Discussion on NPSH of centrifugal pump
Cavitation is a common problem in the operation of centrifugal pump, which will cause the increase of pump vibration and noise, the decrease of performance, and the serious damage of pump parts. Only a comprehensive and correct understanding of the cavitation performance of centrifugal pump and a reasonable choice of safety margin can ensure its safe and stable operation and achieve the expected service life.
The safety NPSH is not only related to the long-term safe and stable operation of the centrifugal pump, but also directly affects the investment cost of the device / equipment. This paper is the second monograph written by the author on the cavitation problem of centrifugal pumps. It will collect and summarize the provisions of different standards / codes and some international counterparts on the cavitation safety margin of centrifugal pumps under different operating conditions, and share the application experience or requirements of the cavitation safety margin of specific operating conditions and main pumps of nuclear power plants with you in combination with some engineering practices for reference only.
1. Some concepts of cavitation
1.1 NPSH
The 11th edition of ANSI / API610 is defined as follows:
NPSH, also known as net positive suction head, is the absolute total suction head of the pump calculated from the reference elevation minus the vaporization pressure of the liquid.
1.2 NPSHA of unit
NPSH of the device, also known as effective NPSH and effective net positive suction head, is a parameter related to the operating system of the pump, which is equal to the residual pressure head of the liquid when the liquid reaches the inlet of the pump minus the vaporization pressure head.
1.3 NPSHr required
NPSH, also known as the necessary net positive suction head, is a parameter related to the structural design of the pump itself. There are three kinds of NPSH:
---NPSH3: the head (first stage impeller) drop of 3% is regarded as the judgment point of cavitation, recommended by ANSI / hi9.6.1, iso9906, ANSI / API610 and other standards, and widely accepted by the global pump industry. The measurement is easy and reliable. It is widely used in various centrifugal pumps.
--NPSH0: it is easy to define, but difficult to measure, that the head is about to start to fall rapidly, but to fall to 0%. Commonly used in axial flow pump and mixed flow pump.
--NOSHI (i.e. npshincipient cavitation incipient): when the first bubble is generated, it is regarded as the judgment point of cavitation, which is more strict than npsh3 and npsh0. The definition is easy, but the measurement technology is complex and the cost is very high. It is commonly used in high suction energy pump with high impeller inlet velocity.
2 . Factors considered in NPSH margin
The difference between npsha and NPSHr is called NPSH margin; the ratio between npsha and NPSHr is called NPSH margin ratio.
In most pump systems, npsha tends to decrease with the increase of flow, while NPSHr tends to increase with the increase of flow. NPSH margin takes many factors into account, as follows:
2.1 impeller inlet velocity
NPSHr can be reduced by reducing the liquid velocity at the impeller inlet, which is a direct function of the speed and the impeller inlet diameter.
2.2 impeller inlet diameter
The NPSHr value can be reduced by increasing the impeller inlet diameter, thus increasing the static head of the impeller in this area. However, larger impeller inlet diameter will aggravate the effect of inlet reflux, so NPSH margin must be increased.
2.3 suction specific speed
The specific speed of suction is calculated by the flow rate at the best efficiency point under the maximum impeller diameter and given speed, which is an index related to the suction performance of centrifugal pump. Suction specific speed is a measure of the sensitivity of a centrifugal pump to internal reflux.
For most pump designs, a suction specific speed value of less than about 145 metric units (7500 US units) is generally considered low, and a value of more than about 250 metric units (13000 US units) is considered high. Compared with the design of lower suction specific speed pump, higher suction specific speed pump is more likely to encounter poor noise and vibration, and the allowable operation range is narrower.
2.4 impact of pumping medium
The abrasives in the pumping medium will significantly increase the wear rate under the cavitation condition.
A small amount of entrained gas (1% to 2%) can buffer the impact force caused by cavitation bubble collapse and reduce the resulting noise, vibration and erosion damage.
Additives in the liquid may cause cavitation damage.
The corrosiveness of liquid will accelerate the damage caused by cavitation.
2.5 operation scope
The impeller is usually designed to operate at a specific flow rate. When it operates beyond the specified flow range, the possibility of cavitation will be greatly increased. Larger NPSH margin / margin ratio is required for pumps that often operate outside the priority work area (POR) but within the allowed work area (AOR).
2.6 materials
Impellers can be made of different materials. Rigid plastics and composites are usually the least resistant to cavitation. Cast iron and brass have the most serious damage in common metals, while stainless steel, titanium and nickel aluminum bronze have much less damage under the same cavitation condition.
2.7 pump size
Compared with smaller pumps, large pumps (impeller inlet diameter more than 450 mm) are more prone to cavitation damage.
2.8 operation
Cavitation damage is related to time. The longer the pump runs under cavitation condition, the greater the damage degree.
