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FMEA of Extruder Gear Pump I. GEAR PUMP : PR 25-6 The gear pump installed in the HDPE-1 plant is model no. PR 25-6, manufactured by MAAG PUMP SYSTEMS AG CH-8023 ZURICH and integrated by Werner & Pfleiderer GmbH with the rest of the extruder system. The number 25 in the model signifies that the gear thickness is 25 x 10 = 250 mm and the number 6 indicates that it has a timing gear whereby both shafts are driven independently. The shaft centerline distance between the 2 gears is 360 mm. It was installed on 12 th February 2000. Its equipment tag number is GPE1-N515. The extruder system can be run without the gear pump, with the twin screw extruder producing the required pressure to force the polymer melt through the die plate. However, operation with the gear pump installed is highly beneficial due to the reasons listed below:
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1. Write-Up on Gear Pump and Associated Systems

Nov 28, 2015

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Anirudh Srikant

gear pump working and process description for extruder
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Page 1: 1. Write-Up on Gear Pump and Associated Systems

FMEA of Extruder Gear Pump

I. GEAR PUMP :

PR 25-6

The gear pump installed in the HDPE-1 plant is model no. PR 25-6, manufactured by MAAG PUMP SYSTEMS AG CH-8023 ZURICH and integrated by Werner & Pfleiderer GmbH with the rest of the extruder system. The number 25 in the model signifies that the gear thickness is 25 x 10 = 250 mm and the number 6 indicates that it has a timing gear whereby both shafts are driven independently. The shaft centerline distance between the 2 gears is 360 mm. It was installed on 12th February 2000. Its equipment tag number is GPE1-N515.

The extruder system can be run without the gear pump, with the twin screw extruder producing the required pressure to force the polymer melt through the die plate. However, operation with the gear pump installed is highly beneficial due to the reasons listed below:

Page 2: 1. Write-Up on Gear Pump and Associated Systems
Page 3: 1. Write-Up on Gear Pump and Associated Systems

As seen in the pressure profile to the left, the maximum attainable pressure is limited to around 250 bar at the end of the barrel (at the throttle valve). Due to the pressure drop in the system, the polymer melt drops to a pressure of around 150 bar by the time it reaches the UG underwater pelletizer.

However, upon adding a gear pump ahead of the diverter valve, the max pressure generated increases to 300 bar. The pressure drops to around 200 bar by the time the melt reaches the underwater pelletizer. This makes it much more efficient at overcoming the resistance to polymer flow offered by the screen pack and the die plate.

The gear pump highly reduces the load on the extruder motor, gearbox and clutch coupling. This helps to ensure a longer life and reduced wear of these components.

Moreover, the energy efficiency of the entire system with a gear pump is also much higher as the gear pump is added as a specialized unit to develop pressure. The volumetric efficiency of a gear pump is in the range of 90-99% depending on the clearances whereas that of the extruder screws is around 40-45%. Without the gear pump, a lot of the input energy is converted to heat due to friction in the screws, which raises the melt temperature in the barrel to approx. 275 deg C. By adding the gear pump, the maximum melt temperature is reduced to around 240 deg C.

THEORY OF OPERATION:

Gear pumps operate on the displacement principle. They comprise of two inter-meshing gearwheels of the same size, encased in a housing with closely defined clearance (clearance between teeth and housing are in the order of 0.1 mm and the clearance between front / back of the gears and the housing is of the order of 0.025 mm). Generally one of the two gearwheels is directly driven by a shaft protruding out of the housing through a shaft seal. The second gearwheel is driven by the teeth of the first one.

For some gear pump applications, however, it may be better for both gearwheels to be driven. This reduces the friction between the gear teeth and distributes forces equally on both gearwheels. For this purpose a special pump drive gearbox is required.

As the gearwheels rotate, the spaces between unmeshing teeth are filled with the inflowing pumping medium. This is then transported along the inside walls of the housing from the suction side to the pressure side. Here the pumping medium is displaced by the meshing teeth and ejected through the pump outlet.

The gear teeth fulfill the following main functions:

Transport of pumping medium from suction to pressure side

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Displacement of pumping medium on the pressure side

Sealing between suction and pressure sides

The polyrex is installed as a booster generating the necessary pressure behind the die plate, relieving the extruder of this work and saving energy. It is installed between the start up / throttle valve and the screen changer unit. It is a conventional gear pump having two gear with spur teeth, thus minimizing product degradation ensuring effective self cleaning. The gears are enclosed in a steel casing with heating jacket. The pump installed with timing gear allows each shaft of the two gears to be driven separately without touching the teeth of each other.

The gearbox nominal power used for its operation is 600 KW allowing a maximum speed by the gears of 33.3 rpm. The bearings can be heated prior to start up and cooled with water during normal operation, thus preventing degradation of melt. It is designed to operate within the following pressure limits:

Suction side, maximum pressure 100 kg/cm2g

Discharge side, maximum pressure 350 kg/cm2g

Differential pressure, maximum 320 kg/cm2g

Specific delivery rate 28.4 l/rev

The main parts of the pump are:

Page 5: 1. Write-Up on Gear Pump and Associated Systems

Housing

The housing has an integral heating jacket for liquid or gaseous heating media.

The housing has an inlet opening on the suction side and an outlet opening on the discharge side. The suction side of the housing is bored with two relief holes, one on the drive side and one on the non-drive side.

The housing is bored to take measuring probe cartridges and temperature probes for housing and bearing temperatures.

Housing covers

The two covers are made of steel and incorporate heating channels for liquid or gaseous heating media.

Each cover has a Vee-shaped relief groove (V-groove) on the inside pointing toward the suction side and matching over the relief hole in the housing.

Both covers have a boring for a temperature probe on top.

Drive shaft and short shaft

These straight-toothed shafts are each mounted in two plain bearings in the pump housing.

The long end of the drive shaft is splined for mounting a hub or toothed coupling.

The non-drive end of projecting drive shafts has a boring for a shaft cooling pipe.

Plain bearings

The drive and non-drive sides of the housing are each fitted with a top and a bottom bearing facing each other. Each pair of bearings is locked by a key against rotation.

The drive side top and non-drive side bottom bearings are identical and thus interchangeable, likewise the non-drive side top and drive side bottom bearings.

The hydrodyanmic plain bearings are lubricated by the pumping medium.

The bearings are designated according to installation position:

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Drive side bearing, top Bearing DST

Drive side bearing, bottom Bearing DSB

Non-drive side bearing, top Bearing NDST

Non-drive side bearing, bottom Bearing NDSB

The bearings have tapped borings for fitting steel shoulder rings and a temperature probe boring on the outside diameter.

Shaft seals

Polymer melt leakage is prevented by viscoseal in the housing covers.

The viscoseals are fitted with cooling water channels. They are connected to the cooling water piping.

In case of double shaft drive, depending on top and bottom shaft rotation, left-hand or right-hand viscoseals are incorporated in the drive and non-drive side housing covers.

The drive side top seal and non-drive side bottom seal are interchangeable, likewise the drive side bottom and non-drive side top seals.

OPERATING MODES:

There are four different operating modes available for the Polyrex installed in an extrusion plant with twin-screw kneader and the following upstream and downstream equipments:

1. Start-up valve flanged to suction side

2. Downstream equipment with screen changer and die-plate, flanged on to pressure side

3. Spaghetti mode (purging of die-plate):

Immediately after starting the Polyrex, polymer melt is extruded through the die-plate on to the floor. This operating mode is known as Spaghetti mode.This is necessary with a new or freshly assembled Polyrex in order to clean out all the Molykote dry lubricant film.

After pauses in operation, Spaghetti mode is necessary in order to purge thermally damaged polymer from the Polyrex and die-plate.

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Normally it takes about 3 to 5 minutes to flush out the Molykote, but purging should be continued until clear polymer emerges from the die-plate.After operation in Spaghetti mode, then the normal operation can be resumed.

4. Start-up mode:

After start-up, following checks should be carried out:

Check the pressure control system

Measure the Polyrex speed with a hand tachometer, and compare with the speed

indicator reading on the process control system.

Check all measuring, display and recording instruments.

Check operating data.

Check the viscoseal shaft seals for leakage.

Check all heating medium supply connections for leakage.

Check for any leakage of the cooling medium connections to the viscoseal shaft seals.

If shaft cooling is installed, check the supply piping for any leakage.

Check all auxiliary systems such as for shaft cooling and lube oil system.

Normal mode

The following operating limits should be maintained during normal operation:

Pumping medium

No. 1 2 3 4 5 6 7

Type HDPE HDPE HDPE HDPE HDPE HDPE HDPE

Designation

GM 6255

GM 6255

GM 9240

GM 4760

GM 7260

GM 9450F

GM 5010

MFI21.6kg/1900C [gr/10’] 2 2 5.6 32 23 - -

Max. throughput [kg/h] 9000 15000 22000 24000 24000 23000 24000

Max.speed Polyrex Rpm 14 23 34 40 41 36 38

Max.Tpumping medium

0C 280 280 270 240 220 270 270

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II. GEARBOX :

The Gearbox for the Polyrex is model no. SPU-36 made by MAAG Gears. It is a 3-stage reduction gear unit with a final timing gear stage. The first reduction stage is a right angle bevel gear stage. The second reduction stage is a planetary gear. The last stage is the timing gear. The 2 shafts of the timing gear synchronize and drive the 2 pump shafts. They ensure contact free meshing of the pump rotors. The coupling used is to be pf MAAG toothed coupling ZFX-8Y type.

TECHNICAL DATA:

Gearbox:

Distance between both driven shafts: 360 mmGear Nominal Power: 600 kWMax Torque: 172 kNmNominal input rpm: 1000 rpmNominal output rpm: 33.3 rpm

Lubricating Oil:

Lube Oil Quality: CLP lube oilsViscosity: ISO VG 320

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Oil flowrate: 90 l/minIII. SHAFT TEMPERING SYSTEM :

The shaft tempering system regulates the temperature of the two shafts of the Polyrex gear pump.

In order to do this, it carries out the following tasks:

It heats the tempering medium up to operating temperature.

It cools the two shafts under certain operating conditions of the Polyrex gear pump.

MAIN COMPONENTS:

The shaft tempering system comprises the following elements:

Two shaft cooling pipes with 1 rotary joint each

One tempering unit for circulating water 25 – 210 deg. C :

Indirectly heated by electrical resistance heater; indirectly cooled by water via heat

exchanger to TEMA C, designed for pressure class PN40.

It also consists of a centrifugal circulation pump of Dickow Pumpen make , NCL type.

One pressurizing and make up water station:

For generating basic system pressure required to achieve the maximum circulation

temperature by means of a high pressure triplex piston pump.

The triplex plunger pump is of type P11/13-100, manufactured by Speck Pumps. It has a

power consumption of 3.9 kW, producing a max pressure of 150 bar @ 1420 rpm. The max.

flowrate is 12.7 lit/min.

One control unit:

For automatic control of the heating / cooling system. Temperature and pressure are

controlled by digital electronic controllers – IP 55, 230 V / 50 Hz

THEORY OF OPERATION:

Tempering Unit:

The temperature control unit heats up the tempering medium to the correct temperature prior to operation and automatically regulates it during operation.

Page 10: 1. Write-Up on Gear Pump and Associated Systems

The Polyrex shafts are either heated by switching on the heating system, or cooled by switching on the cooling system.

Heat is transferred by feed water, circulated by a pump through the two shafts of the Polyrex.

Heating is done by electrical resistance heating element.

Cooling is done indirectly by heat exchanger.

Control System

The plain bearing temperature control system maintains the Polyrex bearing temperature (controlled variable) at the set-point (reference variable). The tempering medium temperature (manipulated variable) is regulated by the bearing temperature control system. If the bearing temperature changes, the tempering medium temperature is adjusted accordingly.

If the bearing temperature rises above the set-point value, the tempering medium is cooled accordingly.

If the bearing temperature falls below the set-point value, the tempering medium is heated accordingly.