Pumps suction system NPSH

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. 

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 ... 

Cavitation of a centrifugal pump, or any pump, develops when there is insufficient NPSH for the liquid to flow into the inlet of the pump, allowing flashing or bubble formation in the suction system and entrance to the pump. Each pump design or family of dimensional features related to the inlet and impeller eye area and entrance pattern requires a specific minimum value of NPSH to operate satisfactorily without flashing, cavitating, and loss of suction flowt... 

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 ... 

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. 

Other pieces may have to be elevated to enable the system to operate. A steam jet ejector with an intercondenser that is used to produce a vacuum must be located above a 34 ft (10 m) barometric leg. Condensate receivers and holding tanks frequently must be located high enough to provide an adequate net positive suction head (NPSH) for the pump below. For many pumps an NPSH of at least 14 ft (4.2 m) H2O is desirable. Others can operate when the NPSH is only 6 ft (2 m) H2O. See Chapter 8 for a method of calculating NPSH. 

The NPSH at the operating point for the pump determines where the pump can be installed in a piping system to ensure that cavitation will not occur. The criterion is that the pressure head at the suction (entrance) of the pump (e.g., the NPSHA) must exceed the vapor pressure head by at least the value of the NPSH (or NPSHR) to avoid cavitation. Thus, if the pressure at the pump suction is Ps and the fluid vapor pressure is Pv at the operating temperature, cavitation will be prevented if... 

On large fire water systems, the location of pumps and storage tanks at various plant areas provides greater reliability of protection and results in less pressure drop between the pump and the area of demand. Net positive suction head (NPSH) requirements and friction loss in the piping should be considered in locating fire water pumps. 

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. 

Cavitation corrosion occurs in pumps that have flow conditions that allow bubble formation on the surface of impellers. These bubbles, upon formation, break with enough force to rupture the protective film of the stainless steel. Plants can prevent this by designing a system that avoids bubble formation (i.e., provide sufficient Net Positive Suction Head - NPSH - for the pump), by polishing rotating parts to remove bubble formation sites and by using alloys with greater corrosion resistance and strength88. 

Organic HTFs oxidize when heated in the presence of air. This starts at about 150°C and the rate increases with temperature. Heat transfer fluid manufacturers usually recommend a blanket of an inert gas such as nitrogen or argon on the expansion tank to prevent oxidation. This inert gas can also be utilized to pressurize the system in case the fluid will be utilized above its boiling point or the net positive suction head (NPSH) of the pump needs to be elevated. 

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. 

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. 

NPSH estimate in the pump suction was in fair agreement with the water test data and indicated that a suction specific speed of 39,300 was achieved before cavitation. Examination of the pumps after the cavitation had occurred revealed that there was no physical damage. It is interesting to note that if the suction line from the tank were increased to 16 in. and an efficient bell mouthed inlet were used in the tank, the suction line losses would be reduced sufficiently to permit operation of the system with the tank ullage vented to atmospheric pressure. 

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. 

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. 

Do not confuse NPSH vdth suction head, as suction head refers to pressure above atmospheric [17]. If this consideration of NPSH is ignored the pump may well be inoperative in the system, or it may be on the border-line and become troublesome or cavitating. The significance of NPSH is to ensure sufficient head of liquid at the entrance of the pump impeller to overcome the internal flow losses of the pump. This allows the pump impeller to operate wfith a full bite of liquid essentially free of flashing bubbles of vapor due to boiling action of the fluid. 

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 ... 

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. 

S] = Friction losses for pipe valves and other system losses, suction side of pump R = Required by pump (NPSH) s = Suction side of pump... 

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