Aeroderivative and Industrial Gas Turbine

Gas Turbine

In oil and gas industries, gas turbine is one of common driver to drive booster centrifugal compressor or generate your power as generator set. Solar turbine, GE and Roll Royse are very known manufacturer of gas turbine.

At this time,It would like to discuss and try to summary what is the basic different between industrial and aeroderivative gas turbine.

Aeroderivative Engines,

It is designed for air craft application and modify to be used in industry. These type of engine requires high thrust at full load, good part load efficiency and low weight. So, multi spool/shaft design with high rotational speeds and a comparatively high compressor pressure ratio.

It has rolling element bearing, fabricated casing,two separate oil systems ( gas turbine and driven equipment), high firing temperature, higher efficiency ( 38 % to 40 %), better fuel consumption, higher NOX and CO.

Industrial Engines,

It is designed for land use where size and weight are less important than aeroderivative design. 

Robust design, hydrodynamic bearing, cast main casing,common lube oil system, lower firing temperatures, lower efficiency ( 30 to 33 %), lower emission than other engines.

The key point in selection gas turbine is based on economical and operational advantage. For the same power, aero derivative engine may offer more compact design compare to industrial gas turbine. Since aeroderivate use more advance material and technology, so it contributes the capital investment / KW Power more expensive than industrial gas turbine. These condition may be the reason why industrial gas turbine is more favourite to be selected and used in Oil and gas Industry. 

Guideline for PERFORMANCE TEST of Centrifugal Pump

Performance test is a critical step to ensure that the purchased pump can deliver the fluid meet with the requirement. The following factors which may help are based on API 610  :

PREPARATION 

• Ideally use water at temperature less than 65 Deg. C for performance test.
• Actual mechanical seal and bearing shall be used.
• The leakage rate of used mechanical seal shall be within acceptable range of ISO 21049 and/or API 682.
• The operating parameter of oil lubrication such as temperature, viscosity and pressure are expected within the recommended values in the Vendor operating instruction.
• Don't miss to prelubricate the bearing prior to testing stage.
• Ensure the tightness of all joints connection and any leaking shall be arrested.
• All warning, protective and control devices shall be checked and adjusted as required.

TESTING

• Test data including head, flowrate, power, bearing temperatures and vibration ( at least five points ) shall be recorded at different condition : Shutoff, minimum continous stable flow, midway between minimum & rated flow, rated flow and max. allowable flow ( as minimum 120% BEP).
• Test point at rated flow shall be within a tolerance band of +/- 5% of rated flow.
• The speed during test shall be within 3% of specified speed in pump data sheet.
• Test data  shall include test curves and a summary of test performance data compared to guarantee points.
• If adjustment such as reduction in impeller diameter of single stage pump requires and it is less than 5%, so no retest required. However, any disassembly of mutli stage pump it call of retest.
• Final performance test shall be run after the correction is made.
• Additional test that can be performed are NPSHR test, Complete unit test, sound level test, Auxiliary equipment test, Bearing housing resonance test and Mechnical run test for 4 hours.

PERFORMANCE TEST TOLERANCES

Different type of SEAL FLUSH Plan

A mechanical seal of Cenrifugal pump requires flushing system to extend the life time. API 682 " Shaft sealing system for centrifugal and rotary pump" provide information of different type Flushing Plan system, as follow :

1. API Plan 01, recommended for clean fluids only.
2. API Plan 02, It is more common in the chemical industry in application with low seal chamber pressures and process temperatures. 
3. API Plan 11,The default seal flush plan for all single seals.
4. API Plan 13. The standard flush plan for vertical pumps that are not provided with a bleed bushing below the seal chamber.
5. API Plan 14. Combination of Plan 11 and Plan 13, it is most commonly used on vertical pump.
6. API Plan 21. Provides a cool flush to the seal.
7. API Plan 23. this plan of choice for all hot water services, particularly boiler feed water, and many hydrocarbon services.
8. API Plan 32. For application which a service containing solids or contaminants where a suitable cleaner or cooler external flush will improve the seal environment.
9. API Plan 41. Combination between Plan 21 and Plan 31 and specify for hot services containing solids.
10. API Plan 52. It is used in a service where no leakage to atmosphere can be tolerated.
11. API Plan 53a, 53b and 53c. Pressurized dual seal systems are used in services where no leakage to atmophere can be tolerated.
12. API Plan 54.Pressurized dual seal system with inner seal leakage into the pumped product.
13. API Plan 62. A quench stream is brought from an external source to the atmospheric side of the seal faces.
14. API Plan 71. Unpressurized dual seals, which utilize a dry containment seal and where no buffer gas is supplied but the provision to supply a buffer gas is desired.
15. API Plan 72. Unpressurized dual seals which also uses a dry containment seal.
16. API Plan 74. Dual pressurized seals, where the barrier medium is a gas.
17. API Plan 75. Unpressurized dual seals, which also utilize a dry containment seal and where the leakage from the inner seal may condense.
18. API Plan 76. Unpressurized dual seals which also utilize a dry containment seal and where leakage from the inner seal will not condense.

Continue with sketch drawings for each Flushing Plan type.

Centrifugal Pump Construction Model

Referring to API 610, it provide several type of centrifugal pumps as per the construction model as listed below:

1. Overhung ( Flexible coupled, rigidly coupled, close coupled ).Flexible coupled consists of Horizontal and Vertical inline with bearing bracket, Rigidly coupled  for vertica inline, and close coupled are used for vertical inline and high speed integrally geared)
2. Between bearing ( 1 & 2 stage and Multi stage ). Axially and Radial Split.
3. Vertically suspended (Single casing and Double casing). Discharge through coloumn and separate discharge, while double casing type has two type : Difusser and Volute.

Table of Pump construction Model.

Cavitation Case on Pump

CAVITATION OUTCOME.

Cavitation is one of the pump failure that very common in Centrifugal pump. At design stage, The system must be verified to have NPSHa ( Net position suction head available ) is higher than NPSHr ( Net Position suction head requreid by pump ). This configuration will allow fluid at suction line has pressure above the vapor pressure of liquid at operation condition, in order to avoid the vaporization of liquid and formation of gas bubbles. 

As addition to that, the other conditions which may lead decrease the fluid pressure :

Minimize any sharp change on the suction line due to it will increase the friction loss.

Operate the pump at best efficiency operating area, so less or more flow will not a case because it will change the friction loss on the system.

Therefore, ensure your selected pump has less NPSHr compare to your NPSHa of the designed system, so cavitation will not your interest.

This failure was iniated the any bubble inside the pump, so the power from driver will use to expanding the bubble instead of bringing more liquid into the pump. so the bubble move from low pressure section to the high pressure zone of the pump, then the bubble can collapse and this will make the liquid strikes the metal parts at the speed of sound. That is way, you will hear abnormal noise on your pump during this phenomena.

The following some options to solve cavitation problem :

• Replace the impeller material with more cavitation resistant material.
• Upgrade the pump which meet to NPSHa and system head.
• Reduce the pump capacity by trim the impeller.
• Throttle the pumps to operate at lower capacity wherein the NPSHa exceeds the NPSHr.
• Use VFD to reduce speed and pump capacity.

Centrifugal Pump : API vs ANSI Pump

In the market and industry, you may find different application and construction of centrifugal pumps. The most frequently find is API 610 Pump and ANSI/ASME B73.1 Centrifugal Pump.

The API 610 pump will be selected for handling Hydrocarbon fluid, while ANSI pump can be used for non critical operation and services.

The overview of each standard requirement is provided below,

API 610 Pump.

• Full faced flange.
• Centerline mounting support.
• Not allowed for Bearing Housing support.
• Bearing type : Duplex, Single row,angular contact.
• Impeller mounting with keyed.
• Wear rings on the casing and impeller.
• Seals system as per API 682.
• Required seal chamber throat bushing.
• Use sock weld or butt weld for auxilliary connections and sch 160 as minimum for the  auxilliary piping.

ANSI/ASME B73.1

• Foot under the casing.
• Bearing Housing support is acceptable.
• Bearing type : not specified
• Impeller mounting with keyed or threaded.
• Wear rings are not specified.
• No specific requirement for sealing system.
• Not specified for the seal chamber throat bushing.
• Use Threaded NPT for auxilliary connections and sch 40 as minimum for the  auxilliary piping.

It is clearly indicated the requirement of API 610 is more stringent than ANSI Pump. As addition to that, ANSI pump is limited for low pressure application.

CENTRIFUGAL API 610 PUMP

SINGLE STAGE CENTRIFUGAL API 610 PUMP (@courtesy GOULD Pump)

The above drawing provides the higlighted internal components as follow :

• Impeller with close type.
• Renewable wear ring ( Front and rear rings control seal chamber flows and pressures ).
• Seal chamber.
• Lubrication system with Ring Oil System.
• Bearing.
• Heavy duty bearing frame.
• Labyrinth Oil Seals.
• Rigid Heavy duty shaft.

Those parts are conform to the design and clearance required as per API 610.

MULTI STAGE CENTRIFUGAL API 610 PUMP (@courtesy GOULD Pump)

These model is between bearing type with radial split casing.  It is used for high pressure and high temperature application.   The highlighted internal parts are :

• Heavy duty thrust bearing.
• Heavy duty shaft.
• Heavy duty radial bearing.
• Renewable wear rings.
• Enlarged seal chamber.
• Opposed impellers arrangement ( Driven by individual key and positioned on shaft by dual locknuts).

The casing is supported excatly in the centerline, it is an unique phisycal identification of the API 610 centrifugal compare to other Standard.