IPE Approach to Centrifugal Pump Sizing and Selection

1. Purpose

This procedure describes centrifugal pump selection.

2. General

The New Hydraulics Program (NHP) and Tool utilize a common pump selection tool for selecting the type of centrifugal pump design and predicting performance. This pump selection chart is based on a cross-section of global centrifugal pump supplier’s published performance curves. It incorporates Inflection Point Engineering and API sizing philosophies, and reflects median hydraulic fit.

The pump selection Tool T-501-03 was developed to facilitate the pump selection process.

The operating conditions and pump performance reported in the pump selection tool are based on 60 Hz performance. For 50Hz performance, the fan laws must be applied with respect to the change in rotative speed. See Inflection Point Engineering Design Manual Attachment 2

3. Pump Selection

The pump selections reflect the following Inflection Point Engineering or API hydraulic requirements:

a. The best efficiency point for the furnished impeller shall be preferably between the rated and the normal point.

• Suction specific speeds are less than or equal to 11,000 ( gpm, ft). (Source: Std. Spec. 5-11 Paragraph 5.1.11).

Note: In , several major pump companies may take exception to this requirement. Inflection Point Engineering shall respond to such deviation requests on a particular project as follows:

“Inflection Point Engineering accepts the deviation to impeller suction specific speed limit provided the contractor investigates the vendor's proven successful experience.

Impellers with low initial incidence angle, an airfoil style leading edge which extends and twists axially and with an optimized pressure distribution on the blade have been shown to deliver good performance while deviating from an Nss < 11,000 suction specific speed limit. It is also essential the pump is properly sized for the desired hydraulic conditions (i.e. not oversized for the application).”

This answer draws heavily from information published in “Influence of Suction Specific Speed on Pump Reliability, IR-No. 1304” by Dr. J.-F. Guelich of Sulzer pumps.

Note 2: In the xylene fractionation column bottoms pump service, and when a fired heater is used as the column reboiler, the allowable suction specific speed limit can be increased to 13,000 (US gpm, ft). This is because these pumps are expected to run at constant flow rates.

c. Limit the maximum impeller diameter for a single-stage overhung pump design operating at 3550 rpm to 15 inches. For a two pole motor connected to a 50 Hz electrical power grid, the impeller diameter may be 17 inches maximum.

Source: (Std. Spec. 5-11 Paragraph 5.6.16.)

d. Pumps shall be capable of at least a 5% head increase at rated conditions by replacement of the impeller(s) with one(s) of larger diameter or different hydraulic design. (Source: API Std. 610 11th Edition, Paragraph 5.1.6)

The pump selection tool is valid for all of the following pump design types:

a. Single Stage Process pumps (OH2)

b. Between Bearing Process pumps (BB2)

c. Multi-stage Process pumps (BB3 & BB5)

d. ASME pumps* (OH1)

e. Sundyne pumps (OH6)

• Canned or Mag-drive Sealless pumps*

* Pump selections and performance for these types of pumps are located on separate tabs in Excel Tool T-501-03.

In the event the operating conditions are beyond the range of the pump selection tool, contact a rotating equipment specialist. Also, certain pump types such as Sunflo and sump pumps are not included. Manually select these types of pumps from the vendor’s published curves.

For API pumps, the selections were based on the following considerations:

• Best hydraulic fit
• Lowest NPSHR
• Lowest initial capital cost
• Lowest energy costs (i.e. efficiency)
• Best turndown
• Number of leak points

The Design Engineer is accountable for comparing the NPSHA to the NPSHR by the pump, reference Procedure ”.

4. Legend for Pump Selection Tool

Example: SSSS, Half, SV, 4, 35, 20

Pump Design Type:

• SSSS: Single Stage, Single Suction
• SSDS: Single Stage, Double Suction
• 2SSS: Two Stage, Single Suction
• MSSS: Multi-Stage, Single Suction
• MSDS: Multi-Stage, Double Suction
• SUN: Sundyne

• Rotational Speed:

• FULL: full speed, 3550
• HALF: half speed, 1750
• THIRD: third speed, 1180
• HIGH: high speed, Sundyne

• Volute Casing Type:

• SV: Single volute
• MV: Multiple volute

Notes:

• Minimum flows for Sundyne pumps are not reported in the pump selection tool. Inflection Point Engineering estimates minimum flow for Sundynes to be 60% of rated flow. Reference Procedure for additional background on selection of Sundyne pumps.

• Volute casing type is not reported for Sundyne pumps. Sundyne pump designs utilize a diffuser type of casing design rather than a volute.

• Pump selections highlighted in gray in the T-501-03 reflect the most recent additions to the pump selection database. Currently, this includes the low flow class of API OH2 style pumps using semi-open Barkse impellers and diffuser casings.