Control valves

Desuperheaters

4.8.19

Detailed example

Steam is required to heat a vessel jacket at 2 bar g and 133.7oC. A low pressure steam supply is available at 10 bar g and 200 °C

for this purpose.

Condensate at 95 °C and 5 bar g is available as the cooling medium.

Minimum steam flow required is 1000 kg / h, maximum flow 5000 kg / h.

Steam supply

Pressure controller

Desuperheater

1

2

3

Pressure regulating

control valve

Cooling water

Method

Non return valve

Temperature controller

As the turndown required equals 5:1 and the application

involves simultaneous pressure reduction, this would

seem to be a good application for a venturi type

desuperheater (VTD). Please note that a steam atomising

desuperheater (SAD) could also be used for this duty, but

the overall installation cost would be higher due to the

greater complexity of the installation.

Temperature regulating

control valve

4

As already stated, it is not possible to control at the

saturated temperature of the steam so any associated

1

temperature controller should have a set point of 3 °C

above the saturation temperature (which is 136.7oC in this

example).

We can use the Spirax Sarco Online Sizing Program to

calculate the unit size (available at www.spiraxsarco.com/

uk with password). The form would be completed as

follows:

The steam temperature at the inlet to the desuperheater

is calculated by the program automatically and is shown

on the datasheet.

Once the calculation button is pressed the software will

calculate the required cooling water pressure and

flowrate. It will also calculate the pressure drop across

the unit. The user will see a summary screen, at which

point the desired flange rating can be selected and then

the 'Save & Email' button can be pressed. You will then

receive an e-mail with a drawing and datasheet to your

chosen e-mail address.

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3

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TI-P475-06 CH Issue 3