Page 213 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Pumping of Liquids 183

fore, the same Jar any fluid through the pump at a given speed of the suction and discharge sides of the pump. Refer to Fig-
rotation and capacity. ures 3-38 and 3-39.
Through conversion, head may be expressed in units
other than feet of fluid by taking the specific gravity of the (3-3)
fluid into account.
The sign of h, when a suction lift is concerned is nega-
(Head in feet), H = (psi) (2.31)/SpGr, for any fluid (3-1) tive, making H = hd - (-hs) = hd + h,
The three main components illustrated in the exam-
Note that psi (pounds per square inch) is pressure on ples are (adapted from [5]):
the system and is not expressed as absolute unless the sys-
tem is under absolute pressure. Feet are expressed as head, 1. Static head
not head absolute or gauge (see later example). Note the 2. Pressure head
conversion of psi pressure to feet of head pressure. 3. Friction in piping, entrance and exit head losses
A pump is acted on by the total forces, one on the sue-
or, (head in ft), H = (psi) ( 144/ p) (3-2)
Lion (inlet) side, the other on the discharge side. By sub-
tracting (algebraically) all the suction side forces from the
where p = fluid density, lb/ cu ft discharge side forces, the result is the net force that the
1 lb/sq in. = 2.31 ft of water al SpGr = 1.0
pump must work against. However, it is extremely impor-
1 lb/sq in. = 2.31 ft ofwater/SpGr of liquid= ft tant to recognize the algebraic sign of the suction side
liquid components, that is, if the level of liquid to be lifted into
1 in. mercury = 1.134 ft of water= 1.134/SpGr liq- the pump is below the pump centerline, its algebraic sign
uid, as ft liquid is negative ( - ) . Likewise, if there is a negative pressure or
vacuum on the liquid below the pump centerline, then
For waler, SpGr = 1.0 at 62°F, although for general use this works against the pump and it becomes a negative
it can be considered 1.0 over a much 'Wider range. For (-). (See discussion Lo follow.)
explanation of vacuum and atmospheric pressure, see
Chapter 2.
c)Butane
Example 3-1: Liquid Heads SpGr= 0.6
lr-
If a pump were required to deliver 50 psig Lo a system, b)Naphtha
for water, the feet of head on the pump curve must read, SpGr= 0.8

2.31 (50) = 115.5 ft
1
For a liquid of SpGr 1.3, the ft of head on the pump a)For water,
curve must read, 115.5/1.3 = 88.8 fl ofliquid. SpGr= 1.0 I d)Carbon
For liquid of SpGr 0.86, the ft of head on the pump 166.7' tetrachloride
curve must read, 115.5/0.86 = 134.2 ft ofliquid. I SpGr=1.50
If a pump were initially selected Lo handle a liquid 125'
where SpGr = 1.3 at 88.8 ft, a substitulion oflight hydro-
carbon where SpGr = 0.86 would mean that the head of
liquid developed by the pump would still be 88.8 feet, but 100' 66.6'
the pressure of this lighter liquid would only be
88.8/[(2.31)/(0.86)1 or 44.8 psi. Nole that for such a
change in service, the impeller seal rings, packing (or
mechanical seal) and pressure rating of casing must be
evaluated to ensure proper operation with a very volatile
fluid. For other examples, see Figure 3-37. a)= 43. .
/ 3 ps1g
The total head developed by a pump is composed of Pressure gauge attached at bottom
the difference between the static, pressure and velocity Figure 3-37. Comparison of columns of various liquids to register
heads plus the friction entrance and exit head losses for 43.3 psig on pressure gauge at bottom of column.