Case StudyNatural gas compressor re-rate

1.0 Background

Three identical natural gas compression trains on anatural gas platform in South East Asia werecommissioned in 2002.

The trains comprised of GE LM2500 driven 25MB4-3Centrifugal Gas Compressors via Speed IncreasingGear-Box.

2.0 Problem definition

All three units experienced failures due to severalreasons soon after commissioning.

The failures on the compressors were associated withfouling and resonance. Metallurgical analyses onfailed impellers suggested that the broken impellersfailed due to fatigue. Modal analyses clearlysuggested that there was impeller natural frequencyinside the speed range of the compressors. Traces ofmercury were found inside the gas although nodirect mercury-related failure was reported.

The resonance issue could not be resolved since itwould require manufacture of a new impeller.Instead, the impellers were scalloped by the OEM(see Figure 1) and the maximum running speed(MCS) was reduced. The excessive scalloping andthe reduced speed range reduced the compressioncapability of the compressors, while decreasingefficiency levels.

In addition to the operational and design relatedissues, the Operator introduced a new gas platform.The natural gas from the new platform would bedirected to the existing platform and the combinedgas would be compressed by the existingcompressor trains. As a result, the aerodynamicdesign of the existing compressors was no longercapable of handling the combined gas.

3.0 Solution

In order to resolve the design related issues and toaccount for the additional gas from the newplatform, the Operator opened a bid for a re-rateto the existing compressors. The re-rate wasrequired to:

Resolve resonance issuesHandle existing productionHandle new productionHandle combined existing & new production

with a maximum available power of 15.5 MW. Afactory test (FAT) and later a site test (SAT) wererequired for verification of the design. The materialsused on the re-rate would have to be NACEcompliant as well as mercury resistant.

Natural gas compressor re-rate in South East Asia

Figure 1. Scalloped impeller on original rotor.

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The solution to the re-rate offered by Weir’sTurbomachinery business was a 7 stage back-to-backdesign with 19 inch impellers. The existing driver,gear-box, couplings and ancillaries were to beretained as is. Existing dry-gas seals and seal systemwould also be retained. Existing journal bearings andthrust bearing were to be utilized upon confirmationby rotordynamic analyses and detailed design work.

To be able to fulfil all duty requirements, thecompressor efficiency had to be high. To achieve thehigh efficiency levels, vane diffusers were introducedat each stage. See Table 1 for performance data. Toprevent any resonance issues, modal analyses of allimpellers would be performed during design andlater verified by ring testing. To delay fouling, therotor wetted surfaces would be coated withadvanced coatings.

The design would be verified by an ASME PTC-10Type II factory test and a site acceptance test (SAT)following commissioning.

All design, manufacture and commissioning workwas to be carried out by Weir’s Turbomachinerybusiness.

In addition, dimensions of existing equipment (e.g.casing, bundle, rotor, housings, etc.) were to bemeasured and amendments to operation andmaintenance manuals were to be provided by Weir.

Project management and regional support was to beprovided by a third party.

Table 1. Re-rate performance data.

Figure 2. Low pressure (LP) section performance map.

Figure 3. High Pressure (HP) section performance map.

Figure 4. Existing vs re-rate LP performance map.

Figure 5. Re-rate design

4.0 Project execution

4.1 Design

A joint project team was formed by Weir’sTurbomachinery business and the third party. Alldesign work was carried out by Weir personnel. Aninitial design was carried out using approximateddimensions of the existing equipment from user-supplied drawings. This design providedapproximate rough stock dimensions for criticalcomponents.

Later, the Design Engineer and the Drafter visited thesite, where a spare rotor and bundle of the existingcompressor was made available.

Figure 6. LP 1st stage modal analysis – 9th mode.

The impeller modal analysis was carried out after allaerodynamic and stress analyses were completed.The modal analysis assured that there were noimpeller structural modal frequencies inside therunning speed range of the compressor. Thecalculated frequencies were later verified by ringtesting.

Inlet scoops were added to each section inlet tobetter guide the flow into the first stage. Low solidityvane diffusers were designed for each diffuser toimprove efficiency levels.

The labyrinth seals were designed with swirl-breaksto reduce destabilizing effects. The labyrinth materialwas chosen as Arlon to protect against mercurydamage, as agreed with the customer.

The rotordynamic analysis was carried out with thenew rotor and stability analysis was also carried out.The train torsional analysis was performed andresults suggested that the existing coupling couldbe re-used.

Thrust analysis suggested that the existing thrustbearing could be re-used. In addition, the existingdry-gas seals and the high-speed coupling werere-used.

4.2 Manufacturing and assembly

Manufacturing was undertaken at Weir facilities orWeir-approved vendors.

The impellers were milled as two pieces; the bladeswere milled into the hub and a shroud with weldslots was manufactured. The blade leading edgeswere welded with stellite weld rods to protectagainst erosion in the field. After heat treatment, theimpeller was finish-machined, NDT inspected,balanced and spun to 115% of the MaximumContinuous Speed.

Hub and shroud weld prep.

Figure 7. Impeller manufacturing steps.

Post weld.

Figure 8. Rotor with advanced coating.

Following assembly and sequential low-speedbalance, the rotor was coated with an advancedcoating to protect against corrosion. The rotor wasthen high-speed balanced.

The inner casing and the diaphragms were alsomanufactured and assembled.

Turbomachinery Engineering

Humber RoadBarton on HumberNorth LincolnshireDN18 5BNUK

Telephone: +44 (0)1652 632 702Facsimile: +44 (0)1652 633 401E-mail: turbomachinery@weirgroup.comWeb: www.weirpowerindustrial.com

Figure 11. Test setup – compressor.

Figure 12. First unit commissioned at site.

Figure 9. Manufactured bundle

4.3 Factory testing

ASME PTC-10 Type II aerodynamic performance andAPI617 mechanical testing was performed at Weir’sTurbomachinery facility in Barton-on-Humber in UK.This was carried out by assembling two purpose-built loops, one for the LP and one for the HPsection. Each loop contained a flow control valve,cooler and flow meter together with theinstrumentation to record the suction and dischargepressures and temperatures. The compressor wasdriven by a variable speed DC motor driving throughtwo gearboxes to achieve the required rotationalspeed. The performance testing was carried outusing Nitrogen at equivalent conditions and the fieldperformance calculated in accordance with theprocedures in PTC10.

Both aerodynamic and mechanical tests weresuccessfully completed for both the machines thatwere tested.

4.4 Commissioning and site testing

The first unit was shipped to site and wascommissioned on the platform in mid-2011.The unit was immediately out on-line, offeringsignificant power savings compared to theexisting units.

Figure 10. LP section FAT performance.

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