"Multiplicity ought not to be posited without necessity."
— Occam's Razor
Split Flow: Innovative API Pump Progress
By W.E. Murray
Split Flow Pumps™ conform with API Standard 610. They may be either overhung horizontal (OH2) or vertical in-line (OH3) or between bearings horizontal two-stage (BB2). The Split Flow design separates or splits an inlet flow stream into two separate outlet flowtreams, each stream for different head-capacity process requirements. They are two-pumps-in-one with a common driver (motor or turbine).
Split Flow Pumps are used for dual service applications, i.e., where liquid from a common source is pumped to two separate destinations, typically high-flow at low-head and low-flow at high-head.
Most API pump services use two, 100% capacity pumps, i.e., one operating and one installed spare. (See Ref. PFD 1.) Since a Split Flow is two pumps in one, a single Split Flow pump will replace two separate pumps.
Alternately, to reduce the equipment count, some system designs use over sized pumps to envelop both the high-head, low-flow and low-head, high-flow streams. (See Ref. PFD 2.) This results in redundant head capacity capability and requires larger drivers than the four-pump designs. The Split Flow option employs a single operating 100% capacity pump (with an accompanying spare). (See Ref. PFD 3.)
Pump sizing criteria and common practice limits the use of overhung pump types. The Split Flow feature of one inlet and two outlets for different head capacity (HQ) conditions, i.e., Hybrid Hydraulics™, is also available for between bearings configurations, i.e., API Standard 610 (BB2), e.g., with impellers in tandem as with overhung design, and also, for larger sizes, with impellers oriented hub-to-hub or eye-to-eye with double-suction availability for primary impellers.
Replacing four pumps with their drivers, associated piping and related control systems with two, is an obvious capital cost benefit. Space is saved and there are also maintenance benefits, i.e., when overhung pumps are used, a total of two sears replace four. When between bearings pumps are used, a total of four seals replace eight. Fewer seals also reduces the risk of hazardous leaks.
API Standard 610 illustrates rotor vibration at various flows with respect to Best Efficiency Point (BEP). Pumps are often run “back on the curve,” i.e., at flows between BEP and minimum safe flow. This results in higher rotor vibration and reduces the Mean Time Between Repair (MTBR). Selection of high-head pumps for flows below 50 to 80 gpm (typical BEP range for 1’’ API pumps is at 3,550 rpm) is difficult. The Split Flow use of Hybrid Hydraulics, i.e., two different impellers in the same pump, enables approximately half of the less efficient low-flow head to be produced by the more efficient, higher flow primary impeller.
The overhung Split Flow design uses drilled hold disk type secondary impellers to shorten the shaft cantilever to produce flow below 50 gpm without the “back on the curve” problems inherent with conventional impellers. This includes the capability of “dead-heading” the secondary impeller, providing the primary impeller is operating above its minimum safe flow.
Commercial operation of Split Flow Pumps began in June, 1996 at the Shell refinery in Martinez, Calif., where a pair of Split Flow Pumps were used in an HGHT unit for fractionator reflux. The 3,600 rpm motor drivers were 60 hp. The oversized pumps considered for a conventional alternate design would require 125 hp motors and be more costly between-bearing type due to impeller diameter exceeding the user’s 13’’ maximum impeller diameter criteria for overhung pumps at 3,550 rpm.
A 2011 revamp removed these pumps from service. The operators documented that the pumps satisfied all premised requirements and reliability during their 15 years of service.
While 15 years of reliable service verified the viability of the Split Flow concept, there is always room for improvement. Toward that end, the number of internal running clearances (wear rings) was reduced from 4 (totalling approximately 250% internal pressure drop), to 3 (totalling approximately 150% internal pressure drop.)*
This change enables a 20% shorter rotor cantilever with resultant reduction of shaft deflection at the seal faces. This enhances seal life. The change also allows for secondary impeller feed by internal rather than from external conduits.
* From the Igor Karassik textbook: “The major potential cause of any reduction in capacity from the original condition is wear at the wearing rings, in other words, the increased leakage through the increased clearances at these rings. The net capacity of a pump at any given hear is reduced by the increase in leakage.”
Article posted July 16, 2015
“Everything should be as simple as possible, but not simpler.”
William of Ockham (1285-1349)