How to build a centrifugal pump with strong reliab

How to build a reliable centrifugal pump

with the development of society and the progress of science and technology, people have higher and higher requirements for health, environmental protection, safety and so on. As a general-purpose machine, the indicators of centrifugal pumps, especially those under important working conditions (such as nuclear power plants, thermal power plants, petrochemical and other key pumps), have also attracted more and more attention, of which the most critical indicator is the reliability of products

this can be embodied in the bidding documents of some large projects in recent years

there are usually such requirements in the technical specifications of many key pumps, such as: the bidder should provide high-quality equipment, which should be mature, reliable and technologically advanced products Ensure that the pump has high operation efficiency and reliability; The equipment provided by the bidder shall be brand new, advanced, mature, complete, safe and reliable...

what is the reliability of products? What are the factors that affect product reliability? Based on relevant standards, specifications and engineering practice experience, this paper will give systematic and comprehensive suggestions on how to build a high reliability centrifugal pump from the aspects of design, processing, manufacturing and assembly, selection of supporting parts, and on-site installation and application of products, which are only for reference of peers

1 nouns and terms

1.1 redundancy

there is more than one means (i.e. artificially adding repeated parts) in the product to perform the same specified function, which is called redundancy. Its purpose is to back up the original single part to improve the reliability of the system

1.2 minimum continuous stable flow

api610 standard [1] is defined as the minimum flow that the pump can work without exceeding the vibration limit specified in this international standard

1.3 suction energy (SE)

suction energy is a measure of liquid momentum at the inlet of centrifugal pump impeller, which is defined as the product of impeller inlet diameter, pump speed, suction specific speed and specific gravity of pumped medium

for end suction pumps, Se value is greater than or equal to 160 × 106 (RPM GPM ft) is defined as high se; For split pump and radial inlet pump, Se value is greater than or equal to 120 × 106 (RPM GPM ft) is defined as high se. When the se value is greater than or equal to 1.5 times the high Se value, it is called very high se

1.4 condition monitoring systems (CMS)

a system that measures specified machine process parameters and predicts trends. Provide alarm, display and analysis tools for detecting and identifying developing faults

2 reliability and its influencing factors

the ability to complete the specified functions under the specified conditions and within the specified time interval is called product reliability. Reliability is a quality index related to time, and the pump can only reach the reliability target value within a certain time range

there are many factors affecting the reliability of centrifugal pumps, which are mainly divided into four categories: pump body design, ● standard punch ball: φ 20 ± 0.05mm product processing, manufacturing and assembly, accessory selection and design, and field installation and application. It can be further divided into:

2.1 pump body design

1) executive standard

2) hydraulic design

3) structural design

2.2 product processing, manufacturing and assembly

1) processing and manufacturing

2) assembly

2.3 pump accessory selection and design

1) bearing type selection and lubrication

2) mechanical seal and system

2.4. Field installation and application

1) field installation

2) field commissioning

3) On site operation

3 executive standards

there are many and miscellaneous standards implemented in China's centrifugal pump industry, including domestic standards (such as national standard GB, Ministry of machinery industry standard JB and enterprise standards), foreign standards (such as European Union standard EN, German standard DIN, Japanese Industrial Standard JIS, American National Standards Association standard ANSI, American Petroleum Association standard API, etc.) and international standards (ISO)

different standards have different requirements for pumps, especially in terms of reliability

generally speaking, API610 standard has the highest requirements - there are strict requirements from hydraulic design to structural design, and then to the selection and design of supporting parts, and the contents involved in this standard basically come from good engineering verification and operation practice. The products that implement this standard have outstanding advantages such as high reliability, long life, strong operability and convenient installation and maintenance. Therefore, it has been widely used in important working conditions at home and abroad Equipment in key occasions

the article will give systematic and comprehensive suggestions on how to improve the reliability of centrifugal pumps based on the relevant provisions and requirements of ansi/api610 11th edition standard (hereinafter referred to as "the standard") and practical engineering experience, hoping to be helpful to colleagues

note: the clauses, tables and drawing numbers involved in this paper refer to the clauses, tables and drawing numbers corresponding to ansi/api610 11th edition standard

4 hydraulic design

excellent hydraulic design should not only meet the required performance (flow, head, cavitation), but also meet the requirements of high efficiency and high reliability

4.1 clause 6.1.8... When determining the NPSH of the device, the buyer and the seller should make clear the relationship between the minimum continuous stable flow and the specific suction speed of the pump. Generally speaking, the minimum continuous stable flow of the pump increases with the increase of the specific suction speed... When selecting the specific suction speed and NPSH safety margin, the existing industrial level and manufacturer's experience should be considered

[interpretation] as for the influence of suction specific speed on the operation reliability of centrifugal pump, international peers have very rich engineering application experience, and give the maximum limit value of suction specific speed. Among them, the limit value of suction specific speed specified in UOP specification [2] has been widely recognized and applied in the world, which stipulates that the suction specific speed of the pump shall not be higher than 13000 (m3/h, RPM, m); When the pumping medium is water or a solution with a water content of more than 50%, and the power of the single-stage impeller of the pump exceeds 75 kW, the suction specific speed shall not be higher than 11000 (m3/h, RPM, m)

[suggestion 1] with the progress of science and technology, due to the emergence of some other design methods that can improve the suction performance of centrifugal pumps, the limit value of suction specific speed is also correspondingly increased. According to the practical engineering experience of international peers, the limit value of BB2 pump is usually controlled at 14400 (m3/h, RPM, m). For details, please refer to literature [3]

[recommendation 2] for high-energy pumps and high suction energy pumps, it is necessary to provide a larger NPSH margin or margin ratio than that recommended in common standards and specifications. For detailed requirements, see references [4] and [5]

4.2 clause 6.1.11 for all application conditions, the pump should preferably have a stable flow head curve (that is, the head curve is continuously rising until the dead point)

[interpretation] it should be avoided that the flow head curve has a hump, which will lead to an unstable operation range of the pump (i.e. the operation condition with small flow), and the operation in this range will lead to a significant increase in vibration and noise and a decline in reliability

[suggestion] for the delivered solid pump, it is necessary to avoid running in the unstable operation range (i.e. the operation condition of small flow); For newly developed products, appropriate measures should be taken in the design process to avoid humps in the flow head curve

4.3 clause 6.1.15 pumps with a single-stage head of more than 200m and a single-stage power of more than 225Kw need special measures to reduce the frequency vibration of impeller blades and low-frequency vibration at small flow. For these pumps, the radial clearance between the guide vane and the outer circumference of the impeller blade is at least 3% of the maximum impeller tip radius (for guide vane pumps) and 6% of the maximum impeller tip radius (for Volute Pumps)

[interpretation] this kind of pump is called high-energy pump. If the vibration exceeds the standard, the above clearance factors may be the main cause

in practical engineering applications, this phenomenon is common in all centrifugal pumps, and it is called "blade runner syndrome" abroad. When the liquid flows through the small channel, the increase of the flow rate of the liquid will cause the decrease of the liquid pressure, local vaporization, the generation of bubbles, and then rupture under high pressure, leading to cavitation and the vibration of the pump

[suggestions] for high-energy pumps, the clearance between impeller blades and volute/guide vanes should not be less than the above recommended value

5 structural design

there are many structural designs that affect reliability, such as the strength of pressure housing, pump shaft and base; Stress and fit of structural members; Balance mode of axial force; Rotor dynamics problems, etc

how to solve the reliability problem of products from the design? Here are some guiding principles [6]:

1) try to use proven and mature technology

2) simplify the structure as much as possible and reduce the number of parts

3) adopt standardized and modular structure as far as possible

4) set the fault monitoring and diagnosis device

5) ensure the design margin (safety factor) of parts

6) adopt redundant design when necessary

7) fail safe design

8) safety life design. A design that ensures full safety without damage in use

9) strengthen the reliability analysis of important parts. FMEA (failure mode effect analysis) and FTA (fault tree analysis) are important means in reliability analysis

10) reliability confirmation test is the only means when there is no ready-made data and available experience

5.1 the 11th edition of ansi/api610 standard adds appendix k.1 about the judgment principle of shaft rigidity of oh2 and oh3 cantilever pumps

Figure 1 shows a simple cantilever pump rotor. D is equal to the shaft diameter at the mechanical seal shaft sleeve (mm), l is the span between the impeller outlet centerline and the radial bearing (mm), and the shaft flexibility coefficient isf=l3/d4

is the relationship between the flexibility coefficient of the cantilever pump shaft and its size factor. In the figure, a limit line for judging the shaft rigidity, namely the rigid line (when the international system of units is adopted, it is determined by the formula ISF, si= 32 × Kt-0.76)

KT in the figure is the "size" factor of the pump, which is equal to the maximum impeller diameter of the pump and the flow Q at the highest efficiency point × Total lift H/speed n (m3/h, m, r/min), related to torque. It can be seen from this figure that smaller pumps have higher l3/d4 than large pumps

[interpretation] the api/iso working group reviewed the records from some users about shaft rigidity and shaft flexibility, which has been widely concerned by manufacturers. Shaft flexibility coefficient has gradually developed into a direct judgment tool for evaluating real API pumps and pumps with API labels that do not meet the design requirements of API standards [7]

l3/d4 is the measurement standard of rotor stiffness and becomes the standard for indirect evaluation or comparison of rotor deflection under load

the smaller l3/d4, the smaller the rotor deflection, which is conducive to improving the reliability of mechanical seals. If l3/d4 is too large, especially when running near the pump dead center and the hydraulic radial load is too large, the pump shaft may break [8]

[suggestion] the use principle of l3/d4 is: for a cantilever pump with a certain size factor, if its flexibility coefficient is below the rigid line, it indicates that the pump is a rigid shaft design, meets the requirements of API pump, and is safe; If the value of l3/d4 exceeds 20% of the rigid line, the user should ask the pump manufacturer to be responsible for the rationality of its design, or provide the operation performance under the same or similar working conditions

5.2 clause 9.2.4.1.1 with the pump set