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...
FOR IT FOLLOWS UNUSUAL LAWS
- First, steam flows through an orifice or control valve at an increasing rate until it meets a limit referred to as Critical pressure. At this point, flow cannot be increased.
- A second factor is supersaturated steam which has less total heat than saturated steam at the same temperature, and this amount must be allowed for.
- Finally, steam selection tables work on the absolute steam pressure as opposed to gauge pressure (absolute pressure is gauge pressure plus atmospheric pressure at 14.7 psi.)
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:
- 300# of steam is needed and the steam supply is 15 psig. (29.7 psia).
- A modulating valve is needed so a line is taken vertically to the recommended pressure drop line and steam flow is read directly as 44#/hr/Cv. Actual Cv required is 300/44 - 6.82 and an ideal candidate would be a 3/4" V21 at 7.0 Cv.
- Work backwards as a double check. 300# passing through a 7.0 Cv valve will require that 42.84# flow through each Cv and the differential pressure of this valve would in fact be 10 psi.
To Download the Steam Chart please click here