Cavitation marginal

The vapor pressure may be dependent on the amount of the dissolved, not the entrained, air in the Hquid. This issue is important to appHcations of cooling-water double-suction pumps (58,59). Because of the unknowns, a safety margin is always recommended for use to minimise the effects of cavitation. 

The definition of NPSHr may change in the future. A pump is in a definite state of cavitation with the 3% total head loss definition. Many pump users want a more explicit definition of NPSHr, and higher NPSHa safety margins to avoid inadequate NPSHa and cavitation altogether. 

Remember from Chapter 2, the NPSHa formula is NPSHa = Ha + Hs - Hvp - Hf - Hi. If you want to raise the NPSHa, it will be necessary to increase the elements (Ha, Hs) that add energy to the fluid, or decrease the elements (Hvp, Hf, Hi) that rob e- rgy from the fluid. Also remember that the NPSHr reading, printed on a pumn c -ve, currently represents a point where the pump is already suffering a 2. ss in function due to cavitation. Some people in the industry are calling for a more precise definition of NPSHr, and higher safety margins on NPSHa. 

The operating parameters used in the experimental work for the optimization exercise include inlet pressure (1 1 kg/cm2), concentration of the controlling reactant species (oxidant i.e. KMnC>4 in this case with concentration varied in the range 0.2-0.5 mol/L per mol of the toluene), type of the orifice plate (without orifice plate, orifice plates of two different geometries). The optimized conditions as obtained in the study are inlet pressure of 3 kg/cm2, 0.4 mol/L of the oxidant (beyond these values, the increase in the cavitational yield is only marginal) and orifice plate with more number of holes (cavitational yield is maximum out of all the geometries considered in the work). 

Note The purchaser should consider an appropriate NPSH margin in addition to the NPSHR specified in 2.1.8 above. An NPSH margin is the NPSH that exists in excess of the pump s NPSHR (see 1.4.30). It is usually desirable to have an operating NPSH margin that is sufficient at all flows (from minimum continuous stable flow to maximum expected operating flow) to protect the pump from damage caused by flow recirculation, separation, and cavitation. The vendor should be consulted about recommended NPSH margins for the specific pump type and intended service. 

When liquids are being pumped, it is important to keep the pressure in the suction line above the vapor pressure of the fluid. The available head measured at the pump suction is called the net positive suction head available (NPSHA). At sea level, pumping 15°C (60°F) water with the impeller about 1 m below the surface, the NPSHA is about 9.1 m (30 ft). It increases with barometric pressure or with static head, and decreases as vapor pressure, friction, or entrance losses rise. Available NPSHA is the characteristic of the process and represents the difference between the existing absolute suction head and the vapor pressure at the process temperature. The required net positive suction head required (NPSHR), on the other hand, is a function of the pump design (Figure 2.121). It represents the minimum margin between suction head and vapor pressure at a particular capacity that is required for pump operation. Cavitation can occur at suction pres-... 

By contrast, ultrasonic probes have the advantage over ultrasonic baths in that they deliver their energy on a localized sample zone, thereby providing more efficient cavitation in the liquid. Also, they are not subjected to any exhaustion restrictions, so they are much more suitable for use in analytical chemistry than are ultrasonic baths. However, probes tend to uncouple as a consequence, cavitation occurs only at the radiating surface and only marginal ultrasound intensity can be detected elsewhere in the surrounding liquid. 

NPSH. The net positive suction head is the most critical factor in a pumping system. A sufficient NPSH is essential, whether working with centrifugal, rotary, or reciprocating pumps. Marginal or inadequate NPSH will cause cavitation, which is the formation and rapid collapse of vapor bubbles in a fluid system. Collapsing bubbles place an extra load on pump parts and can remove a considerable amount of metal from impeller vanes. Cavitation often takes place before the symptoms become evident. Factors that indicate cavitation are increased noise, loss of discharge head, and reduced fluid flow. 

A feedwater collector is located at the bottom of each drum separator. From these, the water which has been separated from the steam-water mixture is mixed with feedwater and is taken via the twelve 312 mm downcomers to a 1020 mm pump inlet header. This creates a cooling effect below satnration temperatnre to give the reqnired cavitation margin at the inlet of... 

Total loss of feedwater flow may be caused by de-energization of the feedwater pump motors and by failure of the oil cooling of these pumps, as well as by inadvertent closing of isolating and/or control valves on the feedwater lines. The transient is characterized by a pressure drop in the coolant circuit and by a reduction of the MCP cavitation margin. 

Low NPSH. Some pumps do not vibrate noticeably when they are marginally short of NPSH—they just lose capacity. Raise the liquid level in the upstream vessel a few feet. If the pump then picks up nicely, you have a partially plugged suction line or too low a liquid level in the upstream vessel. See the section on "Origins of Cavitation."... 

Hence an increase in the pump rotation frequency requires a rise in the reactor pressure (to the NPSH value) or the development of a special flow path with low NPSH. Increasing the reactor cover gas pressure complicates the design and makes it more expensive, while creation of a special flow path with low cavitation margin complicates the pump design. 

The NPSHr characteristics should be requested from the manufacturer who usually evaluates these with water using the well-known 3% A//-criterion (Figure 9.29a). For solvent close to their vapour pressure the manufacturer should be asked for special recommendations as general information [30] may not be sufficiently reliable. As centrifugal pump cavitation is rather dangerous for pump life, the actual NPSH should include a safety margin. A factor between 1.5 and 2.0 is recommended. 

To ensure operation without cavitation, it is required that the absolute pressure at the pump impeller eye exceeds the vapor pressure by a certain margin. By convention the required absolute pressure at the pump entrance is expressed in terms of head of the mixture being pumped, using the term Net Positive Suction Head Required, NPSHR. The NPSHR is usually specified by the pump manufacturer based on tests with water. Cavitation curves for various rotary speeds are given in Fig. 3 for the pump used here. 

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