5 Basic Design
5.1 General Requirements
5.1.1 Heavy duty centrifugal pumps(including accessories)shall be,as a design criterion,designed and constructed for a minimum service life of 20 years and at least 3 years of uninterrupted operation.Unless otherwise approved by the purchaser in written form,vendor supplied equipment shall have the specifications within the vendor’s experience for design and fabrication,and shall be proven reliable for use in the similar service conditions.Two years of proven operating experiences shall be available for at least one pump unit of the same size under the same or similar process conditions.
5.1.2 Unless otherwise specified,the vendor shall assume full responsibilities for the whole unit including pump,gear box(if required),driver,oil system,controls and instrumentation,couplings,associated accessories and piping systems,and the proper configuration between them.The scope of supply by vendor refer to Annex A in this specification.
5.1.3 Unless otherwise specified in datasheet which refer to Annex B in this specification,pumps,drivers and their accessories shall be suitable for outdoor startup and continuous operation under all specified environmental conditions.
5.1.4 Pumps shall be capable of at least 5% head increase at rated conditions by replacement of the impeller(s)with one(s)of larger diameter or different hydraulic design.
5.1.5 For variable speed pumps,clause 5.1.7 and 5.1.8 in ISO 13709-2003(API Std 610-2004)shall be conform.
5.1.6 To achieve high efficiency and low vibration level,pump should run within preferred operating region(70%-120% of the best efficiency flowrate),and normal flow should be less than the best efficiency flowrate.The pump with best efficiency flowrate close to the normal flowrate required by process is preferred.
5.1.7 The pump shall have stable head/capacity curves showing continuous head rise from the rated point to the shutoff point and the head rise shall be at least 5% of the rated head.If operation in parallel is specified,the head rise from rated point to shutoff shall be at least 10% of the rated head.
5.1.8 Unless otherwise specified by the purchaser in written form,the head at shutoff point shall not exceed 120% of the rated head.
5.1.9 The net positive suction head required(NPSHr)at rated point shall be at least 0.6m less than the net positive suction head available(NPSHa),and a correction factor for hydrocarbon liquids shall not be applied.The datum elevation is the shaft centerline for horizontal pumps,the suction centerline for vertical in-line pumps,and the top of the pump foundation for other vertical pumps.
5.1.10 For pump without inducer,it shall have a suction specific speed(S)not more than 12780.When S is more than 12780,alternative design can be provided by the vendor if he can offer sufficient data for reliability proving,however,such alternative design shall be approved by the purchaser.The value S can be calculated as per the following Formula(1):
Where:N—Rotative speed,r/min;
Q—Flowrate per impeller eye:total flowrate for single-suction impellers,and one half for doublesuction impellers,m3/h;
NPSHr—Net positive suction head required for the pump,m.
5.1.11 Inducer should not be used except for high speed pumps.When an inducer is provided,all the requirements of 5.1.11 a)through 5.1.11 d)shall be met:
a)The value of suction specific speed(S)shall be within the range proven of vendor’s experience;
b)The design capacity of the inducer shall be near to the rated point of the pump;
c)The inducer shall have stable mechanical performance on vibration,and shall have high durability against erosion and any other mechanical failures;
d)Stable operation range of the inducer shall be counted into the stable minimum continuous flow,which is specified in the data sheet.
5.1.12 For pumps with heads greater than 200 m per stage and with more than 225kW per stage,the radial clearance between the diffuser vane or volute tongue and the periphery of the impeller blade shall be in accordance with clause 5.1.17 in ISO 13709-2003(API Std 610-2004).
5.1.13 Unless otherwise specified,pump cooling system shall be designed in accordance with clause 5.1.19 through clause 5.1.22 in ISO 13709-2003(API Std 610-2004).
5.1.14 The arrangement and maintenance of equipment shall be in accordance with clause 5.1.23,5.1.26 and 5.1.27 in ISO 13709-2003(API Std 610-2004).
5.1.15 Unless otherwise specifically approved by Purchaser,use of overhung type pump shall be avoided for the following case:
a)Two-stage pumps;
b)Double-suction pumps;
c)Single-stage pumps with large diameter impellers,which are out of the vendor’s proven experience for overhung pumps design.
5.1.16 A single volute casing can be used for the pump with discharge nozzle size of DN 100(4 in)or less.When a single volute casing is applied for the pump with a discharge nozzle size of DN 100(4 in)or larger,it shall be subject to Purchaser’s written approval.
5.1.17 The use of vertical in-line pumps shall be limited to the following conditions:
a)The impeller tip speed shall be less than 50m/s;
b)The maximum working temperature for the pumps shall be below 150℃.
5.1.18 Unless otherwise specified,horizontal and vertical in-line pumps shall be back pullout construction and furnish spacer-type couplings.
5.1.19 For vertical,in-line,integral-geared and high speed pumps,steel structure support for motors shall be provided when the rated power of the motor driven pumps is 150 kW or larger or the motor weight is 1600kg or above.
5.1.20 For vertical sump pumps,the vendor shall provide a suction screen with proper mesh size.