanes for its CPX range of chemical process pumps. JOHN BOWER Engineering Manager explains why it’s important.

Though single stage centrifugal end suction pumps are widely used and are available in many different types none are specifically designed to meet applications with a low flow high total head requirement.

In most cases a low flow duty has been met with a pump sized for more flow than is required by the intended application. This provides the required pump capacity but it means that the pump has to operate away from its optimum design condition. The most common pump design has an impeller with a narrow outlet width and low number of vanes and needs a larger impeller diameter. This produces a larger heavier pump with only a fair suction performance.

Some special impeller designs for this duty have a large number of single curvature or even straight vanes with radial discharge. This can produce a high total head from a smaller diameter but inevitably has poor suction performance and low efficiency.

The problem arises from localised impeller inlet and outlet geometry athat is mutually conflicting. An optimised design for the impeller exit is compromised by the optimised design for the inlet and vice-versa.

Now this has been overcome in a new impeller from Ingersoll-Dresser Pumps by creating a design with a double row of independently optimised vanes. The first row geometry is matched to the suction flow conditions and is separated at the exit by an offset from the second row. This row in its turn is optimised for the discharge system conditions and therefore is not compromised by the impeller inlet. This fully harmonises the design and system needs leading to improved efficiency and reliability.

A team of engineers from Ingersoll-Dresser Pumps operations around the world developed the new impeller. Led by Bruno Schiavello Director of Fluid Dynamics at the Ingersoll-Dresser Pumps plant in Phillipsburg New Jersey. The other members of the team were Alan Paton Hydraulics Engineer at the Ingersoll-Dresser Pumps plant at Desio in Italy. The impeller design features created by the team have now been incorporated in a one-piece precision casting. This impeller already has a US Patent with the disclosure for a European Patent in process.

Ingersoll-Dresser Pumps has also launched the CPXP range of self-priming pumps which operate on the reflux principle and are therefore suitable for self-priming applications in all industries. Designed to meet the ISO 5199 standard the CPXP has a different casing to but shares the majority of parts with the CPX pump design. The casing is the key to the self-priming capaiblity and uses liquid flow back to prime the pumps removing the need for internal valves and external priming devices or foot valves.

CPXP pumps share the same footprint as CPX models which can be retrofitted with self-priming casings.

Q: Single stage pump impellers are not specifically designed to meet applications with low flow and high total head requirements. To what extent can the new impellers handle these conditions without compromising efficiency?

A: The particularly novel feature of the impeller is the ability to produce very high heads at low flow and at the same operate at low NPSH available. The design also leads to cost savings in the pump and motor.

At 30m3/h the NPSH required for the CPX impeller is less than 1m whereas most pumps have an NPSH requirement of about 3m. The head generated at 2900 rpm is 170m compared to 140 for comparable pumps with the same diameter of impeller. Efficiency is comparable despite the increased output.

Q: How is the efficiency suction performance and reliability improved?

A: The CPX impeller overcomes previous limitations by using five twisted vanes at the eye for low NPSH and minimum blockage and 20 nearly-straight vanes in the outer set for generating maximum head. Having the radial gap and offsetting the outlets and inlets of the two sets of vanes enhances the performance.

Q: Besides these types of pump are there any other pumps that are designed to handle these conditions?

A: To generate a high head at low flow from a given impeller diameter it is usual to use a large number of vanes but on a low flow design these vanes are too numerous and block the eye. In some designs these problems are avoided by using an over-sized impeller and the pump is operated at partial flow for the low flow duties. But this leads to higher hydraulic loads worse erosion and greater motor power than for a correctly sized pump.

Q: How does the cost of the impeller affect the overall cost of the pump?

A: Avoiding over-sized pumps for low flow duties reduces end-of-curve power absorbed and will generally drop the motor power required by one size. The CPX impeller is ideal for manufacture as an investment casting to achieve the precise geometry and surface finish. The compact size for the output achieved not only reduces the weight and cost of the impeller but also of the pump casing. Pump costs savings are between 5 and 10%

Examples of applications:

• John Bower
• Ingersoll-Dresser Pumps
• 01636 705151

May 1998