Walchem WebMaster Instruction Manual download pdf (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:

2000

4000

6000

8000

10000

12000

14000

16000

18000

10 20 30 40 50 60 70 80 90 100 200 300

Lbs/hr

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

1 2 ... 11 12 13 14 15 16 17 18 19 20 21 ... 179 180

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Walchem WebMaster Instruction Manual Page 16