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Pump Engineer

It is common practice for most American Petroleum Institute (API) pump installations to incorporate 100 percent capacity spares. It is also common practice to install two pairs of pumps, taking suction from the same source, for different process services (see Figure 1, PFD 1).

Two-pump systemFigure 1. PFD 1 (top): Two-pump system. PFD 2 (middle): Single oversized pump system with pressure-control valve. PFD 3 (bottom): Single pump system using split flow feature. (Graphics courtesy of the author)

Process designs using two 100 percent capacity pumps, rather than four, are sometimes used. For example, some operations oversize pumps to envelop two different process conditions—typically low flow, high head and high flow, low head (see Figure 1, PFD 2). The larger motors required for this option waste energy compared with four-pump designs. Another option is to use two 100 percent capacity pumps with the split flow feature (see Figure 1, PFD 3).

What Are Split Flow Pumps?

Split flow pumps separate or split a pump inlet flow stream into two separate outlet flow streams using hybrid hydraulics—for example, different impellers in the same pump casing (see Figure 2). The first stage is selected for the total flow of both outlet streams at the head of the first outlet stream. The second stage is selected for the lower flow at the higher additional head of the second outlet stream.

Criteria for use of split flow feature for dual-service API standard 610 pumpsFigure 2. Criteria for use of split flow feature for dual-service API standard 610 pumps; objective is to use smaller motors and reduce number of installed pumps.

Split flow pumps conform with API Standard 610 and may be either overhung horizontal (OH2) (see Figure 3) or vertical inline (OH3) with low-flow secondary impellers, or between-bearings horizontal two-stage (BB2) (see Figure 4). Split flow pumps are “two pumps in one” with a common driver (motor or turbine).

Overhung horizontalFigure 3. Overhung horizontal (OH2).

The overhung split flow design uses drilled-hole, disk-type secondary impellers. This shortens the shaft cantilever (compared with conventional impellers) and produces flow below 50 gallons per minute (gpm) without the “back-on-the-curve” problems inherent with conventional impellers. This includes the capability of dead-heading the secondary impeller, assuming the primary impeller is operating above its minimum safe flow.

Pump-sizing criteria and common practice limit the use of overhung pump types. The split flow feature of one inlet and two outlets for different head-capacity (H-Q) conditions is available for between-bearings configurations, including tandem as with the overhung design, and for larger sizes, with impellers oriented hub-to-hub or eye-to-eye and with double suction availability for primary impellers (see Figure 4).

Between-bearings horizontal two-stageFigure 4. Between-bearings horizontal two-stage (BB2)

When & Why Are Split Flow Pumps Used?

Split flow pumps are used for dual-service applications—for example, where liquid from a common source is pumped to two separate dispositions (see Figure 2).

Most API pump services use two 100 percent capacity pumps—one operating and one installed spare (see Figure 1, PFD 1). Alternatively, to reduce the equipment count, some system designs use oversized pumps to envelop both the high-head, low-flow and low-head, high-flow streams (see Figure 1, PFD 2). This results in redundant H-Q capability and requires larger drivers than the four-pump designs.

Because the split flow design is actually two pumps, one split flow pump can replace two separate pumps (see Figure 1, PFD 3).

What Are the Benefits of Using Split Flow Pumps?

Replacing four pumps with their drivers, associated piping and related control systems with two pumps provides capital cost and space savings.

This design also provides maintenance benefits. When overhung pumps are used, two seals replace four. When between-bearings pumps are used, four seals replace eight.

Having fewer seals reduces the risk of hazardous leaks.

API Standard 610 illustrates rotor vibration at various flows with respect to the best efficiency point (BEP) (see Figure 5). Pumps are often run “back on the curve,” meaning at flows between the BEP and minimum safe flow. This results in higher rotor vibration and reduces the mean time between repairs (MTBR).

Relationship between flow and vibrationFigure 5. Relationship between flow and vibration

Selecting suitable high-head pumps for flows below 50 and 80 gpm (typical BEP range for 1-inch API pumps at 3,550 revolutions per minute [rpm]) is a challenge. The split flow use of hybrid hydraulics enables approximately half of the less efficient, low-flow head to be produced by the more efficient, higher-flow primary impeller (see Figure 6).

Pump selection comparison for debutanizer reflux/product serviceFigure 6. Pump selection comparison for debutanizer reflux/product service

How & Why Were Split Flow Pumps First Used?

The first pair of split flow pumps began commercial operation in a California refinery in June 1996. The service was fractionator reflux with an HGHT unit.

The 3,600-rpm motor drivers were 60 horsepower (hp). The oversized pumps considered for a conventional alternate design would require 125-hp motors.

The pumps also would need to be the more costly between-bearings type because of an impeller diameter exceeding the user’s 13-inch maximum impeller diameter criteria for overhung pumps at 3,550 rpm (see Figure 6).

A 2011 process revamp removed these pumps from service. The operators documented that the pumps satisfied all premised requirements, including reliability, during their 15 years of service.

Improvements to the Prototype Overhung Design

While 15 years of reliable service verified the viability of the split flow concept, there is almost always room for improvement.

The number of internal running clearances (at wear rings) was reduced from four (totaling approximately 250 percent internal pressure drop) to three (totaling approximately 150 percent pressure drop).

Deleting hub wear rings and balance holes from the primary impeller (a standard feature with most OH2 designs, including the original split flow prototype) enhances both hydraulic efficiency and net positive suction head (NPSH) margin by precluding bypass into the impeller eye.

This design enables hydrodynamic axial balance of the rotor assembly with fewer running clearances. It also allows a 20 percent shorter rotor cantilever and secondary impeller feed by internal rather than from external conduits.