Walchem WebMaster ONE Instruction Manual page 16

3. Determine the Required Blowdown Rate in Pounds Per Hour
Blowdown Rate = Steam Production / (Concentration Ratio –1)
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:
18000
16000
14000
12000
10000
8000
6000
4000
2000
0
10 20
Flow Rate in Lbs/hr for Various Orifices
30 40 50 60
Pressure PSI
70 80 90 100
12
1/8 inch dia
3/16 inch dia
1/4 inch dia
5/16 inch dia
200 300