A. INTRODUCTION
This topic cover design consideration for some rotating equipment such as pumps, compressors, gas turbines and steam turbines and their auxiliaries. This guideline, of course, as a general use and consideration, so specific consideration and detail requirement shall refer to owner specification and/or project specification for each project.
The important thing for rotating equipment is material selection. This selection shall be based on process system (if material selection chart is available, it should be for reference) and applicable International Codes & Standards. Manufacturer may offer superior base material based on their experience. For items where material selection basis and/or any International Codes & Standards are not available, engineer shall consult with material engineer/specialist.
The other important matter is the equipment itself shall be of proven design equipment. As general rules, the definition of proven design shall be that at least one equipment of similar capacity and comparable operating conditions have been supplied, installed and operated in similar applications with 8000 hours in operation (16000 hours for steam turbine, gas turbine and centrifugal compressors) of uninterrupted operation.
B. PUMPS DESIGN BASIS
B.1 General
B.1.1 Pump manufacturer experience in similar service should be checked carefully. Some parameters such as, pump type/model, pumped fluid, capacity, head, suction & discharge pressures, pumping temperature, material of major components, driver rating, speed, manufacturing date
B.1.2 NPSH Margin
Available NPSH shall be higher than the required NPSH at least 0.6 m to prevent detrimental cavitations, unless client or licensor is specified.
For Boiler Feed Water pump, NPSHA shall be higher than 1.5 -1.8 times NPSHR because load fluctuation for BFW pump causes pressure fall of suction drum (de-aerator) below the vapor pressure of hotter liquid still being in suction line. It is also confirmed that this margin should be satisfied with all operating range of pump operation
B.1.3 Suction Specific Speed (SSS)
The suction specific speed shall not exceed 11,000 (USGPM-ft-rpm), 13000 (m3/hr-m-rpm). Suction specific speeds of up to 24000 (USGPM, ft) are acceptable for high speed integrally gear driven type pumps (Pump type OH6) and cryogenic pump
B.1.4 Pumps for parallel operation shall have a similar percentage head rise to shut-off and at least 10% head rise to shut-off
B.2 API 610 Designed Centrifugal Pumps
API 610/ISO 13709 designed pumps shall be the primary selection for hydrocarbons service, toxic service, solvent service such as Amine, Sulfiinol & Carbonate service. Alternative pump (ANSI/ASME B73.1, B73.2 or ISO 5199 Class 2) selection may be considered when operating condition and/or pumping liquid meet the requirement in specified below:
- Handling liquid: Hydrocarbon > S.G 0.75, chemical and water
- Operating temperature: 0 ~ 150 deg C
- BHP: less than 100kw
- Head: less than 120 m
- Suction pressurere: less than 5 barg
- Discharge pressure: less than 19 barg
- ANSI flange pressure rating maximum 150#
- Max rotating speed : 3600 rpm
B.3 Pumps for Water Service
B.3.1 Boiler Feed Water Pumps
Boiler Feed Water pumps shall comply with API 610. API 610 Type BB4 (Ring Section Pumps) may be used only when owner/client approve.
B.3.2 Horizontal Cooling Water Pumps
In general, if BHP is equal or less than 100 kW, general service centrifugal pumps (non-API pump) may be applied. If BHP is over 100 kW, API 610 shall be applied and it should be considered to confirm with vendor/manufacturer if vibration criteria specified in API 610 can be complied especially large capacity application.
B.3.3 Vertical Sea Water Intake Pumps
Vendor standard designed is acceptable with the following conditions unless otherwise specified in project specification.
a) Vibration level shall refer to Fig.9.6.4.14 of ANSI HI 964 (for pump power less than 2.2 MW) and
7.11 mm/s RMS unfiltered for pump over 2.2 MW
b) Pumps shall have a stable head/capacity curve rising continuously to shut-off. The pump shut-off head shall be at least 110% of the head at the rated capacity.
c) Normal operating point should be range of between 70% and 120% of BEP (Best Efficiency Point)
d)For maintenance purpose, the motor shall be supported by a stool mounted on the discharge head. So it will give adequate access to the shaft seal and coupling without removing the motor.
e) Pumps shall be tested at the minimum liquid level stated in the equipment data sheet.
f) If pump is shortened to suit the vendor’s test facility. Corrections shall be made in the head produced due to the reduced length and friction losses.
g) When operated at rated speed and rated capacity, pumps tesst tolerances shall be in accordance with the performance tolerances defined in API 610
B.4 Seal Less Pumps:
Seal less pumps shall comply with ANSI/ASME B73.3. API 685 may be applied only when client/owner specified.
Seal less pumps may be applied for pump applications within following operation conditions:
a) Handling liquid: hydro carbon, chemical and water and comply with the following condition:
- the liquid does not contain any solid particles
- the liquid does not crystallize
- the liquid kinematic viscosity is not greater than 40 mm2/s
b) Flow rate: less than 150 m3/hr
c) Operating temperature: between minus (-) 80 deg C and 200 deg C
d) Head: less than 200 m
e) Casing max. allowable working pressure: less than 80 barg
B.5 Vertical Sump Pumps and Self Priming Pumps:
B.5.1 Vertical Sump Pumps
Vertical sump pumps are cantilever pumps (VS5), wet pit single stage casing volute line-shaft pumps (VS4), Wet pit, two stage and multi stage pumps including ring-section pumps. Vertical sump pumps shall comply with ISO 5199 class II.
B.5.2 Self Priming Pumps
Vendor standard pump is acceptable for this pump type. As a general guideline, self priming pumps should be applied when suction lift does not exceed 6 m. Maximum suction lift based on operating capacity and Head shall be checked with vendor before finalizing pump selection.
In practical application that shaft length of cantilever pumps and suction lift of self priming pumps have limitations, so pit depth including low liquid level shall be carefully checked for the pump selection.
B.6 Submerged Motor Cryogenic Pumps:
The Submerged motor cryogenic pumps shall be applied for cryogenic liquid service, for example pump minimum operating temperature is below minus 80 degC, and minimum metal design temperature is below minus 100 degC.
B.7 Fire Water Pumps:
Fire water pumps shall comply with NFPA 20. UL listed/FM approved pumps shall be selected.
Diesel engine driven fire water pumps shall be string tested on load condition at vendor shop.
The fire jockey water pump is exempted from the Fire water pumps, however shall comply with ANSI/ASME B73.1, B73.2 or ISO 5199 Class 2
B.8 Control Volume & Reciprocating Pumps:
B.8.1 Control Volume Pumps
Control volume pumps are classified as diaphragm type and plunger type. Control volume pumps shall comply with API 675. This pump shall only be used in metering or chemical injection service. Double diaphragm pumps shall be specified for toxic and hazardous service. Diaphragm failure indication shall be provided.
B.8.2 Reciprocating Pumps
Reciprocating pumps are classified as direct-acting type and power-frame type. Reciprocating pumps shall comply with API 674.
B.9 Rotary Pumps:
Rotary pumps are classified as Screw pumps and Gear pumps. Rotary pumps shall comply with API 676 unless manufacturer standard is approved by client
B.10 Deep Well Water Pump
Vendor standard pump is acceptable. The following auxiliaries should be included in vendor scope of supply. Check valve, Bleed valve, Level switch, Cable, Riser pipe, Local control panel (LCP) including motor starter, Well head/Discharge head
B.11 Submersible Pump
Vendor standard pump is acceptable. This pump is usually be used for sewage/sanitary, water drainage, etc. Guide rail, discharge adopter, lifting chain should be included in vendor scope of supply. Scope of riser piping for submersible pumps shall be mutually agreed between vendor and contractor.
C. COMPRESSORS DESIGN BASIS
C.1 General
Selection of compressor (axial, centrifugal, reciprocating or rotary) depends primarily on the required flow to be compressed (see fig. 1 below). Compressors in hydrocarbon service shall be centrifugal compressor as the primary selection when operating capacity and pressure are within centrifugal compressor area in Fig 1 Compressor Coverage Chart, unless specified otherwise in the process data sheet. There is an overlap of centrifugal and reciprocating compressors on the low end of the flow range (see Fig. 1). On the higher end of the flow range an overlap with the axial compressor exists. Before a technical decision could be reached as to the type of compressor that would be installed, the service, operational requirements, and economics would have to be considered.
C.2 Centrifugal Compressors
Centrifugal Compressors shall comply with API 617 (chapter 1 & 2). In common practice, the maximum discharge temperature shall be kept below 200 °C under all operating conditions (including full recycle and starting conditions), unless otherwise specified. Cracked gas compressor in ethylene plant, normal operating discharge temperature should be maintained below 90 °C for cracked gas compressors to avoid polymerization or coking.
It is recommended that single section compressors with in-line impeller arrangements shall have no more than 9 impellers per casing and two section compressors shall have no more than 8 impellers per casing.
At bidding stage /proposal stage, vendor experience shall be checked for aerodynamics, rotordynamics and construction features in similar service. In general, proven experience shall be checked for their performance (flow coefficient, inlet mach number at impeller eye, machine mach number, polytrophic head coefficient and head developed at each stage, Impeller tip speed), number of impellers per similar casing construction, bearing span, rotor dynamics and critical speed, Rotor stiffness ratio, casing information (suction/discharge /design pressure, suction/discharge temperature, and casing construction), casing and impeller material
Where more than one compressor casing is included in the compression train, Selected compression train configuration should have maximum of four individual equipment in the coupled String and Maintenance method should be discussed with Supplier.
C.3 Reciprocating Compressors
C.3.1 General
If the required flow is too small or head required too high for a centrifugal compressor, then a reciprocating compressor shall be considered. Vendor experience shall be checked for construction features of offered in similar service. As a minimum proven experience shall be shown for Average Piston Speed, Rotating Crank Speed, Combined Rod Load, Cylinder/Piston Diameter and its material & construction, Hardened method of Connecting rod and rod packing material, Piston ring and rider ring material, Frame information (suction/discharge/design pressure, suction/discharge temperature, absorbed power, Lube or Non-lube, handing gas)
Reciprocating compressors shall comply with API 618 except high speed reciprocating compressor (refer to API 11P), diaphragm compressor and Packaged reciprocating compressor (refer to API 680)
Non-lubricated type reciprocating compressors shall be selected for air and oxygen service. In critical services, , a spare unit shall be considered when operational interruption for maintenance cannot be accepted.
C.3.2 Horizontal-balanced Opposed Type Reciprocating Compressor
The maximum average piston speed in lubricated compressor shall not exceed the following values:
- 150 kW and lower 5 m/s
- 150 – 250 kW 5 m/s
- 250 kW and higher 4 m/s
The average piston speed in compressors handling saturated hydrocarbon gases and in non-lubricated compressor cylinders shall not exceed 3.5 m/s.
C.3.3 Vertical Labyrinth Piston-type Compressors
The maximum speed of vertical labyrinth piston-type compressors shall be 420 rpm, irrespective of the installed driver power.
4.3.4 High Speed Reciprocating Compressors
High speed reciprocating compressors shall comply with API 11P. This compressor type shall be limited to a maximum speed of 1800 rpm. Since operating speeds over 1,000 rpm will have a major impact on machine wear and will reduce the time between maintenance. If for some reasons a speed above 1,000 rpm is selected, this shall be supported by sufficient references/experience and impact on maintenance shall be included in the evaluation.
C.3.5 Packaged Reciprocating Compressor
Packaged Reciprocating compressors shall comply with API 680
C.3.6 Diaphragm Type Compressors
For low flows, typically 88 cfm and less, a diaphragm type compressor should be considered.
C.4 Screw Compressors
Screw Compressors shall comply with API 619.
C.5 Integrally Geared Compressors
Integrally Geared Compressors shall comply with API 617 Chapter 1 & 3. It is recommended that bending and torsional blade natural frequencies under both static and operating condition shall not coincide with any source of excitation from 10% below minimum allowable speed to 10% above maximum continuous speed (verified by Campbell diagram)
If this is not feasible, blading shall be designed with stress levels low enough to allow unrestricted operation within operating speed (verified by Goodman diagram)
Sundyne compressor (high speed integrated gear in-line compressor) may be used when operating conditions are on the conditions: maximum motor rating 300 kW, maximum suction pressure and discharge pressure are 70 kg/cm2g and 99 kg/cm2g, respectively.
C.6 Expander-Compressors
Expander-Compressors including expander for generator drive shall comply with API 617 Chapter 1 & 4. Active magnetic bearing shall be adopted to eliminate possible oil migration into process side, unless otherwise specified. Shaft seal type should be labyrinth as first selection. External seal gas with enough margin of the required pressure should be used for the sealing system even on normal operation. In case compressor discharge gas is applied as seal gas, the feasibility study shall be verified and confirmed with vendor/manufacturer, due to the less margin between available pressure and required pressure.
C.7 Packaged, Integrally Geared Centrifugal Air Compressors
The Compressors shall comply with API 672. Packaged, integrally geared centrifugal air compressors shall be considered such as, for instrument air and plant air services.
C.8 Packaged, Screw Plant and Instrument Air Compressors
The Compressors shall comply with ISO 10440 Part-2 and ISO 1217. Packaged, screw plant and instrument air compressors may be considered if the required flow rate is less than 2500 Nm3/hr at discharge pressure of 10 kg/cm2a
D. FAN & BLOWER
D.1 Fan
Centrifugal fans (except fired-heater service and incinerator service) that develop less than 0.25 kg/cm2 g shall comply with API 673. Centrifugal fans for fired-heater service and incinerator service shall comply with API 560, Appendix E.
D.2 Blower
In general use, API673 shall be applied to centrifugal blowers that develop greater than 0.25 kg/cm2g and less than 0.35 kg/cm2g
(to be continued….)
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