Net Positive Suction Head. It is the difference between hydraulic pressure and vapor pressure, usually of concern at the inlet of a pump. Without sufficienty high NPSH at a pump inlet, there will be cavitation in the pump. Within a centrifugal pump there can be pressures lower than at the inlet as the fluid is moved by the impeller. If any of those pressures is lower than the vapor pressure, vapor pockets- bubbles- form and you get cavitation. A more complete explanation: http://en.wikipedia.org/wiki/NPSH
first of all the term NPSH should be clear.It is pressure which should be available at the eye of the pump impeller,so as to avoid vaporisation of liquid. Second which arises ,how this liquid will vaporise?Ans-if a vapour pressure of a liquid falls at constt temperature or temperature of liquid is raised at constt pressure it vaporises.In case of pump it's mostly the first case. Now,how to manitain NPSH. NPSHa> NPSHr (always) where NPSHa= available NPSH NPSHr = Required NPSH NPSH = Hps+Hsl-Hvp-Hfl Hps= pressure acting on the eye due to pressure in the suction drum. Hsl = pressure acting due to height of liquid in the suction line. Hvp= vapour pressure of the liquid Hfl= head loss due to friction losses Thus, to maintain the NPSH, variables in hand are,Hps and Hsl.increase the height of the suction line or increase the pressure of the suction drum.
Use full bore for applications where pressure drop should be minimum (such as in the suction pipe of a pump with required NPSH very close to available NPSH), or for pipes which could be cleaned with a pig (so that the pig doesn't get stuck on the valve). In other applications a reduced bore should be OK.
there are two main parameters: Pressure and flow. You request them for a single point or more. the you can define the kind of motor/diesel engine, NPSH, size of suction oand impulsion... ¡¡there are many things involved on a pump specification!!!
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to develop NPSH in BFP
Net Positive Suction Head. It is the difference between hydraulic pressure and vapor pressure, usually of concern at the inlet of a pump. Without sufficienty high NPSH at a pump inlet, there will be cavitation in the pump. Within a centrifugal pump there can be pressures lower than at the inlet as the fluid is moved by the impeller. If any of those pressures is lower than the vapor pressure, vapor pockets- bubbles- form and you get cavitation. A more complete explanation: http://en.wikipedia.org/wiki/NPSH
first of all the term NPSH should be clear.It is pressure which should be available at the eye of the pump impeller,so as to avoid vaporisation of liquid. Second which arises ,how this liquid will vaporise?Ans-if a vapour pressure of a liquid falls at constt temperature or temperature of liquid is raised at constt pressure it vaporises.In case of pump it's mostly the first case. Now,how to manitain NPSH. NPSHa> NPSHr (always) where NPSHa= available NPSH NPSHr = Required NPSH NPSH = Hps+Hsl-Hvp-Hfl Hps= pressure acting on the eye due to pressure in the suction drum. Hsl = pressure acting due to height of liquid in the suction line. Hvp= vapour pressure of the liquid Hfl= head loss due to friction losses Thus, to maintain the NPSH, variables in hand are,Hps and Hsl.increase the height of the suction line or increase the pressure of the suction drum.
Capacity, Net Positive Suction Head (NPSH), and Efficiency
Use full bore for applications where pressure drop should be minimum (such as in the suction pipe of a pump with required NPSH very close to available NPSH), or for pipes which could be cleaned with a pig (so that the pig doesn't get stuck on the valve). In other applications a reduced bore should be OK.
By putting deaerator at height, vapour doesn't form at the suction area of feed water pumps so cavitation phenomenon in the impeller can be prevented. also to get NPSH.
there are two main parameters: Pressure and flow. You request them for a single point or more. the you can define the kind of motor/diesel engine, NPSH, size of suction oand impulsion... ¡¡there are many things involved on a pump specification!!!
1. PRIMING LOSS: This is the suction loss for a pump whose NPSH is (-ve)2. CAVITATION: It is also suction loss by forming cavity of a volatile liquid whose NPSH(a) is belowNPSH(r) eg..Choking or blockage of suction strainers, closing of suction valve of apump when running.3. VAPOUR LOCK: This is also loss of pumping due to blockage in discharge of a pump..closing of discharge valve and running for more time..running the pump with in adequate minimum flow..liquid above boiling pointHence Cavitation, Vapour Lock and Primimng loss are different
Pump Selection - Things to Consider•Pump location■ Equipment room or plenum?■ Close to occupied areas?•Available space■ Installation footprint■ Maintenance footprint•Maintenance requirements■ Parts availability■ Special skills, tools etc.•Reliability•Hydraulic requirements■ Flow rate, head, efficiency, horsepower■ NPSH
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