Sizing Steam Valves
When working with steam the maximum operating pressure of a valve must be carefully considered. The maximum static pressure ratings of control valves are for water and steam, but only up to a given rated temperature. If the valve is rated to a maximum temperature of 100°C (230°F) then by checking the temperature/pressure table at the bottom of steam chart ‘C’ it will be seen that this valve would be limited to 5 psig steam. Likewise, 130°C (266°F) = 15 psig and 135°C (275°F) = 30 psig. | |
Many valves rated at 6 psig steam (230°F) will operate for decades at 30 psig (275°F) without failure if properly sized. However,better life expectancy can be anticipated if valves used on high temperature mediums are mounted with the operator to the side of the pipe rather than on top of it. In this way, the valve operator is not located in the high temperature air directly above the high temperature steam pipe, and longer operator life can be expected. (Diaphragm life of a typical VP-2170 on 30 psi steam is shortened to 5 – 6 years when located above the pipe as in Fig. ‘A’. Installed as shown in Fig. ‘B‘ life expectancy can be measured in decades.)
STEAM IS A UNIQUE MEDIUM TO CONTROL…
Using Figure 3 you can calculate the amount of steam which will pass through a valve with a Cv of 1.0. For other sizes of valves, multiply by the Cv. To calculate Cv, follow the example on the chart as follows: example:
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