Page 218 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

I90 Applied Process Design for Chemical and Petrochemical Plants

texing. Since the forces involved are severe in vor- Total Suction Lift (as water at 70°F) = NPSH, (calcu-
texing, the vortex breaker must be of sturdy con- lated for fluid system) - 33 feet. The vapor pressure of
struction, firmly anchored to the vessel. water at 70°F is 0.36 psia.
8. Nozzle size on liquid containing vessel may create
severe problems if inadequate. Liquid suction veloc- Example 3-3: Suction Lift
ities, in general, are held to 3-6.5 feet per second.
Nozzle losses are important to recognize by identify- What is the Suction Lift value to be used with the pump
ing the exit design style (see Chapter 2). Usually, as curves of Figure 3-36A, if a gasoline system calculates an
a guide, the suction line is at lea,st one pipe size larg- NPSH of 15 feet available:
er than the pump suction nozzle. Total Suction Lift (as water) = 15 - 33 = -18 feet.
Therefore, a pump must be selected which has a lift of at
The NPSHA available from or in the liquid system on least 18 feet. The pump of Figure 3-36A is satisfactory
the suction side of a pump is expressed (corrected to using an interpolated Suction Lift line between the dotted
pump centerline) as: curves for 16 feet and 21 feet of water. The performance
of the pump will be satisfactory in the region to the left of
the new interpolated 1 8-foot line. Proper performance
should not be expected near the line.
NPSH, = S + (Pa - PT) (2.31/SpGr) - hSL (3-1 0)
If the previous system were at sea level, consider the
same pump with the same system at an altitude of 6000
Where pla or Pa represent the absolute pressure in the ves- feet. Here the barometric pressure is 27.4 feet of water.
sel (or atmospheric) on the liquid surface on the suction This is 34 - 2’7.4 = 6.6 feet less than the sea level instal-
side of the pump. lation. The new NPSHA will be 15 ft - 6.6 ft = 8.4 feet
p’, or P, represent the absolute vapor pressure of the available. Referring to the pump curve of Figure 3-36A it
liquid at the pumping temperature. is apparent that this pump cannot do greater than 21 feet
hSL is the suction line, valve, fitting and other friction suction lift as water or 12 feet NPSHR of liquid (fluid).
losses from the suction vessel to the pump suction flange. Total Suction Lift as water = 8.4 - 33 = -24.6 feet.
S may be (+) or (-) depending on whether static head The pump curves show that 21 feet suction lift of water is
or static lift is involved in the system. all the pump can do, hence the 24.6 feet is too great. A
This available value of NPSHA (of the system) must different pump must be used which can handle this high
always be greater by a minimum of two feet and preferably a suction lift. Such a pump may become expensive, and it
three or more feet than the required NPSH stated by the may be preferable to use a positive displacement pump
pump manufacturer or shown on the pump curves in for this high lift. Normally lifts are not considered rea-
order to overcome the pump’s internal hydraulic loss and sonable if over 20 feet.
the point of lowest pressure in the eye of the impeller.
The NPSH required by the pump is a function of the phys- Example 3-4: NPSH Available in Open Vessel System at
ical dimensions of casing, speed, specific speed, and type of Sea level, Use Figure 3-38
impeller, and must be satisfied for proper pump perfor-
mance. The pump manufacturer must always be given com-
Conditions: at sea level, atmospheric pressure, Pa =
plete suction conditions if he is to be expected to recom- 14.7 psia.
mend a pump to give long and trouble-free service.
As the altitude of an installation increases above sea Assume liquid is water at %OF, vapor pressure = P, =
level, the barometric pressure, and hence pla or Pa 0.6 psia.
decreases for any open vessel condition. This decreases Assume tank liquid level is 10 feet above center line of
the available NPSH. pump, then S = +10 feet.
Figure 3-36A represents a typical manufacturer’s per- Assume that friction losses have been calculated to be
formance curve. The values of NPSHR given are the min- 1.5 feet, hsL = 1.5
imum values required at the pump suction. As men-
tioned, good practice requires that the NPSHA available Then: NPSH, available = S + (Pa - Pyp) (2.31/SpGr) - h,,
be at least two feet of liquid above these values. It is impor- = + 10 + (14.7 - 0.6)(2.31/0.997) - 1.5
tant to recognize that the NPSHR and Suction Lift Values = 41.2 ft (good) (3-1 0)
are for handling water at about 70°F. To use with other liq-
uids it is necessary to convert to the equivalent water suc- Note: For worst case, which is an empty tank, “S”
tion lift at 70°F and sea level. becomes Sw on the diagram.