Page 412 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

380 Applied Process Design for Chemical and Petrochemical Plants

Absolute Pressure where Pct = piston displacement cu fl/min
Controller
0 V = system volume, cu ft
t = evacuation time, min
Steam P 2 = absolute discharge pressure of pump, psia
Pc = absolute intake pressure of pump with closed intake
P 1 = absolute intake pressure of pump
Process
§�
The relation above is theoretical, and does not take
Discharge into account any inleakage while pumping. It is recom-
mended that a liberal multiplier of perhaps 2 or 3 be used
Figure 6-30. Single-stage ejector control. to estimate closer to actual time requirements.
An alternate relation for calculating evacuation time is
from the Heat Exchange Institute [11]:

(6- 25)
Steam
Alternate Pumpdown to a Vacuum Using a Mechanical
Process Pwnp
§_ystem
For large process systems of vessels, piping, and other
Discharge equipment, the downtime required to evacuate the system
before it is at the pressure (vacuum) level and then to
Figure 6-31. Single-stage ejector control with varying load.
maintain its desired vacuum condition, can become an
important consideration during start-up, repair, and
Absolute Pressure 'Atmospheric Air Bleed restart operations.
or Tem�eroture Waler I
Controller I Reference [26] suggests an improved calculation:
Absolute O I
Pressure -- - --'----Steam
Controller �----Pressure-- Low Vacuum
i -
; si� (6- 26)
-,-- Discharge
cJndenser
i
where t, = pumpdown time, sec
0
Process p" = final pressure in vessel or system, torr
§.rstem Toil pipe
P" 0 = starting pressure in vessel or system, torr
Rps= pump speed, rotations (or strokes)/sec
V' = volume of vessel or system, liters
Figure 6-32. System handling large quantities of condensable V' 0 = volume of pump chamber, liters
vapors. S = pump speed, liters/sec
S 0 = pump speed, at P" liters/sec
0,
Time Required For System Evacuation Sn= pump speed at P" liters/sec
0,
It is difficult to determine the time required to evacu- Example 6-13: Determine Pump Downtime for a System
ate any particular vessel or process system including pip-
ing down to a particular pressure level below atmospher- Calculate the pump downtime for a system of vessels
ic. When using a constant displacement vacuum pump and piping with a volume of 500 liters. The final pressure
this is estimated by O'Neil [31]: is to be 0.01 torr, starting at atmospheric. From the speed-
pressure curve of a manufacturer's pump at 0.01 torr,
0
v p - p c speed is 2.0 liters/sec. At atmospheric pressure, S = 2.75
2
Pd= -log (6-24) liters/sec with P" 0 = 760 torr. From the manufacturer's
t c p - p
I c data, Rps = 15 and V' 0 = 0.5 liters.