Size Liquid Motive for Evacuating Gases
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| The times given in the ML, MLE evacuation charts
below are the time in minutes to evacuate 1 Ft 3 to the pressure
(vacuum) listed, from atmospheric pressure. |
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| Step 1 |
| Before beginning to do the actual sizing, convert
all pressures and flows to the units used in the sizing table.
(If sizing is being done regularly with other input units, request
a special sizing table from your representative.) |
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| Step 2 |
| Divide the desired time to evacuate the volume by
the number of Ft' to be evacuated. |
| Desired
evacuation Time |
=
Desired Time per Ft3 |
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| Ft' to be
Evacuated |
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| Step 3 |
| Locate the value equal to or less than the available
flowing pressure for W your system in the Motive Pressure (Pm)
column. If your available system motive pressure is between the
listed pressures, you may interpolate between them. Choosing a
lower pressure will yield a more conservative estimate. |
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| Step 4 |
| Locate the Suction Pressure (Ps) row that corresponds
to the suction pressure that you need. If your desired suction
pressure is between the listed pressures, you can interpolate between
them. This will result in a more accurate sizing. At the intersection
of the row you have located and the column from Step 3 is the time
per Ft3 for a 1-1/2" unit. |
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| Step 5 |
| Choose a S.F. that when divided into the time/Ft'
from Step 4 will yield a time/Ft 3 equal or lower than the Desired
time/Ft' from Step 2. |
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| Step 6 |
| To determine the time required, multiply the Ft 3
of the suction vessel by the time per Ft 3 determined from the
table in Step 5. This will give the time required to evacuate the
vessel under actual conditions. |
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| Step 7 |
| To determine the motive water required, refer to
the same column in the row labeled Motive Flow(Qm). Multiply this
number times the Sizing Factor (S.F.) for the unit selected. The
result will be the GPM of motive water required to operate the
selected unit. |
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| Note: Eductors for evacuations are sized at atmospheric
outlet pressure. For liquid motives, these values calculated can
be used with higher outlet pressures provided they do not exceed
5% of the motive pressure or 5 PSIG, which ever is lower. If higher
outlet pressure is being used, consult the factory or your representative. |
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| Adjusting Formulas for Priming |
| Step 1 |
| Multiply the Ft3 to be evacuated by 2, then go to
Step 1 under Evacuation and proceed through the procedure described
above. |
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| Example: |
| Required Time
to Evacuate Vessel |
4 Minutes |
| Motive Pressure, (Pm) |
60 PSIG |
| Size of Vessel to be Evacuated |
45 Ft 3 |
| Outlet Pressure, (Po) |
1 PSIG |
| Desired Final Suction Pressure,
(Ps) |
10 In Hg Abs |
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| Step 1 |
| All units are in pressures and flows used in charts. |
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| Step 2 |
| 4 minute desired evacuation time/45 Ft 3 = .089
minute/Ft3 |
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| Step 4 |
| Go to the ML and MLE table to find an MLE operated
at 60 PSIG and pulling down to 10 In Hg Abs will evacuate each
Ft 3 in 0.14 minutes. |
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| Step 5 |
| The S.F. for a 2" MLE gives. 14/1.82 = .077
minutes per Ft 3 , the first number below .089, the desired time
from Step 2. |
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| Step 6 |
| The actual time to evacuate will be 45 Ft 3 x .077
minutes = 3.47 |
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| Step 7 |
| The motive flow (Qm) required will be 26.2 gal.
x 1.82 S.F. 47.7 GPM. |
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| Will the outlet pressure effect the unit" 60
x .05 = 3 PSIG. |
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| Therefore, as the outlet pressure (Po) does not
exceed 5% of the motive pressure or 5 PSIG it should have little
effect on the performance of the unit. |
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| Liquid Motive Evacuating Gases in Times |
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ML,
MLE
|
Motive
Press, Pm (psig)
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Minutes
per cubic foot
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|
1-1/2" Unit
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|
Suction
Press
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20
|
40
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60
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80
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100
|
140
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Ps,
(In Hg Abs)
|
ML
|
MLE
|
ML
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MLE
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ML
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MLE
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ML
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MLE
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ML
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MLE
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ML
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MLE
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30
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0.00
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0.00
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0.00
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0.00
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0.00
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0.00
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0.00
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0.00
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0.00
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0.00
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0.00
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0.00
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25
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0.07
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0.14
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0.02
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0.02
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0.01
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0.01
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0.01
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0.01
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0.06
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0.01
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0.01
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0.01
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20
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0.25
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0.14
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0.10
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0.06
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0.05
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0.04
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0.03
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0.03
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0.02
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0.03
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0.01
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0.02
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15
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0.51
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0.30
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0.24
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0.14
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0.01
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0.08
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0.07
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0.06
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0.06
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0.05
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0.03
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0.04
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10
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0.95
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N/A
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0.45
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0.26
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0.28
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0.14
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0.17
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0.10
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0.11
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0.08
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0.07
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0.06
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S
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N/A
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N/A
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0.91
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0.47
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0.59
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0.25
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0.41
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0.17
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0.29
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0.14
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0.13
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0.11
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Motive
Flow,
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18.9
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16.3
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25.3
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21.8
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30.4
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26.2
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34.8
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30
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38.7
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33.3
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45.4
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39.2
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Qm
(GPM)
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