The NPSH requirements of centrifugal pumps are normally determined on the basis of handling water at or near normal room temperatures. Operating experience in the field has indicated, and a limited number of carefully controlled laboratory tests have confirmed, that pumps handling certain hydrocarbon fluids, or water at significantly higher than room temperatures. will operate satisfactorily with less NPSH available than would be required for cold water.
Figure 61 is a composite chart of NPSH reductions which may be expected for hydrocarbon liquids and high temperature water based on available laboratory data from tests conducted on the fluids shown, plotted as a function of fluid temperature and vapor pressure at that temperature.
·Limitations for Use of Chart for Net Positive Suction Head Reductions (Fig. 61)
The following limitations and precautions should be observed in the use of Fig. 61.
Until specific experience has been gained with operation of pumps under conditions where this chart applies, NPSH reductions should be limited to 50% of the NPSH required by the pump for cold water.
This chart is based on pumps handling pure liquids. Where entrained air or other noncondensable gases are present in a liquid, pump performance may be adversely affected even with normal NPSH available (see below) and would suffer further with reductions in NPSH. Where dissolved air or other noncondensables are present, and where the absolute pressure at the pump inlet would be low enough to release such noncondensables from solution, the NPSH required may have to be increased above that required for cold water to avoid deterioration of pump performance due to such release.
For hydrocarbon mixtures, vapor pressure versus temperature relationships may vary significantly with temperature, and specific vapor pressure determinations should be made for actual pumping temperatures.
In the use of the chart for high temperature liquids, and particularly with water, due consideration must be given to the susceptibility of the suction system to transient changes in temperature and absolute pressure, which might necessitate provision of a margin of safety of NPSH far exceeding the reduction otherwise available for steady state operation.
Because of the absence of available data demonstrating NPSH reductions greater than ten feet, the chart has been limited to that extent and extrapolation beyond that limit is not recommended.
·Instruction for Using Chart for Net Positive Suction Head Reductions (Fig. 61)
Enter Figure 61 at the bottom of the chart with pumping temperature in degrees F and proceed vertically upward to the vapor pressure in psia. From this point follow along or parallel to the sloping lines to the right side of the chart, where the NPSH reductions in feet of liquid may be read on the scale provided. If this value is greater than one half of the NPSH required on cold water, deduct one half of the cold water NPSH to obtain corrected NPSH required. If the value read on the chart is less than one half of the cold water NPSH, deduct this chart value from the cold water NPSH to obtain corrected NPSH required.
Example: A pump that has been selected for a given capacity and head requires a minimum of 16 feet NPSH to pump that capacity when handling cold water. In this case the pump is to handle propane at 55 F, which has a vapor pressure of 100 psia. Following the procedure indicated above, the chart yields an NPSH reduction of 9.5 feet, which is greater than one half of the cold water NPSH. The corrected value of NPSH required is therefore one half the cold water NPSH or 8 feet.
Example: The pump of example above has also been selected for another application to handle propane at 14 F, where it has a vapor pressure of 50 psia. In this case, the chart shows an NPSH reduction of 6 feet, which is less than one half the cold water NPSH. The corrected value of NPSH is therefore 16 feet less 6 feet, or 10 feet.
·Use of Chart for Net Positive Suction Head Reductions (Fig. 61) for Liquids Other Than Hydrocarbons or Water.
The consistency of results which have been obtained on tests which have been conducted with both water and hydrocarbon fluids suggests that NPSH required by a centrifugal pump may be reduced when handling any liquid having relatively high vapor pressure at pumping temperature. However, since available data are limited to the liquids for which temperature and vapor pressure relationships are shown on Figure 6.1, application of this chart to liquids other than hydrocarbons and water is not recommended except where it is understood that such usage can be accepted on an experimental basis.
·Centrifugal Pumps Handling Entrained Air or Gas
Under a number of different circumstances, centrifugal pumps may be required to handle a mixture of air and water or similar mixtures. It is known that this reduces the head-capacity and efficiency of a centrifugal pump, even when relatively small percentages of air or gas are present.
Deterioration of performance for a given percentage of air or gas varies from pump to pump depending on rotating speed, specific speed, pump size, suction pressure, discharge pressure, number of stages and various special design features. These mixtures may also have a detrimental effect on the mechanical operation of the pump. An explanation and evaluation of the effect of these factors is beyond the scope of this article.
·Determination of Pump Performance When Handling Viscous Liquids
The performance of centrifugal pumps is affected when handling viscous liquids. A marked increase in brake horsepower, a reduction in head, and some reduction in capacity occur with moderate and high viscosities.
Figs. 62 and 63 provide a means of determining the performance of a conventional centrifugal pump handling a viscous liquid when its performance on water is known. They can also be used as an aid in selecting a pump for a given application. The values shown in Fig. 62 are averaged from tests of conventional single stage pumps of 2-inch to 8-inch size, handling petroleum oils. The values shown in Fig. 63 were prepared from other tests on several smaller pumps (1" and below). The correction curves are, therefore, not exact for any particular pump.
When accurate information is essential, performance tests should be conducted with the particular viscous liquid to be handled.
·Limitations on Use of Viscous liquid Performance Correction Chart
Reference is made to Fig. 62 and Fig. 63. Since these charts are based on empirical rather than theoretical considerations, extrapolation beyond the limits shown would go outside the experience range which these charts cover and is not recommended.
Use only for pumps of conventional hydraulic design, in the normal operating range, with open or closed impellers. Do not use for mixed flow or axial flow pumps, or for pumps of special hydraulic design for either viscous or non-uniform liquids.
Use only where adequate NPSH is available in order to avoid the effect of cavitation.
Use only on Newtonian (uniform) liquids. Gels, slurries, paper stock and other non-uniform liquids may produce widely varying results, depending on the particular characteristics of the liquids.
·Symbols and Definitions Used in Determination of Pump Performance When Handling Viscous Liquids
These symbols and definitions are:
Qvis = Viscous capacity in gpm
The capacity when pumping a viscous liquid
Hvis = Viscous head in feet
The head when pumping a viscous liquid
Evis = Viscous efficiency in percent
The efficiency when pumping a viscous liquid.
bhpvis = Viscous brake horsepower
The horsepower required by the pump for the viscous conditions
Qw = Water capacity in gpm
The capacity when pumping water
Hw = Water head in feet
The head when pumping water
Ew = Water efficiency in per cent
The efficiency when pumping water
spgr = Specific gravity
CQ = Capacity correction factor
CH = Head correction factor
CE = Efficiency correction factor
1.0 Qw = Water capacity at which maximum efficiency is obtained.
The following equations are used for determining the viscous performance when the water performance of the pump is known:
Qvis = CQ x Qw
Hvis = CH x Hw
Evis = CE x Ew
bhpvis =[Qvis x Hvis x Spgr] / [3960 x Evis]
CQ, CH and CE are determined from Fig. 62 and Fig. 63 which are based on water performance. Fig. 62 is to be used for small pumps having capacity at best efficiency point of less than 1.00 GPM (water performance).
The following equations are used for approximating the water performance when the desired viscous capacity and head are given and the values of CQ and CH must be estimated from Fig. 62 or 63 using Qvis and Hvis, as:
Qw (approx.) = Qvis/CQ
Hw (approx.) = Hvis/CH
·Instructions for Preliminary Selection of a Pump for a Given Head-Capacity-Viscosity Condition
Given the desired capacity and head of the viscous liquid to be pumped, and the viscosity and specific gravity at the pumping temperature, Figs. 62 or 63 can be used to find approximate equivalent capacity and head when pumping water.
Enter appropriate chart at the bottom with the desired viscous capacity, (Qvis) and proceed upward to the desired viscous head (Hvis) in feet of liquid. For multistage pumps use head per stage. Proceed horizontally (either left or right) to the fluid viscosity, and then go upward to the correction curves. Divide the viscous capacity (Qvis) by the capacity correction factor (CQ) to get the approximate equivalent water capacity (Qw approximately). Divide the viscous head (Hvis) by the head correction factor (CH) from the curve-marked "1.0 x Qw" to get the approximate equivalent water head (Hw approximately). Using this new equivalent water head-capacity point, select a pump in the usual manner. The viscous efficiency and the viscous brake horsepower may then by calculated.
This procedure is approximate as the scales for capacity and head on the lower half of Fig. 62 or Fig. 63 are based on the water performance. However, the procedure has sufficient accuracy for most pump selection purposes. Where the corrections are appreciable, it is desirable to check the selection by the method described below.
EXAMPLE: Select a pump to deliver 750 gpm at 100 feet total head of a liquid having a viscosity of 1000 SSU and a specific gravity of 0.90 at the pumping temperature.
Enter the chart (Fig. 63) with 750 gpm, go up to 100 feet head, over to 1000 SSU, and then up to the correction factors:
CQ = 0.95
CH = 0.92 (for 1.0 Qnw)
CE = 0.635
Qw =[750/0.95] = 790 gpm
Hw = [100/0.92] = 108.8 (~ 109 feet head)
Select a pump for a water capacity of 790 gpm at 109 feet head. The selection should be at or close to the maximum efficiency point for water performance. If the pump selected has an .efficiency on water of 81 per cent at 790 gpm, then the efficiency for the viscous liquid will be as follows:
Evis = 0.635 x 81% = 51.5 per cent
The brake horsepower for pumping the viscous liquid will be:
bhpvis = [750 x 100 x 0.90]/[3960 x 0.515] = 33.1 hp
For performance curves of the pump selected, correct the water performance as discussed below.
·Instructions for Determining Pump Performance on a Viscous liquid When Performance on Water is Known
Given the complete performance characteristics of a pump handling water. determine the performance when pumping a liquid for a specified viscosity.
From the efficiency curve, locate the water capacity (1.0 x Qw) at which maximum efficiency is obtained.
From this capacity, determine the capacities (0.6 x Qw), (0.8 x Qw) and (1.2 x Qw).
Enter the chart at the bottom with the capacity at best efficiency (1.0 x Qw), go upward to the head developed (in one stage) (Hw) at this capacity, then horizontally (either left or right) to the desired viscosity, and then proceed upward to the various correction curves.
Read the values of CE and CQ and of CH for all four capacities.
Multiply each head by its corresponding head correction factor to obtain the corrected heads. Multiply each efficiency value by CE to obtain the corrected efficiency values which apply at the corresponding corrected capacities.
Plot corrected head and corrected efficiency against corrected capacity. Draw smooth curves through these points. The head at shut-off can be taken as approximately the same as that for water.
Calculate the viscous brake horsepower (bhpvis) from the formula given above.
Plot these points and draw a smooth curve through them which should be similar to and approximately parallel to the brake horsepower (bhp) curve for water.
EXAMPLE: Given the performance of. a pump (Fig. 64) obtained by test on water, plot the performance of this pump when handling oil with a specific gravity of 0.90 and a viscosity of 1000 SSU at pumping temperature.
On the performance curve (Fig. 64) locate the best efficiency point which determines Qnw. In this example it is 750 gpm. Tabu1ate capacity, head and efficiency for (0.6 x 750), (0.8 x 750) and (1.2 x 750).
Using 750 gpm, 100 feet head and 1000 SSU, enter the chart and determine the correction factors. These are tabulated in Table of Sample Calculations. Multiply each value of head, capacity and efficiency by its correction factor to get the corrected values. Using the corrected values and the specific gravity, calculate brake horsepower. These calculations are shown below. Calculated points are plotted in Fig. 64 and corrected performance is represented by dashed curves.
Fig. 62 is used in the same manner as fig. 63 except that only one point on the corrected performance curve is obtained. Through the corrected head-capacity point, draw a curve similar in shape to the curve for water performance and having the same head at shut-off. If the capacity correction CQ is less than 0.050, the corrected head-capacity curve should be a straight line. The corrected-efficiency point represents the peak of the corrected efficiency curve, which is similar in shape to that for water. The corrected brake horsepower curves are generally parallel to that for water.
Radial Thrust in Single Volute Pumps Single volute pump casings in the specific speed range between 500 and 3500 may be designed for uniform pressure around the volute casing at the design or best efficiency point capacity. For pumps in applications normally operating at or near the best efficiency point capac.ity, the thrust factor may approach zero. On either side of the best efficiency point capacity, pressure distripution is not necessarily constant, resulting in radial thrust. The radial thrust at shutoff may be approximated, for this type of design, using the following expression:
Rso = Kso x [(Hso x spgr)/2.31] x D2 x B2
Thrust values R at capacities other than shut- off may be approximated by the following formula:
R = (K/Kso) x (H/Hso) x Rso
where K = Kso x [1-(Q/Qn)x]
and
Rso = Radial thrust in pounds, at shut-off
R = Radial thrust in pounds, at operating condition
Kso = Thrust factor at shutoff from Fig. 65
K = Thrust factor at operating condition
Hso = Total head at shutoff in feet
H = Total head at operating condition in feet
spgr = Specific gravity of the liquid
D2 = Impeller diameter in inches
B2 = Impeller width at discharge, including shrouds in inches
Q = Capacity at operating condition, in gpm
Qn = Capacity at best efficiency point in gpm
x = Exponent, varying between 0.7 and 3.3 established by test. In the absence of test data, the exponent may generally be assumed to vary linearly between 0.7 at specific speed 500 and 33 at specific speed 3500.
