E. DESIGN BASIS OF TURBINES
Vendor experience of steam turbines shall be checked in similar service for the following (for API 611 turbines, only a, b and c to be checked):
a. Inlet pressure, temperature, volume flow rate
b. Output power, Turbine speed
c. Exhaust pressure, temperature (for back pressure turbine/non-condensing)
d. Type of the steam turbine (impulse or reaction turbine)
e. Number of stage
f. Bearing span
g. Driven equipment (compressor, Generator etc.)
E.1 API 611 Steam Turbines – General Purpose
Steam turbines shall comply with API 611 and refer to API 611, maximum steam pressure is 49 kg/cm2g, maximum steam temperature is 400 deg C, maximum rotation speed is 6,000 rpm.
It is recommended to apply this type of steam turbine for only single stage, horizontal, back pressure (non-condensing) steam turbine. Grease lubrication of bearings may not be applied unless otherwise specified. The lubrication may be pressurized, non-pressurized (ring-oiled) with cooling or non-pressurized (ring-oiled) without cooling depends on the following, as general rules:
- Steam turbines with rated speeds above 3,600 rpm, pressurized lubrication should be specified.
- Steam turbines with rated speeds 3,600 rpm and below, the selection of lubrication system depends mainly on the inlet steam temperature and the developed power (up to 325 deg C and power up to 150 kW may not without cooling, from 150 to 500 kW and from 325 deg C to 400 deg C will be with cooling, outside this range shall be pressurized)
E.2 API 612 Steam Turbines – Special Purpose
The steam turbines shall comply with API 612/ISO 10437. Steam turbine rated power and speed shall not exceed the limits of the vendor’s proven design and well within the range of the vendor’s successful experience.
It is recommended that an automatic engaging turning gear shall be specified, particularly if there is a requirement to re-start quickly within 8 hours after shutdown, or if there is a risk of rotor sag during cool down or standstill. Motor driven turning gear shall be provided with emergency power supply.
E.3 Gas Turbines
There are two types of gas turbine as follows:
1. Industrial heavy duty gas turbine
High power required, over 26 MW. Some advantage are less frequent maintenance, burn wider variety of fuels and available in larger power size
2. Aero-derivative gas turbine.
This type of gas turbine is based on an aircraft engine design which has been adapted for industrial use. Some advantages are higher efficiency, quick overhaul capability, lighter and more compact.
Gas turbines design is also categorized by shaft configuration, i.e single shaft and dual shaft. In single shaft, all rotating equipment of the gas turbine are mounted on one shaft. In dual shaft design, the air compressor rotating components are generally mounted on one shaft, and the power turbine rotating component is mounted on another shaft. The driven equipment is connected to the power turbine shaft. Single shaft design is simpler, requiring fewer bearings and generally used where the speed range of driven equipment is narrow or fixed. But it requires a powerful starting system since all rotating component (including driven equipment) must be accelerated to idle speed during the start cycle. Dual shaft design is advantageous where the driven equipment has a wide speed range or high starting torque. The air compressor is able to run at its most efficient speed while power turbine speed varies with the driven equipment. Dual shaft design allows a much smaller starting system since only the air compressor shaft is accelerated during start cycle.
The gas turbines shall comply with API 616. Power vary depending environmental condition, so site rating should be adequate to cover losses due to gas turbine losses: air compressor fouling (about 2%), ageing (about 3%), inlet system and outlet losses, waste heat recovery units in exhaust WHR loss to be specified in data
In aero-derivative gas turbine, some items should be considered: synthetic lube oil may be required in addition to mineral oil for driven equipment, so two separate lube oil system may be required, spare engine core is recommended in order to minimize maintenance duration
The following item shall be considered to be included in vendor scope:
- Oil mist eliminator with twin motor driven fans (one for spare) and manual by-pass
- Twin de-mister/separators with both manual and automatic drain valves (fuel gas scrubber skid)
- Automatic stop/bleed valves installed outside of noise enclosure.
The others consideration related to environmental condition are as follows:
a) Desert area with a high probability of dust/sand storms, installation of self-cleaning filters with inertial separators is recommended. In such applications it is particularly important to elevate the air intake as high as possible since airborne dust/sand will typically be confined to five meters above grade.
b) Coastal and marine area with a high probability of salt contaminant, installation of pre-filter and high efficiency filter is recommended.
c) Tropical area with a high probability of heavy raining, installation of weather hood, vane separator, pre-filter and high efficiency filter is recommended.
Installation of gas turbines in Zone 0 and Zone 1 areas is prohibited. The area classification inside of the enclosure shall be Zone 2 area. In order to achieve Zone 2 area, the following design consideration shall be considered: Air inlet of intake air filter shall be located in non-hazardous area and flammable-gas detectors shall be located as far upstream as practicable in the air intake filter, gas turbine shall be tripped and any remaining fuel gas in the fuel gas system inside the enclosure shall be vented out immediately when the enclosure ventilation is out of operation, ventilation outlets for the enclosure shall be extended to non-hazardous area, but when vendor can provide all electric equipment and instrument which are suitable for Zone 1 area, this requirement is not be required.
F. ACCESSORIES OF ROTATING EQUIPMENT
F.1 Shaft Seals (mechanical seal, gland packing, lip seal) for centrifugal and rotary pump
As general rule, mechanical seals shall be applied for all pumps, except fire pumps, cooling water pumps, sea water intake pumps and vertical sump pumps (VS4 / VS5). However, mechanical seal should be applied to vertical sump pump whose pumping liquid include toxic, hazardous and etc. Mechanical seals shall be in accordance with API 682 unless otherwise specified. Category 2 for API 610 pumps and category 1 for Non-API 610 pumps.
Gland packing can be installed for fire pump, cooling water pumps and vertical sea water intake pumps, Lip seal or gland packing can be installed for vertical sump pumps in non-hydrocarbon service.
F.2 Shaft End Seals for Compressors (Dry Gas Seals)
Dry gas seals shall be in accordance with API 617and/or API 614 Part-4. Dry gas seals shall be applied for all centrifugal compressors including integrated gear type except for air and nitrogen gas service.
A dual tandem self-acting dry gas seal arrangement with an intermediate labyrinth and an inert clean buffer gas between the labyrinth and the secondary seal shall be supplied as the first choice. Nitrogen gas shall be used for secondary seal gas.
The seal gas source shall be available at sufficient pressure to cover entire operating range of the compressor, including start-up, shut-down, or idle, and all other static conditions. A common practice is seal gas supply is from the compressor discharge. In case compressor does not develop enough pressure rise or not running, an external source of seal gas supply system shall be considered.
In refrigerant compressor service, seal gas pressure boosting up system is preferable to eliminate contamination by other gas other than refrigerant gas.
A labyrinth type separation seal with dry nitrogen gas as separation gas shall be provided between the DGS assembly and the bearing housing, to protect the DGS from possible oil contamination unless otherwise specified.
Contact carbon segments type separation seal shall be prohibited to use. Non contact carbon segments type separation seal may be used only when nitrogen gas consumption is limited to use or Client specify. However in this case required dew point of nitrogen gas by separation seal supplier shall be met with dew point of nitrogen gas supplied to separation seals.
Other seal types, such as, labyrinth type seals or a restrictive ring may be selected for compressors handling non-toxic and nonflammable gases such as air. In the case of non-flammable, non-corrosive and non-toxic services at ambient temperature
F.3 Forced Lube Oil Systems and Oil Mist System
Forced lube oil system for API 610 designed pump services shall be in accordance with API 614 Part 1 and API 614 Part 3 (General purpose oil system), except for OH6 pump (Sundyne) and API 618 reciprocating compressor.
Forced lube oil system for compressors, gas turbines, expanders and special purpose steam turbines shall be in accordance with API 614 Part 1 and Part 2 (Special purpose oil system).
The design shall allow for transfer between operation and shutdown of the main and spare components of the system for maintenance without interrupting the operation of the system or equipment the system serves.
Oil mist lubrication system shall be applied only when specifically required by project requirement. There are two types of oil mist system, i.e. pure oil mist (dry sump) or Purge oil mist (wet sump).
Pure oil mist utilize a continuous oil mist flow through the bearing housing to deliver clean oil directly to the bearings and maintain an outward flow of air from the housing to prevent the ingress of moisture and other corrosive contaminants.
Purge oil mist use reclassified oil to maintain a constant level of oil in the sump to lubricate the bearings. The air purges the bearing housing and prevents the ingress of moisture and other corrosive contaminants. Purge oil mist shall apply to sleeve or journal bearings. For equipment handling liquid or gas temperature more than 300 deg C, bearing housings shall be protected from being overheated by heat transfer from the equipment, so specail features may be required.
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