1.1minimum continuous thermal flow

The minimum flow rate that the pump can maintain without being damaged by the temperature rise of the pumped liquid.

1.2 minimum continuous stable flow

The minimum flow rate at which the pump can work without exceeding the vibration limit specified in the specified standards/technical specifications, etc.

1.3 allowable operating region，AOR

API610 11th edition standard [1] is defined as follows:
When operating at a flow rate in this area, the pump's vibration is high, but still at an "acceptable" level.

1.4 energy density

Energy density is also called energy intensity, which is the product of the pump's rated power kW and rated speed r/min. ANSI/API 610 11th edition standard stipulates: If the energy intensity of the pump is 4 million or more, fluid dynamic pressure radial bearings and thrust bearings should be used. The 12th edition of API 610 stipulates that all services other than the recommended higher energy density level of 10.7×106 for pipeline services still need to meet this requirement.

1.5 suction specific speed

The suction specific speed is also called the cavitation specific speed. It is the necessary cavitation margin when the head drops by 3% at a given speed. It is calculated by the flow rate of the best efficiency point (BEP). Performance related index. The specific suction speed is a measure of the sensitivity of a centrifugal pump to internal reflux. The formula is defined as follows:
In the formula: n = pump speed, unit r/min;
Q = flow at the best efficiency point, in m3/s; for single-suction impellers, Q is the total flow, for double-suction impellers, Q is half of the total flow;
NPSH3 = The required NPSH when the head drops by 3% at the flow rate at the best efficiency point and the maximum diameter of the first stage impeller, in m.

1.6 suction energy

Inhaled energy is defined as: Inhaled energy = De × N × S × Sg
Where: De = the diameter of the impeller inlet. In practical engineering applications, the size of the pump inlet pipe (diameter) is usually used instead, in;
N = pump speed, rpm;
S = suction specific speed, (gpm, ft);
Sg = the specific gravity of the fluid.
For end suction pumps, high suction energy starts at 160 × 106; for horizontal split pumps, it starts at 120 × 106. The high suction energy is 1.5 times of the high suction energy.
Figure 9.6.1.3 in the ANSI/HI 9.6.1-1998 "Centrifugal and Vertical Pumps for NPSH Margin" standard shows a simplified method for identifying high suction energy pumps.

1.7 high energy pump

The 11th edition of the API 610 standard defines a pump with a single-stage head greater than 200 meters and a single-stage power greater than 225 kW as a high-energy pump.

There are many factors that affect the allowable working area of a centrifugal pump, mainly in the following aspects.

2.1 vibration

The vibration of the centrifugal pump changes with the flow rate, usually the minimum vibration near the best efficiency point, and increases with the increase or decrease of the flow rate. Starting from the best efficiency point flow, the change of vibration with flow depends on the increase of pump energy density, specific speed, and suction specific speed. Vibration testing can be used to help evaluate AOR.

2.2 noise

Any pump will produce a certain amount of noise. The generation of noise includes mechanical factors (such as friction between moving/static parts) and hydraulic factors (such as cavitation). High and very high suction energy pumps generally operate at higher noise levels. At higher and lower flow rates and lower NPSH margins, noise will increase significantly. For this, high specific speed pumps are more sensitive than low specific speed pumps. In addition, noise usually accompanies the appearance of vibration. Excessive noise usually causes mechanical damage and limits AOR. Noise testing can also be used to help assess AOR.

2.3 Any pump will produce a certain amount of noise. The generation of noise includes mechanical factors (such as friction between moving/static parts) and hydraulic factors (such as cavitation). High and very high suction energy pumps generally operate at higher noise levels. At higher and lower flow rates and lower NPSH margins, noise will increase significantly. For this, high specific speed pumps are more sensitive than low specific speed pumps. In addition, noise usually accompanies the appearance of vibration. Excessive noise usually causes mechanical damage and limits AOR. Noise testing can also be used to help assess AOR.

Manufacturers will limit the AOR of pumps designed for continuous operation to operating conditions where the life of the bearing system is greater than or equal to 16,000 hours [1]. Pumps designed for intermittent operation can have a shorter calculated bearing life; vertical diffuser pumps and pumps with hydrodynamic bearings usually do not have bearing life calculated relative to flow, but when calculating bearing rotation and maximum load flow You can consider flow restrictions [2].
Excessive deflection (deflection) of the shaft at the mechanical seal surface will shorten the life of the seal. In order to obtain a good sealing effect, under the most severe dynamic conditions (maximum impeller diameter and specified speed, specified medium conditions), the pump manufacturer limits the AOR to the main sealing surface and the total deflection of the shaft does not exceed 0.05 mm. The limitation of shaft deflection here can be achieved by a combination of shaft diameter, shaft span or cantilever length, and housing design (including the use of double volutes or guide vanes) [1].

2.4 Temperature rise

The temperature rise when the pumped medium flows from the pump inlet to the pump outlet is called temperature rise. The temperature rise of the liquid decreases as the flow rate of the pump increases. When the pump is running at or near the dead center, most of the input power is converted into heat energy, causing the liquid temperature to rise sharply. The over-current parts of the pump may expand and deform due to the rising temperature, resulting in eccentricity between the pump shaft and the drive shaft, friction between moving and static parts, and even seizure, which may damage the pump. The temperature rise directly affects the AOR of the pump.

2.5 NPSH margin

The difference between NPSHA and NPSHR is called NPSH margin. The size of NPSH margin depends on the size, design, application and material of the pump, and it will directly affect the flow operating range of the pump. GB/T 16907-2014 "Technical Conditions for Centrifugal Pumps (Class I)" stipulates: NPSHA should have a margin 10% larger than NPSHR (NPSHR here is NPSH3), and the margin should not be less than 0.5 meters. Suitable for most common centrifugal pumps.

2.6 Power limit

The power curve of low specific speed centrifugal pumps usually increases with the increase of flow rate, while the power curve of high specific speed centrifugal pumps increases with the decrease of flow rate. The power of the motor and the starting conditions (such as open valve or closed valve start) limit the AOR. The pump manufacturer shall provide a flow limit with sufficient torsional stress safety factor.

2.7 Inlet reflux

Inlet reflux means that when the flow rate of the pump is lower than a certain value, the flow rate in the inlet area of the impeller will be separated from the blades (outflow) and a circulating vortex will be formed. As the pump flow rate decreases further, the circulation intensity increases, which in turn will cause cavitation, noise and fluid pulsation. Experience has shown that the occurrence of inlet reflux is closely related to the suction specific speed. The flow rate when inlet reflux occurs increases with the increase of the impeller inlet diameter and suction specific speed, which will greatly compress the AOR of the pump.

2.8 The shape of the flow-lift curve

For centrifugal pumps with medium and low specific speeds, the flow-head curve is prone to "hump"; for pumps with high specific speeds, the middle of the flow-head curve may appear "sinking" (ie saddle shape). In practical engineering applications, avoid running on the left side of the hump and saddle area, both of which will limit AOR.

2.9 Internal mechanical contact

Both the manufacturer and the user hope that the pump will always run with its BEP. At this flow rate, the volute pump generates the least hydraulic load. In actual engineering applications, the pump is rarely in its BEP operation all the time. The hydraulic load changes with changes in operating flow. As the load increases, the deflection of the rotor may become large, resulting in contact between the rotating and stationary parts. Pump manufacturers should evaluate their design and operating experience to determine whether to impose necessary restrictions on AOR.

In engineering practice, the maximum allowable working flow of most centrifugal pumps is usually 120% ~ 125% of the flow at the highest efficiency point of the pump, which is mainly determined by the following factors.

3.1 Specific speed

The specific speed directly affects the development trend (normal, gentle, steep drop) of the performance curve (flow-head, flow-efficiency, etc.). For low specific rotation centrifugal pumps, the head curve usually drops faster after the flow rate at the highest efficiency point, and the flow rate may not reach 105% to 110% of the BEP. In this case, the seller should give a maximum flow limit on the bid performance curve。

3.2 NPSH margin

In most pumping systems, NPSHA tends to decrease with increasing flow, while NPSHR tends to increase with increasing flow. In the case that the height of the system device is determined, the reasonable operating range of the pump should be determined according to the size of the NPSH margin. This NPSH margin is sufficient to protect the pump from backflow and cavitation at all flows (from the minimum continuous stable flow to the maximum allowable working flow).
What needs special explanation is: for "special purpose" high-energy pumps (such as 500 bar high pressure, 6000 rpm high speed, single stage head 500 m water injection pump; high pressure ethylene pipeline pump; high pressure boiler feed water pump; there may even be no spare 3 to 4 MW's refinery charge pumps, etc.), the 12th edition of the API 610 standard stipulates that “the appropriate NPSH margin should be determined based on the initial cavitation (NPSHi), not just the general NPSH3”. The selection of NPSH margin can be based on actual engineering application experience or refer to the recommended value in ANSI/HI 9.6.1-2012 "Rotodynamic Pumps Guideline for NPSH Margin".

3.3 Power limit

The power of the driver directly limits the AOR. Under the maximum allowable working flow, it should be ensured that the driving machine will not be overloaded (normal bearing temperature, normal driving machine vibration and noise). The 11th edition of API610 standard clearly stipulates the matching power of centrifugal pump motors for petroleum, petrochemical and natural gas industries: when the shaft power of the pump is less than 22kW, the matching power of the motor is selected at 1.25 times; when the shaft power of the pump is 22kW When ~55kW, the power of the motor is selected according to 1.15 times; when the shaft power of the pump is greater than 55kW, the power of the motor is selected according to 1.10 times. In engineering practice, for pumps used in some important working conditions (such as conventional island main feed pumps and condensate pumps in nuclear power plants), it is usually required that the power of the driver is not less than 1.15 times the shaft power of the driven equipment under the maximum operating conditions.

3.4 summary

The minimum value of the flow rate obtained under the above influencing factors is the maximum allowable flow rate of the pump in the specified device.

When the pump is running at a small flow rate, it may cause the following problems: increase in the temperature of the pumped liquid, additional radial force (single volute pump), inlet return, cavitation, etc., which may cause mechanical vibration, increased noise, and The life of bearings and mechanical seals is reduced. Therefore, for the specified device, the manufacturer should give the minimum continuous stable flow limit of the pump.
In engineering practice, the minimum continuous steady flow of most centrifugal pumps is usually 25% to 30% of the flow at the highest efficiency point. Small centrifugal pumps are relatively small, while large centrifugal pumps may reach more than 35% of the flow at the highest efficiency point. Mainly determined by the following factors.

4.1Pump size

Compared with smaller pumps, large pumps (such as the impeller inlet diameter exceeding 450 mm) are more prone to cavitation damage, and the minimum continuous stable flow value is correspondingly larger. For example, EBARA company OH2 UCW pump, when the inlet/outlet diameter is less than 50×40, the minimum continuous stable flow is usually 12% of the BEP point flow; when the inlet/outlet diameter is equal to 50×40, it is 15% of the BEP point flow. %; When the inlet/outlet diameter is greater than or equal to 100×80, it is 25% ~ 30% of the flow rate at the BEP point.

4.2 Specific speed

For centrifugal pumps with medium and low specific speeds, the flow-head curve is prone to hump; for high-speed centrifugal pumps, the flow-head curve usually has a saddle shape, which will greatly limit the AOR of the pump. When a hump and saddle-shaped flow-head curve appears, the minimum continuous stable flow should be the flow value corresponding to the maximum head in the area.

4.3 Inlet reflux

The inlet reflux is related to the pump suction specific speed and suction energy, and the inlet reflux will directly affect the determination of the minimum continuous stable flow of the pump. Generally, the minimum continuous steady flow rate increases with the increase in suction specific speed or suction energy. In order to avoid inlet backflow (causing a significant increase in pump vibration and noise), people usually set a limit value for the suction specific speed. It is widely recognized in the global petrochemical industry that the UOP 5-11-7 specification [3] stipulates: the suction specific speed of the pump shall not be higher than 13000 (m3/h, m); when the pumping medium is water or the water content exceeds 50% solution, and the pump's single-stage impeller power exceeds 75 kW, the suction specific speed must not be higher than 11000 (m3/h, m).

4.4 Temperature rise

The efficiency of a pump is the ratio of the work done by the pump to the fluid (effective power) and the power delivered to the pump shaft (shaft power), expressed as a percentage. The difference between the two powers is the power loss caused by the internal hydraulic power of the pump, the friction of the bearing and the mechanical seal, and the leakage (including the balance return water). Except for less power loss in leakage, bearings and mechanical seals, all other energy (power) losses are converted into heat and then transferred to the pump through the fluid. The specific performance is the temperature rise of the pumped liquid, and the relationship between it and the total lift and efficiency of the pump is as follows [4]:
In the formula (metric unit): = temperature rise, ℃;
H = total pump head corresponding to the used flow, m;
102 = constant;
Cp = the specific heat of the medium at the pumping temperature, kJ/(kg·K), the specific heat of water is 4.18 kJ/(kg·K);
η= The pump efficiency corresponding to the flow rate, expressed as a decimal number
In order to prevent the pump from overheating, each pump will provide an appropriate minimum continuous thermal limit flow value, which is usually less than the minimum continuous stable flow rate of the pump (about 10% of the flow rate at the best efficiency point). It is generally believed that the limit of the temperature rise of the liquid passing through the pump is 8°C. In most installations, when the temperature rise through the pump is considered at 8°C, this appropriate minimum continuous thermal limit flow can be estimated by the following formula [4]:
Estimate the minimum continuous thermal limit flow according to the allowable temperature rise [5]:
In the formula (metric unit): Pp = shaft power at the minimum flow point of the pump, kW
Pa = Shaft power at the rated point of the pump, kW
433 = constant
ρ= density of medium, kg/m3
HS = head of the pump at dead point, m
g = 9.81 m2/s
When NPSHA is much larger than NPSHR, the allowable temperature rise of the pump is determined by the comprehensive factors such as pump material, medium characteristics and sealing conditions; when NPSHA and NPSHR are close or when transporting easily vaporized medium (such as liquid hydrocarbon), the allowable temperature rise of the pump Liter is determined by cavitation conditions. In general estimation, the allowable temperature rise of the pump is selected according to the empirical value given in Table 1 [6].
Table 1: Reference value of allowable temperature rise of centrifugal pumps for different purposes (unit: ℃)

4.5 summary

The maximum value of the flow rate obtained under the above influencing factors is the minimum continuous stable flow rate of the pump in the specified device.

The actual minimum continuous stable flow value of the pump is usually obtained from the factory test/field operation test, and the final minimum continuous stable flow value provided to the user in the bidding document is usually (relatively conservative) larger than the value obtained from the test.

Although there are many factors related to the allowable working area of a centrifugal pump, there is only one principle for its determination, that is: when operating in all prescribed allowable working areas, the reliable operation and service life of the pump (group) will not be affected.

references
[1] ANSI/API STANDAED 610'Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries', ELEVENTH EDITION, SEPTEMBER 2010; ISO 13709: 2009 (Identical)
[2] ANSI/HI 9.6.3-1997, American National Standard for Centrifugal and Vertical Pumps for Allowable Operating Region, Hydraulic Institute, Parsippany, www.pumps.org
[3] UOP 5-11-7, CENTRIFUGAL PUMPS, STANDARD SPECIFICATION, 2005, Page 2 of 9
[4] ANSI/HI 1.3-2009, American National Standard for Rotodynamic (Centrifugal) Pumps for Design and Application, Hydraulic Institute, www.pumps.org
[5] Chen Wei, Huang Shuilong, etc. Industrial pump selection manual [M]. Beijing: Chemical Industry Press, 2010.4
[6] Guan Xingfan. Modern Pump Theory and Design [M]. Beijing: China Aerospace Publishing House, 2011.4