Why Split-Range Control Valves?
Why do we need split-ranging arrangement in process Control applications? Can't we keep things simple and use a single Control Valve to control process?
Actually there are some product requirements when there is no other feasible/efficient solution than split-ranging Control Valves.
Based on application, mainly 3 types of split-range arrangements exist:
- Complementary Split-range
- Exclusive Split-range
- Progressive Split-range
Complementary:
Let's say two liquids A&B are required to be mixed in a ratio(say 60/40% respectively). Each liquid line ends in a tank. Liquid A line will have a Air-to-Open Control Valve. Liquid B line will be Air-to-Close Control Valve. For a 60/40% ratio set by Set-point option on Controller, a 10.2psi signal will be applied by Controller due to which A line CV will open 60% and B line CV will result in 40% open.
Controller O/P - A CV % - B CV %
3psi Full open Full Close
6psi 75% open 25% open
9psi 50% open 50% open
10.2psi 40% open 60% open
12psi 25% open 75% open
15psi Full close Full open
There is one other way also to get the same result as above is by using one three-way Control Valve rather than two complementary split-ranged CVs. Between the two, which method to use, it can be decided upon by considering the factors such as rangeability, accuracy and cost, etc.
Now if, let's say we have an acid line and an alkali line both coming to a tank.
Controller O/P - Acid CV % - Alkali CV %
3psi 100% open Fully Close
6psi 50% open Fully close
9psi Fully close Fully close
12psi Fully close 50% open
15psi Fully close 100% open
This CV's arrangement is said as Exclusive split-ranging. Here, let's say we want to keep PH of water in a tank at 7. The water supply coming into tank is mostly alkaline so then Acid line CV is open to add acid into the tank until the PH gets 7 and Acid CV fully closes at 9psi i.e. as the PH gets closer to 7 so does the Controller signal increases from 3psi to 9psi.
If the water in tank becomes more alkaline, then Controller signal can increase from 9psi till maximum 15psi to open Acid CV. In this time i.e. from 9psi to 15psi signal value, Acid CV will remain fully closed.
Because if both Acid & Alkali CVs are open at same time, it will be a wastage as acid will keep neutralizing the alkali in tank while both acid and alkali coming into tank and in overall tank PH will not be affected. So this is why and where, a progressive split-range type CV arrangement becomes a need.
Progressive Split-ranging:
Thirdly, the need for progressive split-ranging arrangement comes when the requirement is that a HIGH URV(Upper Range Value) of process can be passed through a Control Valve. Due to which, a large-sized Control Valve is required. But the problem with large-sized Control Valve is that then needing to pass exact very small amounts of process through the CV becomes erroneous for product requirements.
Examples are when automotive engineers sized a single carbeurator butterfly valve(that inlets air+fuel mixture based on pressure drop/vacuum suction of fuel). Problem with this single large sized Butterfly valve was that it worked okay when high speed of car was required but this butterfly Valve caused inefficiency when car was on 1st,2nd gear i.e. on low speed. So a 2nd butterfly valve was introduced in car which throttled on upper range.
Below table will clear it more:
Signal CV1 Status CV2 Status
3psi Fully close Fully Close
6psi 25% open Fully close
9psi 50% open Fully close
12psi 75% open 50% open
15psi 100% open 100% open
Hence, as clear from above table, the lower range Controller Set-point will be throttled by CV1 only. When Controller Set-point value is set to above a certain value, the CV2 also opens to throttle for higher input and output. Accuracy in this application cannot be attained using a single Control Valve arrangement where both accuracy and High URV(of car speed, etc) is required.
Another example of Progressive Split-ranging is of Boiler Feedwater Control in Boiler Stand-by(Low Load) and Full Power(Full Load) conditions. When Boiler is in Low Load(Stand-by) state i.e. Steam and thus Feedwater requirement is less, CV1 throttles for accurate lower range control. In Full Load(Full Power) i.e. when Steam and thus Feedwater requirement is high above a certain value which we can set, CV2 then also opens up and throttles for high range input/output control along-with CV1 open 100% in parallel.
Actually there are some product requirements when there is no other feasible/efficient solution than split-ranging Control Valves.
Based on application, mainly 3 types of split-range arrangements exist:
- Complementary Split-range
- Exclusive Split-range
- Progressive Split-range
Complementary:
Let's say two liquids A&B are required to be mixed in a ratio(say 60/40% respectively). Each liquid line ends in a tank. Liquid A line will have a Air-to-Open Control Valve. Liquid B line will be Air-to-Close Control Valve. For a 60/40% ratio set by Set-point option on Controller, a 10.2psi signal will be applied by Controller due to which A line CV will open 60% and B line CV will result in 40% open.
Controller O/P - A CV % - B CV %
3psi Full open Full Close
6psi 75% open 25% open
9psi 50% open 50% open
10.2psi 40% open 60% open
12psi 25% open 75% open
15psi Full close Full open
There is one other way also to get the same result as above is by using one three-way Control Valve rather than two complementary split-ranged CVs. Between the two, which method to use, it can be decided upon by considering the factors such as rangeability, accuracy and cost, etc.
Now if, let's say we have an acid line and an alkali line both coming to a tank.
Controller O/P - Acid CV % - Alkali CV %
3psi 100% open Fully Close
6psi 50% open Fully close
9psi Fully close Fully close
12psi Fully close 50% open
15psi Fully close 100% open
This CV's arrangement is said as Exclusive split-ranging. Here, let's say we want to keep PH of water in a tank at 7. The water supply coming into tank is mostly alkaline so then Acid line CV is open to add acid into the tank until the PH gets 7 and Acid CV fully closes at 9psi i.e. as the PH gets closer to 7 so does the Controller signal increases from 3psi to 9psi.
If the water in tank becomes more alkaline, then Controller signal can increase from 9psi till maximum 15psi to open Acid CV. In this time i.e. from 9psi to 15psi signal value, Acid CV will remain fully closed.
Because if both Acid & Alkali CVs are open at same time, it will be a wastage as acid will keep neutralizing the alkali in tank while both acid and alkali coming into tank and in overall tank PH will not be affected. So this is why and where, a progressive split-range type CV arrangement becomes a need.
Progressive Split-ranging:
Thirdly, the need for progressive split-ranging arrangement comes when the requirement is that a HIGH URV(Upper Range Value) of process can be passed through a Control Valve. Due to which, a large-sized Control Valve is required. But the problem with large-sized Control Valve is that then needing to pass exact very small amounts of process through the CV becomes erroneous for product requirements.
Examples are when automotive engineers sized a single carbeurator butterfly valve(that inlets air+fuel mixture based on pressure drop/vacuum suction of fuel). Problem with this single large sized Butterfly valve was that it worked okay when high speed of car was required but this butterfly Valve caused inefficiency when car was on 1st,2nd gear i.e. on low speed. So a 2nd butterfly valve was introduced in car which throttled on upper range.
Below table will clear it more:
Signal CV1 Status CV2 Status
3psi Fully close Fully Close
6psi 25% open Fully close
9psi 50% open Fully close
12psi 75% open 50% open
15psi 100% open 100% open
Hence, as clear from above table, the lower range Controller Set-point will be throttled by CV1 only. When Controller Set-point value is set to above a certain value, the CV2 also opens to throttle for higher input and output. Accuracy in this application cannot be attained using a single Control Valve arrangement where both accuracy and High URV(of car speed, etc) is required.
Another example of Progressive Split-ranging is of Boiler Feedwater Control in Boiler Stand-by(Low Load) and Full Power(Full Load) conditions. When Boiler is in Low Load(Stand-by) state i.e. Steam and thus Feedwater requirement is less, CV1 throttles for accurate lower range control. In Full Load(Full Power) i.e. when Steam and thus Feedwater requirement is high above a certain value which we can set, CV2 then also opens up and throttles for high range input/output control along-with CV1 open 100% in parallel.
Comments
Post a Comment