Revitalize older pump installations

A cartridge-style screw pump may be the way forward.

By Sven Olson, Leistritz ATC December 6, 2017

The capital costs of purchasing and installing pumps for vintage oil fields and process facilities have increased steadily over the years. These costs are often an obstacle to extending the life or upgrading these plants. Yet these stopgap measures may be the only alternative available as regulatory and economic demands make new construction hard to justify.

Pump component costs, however, as shown in industry indices like the Nelson-Farrar Cost Index, are not the driving factor behind the rising costs involved. The fact is, competition among manufacturers, along with moderate labor and material cost increases, have kept pump prices stable over the years.

The cost increases are related to the installing and commissioning of pumps. These costs have shown significant increases, especially as more and more third-party installation and service contractors are tapped to perform the work. These costs, in turn, are driven by the lack of a skilled workforce and new regulatory demands.

All these factors can combine to be especially challenging when upgrading or renewing a pump installation in an existing facility where the original pump is no longer available in the market or the replacement parts are excessively expensive.

This happens more than you might think. The manufacturer of the original pump may have stopped production of a given model or replaced it with a new model that does not fit the existing installations’ footprint. Even worse, a pump manufacturer might have been acquired and reorganized. In that case, it may be that no one knows anymore what the origins of a pump are, with no sure way to recover them. 

An alternative suggestion

One alternative method for replacing a pump in an existing installation is to do so with a "re-engineered" pump. Note that while a "re-manufactured" pump is an upgraded, modified, or repaired older pump, the reengineered pump is brand new.

The improved value proposition with a re-engineered pump comes from avoiding modification of the original installation. The existing piping connections, locations, insulation, and instrumentation, as well as the original driver and mounting configuration, are maintained. The reengineered pump fits into the original envelope, with the mounting pads, nozzles, and drive shaft suited to the location of the original pump installation. Thus, the overall installed costs of a replacement pump are significantly lower than they might have otherwise been.

One type pump, typically well-suited to a reengineering approach, is the screw pump, a common type of positive displacement pump. It is known for its good suction lift, for use with viscous liquids, and for low fluid pulsations.

A screw pump’s flow rate is directly proportional to the speed of the pump. Therefore, when speed control is realized using a variable speed drive, flow can be controlled closely. This saves power.

Use of a direct drive from a synchronous motor speed eliminates the need for speed reducers. With internal hydraulic rotor balancing, the thrust load is compensated for without using thrust bearings. It is a straightforward design, which makes it easy to install, operate, and maintain.

One type of screw pump often reengineered is a three-screw pump design. The reengineered components come as a standard pump element, called a cartridge, which can be installed in a fabricated pump casing.

The cartridge is sized based on the hydraulic data of the installation and separately tested in the factory. The casing, most often made of fabricated steel, allows for almost infinite variations of nozzle locations and mounting feet. A modern CAD system can import dimensions and locations of an existing pump installation and apply it to a new casing design, customized to fit an older installation. 

Customized casings

The casing can be customized with a pressure relief valve. This can be a great help for any existing installation equipped with a separate relief valve, if its function is questionable or the operating data has been modified. Installing the relief valve integral to the pump saves the expense of piping modifications and troubleshooting.

Uses for reengineered screw pumps are typically in some form of mineral oil service in refineries, pipelines, terminals, or power plants. The original pump most often is another positive displacement pump, such as a gear, vane, piston, or screw pump. Liquids that reengineered pumps handle include everything from distillate to No. 6 oil, pitch, asphalt, crude oil, diesel or regular lubrication and hydraulic oils. Applications can include charge and transfer applications, pipeline and loading service, as well as general lubrication of turbines and reduction gears.

Besides positive displacement pumps, a reengineered pump can also replace centrifugal pumps. The need for replacement may follow from pumps that were incorrectly sized or misapplied for a certain service. Other times, when the original process conditions change, the pump no longer works.

In one example, a vertical centrifugal pump for lubrication oil service of major rotating machinery was replaced because of its difficulties maintaining the oil pressure when air was present in the oil. Matching the dimensions and nozzle location of the original pump, a direct replacement screw pump was dropped in the lube-oil tank without system modifications.

The original centrifugal pump did not require any pressure relief valve. The screw pump, however, needed to have this valve, which was installed on the pump mounting plate and connected back to the tank. The new installation doesn’t interfere with existing structures or expand outside the original pump envelope. 

How it’s done

A reengineered pump can match an existing pump’s envelope, eliminating the need to redo piping or foundations.

Typically, when users need a reengineered pump, they need it "yesterday." The need for a quick turnaround is a key feature in any reengineered pump program.

Leistritz stocks cartridges for most flow rates in the range between 50 and 400 GPM and pressure up to 600 psi. Hardened and finish ground carbon steel rotors, cast or ductile iron liners, and use of mechanical seals are standard. Full-flow cartridge testing is done on a factory test stand, ensuring a cartridge from inventory is ready to be installed in the customized casing.

Pump casing specifications are based on user input data, typically including the maker and model number of the original pump. Past operating data, motor information, and existing instrumentation complete the picture. If an integral relief valve is needed, the pressure setting and location on the pump body must be decided.

Fabrication takes place in a certified shop, where the rotor liner bore is machined; mounting feet and flanges are installed; and surface preparation and finish painting are done. ASTM standard pipe material, flanges and structural steel are used. The pump service determines the pressure rating of the casing, which is hydro tested per API Standard 676, Third Edition. If necessary, additional pipe spools with flanges and instrument tabs are provided. Shims or a pump bedplate can be offered, if the motor or driver needs to be modified.

In times of constrained budgets, a reengineered pump offers a competitive alternative that simplifies the change-out of old, obsolete, and inefficient pumps. A cartridge style screw pump is ideal for mating with the customized casing for quick turnaround and easy direct fit with a minimum of on-site disruption and installation work.

Sven Olson is a senior consultant and former CEO of Leistritz ATC.

Original content can be found at Oil and Gas Engineering.