Last year valve manufacturer CIRCOR International Inc completed the acquisition of Colfax Corp’s Fluid Handling business.
Three-screw pumps can increase reliability, which means increased uptime, longer maintenance intervals, and longevity. This means savings for machinery lubrication, crude oil transport, hydraulic power transfer, fuel oil transport and machine coolant applications.

The modern-day three-screw pump design was invented in 1923 by Carl Montelius who, along with Swedish financier Bengt Ingestrom, founded Imo AB (today a part of CIRCOR). The pump operates on the same principle of intermeshing screws as the two-screw pump, but the three-screw design uses a centrally located primary drive screw, or power rotor, which intermeshes with two secondary sealing screws, or idler rotors.

The intermeshing of the threads of these rotors, along with the close fit of the surrounding housing, creates a moving labyrinth seal, which captures the fluid and transports it axially. The enclosed area containing the fluid is referred to as a fluid closure (see Figure A). The pumping element does not pre-compress the fluid, but rather transports it from the suction side of the pump to the discharge port in a smooth, continuous manner. It is the fluid closure concept that provides the three-screw pump with its positive displacement capability.

Symmetrical pressure loading on the power rotor with such small forces eliminates the need for bearings to absorb radial forces. The idler rotors generate a hydrodynamic film providing radial support similar to journal bearings (See Figure B). Axial loads on the power rotor and idler rotors, created by differential pressure, balance them hydraulically with such small forces that they can be handled by a single ball bearing. This dramatically increases the life of the pump and simplifies the service.

Three-screw pumps come in many configurations and sizes, enabling them to serve as a replacement when other pumps are not performing. They can be easily retrofitted into existing systems.

Three-screw pump design

Three screw pumps are ideal for clean, lubricating fluids. Fluid film thickness/operating clearance is typically smaller than that of a two-screw design, giving higher efficiencies and higher pressure capabilities than a two screw pump, providing that the viscosity is adequate and the fluid is clean enough to avoid disruptions to the internal fluid film.

The simple design of a three-screw pump: just three rotating parts and one shaft seal, offers advantages for a variety of industries and applications. Some of the primary advantages to three-screw pumps include:

• Pulse-free flow with extremely low vibration and noise levels. The pumping element geometry eliminates the requirement for pulsation dampeners often found in systems employing other pumping technologies. Pulsation-free flow allows output condition management that’s critical to a variety of applications, such as precision hydraulic controls and fuel metering for gas turbine atomization.

• Low noise. The rotor profile in the screw pump provides a smooth and continuous output flow that greatly reduces pressure pulsations. The result is lowered airborne, fluid-borne and structure-borne noise, typically less than 75 db(A).

• Higher-pressure boost capabilities, even when handling low-viscosity fluids as low as one centistoke. This advantage is due to the smaller operating clearances than are found with two-screw designs.

• Highly energy efficient design. Tight internal clearances, coupled with a design that allows for minimal input power in relation to output power, results in high energy efficiency.

• Long service life due to non-contacting pumping elements by means of hydrostatic and hydrodynamic fluid films, axially balanced rotors, and top-notch metallurgy.

• Excellent suction lift compared to other pump designs with similar output flows. This is due to the small peripheral diameter of the rotors and low axial velocity of the fluid. The three-screw pump has an inherent advantage in negative suction pressure applications, allowing the pumping of higher viscosity fluids at much higher speeds than other pumping technologies.

• Low horsepower consumption while operating on high viscosity fluids. This is due to the small peripheral diameters of the rotors and low fluid axial velocities reducing fluid shearing within the pump.

Maintenance Tips

Because wear is a natural occurrence with rotating equipment, to ensure maximum equipment life, plant personnel should attend to the following:

a. Filters and strainers: Periodically check for cleanliness and clean or replace as necessary to protect equipment from damage due to pressure drops across clogged or dirty elements.

b. Foundation and hold-down bolts: Check for tightness at least every six months.

c. Alignment of pump and driver: Check and correct, if necessary, every six months, or more often if your system experiences an unusual amount of vibration or large variations in operating temperatures.

d. Bearings: Lubricate as specified in instruction manual. Check often for noise and/or abnormal vibrations and rough operation; if noted, stop operation and replace bearings.

e. Packing: Repack when all packing gland travel is exhausted or damaged.

f. Shaft seals: Recognize that a small amount of seal leakage (about 10 drops per hour per seal) is normal and necessary, visually check equipment for signs of damage or leakage from shaft seals, gaskets and O-rings and be sure all connections are tight. If the 10 drops per hour per seal is exceeded, shut down equipment and repair or replace with a seal compatible to the pump’s operating conditions, as shaft seals have a finite life.

Case study

The Three Gorges Dam is a hydroelectric river dam spanning the Yangtze River in Yichang, Hubei province, China. It is the largest hydroelectric power station in the world, with a total generating capacity of more than 22,400 MW. The project is comprised of 32 power generator units (14 on the left bank, 12 on the right bank, and six underground).

The pumps for the governing system at the hydropower plant operate continuously, requiring them to possess the utmost in quality and durability to withstand the enormous wear and tear they are subjected to, as well as to perform safely. The dam’s unique circumstances, however, presented some additional challenges, especially variations in outlet pressure. While it is relatively simple to design components to operate under steady pressure, frequent pressure variations can damage the pump’s housing as well as the rotating and bearing parts of the pump. The original pumps used for this application failed to meet these unique demands, and did break the housings. The China Three Gorges Project Corporation needed a better solution.

As the authorized CIRCOR distributor for the Three Gorges Dam project, Wuhan KLF Pump Co. recommended a retrofit package of 78 Allweiler SN and SM three-screw pumps based on their proven safety, compliance with quality standards and available local technical support from a full-service manufacturing and service center in Wuxi. Three-screw pumps are a technology well suited to powering hydraulic machinery because they move liquid continuously, and virtually without pulsation, turbulence, crushing, or loss of lubrication.

Ordinarily, there are compatibility hurdles to be overcome in replacing pumps within an existing setup, but once having a firm grasp on the precise operating conditions presented, the team could determine the most reliable pump setup, including engineering a custom-designed baseplate. Designed in collaboration with the Changjiang Water Resources Commission, the malfunctioning pumps on the hydropower plant’s left bank were replaced with 16 units of SM pumps.

Subsequently, another 47 units were installed in the new-build right bank portion of the plant. In addition to the governor pumps, low-pressure pumps were also installed for the lube oil system at the plant, a total of 14 units of SN pumps.

One of the unique applications at the dam is pumping hydraulic oil to open and close the heavy floodgates and regulate the water inlet. The SM governor pumps (lube oil feed/boost pump) were selected due to their heavy-duty design. They are used in the hydraulic system for opening and closing the water turbine’s inlet blade. The SN lube oil circulation pump and the lube oil drain pump were configured to cool the water turbine bearing.

To this day, the Allweiler three-screw pumps continue their smooth operation. Wuhan KLF’s service team has always offered support to address small issues as they arise.

Conclusion

Three-screw pumps can optimize performance and efficiency in a broad spectrum of applications while delivering benefits across a very long lifecycle. With an inherent advantage in negative suction pressure applications, higher viscosity fluids can be pumped at much higher speeds than with other pumping technologies. The pumps’ simple design provides significant maintenance advantages that further enhance their value in critical applications.

Close collaboration between Sulzer specialists in Asia, Europe and the USA was key to a complex project that allowed twelve sophisticated pumps from three individual designs to be manufactured at the company’s plant in China for the first time, meeting demanding technical goals.

Using equipment manufactured in their own country wherever possible is an important part of a Chinese refinery’s procurement strategy, as it seeks to localise its supply chain and ensure ease of maintenance and support. Sulzer’s ability to bring the new models into production quickly allowed its customer to meet tight project timelines, as well as its technical and commercial goals.

Continuous investment Market demand for fuel and products derived from oil and gas is being met by expanded exploration and the use of new technologies to maximise the potential of existing wells. The introduction of new technology is improving efficiency for the petrochemical sector, which in turn helps to reduce the cost of everyday items, from recycling bins to toothbrushes and the tyres on cars.

China’s hydrocarbon processing industry is the largest and fastest growing in the world. To meet demand and maintain competitiveness, companies across the sector are continually investing in new capacity and advanced technologies. For one refinery, that included a major project to upgrade and optimise its facilities.

Challenging start In 2016, the refinery took the decision to introduce new equipment across its plant, including a new ebullated bed hydroprocessing system using H-OilRC technology from a French process licenser. This system is designed to add value in the refining process by extracting valuable hydrocarbon products from heavy feedstock residues, which may contain high levels of solids and other contaminants.

When it came to the selection of pumps for the new system, the refinery was aware that the nature of the various feedstocks created demanding operating conditions. The internal components can be exposed to high levels of wear and there is a significant potential for flow to be disrupted by large solid particles.

The refinery knew that it was possible to obtain pumps designed specifically to perform well in these conditions, but it was unable to find a local source of supply, an important part of its procurement strategy.

As a global pump specialist, Sulzer already had a significant presence in China, producing pumps in the Dalian factory since 1999 and at a second plant in Suzhou since 2010. Sulzer also has a long-standing relationship with the parent company of the refinery, supplying and supporting pumps at many of its facilities in the country. After discussing the requirements, oil and gas industry experts at Sulzer recommended two proven technologies from the company’s portfolio. Equally as important, they also agreed to manufacture the critical pumps in China.

Like any major facility upgrade project, the ebullated bed system was planned to a tight schedule. So once the agreement was in place, Sulzer moved quickly to bring the new products to life in its China plants.

Maximum performance The first of those products was the CP multistage barrel pump designed to API 610 type BB5 specifications. The Sulzer CP design incorporates an axial-split volute casing, which is less susceptible to wear than alternative designs when a highlevel of abrasive solids is present. Sulzer’s design and manufacturing specialists in China worked closely with their colleagues from the USA, who hold the design responsibility for the CP, to allow for some important design modifications. This particular application required a low flow, high head, high speed solution, so Sulzer had to pay special attention to the surface characteristics of the fluid channels within the pump in order to minimise losses and achieve the target performance.

The second key technology was a solution to the problem of pump performance impairment by large solid particles. For applications like these, Sulzer has developed a range of pumps with a special coke crusher unit at the pump inlet stage. This technology uses a rugged, bladed rotor that will break up solids to ensure they pass smoothly through the rest of the pump. Sulzer in China worked with colleagues in Germany and the USA to integrate the coke crusher technology into two pump types for the project: the BBT, an API 610 type BB2 centrifugal pump, and the ZF, an API 610 type OH2 centrifugal pump.

Delivering under pressure In addition to building up the necessary in-house manufacturing processes, Sulzer in China also had to qualify suitable local suppliers for key components of the new pumps. The company’s sourcing teams conducted detailed supplier audits to ensure that each one met Sulzer’s global quality standards, especially when it came to critical parts such as the inner casing of the CP pump and the coke crusher components for the BBT and ZF units.

During the development and manufacturing process, representatives from the refinery made regular visits to Sulzer’s plants, giving them the confidence that work was proceeding on schedule and to the highest quality level.

They were also present to see all the new pumps successfully meet their performance specifications in testing. In total, 12 of the newly localised pump models were delivered to the customer: three CP units, six BBT units with the coke crusher installed, and three coke crusher-equipped ZF units.

“This project was an important next step for our partnership with the refinery and for our support of China’s petrochemical sector as a whole,” says Zhang Jianfeng, managing director of Sulzer’s Suzhou manufacturing facility. “We have demonstrated that we can provide our customers with all the benefits of local manufacturing combined with Sulzer’s excellence in technology, quality and reliability.”

Engineers at a European power station’s FGD plant wanted to reduce SO2 emissions at the same time as maintaining consistent site efficiency. Installing two large motors to drive a booster fan and an absorber recirculation pump helped them ensure that the station’s reputation for environmental responsibility was upheld.

Figure 1. Coal is delivered to the plant via automated rail wagons.
Figure 1. Coal is delivered to the plant via automated rail wagons.
Figure 2. The second motor installed is also a MGF high-voltage unit from WEG’s M-Line of high voltage units. It drives an absorber recirculation pump on one of the FGD towers.
Figure 2. The second motor installed is also a MGF high-voltage unit from WEG’s M-Line of high voltage units. It drives an absorber recirculation pump on one of the FGD towers.
Figure 3. The largest of the WEG motors is an IEC 1000 frame size 6150 KW, 10 pole, 11 KV M-line unit.
Figure 3. The largest of the WEG motors is an IEC 1000 frame size 6150 KW, 10 pole, 11 KV M-line unit.
A pdf version of this article is avilable on the right-hand side under ‘Downloads’.

Ratcliffe Power Station, situated near the River Soar in Nottinghamshire is recognised as one of the most efficient coal fired power stations in the UK. The site has a generation capacity of 2,000 MW from four 500 MW units, enough to power around 1.5 million homes. The site covers 700 acres and has been producing electricity for over forty years.

The site takes its environmental responsibility seriously, ensuring that its impact on the surrounding area is kept as minimal as possible. Large numbers of native trees and shrubs have been planted and a biodiversity action plan exists to enhance the ecological value of the land. Its flue gas desulphurisation (FGD) plant, commissioned in 1993, is the centrepiece of the drive to reduce SO2 emissions while also creating commercial quality gypsum.

The site is run with efficiency as a top priority and the main criteria when sourcing motors were energy efficiency and reduced maintenance requirements.

“It is always surprising to people who visit the site that we are so efficiency conscious, but they’re often even more surprised that we have to pay for the power we use during the generation process so it’s essential we don’t use more energy than we actually need,” said Chris Bennett, a senior electrical engineer at Ratcliffe. “Even more important is minimising the environmental impact of the power station and ensuring the reliability of the plant; outages have to be avoided where possible so critical equipment such as the motors have to be reliable.”

Smooth running
The Ratcliffe Power Station owner, power and gas specialist E.ON, decided to install two new large WEG motors. The first motor to be installed is a 6150 kW, 10 pole, 11 kV WEG M-line MGF 1000 unit used to drive the booster fan on the FGD plant. The motor is an IEC 1000 frame size with an output torque of over 100,000 Nm and is the largest of the WEG motors currently installed at the power station. The booster fan is used to power the flue gas through the FGD plant.

The second motor installed is also a MGF high-voltage unit from WEG’s M-Line of high voltage units. It has a smaller IEC 400 frame size MGF 400E 750 kW, 4 pole 3.3 kV motor. This motor drives an absorber recirculation pump on one of the FGD towers. These pumps are used to transfer up to 8,000 tonnes of limestone slurry an hour to the top of the 50 m high towers to be sprayed into the flue gas.

Both motors are reportedly highly efficient, the larger unit up to 96.4% at peak efficiency and the smaller motor at 95.4% efficiency. WEG motors are designed to be compact and robust, using cast iron frames and corrosion resistant finishing and galvanised mounting components. The windings are sealed using a Vacuum Pressure Impregnation (VPI) system which offers high quality insulation and stator coil protection through the application of an epoxy based resin.

“We opted to install the WEG motors as they run smoothly and we could be confident that they would require a low level of maintenance,” said Bennett. “It was important that we install robust motors as they are running continuously on a high load in conditions that can become harsh at times. As well as the two large motors installed on the fan and the pump we have several smaller WEG motors throughout the power station.”

The FGD process
The desulphurisation process passes flue gas through a heat exchanger to cool it below 90°C. The gas is then drawn into the tower where it comes into contact with the limestone slurry. This creates a chemical reaction, removing the SO2 and converting the limestone into calcium sulphite (which is in turn processed to create commercial quality gypsum). During this process, 95% of the hydrogen chloride is also removed. Once the SO2 has been removed the gas is reheated to allow it to rise through the chimney.

As the FGD plant plays such a vital role in cleaning the emissions from the power station it is vital that down time is kept to a minimum. This means that all equipment in the plant must have the longest operating life possible with low maintenance requirements. It was important that the new motors were able to survive in an outside environment for a long period, working at a continuously high load.

Operating temperature is crucial to both the efficiency and the longevity of electric motors, and so the WEG units employ a combination of an internal cooling circuit, external cooling fins and cooling fans to guarantee an optimum temperature gradient within the motor. These, coupled with specifically dimensioned bearings help extend the running life of the motors. The WEG motors also have dynamic balancing at motor operating speeds. In this case the standard balancing is on two planes. Both are capable of very low vibration levels.

WEG also employed a pressure grease relief system enabling old grease to be removed and replenished with new while the motor is still running, reducing the need for maintenance down time.

ummer 2009 sees a new challenge to the land speed record for a steam car, ‘Inspriation’. Technology from CAT PUMPS is playing a critical part in this record-breaking attempt.

The three-ton steam-powered car ‘Inspiration’ has enough fuel, compressed air and water to run for three minutes.
The three-ton steam-powered car ‘Inspiration’ has enough fuel, compressed air and water to run for three minutes.
Summer 2009 sees an attempt to break the land speed record for a steam car. Powered by a steam turbine, the car’s 12 boilers need to be carefully primed with demineralised water. CAT PUMPS provided a critical solution to provide continuous forward liquid flow for a smooth operation.

History
Back in 1906, the fastest car in the world was the Stanley Steamcar which set a world record speed of 127 mph at what is now Daytona Beach, Florida. In comparison with today’s production road cars that is a relatively modest speed, but it is an official FIA record that has never been broken by another steam powered car.

In 1985, a similar car powered by a steam turbine car did reach a speed of 145 mph, but this was not recognised by the FIA because it did not comply with their regulations, these being two runs over a measured mile within one hour. Since then, there has not been a serious attempt to set a new record.

The Steam Car Company
In 2009, this 103-year-old land speed record could well be broken as a challenge is mounted by an innovative group of engineers from the UK operating under the auspices of The Steam Car Company. Their attempt with a steam car named ‘Inspiration’, taking place at the Edwards Air Force Base in California, USA, will be the culmination of 10 years’ work.

At 25 ft (7.6m) long, weighing just over three tons and using a ton of water every 25 minutes, Inspiration is no ordinary vehicle. When the car is moving, the 40 bar water pressure is maintained using compressed air to fill two 70-litre hydraulic accumulator bladders. The boilers contain more than 1.86 miles (3 km) of tubing and are insulated with advanced ceramic-and-silica cloth.

The burners develop three megawatts of heat that raises the water to 250°C, and because it is at 40 times atmospheric pressure it’s referred to as ‘wet’ steam. It is then super-heated to 400°C ‘dry’ steam, which is directed down the car via heavily lagged pipes and two industrial steam valves into a two-stage turbine.

The steam is injected into the turbine at more than twice the speed of sound and the turbine spins at up to 13,000 rpm. The turbine then drives the rear wheels, crown wheel and pinion reduction gear. The machine actually runs for less time than it takes start. It takes eight minutes to get going and has enough fuel, compressed air and water to run for three minutes – the record can be achieved in less than two minutes running time.

Speed build up
While the record is measured over a distance of one mile, the car needs a straight runway of almost seven miles in order to build up speed and then slow down. At the end of the first run, additional water has to be injected to carry on for the second run and then to cool down.

In order to generate the required stream pressure for a run, the steam car’s two 70 l hydraulic accumulator bladders, which feed the 12 onboard boilers, need to be carefully primed with demineralised water. The water is supplied from holding tanks located on two purpose-built ‘start’ and ‘turnaround’ rigs at a pressure of 40 bar. This essential task has been allocated to two 1520C triplex positive displacement plunger pumps from Cat Pumps, which feed the car at a rate of between 50 and 40 litres per minute.

The pumps have played an important role throughout the development of the steam-powered car, not just on the rigs but also on the test beds where the boilers have been designed. “When the car is running, all the steam generated must go to the wheels so there is a lot of accumulated power onboard,” says consulting engineer Matthew Candy.

“The design of the system means that the water must go seamlessly from the displacement to the car’s accumulator systems, as is it essential to have water in the boilers at all times, for if we lose the water the boiler tubes will fail very quickly. The Cat 1520C pump meets this requirement.”

CAT PUMPS
Cat Pumps’ 1520C model is a triplex reciprocating positive displacement piston type pump and is capable of generating pressures up to 70 bar. This type of pump takes in a fixed volume of liquid on each inlet stroke and physically moves it through and out of the pump with virtually no losses or inefficiencies.

Cat Pumps’ triplex pumps contain three synchronised pistons or plungers which produce a low-pulsation flow that is sufficiently smooth for most applications. The flow is highly stable and predictable because the flow rate is determined almost exactly by the running speed of the pump and is virtually unaffected by back-pressure, liquid viscosity or specific gravity.

Recognised for its reliable operation and ease of maintenance, this pump can be found in many situations where a continuous forward liquid flow is required for a smooth operation. A significant bonus for the steam car application is that is capable of handling the non-inherent lubricity properties of demineralised water.

Boiler development
The development of ‘Inspiration‘s’ boilers has taken several years, moving from an original plan of four units to a final complement of 12 units, and the 1520C pump has been used on the boiler tests for all that time.

“The versatility of the pump’s output has enabled all testing to be undertaken with just one pump,” continues Candy. “When powered by an electric motor in the boiler test cell we found the pump easy to control and highly sensitive to the task in hand. We were able to run the flow down to as low as 4 l/min in order to test each boiler.

For the next stage of the boiler development programme, we shall use a smaller unit for a new test cell. An electric motor is not feasible for powering the pumps on the rigs, so each of the pumps is powered by a Briggs & Stratton motor which does reduce the performance variability slightly, but with no detriment to the duty. The pumps are rugged, compact and come with a pressure relief system to take the bounce out of the flow.”

When considering the steam car development programme in its entirety, Cat Pumps’ 1520 C pumps are a relatively small component. That said, their input is critical to the success of the challenge during the start-up sequence for the first run and once again before the second run. The steam cannot be generated by the car’s boilers without the controlled high pressure inflow of water from the pumps and without steam at the right temperatures and pressures, Inspiration is a non-starter.

The TowerPack electric pump from Boltight Ltd delivers a maximum pressure of 2000 bar/29,000 psi and is suitable for tough use in difficult conditions. Boltight says that the pump will offer durability and reliability when working in wet, windy, remote and harsh environment; it is IP56 protected.

The Towerpack pump is built around a rugged steel frame with an integral swivel lifting point. The frame itself has been balanced to keep vibration to a minimum. It features a full pendant control interface with a digital pressure read out. High oil flow helps the pump reach the desired operating pressure quickly.

The pump offers on board diagnostics, high and low pressure filtration with in-built sensors and a high-pressure quick connect port. The user can select the optional constant pressure operation if required.