Powers 595 Series 11 Self-Operating Temperature Regulators - Type ST Stainless Trim - Manual (Page 3)

3
Sizing and Selection
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Proper sizing of the Regulator is essential for correct system
operation. An undersized Regulator will not allow suffi cient fl ow
at maximum load. An oversized Regulator may cycle and will not
utilize the full valve stroke for effi cient modulation of fl ow. This
results in poor control and shortened valve life (quicker deterio-
ration of valve plug and seat). For these reasons, the correct siz-
ing of the Regulator for actual expected conditions is considered
essential for good control.
NOTE: Select a bulb (see chart on page 14) that has the set
point in the upper third of the temperature range for best valve
performance.
Caution: Do not exceed maximum pressure differentials for given valve sizes. The maximum differential is the pressure the valve
has against it at shutoff. Too large a differential can cause valve chatter and/or prevent shutoff.
Water Capacities
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Use a pressure drop of at least 25% of inlet pressure when sizing valves for water applications.
Size the #11 Regulator for actual rather than maximum condi-
tions. DO NOT size according to pipe size; piping systems are
designed for different criteria than process controls. Refer to
Powers document AE-1—“Valve Selection and Sizing”—for fur-
ther recommendations.
Maximum Operating Pressure Differential (differential for fl uid
fl ow): In order for the process medium to fl ow, a pressure drop
must exist across the valve. “Pressure differential” is the differ-
ence in valve pressure between the inlet and outlet under fl ow
conditions. The greater the differential, the greater the fl ow at
any given plug position.
Though the regulator should be sized for actual conditions, you
need to know the available differential at maximum fl ow. For
optimum control, take as much differential as possible across
the valve.