Available NPSH

It is difficult to determine exactly the areas of localized pressure reductions inside the pump, although much research has been focused on this field. It is easy, however, to measure the total fluid pressure (static plus dynamic) at some convenient point, such as pump inlet flange, and adjust it in reference to the pump centerline location. By testing, it is possible to determine the point when the pump loses performance appreciably, such as 3% head drop, and to define the NPSH at that point, which is referred to as a required NPSH (NPSHR). The available NPSH (NPSHA) indicates how much suction head... 

Item No. No. of Units Service Liquid Oper. Temp. F So. Gr.GPM Avail. NPSH, FI. Ditchorge Pretf. PSI6 Speed RPM BHP Pump H.P. Driver Driver Type File Ref. 

For low available NPSH (less than 10 feet) the pump suction connection and impeller eye may be considerably oversized when compared to a pump not required to handle fluid under these conditions. Poor suction condition due to inadequate available NPSH is one major contribution to cavitation in pump impellers, and this is a condition at w hich the pump cannot operate for very long without physical erosion damage to the impeller. See References [11] and [26]. 

As the altitude of an installation increases above sea level, the barometric pressure, and hence p or P decreases for any open vessel condition. This decreases the available NPSH. 

The pump selected for this application (water boiling at 0.98 psia) must have a required NPSH less than 4.7 ft, preferably about 3 to 3.5 ft. This is a difficult condition. If possible the vessel should be elevated to make more head (S) available, which wll raise the available NPSH. 

Using the example of Reference [6], assume a pump with characteristic curve and added temperature rise data as showm on Figure 3-59 is to handle boiler feed water at 220°F, with a system available NPSH = 18.8 feet. The v apor pressure of w ater at 220°F is 17.19 psia from steam tables and the SpGr = 0.957. Correcting the 18.8 feet NPSHa psia = 18.8 (l/[2.31/0.957)] = 7.79 psia at 220°F. 

Substituting for hs from equation 4.2, the available NPSH is given by... 

The available NPSH given by equations 4.8 and 4.9 must exceed the value required by the pump and specified by the manufacturer. The required NPSH increases with increasing flow rate as discussed below. 

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. 

Figure 4.5 shows a typical relationship between the available NPSH in the system and the NPSH required by the pump as the volumetric flow rate of liquid or capacity Q is varied. The NPSH required by a centrifugal pump increases approximately with the square of the liquid throughput. The available NPSH in a system can be calculated from equation 4.9 having substituted for hfs... 

Equation 4.15 shows that the available NPSH in a system decreases as the liquid throughput increases because of the greater frictional head losses. 

A centrifugal pump will operate normally at a point on its total head against capacity characteristic curve until the available NPSH falls below the required NPSH curve. Beyond this point, the total head generated by a centrifugal pump falls drastically as shown in Figure 4.6 as the pump begins to operate in cavitation conditions. 

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 20 ft matches the level of liquid in the drum above the suction line of the pump, shown in Fig. 25.1, and equals the available NPSH to the pump. 

The required NPSH of the pump may be read from Fig. 25.2 (regardless of the SG of the liquid being pumped). It shows that at 250 GPM, the required NPSH of 20 ft, will equal the available NPSH of 20 ft. Therefore, at a flow rate of 250 GPM, the pump will cavitate. This calculation has neglected frictional losses in the suction line, which should be subtracted from the available NPSH. 

One way of getting this extra suction pressure, or NPSH, is to raise the liquid level in the drum. For a liquid of 0.58 SG, with every 4 ft we raise the level in the drum, the suction pressure will increase by 1 psi, and the available NPSH will increase by 4 ft. But, unfortunately, the drum shown in Fig. 25.1 is already almost full. 

Again, the available NPSH is the physical pressure at the suction of the pump, minus the vapor pressure of the liquid at the suction of the pump. If both pressures increase by 5 psi, the net gain in NPSH is zero. 

Answer—yes But why Well, the liquid is cooled by 5°F after it leaves the drum. The cooled liquid is not in equilibrium with the vapor in the drum. It has been subcooled by 5°F. This means that the bubble-point liquid has been cooled, without altering its composition. The vapor pressure of the liquid has been reduced. As can be seen in Fig. 25.3, subcooling this particular liquid by 5°F reduces its vapor pressure by about 2 psi. As the specific gravity of the liquid is 0.58, this is equivalent to an increase in the NPSH by 8 ft. Once again, our objective is to increase the flow from 250 to 300 GPM. Figure 25.2 tells us that the required NPSH increases from 20 to 26 ft. However, when we subcool the liquid by 5°F, the available NPSH increases from 20 to 28 ft. As the available NPSH now exceeds the required NPSH by 2 ft, the flow can be increased without risk of pump cavitation. 

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 ... 

Answer suddenly increase the pressure in the drum, by partly closing the backpressure-control valve shown in Fig. 25.4. This will instantly increase the pressure at the suction of the pump. It is true, as we said before, that raising the pressure in a drum does not increase the available NPSH, assuming that the vapor and liquid are at equilibrium. The idea of equilibrium assumes that the vapor is at its dew point and the liquid is at its bubble point. 

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

They lack sufficient available NPSH to overcome the frictional losses in the suction piping and the drain or draw nozzle. 

The most likely explanation for this head loss of 7 ft is frictional loss in the suction line. This reduces the available NPSH from 46 to 39 ft. But this is still a lot more available NPSH than the 14 ft of required NPSH needed to pump 110 GPM. 

The boiling-point pressure of the water is equal to 30 psig that is, we can assume that the water draw-off is at its bubble-point pressure. At 36 psig pump suction pressure, the available NPSH is then... 

Lack of available NPSH may also be caused by high frictional loss in the suction piping. If this is the case, a small reduction in flow will not noticeably increase the pressure at the suction of the pump. A properly designed suction line to a centrifugal pump should have a frictional head loss of only a few feet of liquid. However, having a large-diameter suction line, and a relatively small draw-off nozzle, usually will lead to excessive loss of available NPSH. 

The 15 ft of head is the available NPSH to this pump. Does this mean that pumps may have a substantial amount of available NPSH, even when their suction pressure is under a partial vacuum Yes, if we are pumping a subcooled liquid. But this is quite common, because the liquid stored in an ordinary atmospheric-pressure storage tank is almost always well below its boiling point—that is, the liquid is subcooled. 

Sump pumps can draw water up from levels as much as 30 ft below the pump s suction. But do such pumps require NPSH Absolutely All centrifugal pumps have some NPSH requirements. What, then, is the available NPSH to the sump pump shown in Fig. 25.8 ... 

This 11.5 ft is the NPSH available to the pump. The pump itself requires only 6 ft of NPSH to pump 1200 GPM of water. Hence, even though the pump s suction is 9 ft above the water in the sump, the available NPSH is twice the required NPSH. 

Where, then, does the available NPSH to a sump pump really come from It comes from atmospheric pressure. Atmospheric pressure, at sea level, is equivalent to... 

The required NPSH, (NPSH)r, is specified by the punqi manufacturer, and the available NPSH, (NPSH)a, is determined by the design of the pun suction piping. To prevent cavitation the available NPSH must be equal to or greater than the required NPSH. 

A somew hat related case was encountered with a pump installed in an oil depot. The pump transferred fuel from a storage tank to oil-delivery trucks. Whenever the oil level in the storage tank was low (i.e., when the available NPSH was low), the pump operated satisfactorily. However, when the storage tank became full (i.e., when the available NPSH was high), the pump operated with extreme noise and vibration. 

The total head against which a pump operates is defined as the difference between the total head existing at a pump s outlet, and the total head available at its inlet. In this particular application, the discharge head tended to be practically constant. This meant that an increase in the available NPSH automatically reduced the total head against which the pump had to operate. 

A pump that had operated satisfactorily at the NPSH values presented in Fig. 2, Curve A, showed satisfactory performance at 40 gpm, when operating at an available NPSH of 5 ft. The same pump, however, continually failed at much lower flowrates (Curve A, dashed line), although previous tests (Curve A, solid line) showed that it could operate, at these flowrates, at NPSH-values significantly lower than the available 5 ft. Its performance at these times is shown by Curve B in Fig. 2 [4 ... 

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