The 14th century philosopher William of Ockham (1285–1349) opined that “Multiplicity ought not to be posited without necessity”, which in contemporary lexicon translates as ‘everything should be as simple as possible, but not simpler’. Occam’s Razor is the inspiration for a unique pump innovation used for petrochemical refining. Split Flow™ simplifies the current practice of selecting multiple or oversize pumps for dual service applications. The Split Flow design incorporates an auxiliary booster impeller with a separate discharge into an American Petroleum Institute (API) Standard overhung centrifugal pump (see Figure 1). This allows the pump to split the discharge into two separate streams. The advantages of this design are: use of smaller motors - energy (kW) savings and thus carbon dioxide (CO2) reduction; use of fewer pumps – capital savings; and better pump-to-system hydraulic fit – longer mean time between repairs (MTBR) for seals and bearings (see Figure 2).
A common design issue encountered in the refining process involves pumping a liquid to two dispositions where a smaller secondary stream requires a considerably higher discharge pressure than the primary stream. An example is the overhead liquid on a naphtha splitter where liquid from the overhead receiver is used as reflux and is also pumped to a stabiliser that operates at a higher pressure than the naphtha splitter. Current practice is to either install two pumps, e.g. a high-flow, low-head pump for the reflux service and a separate low-flow, high-head pump for the product/export service, or to install a single pump oversized to pump all of the liquid at the higher discharge pressure and throttle the discharge flow (by control valve) to produce the required lower head for the primary reflux stream. The two-pump system requires that one pump be sized for low-flow, high-head; this frequently poses selection and reliability issues. The single oversized pump system requires a larger pump and motor driver and involves throttling of the primary flow; this wastes energy. Another option employs a low-head primary pump sized for both streams with a separate low-flow booster pump to increase the head of the product stream.
Considering Occam’s Razor, it is logical to incorporate the booster pump performance into the primary pump (see Figure 3). The single-pump system using Split Flow simply incorporates the booster pump hydraulics into the primary pump, with obvious benefits, including no multiplicity, no throttling with attendant energy waste and optimisation of the pump hydraulic performance for both streams.
Figure 1 illustrates the prototype pump with a top suction orientation option (the primary top discharge is not shown). The pump case, primary impeller and bearing bracket are standard. The shaft and case cover are modified to incorporate the booster impeller. The prototype pumps were designed with external conduits to feed the auxiliary impeller from the primary impeller discharge. In keeping with Ockham’s simplicity premise, subsequent designs provide for internal feed from the primary to the booster impeller. To simplify the adaptation of a custom-designed, low-flow booster impeller to API Standard pumps, a drilled-hole disc type design (similar to the ‘kicker stage’ used with boiler feed pumps) was selected for ease of manufacture, lowest axial space requirements and robust characteristics. The modified pump design has proved its reliability, with minimal maintenance of seals and bearings. Where process conditions require secondary flows exceeding practical limits for drilled-hole disc type impellers, suitable impellers are available from various other API Standard pump types.
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