Valve World Magazine November 2022 | Page 26

End user feature

Nowadays control valves are operated through positioners for the control function via the 4-20 mA from DCS . When the control valve is also part of shutdown operation then a solenoid may be introduced between the positioner and actuator to carry out the trip function . In earlier design control valve without an exclusive trip SOV used same 4-20 mA analog signal from DCS to both throttle as well as either open or close the process depending upon the process requirement . Such example of introduction of solenoid in valve loop is shown in Figure 1 for reference purpose . In case of doubleacting actuator , the configuration is more complex with additional accessories . New plant is designed considering as throttling and on-off service separately and never considered as single valve for both purposes . Be that as it may , it is interesting to list and compare the pros and cons of using a separate valve for control and on-off function or to use a single valve for both duties . Please see Table 1 .

S 1 2

Fig . 1 : Control valve with spring and diaphragm actuator for emergency trip solenoid

3

SOLENOID

VENT

AIR

AIR FILTER

There are of course unique requirements for dedicated control and on-off valves . On-off valve specific requirements can include the need for open channel handling , abrasive service , low pressure drop inverse huge flow , etc . On other hand , control valve has also unique requirements which are high Dp inverse low flow requirement , critical valve problems like cavitation , high noise , flashing , etc . Such specific requirement may opt for dedicated valve instead of a dual-purpose valve . Apart from such general categorizing of valve types , valve selection will also depend on the specific process and plant requirements . In plants built forty or fifty years ago it is often possible to find valves with tags bearing acronyms such as EFV and ELV . These indicate the valve is normally operated as flow or level control valve and in emergency case can be used as an emergency block valve . Service in such cases would have the requirement of normal

operation as throttling requirement and in emergency case to depressurize pressure containing equipment . Provided the valve has been properly manufactured using appropriate materials , has been designed for rugged use , has been fitted with an actuator capable of handling the thrust requirements associated with control and on-off duty , and has been fitted with internals ( plug / ball and seat ) that can close properly , then such a valve can deliver good value addition such as cost saving , cabling , IO assignment , carbon footprint reduction , etc . In earlier plant designs use has been made of locally operated control loops having pneumatic controllers and , in such cases when DCS monitoring was not available , operation of such local loops also used a single valve for dual control and on-off application obtaining trip from a nearby instrument which would be the governing parameter . Today , many existing plants continue to use such dual-purpose valves . Normally , conversion to a situation with two separate valves is only considered if a situation arises such as passing , vibration , etc . And in such cases these changes would cause major field contractor attention , and typically all repair / replacement work would be scheduled for a regular plaint shut-down . The disadvantage of valve used as dual purpose is that the valve could lose the inherent mechanical characteristics by trim deterioration , could suffer valve seat damage , controllability during normal operation , cannot be added to SIS loops in future , sluggish response over a period of prolonged usage as dual purpose , reliability of process stability etc . Leakage of such design is again questionable since for a control valve leakage as

Table 1 : Comparing the use of a single valve or separate valves for control and on-off functions

Description Separate valve for control / on-off Single valve for dual purpose Mechanical Integrity

Dedicated valve will have stable mechanical condition since it is used for dedicated operation .

Operation Control valve with a positioner or on-off valve with SOV . Positioner with or without SOV . Design

Separate valve is dedicated design for either control or on-off .

Dual purpose valve will have to withstand dual purpose operation with more chance the valve internals could lose rigidity . The trim in particular could become imbalanced over a period of continuous operation .

Single valve design is used for both purposes .

Cost Initial cost is higher . Initial cost Is lower .

26 Valve World November 2022 www . valve-world . net