Honeywell MS4103 manual Linear vs. Nonlinear System Control

Appendix A: Valve Selection and Sizing

Linear Valve

	NONLINEAR SYSTEM
100%	RESPONSE
PERCENTAGEOF COOLINGFULL	RESULTANT
	LINEAR SYSTEM
	CONTROL
	EQUAL PERCENTAGE
	CONTROL VALVE

0%100%

TEMPERATUREC2330

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.

100%

90%

QUICK-OPENING

CONTROL VALVE

FLOW

0%	20%	100%
	STEM TRAVEL	C2331

Fig. 9. Flow vs. Stem Travel Characteristic of a Quick-

Opening 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.

100%			100%			100%
90%			90%			90%
HEAT OUTPUT			FLOW			HEAT OUTPUT
20%			20%			20%
0%	20%	90% 100%	0%	20%	90% 100%	0%	20%	90% 100%

FLOW	STEM TRAVEL	STEM TRAVEL
GRAPH A	GRAPH B	GRAPH C	C2332
Fig. 10. Heat Output, Flow, and Stem Travel Characteristics of a Linear Valve.
EQUAL PERCENTAGE VALVE	equal percentage, regardless of flow rate. In mathematical
An equal percentage valve includes a contoured plug or	terms, this is an exponential response.
contoured V-port shaped so that similar movements in stem

travel at any point in the flow range change the existing flow an

APPENDIX

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