Suction system NPSH available

Typical NPSH calculations keep the pump s lowest pressure below the liquid s vapor pressure as illustrated by the following three examples  

WHERE PRESSURE IN FEET (PRESSURE. PSIA)(2 3l) (SPECIFIC GRAVITY) 

NPSH AVAILABLE MUST BE GREATER THAN NPSH REQUIRED BY THE PUMP 

ABSOLUTE PRESSURE GAUGE PRESSURE 4 ATMOSPHERIC PRESSURE - GAUGE PRESSURE 14 7 

ABSOLUTE. VAPOR LINE 4 DIFFERENCE available PRESS,ft PRESS.FT LOSS,ft in ELEV.FT 

AaSOLOTE VAPOR LINE 0 FFE ENCE AVAILABLE PRESS,PT PRESS FT LOSS.FT L V.Ff 

leSOLUTC V P0B LINE 4. OifFERENCt 4V4ILAai PRESS,FT PRESS FT LOSS.FT HELEV.Fr 

WHERE PRESSURE IN FEET - [PRESSURE, PSIFIIZSII (SPECIFIC ORFVITYI 

AfisoLirre paessurc gauge fressuae atmospheric pressure 

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NPSH calculations might have to be modified if there are significant amounts of dissolved gas in the pump suction liquid. See Suction System NPSH Available" in this handbook for calculations when dissolved gas does not need to be considered. In that case the suction liquid s vapor pressure is a term in the equation. With dissolved gases, the gas saturation pressure is often much higher than the liquid s vapor pressure. 

Methods for the calculation of pressure drop through pipes and fittings are given in Section 5.4.2 and Volume 1, Chapter 3. It is important that a proper analysis is made of the system and the use of a calculation form (work sheet) to standardize pump-head calculations is recommended. A standard calculation form ensures that a systematic method of calculation is used, and provides a check list to ensure that all the usual factors have been considered. It is also a permanent record of the calculation. Example 5.8 has been set out to illustrate the use of a typical calculation form. The calculation should include a check on the net positive suction head (NPSH) available see section 5.4.3. 

The rotative speed of a pump is dependent upon the impeller characteristics, type fluid, NPSH available and other factors for its final determination. The most direct method is by reference to manufacturer s performance curves. VVTien a seemingly reasonable selection has been made, the effect of this selected speed on the factors such as NPSH required, suction head or lift, fluid erosion and corrosion, etc., must be evaluated. For many systems these factors are of no concern or consequence. 

Figure 32.51 illustrates how system NPSH or NPSH-available is calculated for the usual suction systems outlined. For a centrifugal pump, the basic NPSH is calculated from ... 

The net positive suction head (NPSH) is the absolute pressure in excess of the liquid vapor pressure that is available at the pump suction nozzle to move the liquid into the eye of the impeller. NPSH must always be calculated using units of absolute pressure and then expressed as head. NPSH is a concept entirely different from a pump s suction pressure. The actual NPSH must exceed the required NPSH for a given pump for adequate pump performance. Without adequate NPSH, cavitation and mechanical damage to the pump can occur. NPSH is an important consideration when selecting a pump required to pump liquids from Systems under vacuum or to handle near-boiling liquids or liquids with high vapor pressures. It is usually not practical to specify values of NPSH less than two feet. 

Net positive suction head (NPSH) A parameter used in pump characterization to predict or prevent cavitation. NPSH is determined from the physical properties of the liquid and the operating conditions at the pump suction. It represents the excess of the total head of the fluid above its vapor pressure at operating temperature. Each pump has its own demand for a minimum value of this parameter this is the required NPSH and is defined by the pump supplier. The available NPSH is determined by operating temperature and pressure, elevation of the fluid above the pump suction, and the design of the piping system. Available NPSH must always be greater than required NPSH this must be assured by proper design and installation of tire system. 

Available net positive suction head The available NPSH is the net pressure available in a given system, based on vessel pressure and static head, minus the liquid vapor pressure and functional losses in the system. The goal is to maintain equipment heights and minimi /se pump. suaion piping to ensure that the available NPSH is greater than tlie required NPSH. Insufficient NPSH can reduce pump capacity and efficiency and lead to cavitation damage. 

Area III describes the fluid above critical pressure conditions and represents the area which may be achieved directly by a pump system. Since pump inefficiencies result in heating the fluid, the boundaries of Area III may shift if pump efficiencies change with flow rate. These boundaries may be furdier controlled within the limits of the NPSH available at pump suction. These boundaries are indicative rather than exact and depend upon the operating characteristics of the pump employed. Some portions shown as Area III may in fact be portions of Areas IV and V. 

To express the quantity of energt available in the lit]iiid entering into the pump, the unit of measure for NPSH is feet of head or elevation in the pump suction. The pump has its NPSHr, or Net Positive Suction Head Required. The system, meaning all pipe, tanks and connections on the suction side of the pump has the NPSHa, or the Net Positive Suction Head Available. There should always be more NPSHa in the system dian the NPSHr of the pump. Let s look at them, beginning with what the pump recgiires ... 

The NPSH v available from or in the liquid system on the suction side of a pump is expressed (corrected to pump centerline) as ... 

This available value of NPSHa (of the system) must always be greater b) a minimum of two feet and preferably three or more feet than the required NPSH stated by the pump manufacturer or shown on the pump curves in order to overcome the pump s internal hydraulic loss and the point of lowest pressure in the eye of the impeller. The NPSH required by the pump is a function of the physical dimensions of casing, speed, specific speed, and type of impeller, and must be satisfied for proper pump performance. The pump manufacturer must ahvays be given complete Suction conditions if he is to be expected to recommend a pump to give long and trouble-free service. 

What is the Suction Lift value to be used with the pump curves of Figure 3-36A, if a gasoline system calculates an NPSH of 15 feet available ... 

In the above discussion it is assumed that the available NPSH in the system is adequate to support the flow rate of liquid into the suction side of the pump. If the available NPSH is less than that required by the pump, cavitation occurs and the normal curves do not apply. In cavitation, some of the liquid vaporizes as it flows into the pump. As the vapour bubbles are carried into higher pressure regions of the pump they collapse, resulting in noise and vibration. High speed pumps are more prone to cavitation than low speed pumps. 

Calculate the available net positive section head NPSH in a pumping system if the liquid density p = 1200 kg/m3, the liquid dynamic viscosity p = 0.4 Pa s, the mean velocity u = 1 m/s, the static head on the suction side 2, = 3m, the inside pipe diameter di = 0.0526 m, the gravitational acceleration g = 9.81 m/s2, and the equivalent length on the suction side SLes = 5.0 m. 

Net positive suction head available (NPSH) is the difference between the total absolute suction pressure at the pump suction nozzle when the pump is running and the vapor pressure at the flowing liquid temperature. All pumps require the system to provide adequate (NPSH). In a positive-displacement pump the (NPSH)a should be large enough to open the suction valve, to overcome the friction losses within the pump hquid end, and to overcome the liquid acceleration head. 

A recirculating pump should be chosen so that the developed head is dissipated as pressure drops through the circuit of the system. It is important that the pump and system be properly matched. The fluid being pumped is at or near its boiling point and, therefore, the required NPSH (net positive suction head) is usually critical. The pump should operate at this design level. If it develops excessive head, it will handle more volume at a lower head. At the new operating point, the required NPSH may be more than is available. 

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