Rotodynamic (Centrifugal) Pumps

Rotodynamic (Centrifugal) Pumps

1.1 Types and nomenclature

Rotodynamic pumps may be classified by such methods as impeller or casing configuration, end application of the pump, specific speed, or mechanical configuration. The method used in Figure 1.1.3a is based primarily on mechanical configuration.

1.1.1 Scope

This standard covers rotodynamic pumps with centrifugal (radial), mixed flow, and axial flow impellers, as well as regenerative turbine and pitot tube type pumps, of all industrial/commercial types except vertically suspended diffuser turbine pumps. It contains description of types, nomenclature, and definitions.

1.1.2 Definition of rotodynamic (centrifugal) pumps

Rotodynamic pumps are kinetic machines in which energy is continuously imparted to the pumped fluid by means of a rotating impeller, propeller, or rotor. The most common types of rotodynamic pumps are centrifugal (radial), mixed flow, and axial flow pumps.

Centrifugal pumps use bladed impellers with essentially radial outlet to transfer rotational mechanical energy to the fluid primarily by increasing the fluid kinetic energy (angular momentum) and also increasing potential energy (static pressure). Kinetic energy is then converted into usable pressure energy in the discharge collector.

1.1.3 Types of rotodynamic pumps

Rotodynamic pumps are most commonly typed by their general mechanical configuration (see Figures 1.1.3a, b, c, and d). The broadest characteristics, which include virtually all centrifugal pumps, are discussed in the following paragraphs:

1.1.3.1 Overhung impeller type

In this group, the impeller(s) is mounted on the end of a shaft that is cantilevered or “overhung” from its bearing supports.

These pumps are either close coupled, where the impeller is mounted directly on the driver shaft; or separately coupled, where the impeller is mounted on a separate pump shaft supported by its own bearings. One variation of this design is the submersible type, where a close-coupled pump/electric motor unit is designed to operate while submerged in the liquid it is pumping or another liquid.

1.1.3.1.1 Close coupled

Close-coupled pumps are commonly characterized by the following attributes:

The pump and driver share one common shaft; the driver bearings absorb all pump thrust loads (axial and radial).

The driver is aligned and assembled directly to the pump unit with machined fits.

1.1.3.1.2 Short coupled

Pumps described as short coupled have a coupling arrangement in which the motor is supplied with a flange adaptor that mounts directly to the casing, or body of the pump, thereby permitting the use of a single or solidly coupled shaft. A variation of this design is a magnetically coupled sealless pump, which uses a series of magnets mounted directly on the motor shaft. (See also Section 1.1.6.8 Special case ASME/ANSI B73.1, C-frame adaptor.)

Short coupled pumps are commonly characterized by the following attributes:

The pump and driver have separate shafts; the pump has an integral bearing housing to absorb all pump thrust loads (axial and radial). The driver is aligned and assembled directly to the pump unit with machined fits.

1.1.3.1.3 Rigidly coupled

Pumps described as rigidly coupled have their shaft rigidly coupled to the driver shaft.

Rigidly coupled pumps are commonly characterized by the following attributes:

The pump and driver have separate shafts connected by a rigid coupling; the pump has an internal product-lubricated radial bearing. The driver is aligned and assembled directly to the pump unit with machined fits. The driver bearings absorb all pump axial thrust loads and residual radial loads.

1.1.3.1.4 Flexibly coupled

Pumps described as flexibly coupled have the pump shaft flexibly coupled to the driver shaft via a flexible element drive coupling. Usually of the spacer type.

Flexibly coupled pumps are commonly characterized by the following attributes:

Pump and driver have separate shafts; the pump has an integral bearing housing to absorb all pump thrust loads (axial and radial). With this arrangement the motor may be mounted on a support that is independent of the pump and not structurally connected to the pump frame.

1.1.3.1.5 High-speed integral gear-driven pumps

High-speed integral gear-driven single-stage overhung pumps have a speed increasing gearbox integral with the pump. The impeller is mounted directly to the gearbox output shaft. There is no coupling between the gearbox and pump; however, the gearbox is flexibly coupled to its driver. These pumps may be oriented vertically or horizontally.

Integral gear-driven single-stage overhung pumps are commonly characterized by the following attributes:

Pump, gearbox, and driver have separate shafts; the pump and gearbox have internal bearings to absorb all thrust loads (axial and radial). The gearbox shaft is flexibly coupled to the driver shaft and the motor mounts on a frame supported by the pump and gear unit.

1.1.3.2 Impeller between-bearings type

In this group, the impeller(s) is mounted on a shaft with bearings at both ends. The impeller(s) is mounted between these bearings.

These pumps may be further identified as single-stage or multistage configurations.

Attributes	Pump type
	Close coupled	Rigidly coupled	Short coupled	Flexibly coupled	Integral gear
Pump and driver share one common shaft	x
Pump and driver have separate shafts		x	x	x	x
Driver bearings absorb pump axial thrust loads	x	x
Driver bearings absorb all pump axial thrust & radial loads	x
Pump has an integral bearing housing to absorb all pump loads			x	x	x
Motor is mounted on a frame structurally attached to the pump	x	x	x		x
Motor may be mounted on a support that is independent of the pump, not structurally connected to the pump frame				x

1.1.3.3 Pumps of other configuration

These pumps operate using the same basic kinetic principles but are configured differently than the conventional rotodynamic designs. The following examples fall within this description.

1.1.3.3.1 Regenerative turbine type

Regenerative turbine pumps are characterized by a low rate of flow and high head. This design uses peripheral or side channel vanes or buckets that are typically manufactured integral with a rotating impeller to impart energy to the pumped liquid. The liquid travels in a helical pattern through the impeller vanes and corresponding flow passages, with the liquid pressure increasing uniformly through the passages from inlet to the discharge.

1.1.3.3.2 Pitot tube type

The pitot tube pump is a variation of a rotodynamic design and uses a pitot tube, in lieu of a volute or diffuser, to capture flow and convert velocity energy to pressure. The primary feature of a pitot tube pump that differentiates it from a conventional rotodynamic pump is that it uses a rotating casing instead of an impeller to impart velocity to the pumped liquid. The pitot tube design follows conventional pump affinity rules; however, it is capable of generating higher head than a comparable rotodynamic design at an equivalent tip speed.

1.1.3.3.3 Hydraulic power recovery turbine

A hydraulic power recovery turbine is a rotodynamic pump that operates by accepting flow in the reverse direction as normal, by virtue of the fact that a differential pressure is applied across its connections. Liquid enters the (normal) discharge connection of the pump and exits the (normal) suction connection. As such, the pump operates as a turbine, producing useable shaft power as a function of speed and differential pressure. Another name used for this type of machine is pump as turbine (PAT). (See ANSI/HI 1.3 Rotodynamic (Centrifugal) Pumps for Design and Application, Appendix A.)

1.1.3.3.4 Vortex (recessed impeller) type

A vortex or recessed impeller pump is a rotodynamic pump designed with large, uniform clearances between the open impeller vanes and the casing shroud. Radial and cup type impellers are used. The impeller is recessed from the liquid flow path, which induces a vortex action to the liquid.

Correspondingly, trash and solids can pass through the pump without impinging on the impeller.

1.1.4 Impeller designs

Impeller designs are classified as radial, mixed, or axial flow, depending on their geometry. These designs are differentiated by specific speed and impeller types as described in the following paragraphs. (Refer to Sections 1.1.4.2, 1.1.4.3, and 1.1.4.4 for additional description.)

1.1.4.1 Specific speed (nS) and suction specific speed (S)

The user is cautioned to check carefully the basis of calculation of specific speed and suction specific speed before making comparisons because there are subtle but significant differences in methods used throughout industry and in related textbooks and literature.

Preferred terms, units, and symbols to be used in the technology of pump applications are shown in Table 1.2a.

US customary units

When calculating the value for specific speed and suction specific speed, the unit of measurement used for rate of flow is defined in US gallons per minute (gpm).

Metric units

When calculating the value for specific speed and suction specific speed, the unit of measurement used within this standard for rate of flow is cubic meters per second (m³/s).

(An alternative method of calculating this value is to use m³/h as the unit of measurement for rate of flow, which then results in a value that is (3600)0.5, i.e., 60 times greater.

Specific speed is an index of pump performance (developed total head). It is determined at the pump's best efficiency point (BEP) rate of flow, with the maximum diameter impeller, and at a given rotative speed. Specific speed is expressed by the following equation:

Where:

n_s = specific speed

n = rotative speed, in revolutions per minute

Q = total pump flow rate, in cubic meters per second (US gallons per minute)

H = head per stage, in meters (feet)

NOTE: When calculating specific speed using units of cubic meters per second for flow rate and meters for head per stage, 51.6 is the conversion factor for specific speed in US gallons per minute and feet (i.e., metric 51.6 = US customary units.)

The usual symbol for specific speed in US customary units is Ns.

An alternative definition for specific speed is sometimes used based on flow rate per impeller eye rather than total flow rate. When applying this alternative method to a double suction impeller pump, the resultant value of specific speed is less by a factor of 1/(2)0.5 (i.e., 0.707 times less).

Suction specific speed is an index of pump suction operating characteristics. It is determined at the BEP rate of flow with the maximum diameter impeller. (Suction specific speed is an indicator of the net positive suction head required [NPSH3] for given values of capacity and also provides an assessment of a pump's susceptibility to internal recirculation.) Suction specific speed is expressed by the following equation:

Where:

S = suction specific speed

n = rotative speed, in revolutions per minute

Q = flow rate per impeller eye, in cubic meters per second (US gallons per minute)

= total flow rate for single suction impellers

= one half total flow rate for double suction impellers

NPSH3 = net positive suction head required in meters (feet) that will cause the total head (or first-stage head of multistage pumps) to be reduced by 3%

NOTE: When suction specific speed is derived using cubic meters per second and meters, the conversion factor to suction specific speed in US gallons per minute and feet is 51.6. The US customary symbol Nss is sometimes used to designate suction specific speed.

The value S is an assessment of a pump's inlet design, including both the stationary casing and the rotating impeller design elements. Higher numerical values of S are associated with better NPSH capabilities. For pumps of typical suction inlet design, values range approximately from 120 to 250 (6000 to 13,000). In special designs, including inducers, S values can be up to 700 (35,000) or higher depending on the connected inlet piping, the pump's suction casing arrangement, the range of flow over which the pump must operate, size and power rating of the machine, and other considerations.

1.1.4.2 Radial flow

Pumps of this type with single inlet impellers usually have a specific speed below approximately 90 (4500) and with double suction impellers, a specific speed below approximately 135 (7000). In pumps of this type, the liquid enters the impeller at the hub and flows radially to the periphery, exiting perpendicular to the rotating shaft. (See Figure 1.1.4.2.)

1.1.4.3 Mixed flow

This type of pump has a single inlet impeller where the flow enters axially and discharges in a mixed axial and radial direction. Pumps of this type usually have a specific speed from approximately 90 (4500) to 200 (10,000) (see Figure 1.1.4.3).

1.1.4.4 Axial flow

A pump of this type, sometimes called a propeller pump, has a single inlet impeller with the flow entering axially and discharging nearly axially. Pumps of this type usually have a specific speed above approximately 200 (10,000) (see Figure 1.1.4.4).

1.1.5 Construction drawings

The construction drawings on the following pages (Figures 1.1.5a – bb) were prepared to provide a means for identifying the various pump types covered by the HI Standards. The drawings also serve as the basis for a common language between the purchaser, manufacturer, and specification writer.

In general, the individual part names on these drawings are numbered such that rotating parts have been assigned even numbers and nonrotating parts have been assigned odd numbers. There are, however, a few exceptions.

In cases where a pump may use two or more parts that are of the same generic type but different geometries (e.g., gaskets), this difference is indicated by the addition of a letter suffix to the item number (e.g., 73A, 73B, etc.).

Hydraulic Institute pump icons represent the general class of pump design. Where available, icons are shown adjacent to the corresponding pump cross sections. Additionally, Section 1.2.9 specifically defines the icons in greater detail.

1.1.6 General information

1.1.6.1 Size of a rotodynamic pump

The standard Hydraulic Institute nomenclature for pump size is “discharge opening size by maximum rated nominal impeller diameter (each indicated in millimeters).” For example, a pump with 80-mm suction, 50-mm discharge openings, and a 160-mm maximum rated nominal impeller diameter, will be referred in SI terms as a 50-160 pump.

Conversely, US nomenclature refers to pumps by using the notation: inlet opening size by discharge opening size by maximum rated nominal impeller diameter, all measured in inches. The pump measured above in US customary units may be referred to as a “3 2 6” pump, i.e., the smaller of the numbers is the discharge size. The pump described above in ISO standards would be referred to as an “80-50-160” pump.

These methods are in compliance with methods used in other reference industry standards such as ISO 5199 and ANSI/ASME B 73.1M.

1.1.6.2 Duplicate performance pump

A duplicate pump is one in which the performance characteristics are the same as another, within the variations permitted by ISO 9906 or ANSI/HI 1.6 test standards, and parts are of the same type. But, by reason of improved design and/or materials, mounting dimensions and parts are not necessarily interchangeable.

1.1.6.3 Dimensionally interchangeable pump

An interchangeable pump is one in which the mounting dimensions are such that the replacement pump can be mounted on the existing bedplate and match existing piping and driver, with hydraulic characteristics and materials to be specified. Interchangeability may involve some variation, not necessarily significant, as a result of manufacturing tolerances.

1.1.6.4 Identical pump (performance and dimensional)

An identical pump is a replica of, and is interchangeable with, a specific pump. Where it is intended that a pump is to be identical in all respects, including parts, mountings, connecting flange dimensions, and materials, it should be identified as identical with pump serial number XXXXXX. An identical pump will replicate the original pump in performance and dimensions as closely as the manufacturing tolerances allow.

1.1.6.5 Definitions for bare rotor and rotating assembly

1.1.6.5.1 Overhung pumps

1.1.6.5.1.1 Liquid end (or wet end) assembly

Term used for overhung pumps to describe the wetted parts, specifically the casing, cover, stuffing box or seal chamber, impeller, and associated fasteners and gaskets. See Figure 1.1.6.5.1.1.

1.1.6.5.1.2 Power end (or frame assembly)

Term used for overhung pumps to describe the assembly that connects the motor to the pump.

The assembly contains: shaft, sleeve (if supplied), thrust bearing, radial bearing, housing, and frame.

See Figure 1.1.6.5.1.2.

1.1.6.5.1.3 Back pull-out assembly

The major assembly for overhung pumps, which includes all parts other than the casing.

The assembly contains: casing cover, stuffing box or seal chamber, mechanical seal assembly or packed gland assembly, adapter (if fitted), power end, and impeller.

The back pull-out assembly is configured as an aid to assembly/disassembly and may be arranged to be installed and removed without disturbing the pump casing or the driver. It thereby provides an aid to improve maintenance by allowing quick change-out of parts and minimized downtime.

See Figure 1.1.6.5.1.3.

1.1.6.5.1.4 Bare rotor

A bare rotor shall consist of the following as an assembly (where used):

• A shaft with all nuts
• Keys
• Impeller
• Impeller ring(s)
• Shaft sleeves

It shall not include such items as mechanical seal(s), gland(s), lantern ring(s), packing, water shield(s), oil thrower(s) or oil ring(s), bearings, bearing appurtenances, coupling, pulley, or sheave.

1.1.6.5.1.5 Rotating assembly

A rotating assembly shall consist of a bare rotor plus casing wearing rings (where used), casing bushings (where used), bearings, and all other stationary or rotating parts required to be assembled over the shaft.

A rotating assembly shall include the following (where used):

• Packing and gland(s)
• Mechanical seal(s) and gland(s)
• Water shield(s)
• Oil thrower(s)
• Oil ring(s)
• Bearing housing cover(s)

It shall not include coupling, pulley, or sheave, nor shall it include bearing housings except when these are of a design that requires that they be assembled prior to mounting of the bearings.

1.1.6.6 Position of casing

The normal position of the discharge nozzle of an end suction horizontal pump shall be top vertical. Optional positions of the discharge nozzle shall be designated by degrees of rotation, measured from the vertical centerline in the clockwise (CW) [c] direction, facing the drive end of the pump (see Figure 1.1.6.6).

1.1.6.7 Rotation of shaft

Pumps are designated as having clockwise (CW) [c] or counterclockwise (CCW) [cc] rotation when facing the drive end of the pump (see Figures 1.1.6.6, 1.1.6.7a, and 1.1.6.7b).

To determine the rotation of a horizontal pump, stand at the driver end facing the pump (Figure 1.1.6.7a). If the top of the shaft revolves from the left to the right, the rotation is clockwise (CW) [c], and if the top of the shaft revolves from right to left, the rotation is counterclockwise (CCW) [cc].

1.1.6.8 Special case ASME/ANSI B73.1, C-frame adapter

The C-frame adapter is cast or fabricated with rabbet fits on either end. One end is mounted to the rabbet of a Cframe NEMA motor, the other end is mounted to a rabbet fit on the bearing frame of the pump. The rabbet fits preclude the need for realignment of the coupling following normal maintenance on the wet end of the pump. Installation requires control of manufacturing tolerances, proper coupling selection, and in some cases, initial motor alignment. Motor weight may also be limited due to the cantilevered construction.

Tolerances cannot always be controlled to ensure that shaft alignment will meet requirements with all pump components, therefore, special components, such as adjustment features and/or soft couplings, must be used to ensure satisfactory operation. Larger motors may also be supported from the baseplate.

They should not, however, be bolted down to the baseplate unless normally used casing bolts are eliminated as this could induce additional misalignment.

Designs differ between manufacturers, so manufacturers’ instructions for installation and operation must be followed to get satisfactory operation and life. Figure 1.1.6.8 depicts a typical configuration of the C-frame adapter to an overhung pump.