Page 211 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Pumping of Liquids 183

fore, the samefor any$& through the $urn$ at a .pen speed of the suction and discharge sides of the pump.
rotation and capacity ures 3-38 and 3-39.
Through conversion, head may lbe expressed in units
other than feet of fluid by taking the specific gravity of the H hd - h, (3-3)
fluid into account.
The sign of h, when a suction Lift is concerned is nega-
(Head in feet), 1-1 = psi) (2.31)/SpGr, for any fluid (3-1) tive, making H = hd - ( -h,) = hd + h,
The three main components illustrated in the exam-
Note that psi (pou.nds per square inch) is pressure on ples are (adapted from [51):
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

or, (head in Et), H = (psi) (P44/p) (3-2) A pump is acted on by the total forces, one on the suc-
tion (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 Bb/sq in. = 2.31 ft of water at SpGr = 1.0 pump must work against. However, it is extremely impor-
1 b/sq in. = 2.31 ft of water/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 ofwater = 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
Foir water, SpGr = 1.0 at 62"F, although for general use this works against the pump and it becomes a negative
it can be considered 1.8 over a much wider range. For (-) . (See discussion to follow.)
explanation of vacuum and atmospheric pressure, see
Chapter 2.

Example 3-1: Liquid Heads c)Butane
SpGr = 0.6
If a pump were required to deliver 50 psig to a system, b)Naphtha
for water, the feet of head on the pump curve must read, SpGr = 0.8

2.31 (50) = 115.5ft

For a liquid of SpGr 1.3, the ft of head on the pump a)For water,
curve must read, 112).5/1.3 = 88.8 ft of liquid. SpGr = 1 .O d)Carbon
For liquid of SpGa 0.86, the ft of head on the pump 66.7 tetrachloride
curve must read, 115.5/8.86 = 134.2 ft of liquid. SpGr ~1.50
If a pump were initially selected to handle a liquid 125'
where SpGr = 1.3 at 88.8 ft, a substitution of light hydro-
carbon where SpCr = 0.86 would mean that the head of
liquid developed by 1,he pump would still be 88.8 feet, but
the pressure of this lighter liquid would only be
88.8/[(2.31)/(0.86)] or 44.8 psi. Note 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 / 3
fluid. For other examples, see Figure 3-37. a j= 43.3 psig b) = 43.3 psig e)
The total head developed by a pump is composed of Pressure gauge attached at bottom
the difference between the static, pressure and velocity
heads plus the friction entrance and exit head losses for Figure 3-37, r;ompanson OT coiumns ot various iiquias KO register
43.3 psig on pressure gauge at bottom of column.