Filter
Top Products
251
APPENDIX
Fig. 8. Linear vs. Nonlinear System Control.
QUICK-OPENING VALVE
A quick-opening two-way valve includes only a disc guide and a
flat or quick-opening plug. This type of valve is used for two
position control of steam. The pressure drop for a quick opening
two-way valve should be 10 to 20 percent of the piping system
pressure differential, leaving the other 80 to 90 percent for the
load and piping connections.Figure 9 shows the relationship of
flow versus stem travel for a quick-opening valve. To achieve 90
percent flow, the stem must open only 20 percent. Linear or
equal percentage valves can be used in lieu of quick-opening
valves in two-position control applications as the only significant
positions are full open and full closed.
Fig. 9. Flow vs. Stem Travel Characteristic of a Quick-
Opening Valve.
Linear Valve
A linear valve may include a V-port plug or a contoured plug.
This type of valve is used for proportional control of steam or
chilled water, or in applications that do not have wide load
variations. Typically in steam or chilled water applications,
changes in flow through the load (e.g., heat exchanger, coil)
cause proportional changes in heat output. For example, Figure
10 shows the relationships between heat output, flow, and stem
travel given a steam heat exchanger and a linear valve as
follows:
— Graph A shows the linear relationship between heat output
and flow for the steam heat exchanger. Changes in heat
output vary directly with changes in the fluid flow.
— Graph B shows the linear relationship between flow and
stem travel for the linear control valve. Changes in stem
travel vary directly with changes in the fluid flow.
NOTE: As a linear valve just starts to open, a minimum
flow occurs due to clearances required to pre-
vent sticking of the valve. Some valves have a
modified linear characteristic to reduce this mini-
mum controllable flow. This modified character-
istic is similar to an equal percentage valve
characteristic for the first 5 to 10 percent of stem
lift and then follows a linear valve characteristic
for the remainder of the stem travel.
— Graph C shows the linear relationship between heat output
and stem travel for the combined heat exchanger and linear
valve. Changes in heat output are directly proportional to
changes in the stem travel.
Thus a linear valve is used in linear applications to provide
linear control.
Fig. 10. Heat Output, Flow, and Stem Travel Characteristics of a Linear Valve.
EQUAL PERCENTAGE VALVE
An equal percentage valve includes a contoured plug or
contoured V-port shaped so that similar movements in stem
travel at any point in the flow range change the existing flow an
equal percentage, regardless of flow rate. In mathematical
terms, this is an exponential response.
100%
0% 100%
NONLINEAR SYSTEM
RESPONSE
RESULTANT
LINEAR SYSTEM
CONTROL
EQUAL PERCENTAGE
CONTROL VALVE
TEMPERATURE
PERCENTAGE OF
FULL COOLING
C2330
100%
0% 100%
STEM TRAVEL
FLOW
C2331
90%
20%
QUICK-OPENING
CONTROL VALVE
C2332
100%
0% 100%
FLOW
HEAT OUTPUT
HEAT OUTPUT
90%
20%
90%
100%
0% 100%
STEM TRAVEL
FLOW
90%
20%
90%
100%
0% 100%
STEM TRAVEL
90%
20%
90%
GRAPH A
GRAPH B GRAPH C
20%
20%
20%
Appendix A: Valve Selection and Sizing