Split-range Control – Basic Principles of Control Valves and Actuators

Split-range Control - Basic Principles of Control Valves and Actuators

There are many course of management purposes within the business the place it’s fascinating to have a number of management valves reply to the output of a standard controller. Control valves configured to observe the command of the identical controller are mentioned to be cut up-ranged, or sequenced.

Split-ranged management valves might take completely different varieties of sequencing. A couple of completely different modes of management valve sequencing are generally seen within the business: complementaryunique, and progressive.

Complementary valve sequencing

The first sort of management valve sequencing is a mode the place two valves serve to proportion a combination of two fluid streams, reminiscent of this instance the place base and pigment liquids are blended collectively to kind colored paint:

Both base and pigment valves function from the identical 3 to fifteen PSI pneumatic sign output by the I/P transducer (AY), however one of the valves is Air-To-Open whereas the opposite is Air-To-Close. The following desk exhibits the connection between the valve opening for every management valve and the controller’s output:

output (%) & (PSI) & (stem place) & (stem place) cr

Controller I/P output Pigment valve & Base valve 0 % 3 PSI absolutely shut absolutely open
25 % 6 PSI 25% open 75% open
50 % 9 PSI half-open half-open
75 % 12 PSI 75% open 25% open
100 % 15 PSI absolutely open absolutely shut

An various expression for this cut up-vary valve behaviour is a graph displaying every valve opening as a colored stripe of various width (wider representing additional open). For this explicit mode of cut up-ranging, the graph would appear like this:

With this way of cut up-ranging, there may be by no means a situation within the controller’s output vary the place each valves are absolutely open or absolutely shut. Rather, every valve enhances the opposite’s place.

An various to complementary valve sequencing in a course of the place two fluid streams combine (or diverge) is to make use of one three-approach valve fairly than a pair of two-approach valves:

{A photograph} of a 3-approach globe valve mixing scorching and chilly water to manage temperature is proven right here:

Exclusive valve sequencing

Other purposes for cut up-ranged management valves name for a kind of valve sequencing the place each valves are absolutely closed at a 50% controller output sign, with one valve opening absolutely because the controller output drives towards 100% and the opposite valve opening absolutely because the controller output goes to 0%. The nature of this valve sequencing is to have an “either-or” throttled path for course of fluid. That is, both course of fluid flows by one valve or by the opposite, however by no means by each on the similar time.

A sensible instance of this way of cut up-ranging is reagent feed for a pH neutralization course of, the place the pH worth of course of liquid is introduced nearer to impartial by the addition of both acid or caustic reagent liquids:

Here, a pH analyzer displays the pH worth of the liquid answer and a single pH controller instructions two reagent valves to open when wanted. If the method pH begins to extend, the controller output sign will increase as effectively (direct motion) to open up the acid valve. The addition of acid to the combination can have the impact of decreasing the combination’s pH worth. Conversely, if the method pH begins to lower, the controller output sign will lower as effectively, closing the acid valve and then opening the caustic valve. The addition of caustic to the combination can have the impact of elevating the combination’s pH worth.

Both reagent management valves function from the identical 3 to fifteen PSI pneumatic sign output by the I/P transducer (AY), however the two valves’ calibrated ranges are usually not the identical. The Air-To-Open acid valve has an working vary of 9 to fifteen PSI, whereas the Air-To-Close caustic valve has an working vary of 9 to three PSI. The following desk exhibits the connection between the valve opening for every management valve and the controller’s output:

output (%) & (PSI) & (stem place) & (stem place) cr

Controller I/P output Acid valve & Caustic valve 0 % 3 PSI absolutely shut absolutely open
25 % 6 PSI absolutely shut half-open
50 % 9 PSI absolutely shut absolutely shut
75 % 12 PSI half-open absolutely shut
100 % 15 PSI absolutely open absolutely shut

Again, we might categorical the 2 valves’ unique relationship within the kind of a graph, with colored stripes representing valve opening:

Exclusive-sequenced management valves are utilized in purposes the place it could be undesirable to have each valves open concurrently. In the instance given of a pH neutralization course of, the objective right here is for the controller so as to add both acid reagent or caustic reagent to “push” the pH worth in both route as wanted. However, concurrently including each acid and caustic to the method can be wasteful, as one reagent would merely neutralize the opposite with no profit to the method liquid itself.

Progressive valve sequencing

The third kind of management valve sequencing is used to increase the working vary of circulation management for some fluid past that which a single management valve may muster. Once once more pH management offers an acceptable instance for example an utility of this way of sequencing.

pH is an particularly difficult utility of course of management as a result of the dynamic vary of the method is big. Each unit of pH worth change represents a ten-fold change in hydrogen ion focus inside the course of liquid. This means the distinction in ion focus between a course of liquid having a worth of 10 pH and a course of liquid having a worth of 7 pH (a pH distinction of 3) is an element of one thousand (103)! Consequently, the circulation price of reagent essential to neutralize a course of liquid stream might differ extensively. It is kind of potential {that a} management valve sized to throttle minimal circulation will merely be too small to fulfill the calls for of excessive circulation when wanted. Yet, a management valve sized massive sufficient to fulfill the utmost circulation price could also be too massive to exactly “turn down” when only a trickle of reagent is required.

This similar normal management downside was encountered by automotive engineers within the days when carburettors had been used to combine gasoline with air previous to combustion in an engine. A carburettor is a mechanical airflow management system utilizing a “butterfly” valve ingredient to throttle airflow into the engine, and a venturi ingredient producing vacuum to aspirate gas droplets into the air stream to create an air-gas combination. A carburettor with a butterfly valve and circulation tube sized to idle effectively and reply to the wants of in-city driving wouldn’t circulation sufficient air to supply good excessive-pace efficiency. Conversely, a big carburettor appropriate for driving at racing speeds would provide poor management at low-pace and idling operation. Their answer to this downside was the progressive carburettor, having two butterfly valves to throttle the circulation of air into the engine. One butterfly valve handed low quantities of airflow solely, whereas a bigger butterfly valve opened up solely when the accelerator pedal was practically at its most place. The mixture of two otherwise-sized butterfly valves – progressively opened – gave drivers the perfect of each worlds. Now, an vehicle engine may carry out effectively each at low energy ranges and at excessive energy ranges.

On a elementary degree, the issue confronted in pH management in addition to by early automotive engineers is similar factor: inadequate rangeability. Some processes demand a larger vary of management than any single valve can ship, and it’s inside these processes {that a} pair of progressively-sequenced management valves is a legitimate answer.

Applying this answer to a pH management course of the place the incoming liquid at all times has a excessive pH worth and have to be neutralized with acid:

Proper sequencing of the small and massive acid management valves is proven within the desk and the graph:

output (%) & (PSI) & (stem place) & (stem place) cr

Controller I/P output Small acid valve & Large acid valve 0 % 3 PSI absolutely shut absolutely shut
25 % 6 PSI half-open absolutely shut
50 % 9 PSI absolutely open absolutely shut
75 % 12 PSI absolutely open half-open
100 % 15 PSI absolutely open absolutely open

With the 2 acid management valves sequenced progressively, the management system can have considerably extra rangeability crucial to control pH below extensively various course of situations.

{A photograph} displaying a pair of progressively sequenced management valves used to manage boiler feedwater to the excessive-stress (“HP”) drum on a big energy boiler is proven right here:

One small management valve (horizontal, with a inexperienced actuator) is the primary to open from 4 to 12 mA. A bigger management valve (vertical, with a pink actuator) is the following to open from 12 mA to twenty mA. Under regular (full-energy) working situations, the inexperienced valve is extensive-open whereas the bigger pink valve throttles feedwater to the steam drum. When the boiler is working in standby (low-energy) mode, the pink valve is shut whereas the smaller inexperienced valve throttles feedwater to the steam drum. The mixture of these two management valves offers higher steam drum degree management over the boiler’s full vary of operation than any single management valve may do alone.

Valve sequencing implementations

In all earlier management valve sequencing examples proven, each management valves acquired the identical pneumatic sign from a standard I/P (present-to-stress) converter. This means every valve acquired the very same stress sign from the transducer for any given controller output worth. Sequencing of the 2 valves (i.e. making each reply otherwise to the identical air stress sign), due to this fact, was a matter of setting every valve to a unique bench-set stress vary.

It must be understood, nonetheless, that establishing two management valves with completely different bench-set ranges shouldn’t be the one solution to cut up-vary a pair of valves. Other methods exist as effectively, every with its personal benefits and disadvantages.

The following illustrations depict a number of various strategies for management valve sequencing (cut up-ranging):

The widespread pneumatic sign method (one controller, one I/P transducer) is easy however suffers from the drawback of gradual response since one I/P transducer should drive two pneumatic actuators. Response time could also be improved by including a pneumatic quantity booster between the I/P and the valve actuators, or by including a positioner to no less than one of the valves. Either of these options works by the identical precept: lowering the air quantity demand on the one widespread I/P transducer.

Wiring two I/P transducers in collection so that they share a standard sign present is one other solution to cut up-vary two management valves. This method doesn’t undergo from gradual response since every valve has its personal devoted I/P transducer to produce it with actuating air. We now have a alternative the place we implement the cut up ranges: we will do it within the I/P transducers (i.e. every I/P transducer having a unique calibration) or within the management valves (i.e. every management valve having a unique bench-set stress vary). Since it’s usually simpler to re-vary an I/P than it’s to rebuild a management valve with a unique spring (to provide it a unique actuating stress vary), this method has the benefit of handy configuration.

An obstacle of the collection-wired I/P technique is the additional burden positioned on the controller’s output sign circuitry: one have to be cautious to make sure the 2 collection-linked I/P converters don’t drop an excessive amount of voltage at full present, or else the controller might have problem driving each gadgets in collection. Another (potential) drawback of collection-linked valve gadgets in a single present loop is the lack to put in “smart” devices speaking with the HART protocol since a number of gadgets on the identical loop will expertise handle conflicts. HART gadgets can solely work in hybrid analogue/digital mode when there may be one system per 4-20 mA circuit.

A preferred solution to implement cut up-ranging is to make use of a number of 4-20 mA outputs on the identical controller. This may be very straightforward to do if the controller is an element of a big system (e.g. a DCS or a PLC) with a number of analogue output channels. If a number of outputs are configured on one controller, every valve can have its personal devoted wire pair for management. This tends to lead to easier wiring than collection-wired I/P transducers or positioners since every valve loop is a typical 4-20 mA circuit similar to some other (non-cut up-ranged) management valve loop circuit. It may be probably the most sensible solution to implement cut up-ranging when “smart” valve positioners are used because the devoted loop circuits enable for regular operation of the HART protocol with no handle conflicts.

An benefit of twin controller outputs is the flexibility to carry out the cut up-vary sequencing inside the controller itself, which is commonly simpler than re-ranging an I/P or calibrating a valve positioner. This approach, the 4-20 mA alerts going to every valve can be distinctive for any given controller output worth. If sequenced as such, the I/P transducer calibration and valve bench-set values could also be commonplace fairly than personalized. Of course, simply because the controller is succesful of performing the required sequencing doesn’t imply the sequencing have to be performed inside the controller. It is feasible to program the controller’s twin analog outputs to ship the very same present sign to every valve, configuring every valve (or every positioner, or every I/P transducer) to reply otherwise to the equivalent present alerts.

A digital adaptation of the twin-output controller sequencing methodology is seen in FOUNDATION Fieldbus techniques, the place a particular software program perform block referred to as “SPLT” exists to supply cut up-ranged sequencing to 2 valves. The “SPLT” perform block takes in a single management sign and outputs two alerts, one output sign for every valve in a cut up-ranged pair. The perform block diagram for such a system seems right here:

In this Fieldbus system, a single PID management block outputs a sign to the SPLT block, which is programmed to drive two distinctive positioning alerts to the 2 valves’ AO (analogue output) blocks. It must be famous that whereas every AO block is exclusive to its personal management valve, the SPLT and even PID blocks could also be situated in any succesful system inside the Fieldbus community. With FOUNDATION Fieldbus, management system features are usually not essentially relegated to separate gadgets. It is feasible, for instance, to have a management valve outfitted with a Fieldbus positioner really carry out its personal PID management calculations and cut up-ranged sequencing by finding these perform blocks in that one bodily system!

Dual controllers are an possibility just for specialised purposes requiring completely different levels of responsiveness for every valve, often for unique or progressive cut up-ranging purposes solely. Care have to be taken to make sure the controllers’ output alerts don’t wander outdoors of their meant ranges, or that the controllers don’t start to “fight” one another in attempting to manage the identical course of variable.

An essential consideration – and one that’s simply missed – in cut up-vary valve techniques are fail-safe mode. As mentioned in a earlier part, the idea of the fail-safe control system design is that the management valve(s) have to be chosen to fail within the mode that’s most secure for the method within the occasion of actuating energy loss or management sign loss. The actions of all different devices within the loop ought to then be chosen to enhance the valves’ pure working mode.

In management techniques the place valves are cut up-ranged in both complementary or unique trend, one management valve can be absolutely closed and the opposite can be absolutely open at every excessive finish of the sign vary (e.g. at 4 mA and at 20 mA). If the sequencing for a set of complementary or unique cut up-ranged management valves occurs after the controller (e.g. completely different actuator actions) the valves should fail in reverse modes upon loss of controller sign. However, whether it is deemed safer for the method to have the 2 valves fail in the identical state – for instance, to each fail closed within the occasion of air stress or sign loss – we might use twin sequenced controller outputs, attaining both complementary or unique management motion by driving the 2 valves with two completely different output alerts. In different phrases, cut up-ranging two management valves so that they usually behave in reverse trend does not essentially imply the 2 valves should fail in reverse states. The secret to attaining correct failure mode and correct cut up-vary sequencing is to rigorously find the place the sequencing takes place within the management system.

As an instance of a cut up-ranged system with reverse valve failure modes, contemplate the next temperature management system supplying both scorching water or chilled water to a “jacket” surrounding a chemical reactor vessel. The function of this method is to both add or take away warmth from the reactor as wanted to manage the temperature of its contents. Chemical piping in and out of the reactor vessel has been omitted from this P&ID for simplicity, so we will focus simply on the reactor’s temperature management system:

Here, the controller has been configured for twin-output operation, the place the output worth drives two equivalent 4-20 mA alerts to the management valve positioners, which straight enter the present alerts from the controller with out the necessity for I/P transducers in between. The scorching water valve (TV-37a) is fail-closed (FC) whereas the chilly water valve (TV-37b) is fail-open (FO). Half-range positioner calibrations present the unique sequencing crucial to make sure the 2 valves are by no means open concurrently – TV-37b operates on the decrease half of the 4-20 mA sign vary (4-12 mA), whereas TV-37a operates on the higher half (12-20 mA).

Consider the consequences of the controller (TIC-37) shedding energy. Both 4-20 mA alerts will go lifeless, driving each valves to their fail-protected modes: scorching water valve TV-37a will absolutely shut, whereas chilly water valve TV-37b will absolutely open. Now contemplate the consequences of air stress loss on each valves. With no air stress to function, the actuators will likewise spring-return to their fail-protected modes: as soon as once more scorching water valve TV-37a will absolutely shut, whereas chilly water valve TV-37b will absolutely open. In each failure occasions, the 2 management valves assume constant states, making certain most cooling to the reactor within the occasion of an output sign or instrument air failure.

However, suppose we desired each of these valves fail within the closed place within the occasion of an output sign or instrument air failure, fairly than have the cooling valve fail open whereas the heating valve fails closed. Clearly this may require each TV-37a and TV-37b to be fail-closed (FC), which might imply we should discover another solution to sequence their operation to realize cut up ranging. Examine this reconfiguration of the reactor temperature management system, utilizing equivalent management valves (sign-to-open, fail-closed) for each scorching and chilly water provide, and a controller with solely-sequenced 4-20 mA output alerts:

Consider the consequences of the controller (TIC-37) shedding energy. Both 4-20 mA alerts will go lifeless, driving each valves to their fail-protected modes: absolutely closed. Now contemplate the consequences of air stress loss on each valves. With no air stress to function, the actuators will spring-return to their fail-protected modes: as soon as once more each management valves are absolutely closed. In each failure occasions, the 2 management valves constantly shut. The failure modes of each valves are nonetheless constant regardless of the character of the fault, however observe how this scheme permits each valves to fail in the identical mode if that’s what we deem most secure for the method.

As with all fail-protected system designs, we start by choosing the right fail-protected mode for every management valve as decided by the security necessities of the method, not by what we’d contemplate the best or best-to-perceive instrument configurations. Only after now we have chosen every valve’s failure mode will we select the opposite devices’ configurations. This consists of cut up-vary sequencing: the place and how we sequence the valves is a call to be made solely after the valves’ fail-protected states are chosen primarily based on course of security.

supply: Split-range Control | Basic Principles of Control Valves and Actuators | Automation Textbook

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