Running NPSH

The longer the suction line and the larger the diameter of the line, the more mass has to be accelerated. This also increases the starting NPSH required. If the sum of the frictional loss in the suction line, plus the running NPSH, plus the starting NPSH, equals the available NPSH, then the pump will cavitate on start-up. 

This pump is presumed to run fine once it is running. The available NPSH is such that it exceeds the running NPSH. So how can I provide a temporary increase in the available NPSH, to satisfy the temporary starting NPSH requirement ... 

They lack sufficient available NPSH to satisfy the conversion of pressure to velocity, in the eye of the impeller (running NPSH). 

The fluid being pumped is hot water. At the desired flow rate of 110 GPM, the manufacturer s pump curve shows that the pump requires 14 ft of NPSH. The elevation difference between the draw-off nozzle, and the suction of the pump is shown on Fig. 25.5, as 46 ft. We really ought to have plenty of running NPSH. But apparently, we do not. 

Running NPSH The required liquid head needed to convert into velocity as the liquid flows into a centrifugal pump. 

Factory tests establish the pressure head, power, efficiency and NPSH over the complete flow range the pump can deliver running at design speed. British Standard, DIN standard or ANSI standard codes or national variations from such main codes lay down the manner of test procedure, and a minimum requirement is quite often defined by industry codes such as API 610. This is not the place to discuss instrument accuracy, as the codes lay down the limits possible from conventional instruments. There are two main classes of test the commercial requirements normally possible in the maker s test plant and high-accuracy tests that are only possible by using substandard instruments and very sophisticated techniques. 

The minimum required NPSH on the pump curves is normally determined using water at 60°F with the discharge line fully open. However, even though a pump will run with a closed discharge line with no bypass, there will be much more recirculation within the pump if this occurs, which increases local turbulence and local velocities as well as dissipative heating, both of which increase the minimum required NPSH. This is especially true with high efficiency pumps, which have close clearances between the impeller and pump casing. 

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. 

The specific speed of an impeller is the rotation in rpm. This is a geometrically similar impeller that would run if it were of such size as to discharge 1 gpm against a total head of 1 ft. Specific speed is used to determine the NPSH required by a given impeller. Those that produced high total dynamic head (TDH) have low specific speeds, and conversely. 

Disturbance-free operation is only possible when the pump runs without cavitation, which requires that the pressure at center of the bladed wheel is higher than the vapor pressure of the pumped medium. In particular, this must be taken into account in the case of sump-draining pumps for rectification columns, which operate dose to the boiling point of the medium. The sensitivity of a rotary pump to cavitation is quantified by the net positive suction head (NPSH Figure 2.4-7), which is defined as the total pressure head of the flow at the center of the bladed wheel minus the vapor pressure head of the liquid (Equation 2.4-23). 

Answer by Author A criterion for NPSH computation reliability is its repeatability when using a wealth of data. A three-thermocouple redundancy per measurement and the multiplicity of test runs pinpointed the temperatures selected. Then, by using equivalent vapor pressures, and by judiciously establishing pump inlet head losses, no erroneously-negative values of NPSH were realized. Since related suction specific speed values were neither imaginary nor infinite, and, because no hypothetical approaches were necessary to calculate their real values, the NPSH values were deemed reliable. 

The high vapour pressure of many solvents means that care in designing suction hoses and pipework is needed and low NPSH (net positive suction head) may be a necessary specification for a centrifugal pump particularly if run at 2900 revolutions/min. 

A pump running with insufficient NPSH vibrates. Eventually, its seal will be damaged. In propane service, running with too low a liquid level for a few hours will often injure the mechanical seal sufficiently to require taking the pump out of service. 

Pumps require more NPSH when their discharge flow is increased. A pump may be running fine at 2,000 B/D. Increase the flow to 2,TOO B/D, and the flow starts to wobble. Put the flow back to a rate of 1,000 B/D, and the pump lines out. The simplest way to correct this difficulty is to raise the level in the upstream vessel a few feet before turning up the flow. 

Low suction pressure. The hydrocarbon in the vessel upstream of the pump may run colder than normal. This lowers the pressure (to bold the liquid at its bubble point) in the vessel. The pump then has to pul up more head and will consequently transfer less volume. This problem has nothing to do with inadequate NPSH. 

Cavitation pump running dry. Follow procedures recommended above for insufficient NPSH problem. 

A pump seal ought to last about six years. Running a seal dry for a few seconds during start-up can reduce the seal life by six months. For a self-flushed pump that loses suction pressure on start-up, there will be little or no seal flush pressure until a normal pump suction pressure is established. (See section on starting NPSH requirements in Chap. 36.) Also, a pump that cavitates on start-up, due to the loss of suction pressure, will vibrate, which also contributes to the damage to the pump s mechanical seal faces. 

Prior to conducting a cavitation run, the water is circulated through the test loop a considerable length of time with the vacuum pump system in operation. In this way it is possible to minimize the air entrainment in the water which, as pointed out previously, appears to exert a considerable influence on the cavitation performance of the pump. For any given cavitation tests, the volume flow rate is held constant as the NPSH is incrementally decreased. As the vacuum on the system is increased it is sometimes necessary to reduce the incremental change in NPSH so that the so-called drop off point is obtained for any given pump test. 

---