8
9
0
5000
10000
15000
20000
25000
20
30
40
50
60
70
80
90
100
150
200
300
Pressure PSI
Flow Control Valve
Maximum Flow Rates in Lbs/hr
1/2" 150 PSI
1/2" 300 PSI
3/4" 150 PSI
3/4" 300 PSI
lbs/hr
3.4 Icon Definitions
Symbol
Publication
Description
IEC 417, No.5019
Protective Conductor Terminal
IEC 417, No. 5007
On (Supply)
O
IEC 417, No. 5008
Off (Supply)
ISO 3864, No. B.3.6
Caution, risk of electric shock
ISO 3864, No. B.3.1
Caution
Example: 3450/(10-1) = 383.33 lbs./hr.
4. Determine if Continuous or Intermittent Sampling is Required
Use intermittent sampling when the boiler operation or loading is intermittent, or on boilers where the required
blowdown rate is less than 25% of the smallest available flow control valve or less than the flow through the
smallest orifice. See the graphs on the next page.
Use continuous sampling when the boiler is operating 24 hours per day and the required blowdown rate is more
than 25% of the smallest applicable flow control valve or orifice. See the graphs on the next page.
Use of a flow control valve will give you the best control of the process, since the flow rate can be easily
adjusted. The dial on the valve also gives you a visual indication if the flow rate has been changed. If the valve
clogs, it can be opened to clear the obstruction, and closed to the previous position.
If an orifice plate is used, you must install a valve downstream from the orifice in order to fine tune the flow
rate and provide additional back pressure in many applications.
Example: An 80 psi boiler has a Required Blowdown Rate of 383.33 lbs./hr. The maximum flow rate of the
smallest flow control valve is 3250 lbs./hr. 3250 x 0.25= 812.5 which is too high for continuous sampling.
Using an orifice, the flow rate through the smallest diameter plate is 1275 lbs./hr. This is too high for continuous
sampling.
5.
Determine the Orifice or Flow Control Valve Size for this Blowdown Rate
Use the following graphs to select a flow control device:
lbs/hr
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
10
20
30
40
50
60
70
80
90
100
200
300
Pressure PSI
Flow Rate in Lbs/hr for Various Orifices
1/8 inch dia
3/16 inch dia
1/4 inch dia
5/16 inch dia
