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Screw pumps are a practical alternative

By Josh Pepper and Michael Moore

In the industrialized world, there may be no more deceptively named mode of transportation than the “very large crude carrier,” or VLCC. Calling an oil tanker that can be nearly half a kilometer in length “very large” is akin to saying that King Kong is a “pretty big” gorilla or that the Great Wall of China is “kinda long.”

In reality, the VLCC — along with its “supertanker” cousin, the somewhat more appropriately named “ultra large crude carrier” (ULCC) — plays a, no pun intended, “large” role in the transportation of crude oil and refined petroleum-based products around the globe. The crude-carrying capacity of a VLCC can range from 160,000 to 320,000 dead weight tons (DWT), with ULCCs able to accommodate upwards of 550,000 DWT in one trip.

Traditionally, centrifugal pumps have been a go-to technology that shipbuilders have utilized on their vessels for various fluid-transfer applications. This white paper, however, will show how an oftentimes overlooked pumping technology — positive displacement twin- and triple-screw pumps — can be a more versatile, reliable and efficient alternative to centrifugal pumps in the critical loading/unloading and transfer operations on the vessels that carry crude oil and finished petroleum-based products for the long-haul shipping industry.

The most obvious challenge — and perhaps the toughest to eliminate — in making screw pumps a front-of-mind option for shipbuilders is convincing them to wean themselves off of a technology that has become defined as the best one for a variety of unique fluid-handling tasks. Generally speaking, the bulk of the fluids that are handled on ships, tankers and barges must be transferred at very high flow rates and volumes, which hits the operational sweet spot of centrifugal pumps.

When meeting the operational needs of the centrifugal pump, though, care must still be taken to ensure that the centrifugal pump is operating at or close to what is known as its “Best Efficiency Point,” or BEP. A centrifugal pump that is able to function between 80% and 110% of its BEP is operating within a tolerable range for efficiency.

However, when the operation of the pump moves too far off its BEP uneven pressure will be applied to the impeller, which can result in increased radial thrust that will cause the pump’s shaft to “deflect.” When this deflection occurs, higher loads will be placed on the bearings and mechanical seal, which can lead to damage to the pump’s casing, back plate and impeller. Because of these potential operational hazards, ensuring that the pump is working within an acceptable BEP range can be a time-intensive task since the pump must be monitored constantly and adjusted as needed, which costs time and money.

There are other considerations that shipbuilder must contemplate before deciding on the best pump technology, including:

• Small footprint on or below decks for installing pumping equipment
• Intermittent service that requires quick, multiple starts with minimal line priming
• Varied shipping media with differing fluid-handling characteristics
• Effective stripping of cargo holds to maximize deliverable working volume while managing gas entrainment without the threat of pump shutdown due to vapor locking
• Strict cycle times regarding incoming and outgoing shipments, with any delays or shutdowns resulting in the incurrence of prohibitive charges or financial penalties that can reduce the shipping company’s profitability
• Level of risk for an explosion due to static charge buildup during filling cycles

The Screw Pump Solution

The design of screw pumps makes them capable of handling liquids with varying levels of viscosity, while today’s screw pump models can offer flow ranges from 220 gpm (833 L/min) to 11,000 gpm (41,635 L/min). The screw pump’s operation sees opposed screws engaged to form a sealed cavity with the pump casing. As the drive screws turn, the fluid is conveyed to the discharge port of the pump, which creates a volumetrically consistent flow rate regardless of the pumping pressure.

Screw-pump technology advantages in shipping applications include:

• A compact design that can be mounted in a vertical or horizontal orientation with minimal deck support while providing a good power-to-density ratio and smooth or nearly pulse-free fluid transfer
• Ability to strip residual fluids from the cargo hull
• Ability to handle a wide range of flows, pressures, liquid types and viscosities without the need to manipulate a BEP position
• No need to pre-heat fluids as high-viscosity fluids can be easily handled without sacrificing performance
• Constant flow, even in the presence of varying system backpressures due to viscosity changes
• Ability to accommodate significant turndown ratios when controlled by an adjustable-speed drive, allowing for a wider flow range without wasteful recycling
• Good suction-lift capabilities that can maximize line-stripping operations while being able to tolerate the intake of entrained air and other gases without vapor locking
• High volumetric and overall operating efficiencies, resulting in reduced operational costs
• Production of high and constant flow rates even when exposed to upstream and downstream pressure fluctuations, ensuring consistent loading and unloading times
• Low internal velocities
• Low mechanical vibration that lengthens service life
• Intrinsically smooth and quiet operation
• Extremely low pulsation reduces stress and prolongs life of associated fluid-transfer components (piping, hoses, etc.)

Two specific types of screw pumps can excel in shipping applications — twin screw with timing gear (WTG) and triple screw. Twin Screw (WTG) pumps have external bearings and a timing-gear transmission, which produces double-suction, self-priming operation with no metal-to-metal contact between the pump’s internal components. Triple screw pumps are designed with a male drive spindle, two female secondary spindles and a case that contains the screws, which allows the fluid to move smoothly and continuously in an axial direction from suction to discharge.

The main operational advantages that screw pumps have — handling a wider window of fluid viscosities at higher rates and pressures, lower energy consumption and no BEP to identify — make them an efficient and versatile alternative to centrifugal pumpers for shipbuilders.

About the Authors:

Josh Pepper is the Global Product Manager — Screw Pumps for Blackmer^®, Grand Rapids, MI, USA, and can be reached at (616) 248-9235 or josh.pepper@psgdover.com. Michael Moore is Blackmer’s Senior Director, Business Development — Screw Pumps and can be reached at (519) 802-2106 or michael.moore@psgdover.com. For more information on Blackmer’s full line of pumps and compressors, please go to blackmer.com or call (616) 241-1611. Blackmer is a product brand of PSG^®, a Dover company, Oakbrook Terrace, IL, USA. PSG is comprised of several of the world’s leading pump brands, including Abaque^®, Almatec^®, Blackmer^®, Ebsray^®, EnviroGear^®, Finder, Griswold^™, Mouvex^®, Neptune^™, Quattroflow^™, RedScrew^™ and Wilden^®. You can find more information about PSG at psgdover.com.