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Poor product quality or plant inefficiency can often be traced back to this control valve and how it is selected and maintained.
Control valves impact the bottom line of a plant in ways that may not be very clear. In most cases, control valves keep a regulated process variable, such as pressure, flow, level or temperature, as close as possible to a desired set point. The control valve manages this by compensating for the load disturbance in the process, and manipulating the regulated processvariable to the desired set point.
The control loop, in general, comprises of a controller, a flow sensor, and a control valve that changes the flow in the loop. The controller senses the error between the desired set point and the regulated process variable, and slowly changes its signal to the control valve, which then corrects the process variable.
Process cycling can be caused by internally created disturbances within the process, or through interaction from other control loops in the plant network that disturbs, and hence influences on the process variable. The control loop works in such a way that if there is no change to the desired set point, the controller would keep changing its output to the control valve. But if the regulated process variable goes past the desired set point, the controller would reverse its signal, causing the control valve todrastically move back in the opposite direction.
Unfortunately, control loops that fluctuate indiscriminately can result in an unstable manufacturing plant. A process variable that is not controlled in accordance to end product specifications could very well mean that energy, resource inputs or feedstock are being wasted. In essence, it is imperative that a plant be able to operate for long periods at a steady state.
These fluctuations in the process can also affect other major components in the plant, such as compressors and pumps, and age this equipment prematurely. This would degrade overall process reliability by wearing out these mechanical equipment, well ahead of their predicted time for maintenance.
To maximize the return on investment of a process control system, it is important to recognize that the control valve is a critical part of the control loop. And many of the problems, attributed to poor product quality or plant inefficiency, can be traced back to thiscontrol valve and how it is selected and maintained.
Rules of thumb
The sizing of a valve is very important if it is to render the performance it is expected of in a control loop. If it is undersized, it will not have sufficient capacity. If it is oversized, the controlled variable may cycle, and the valve seat and disc will be subject to damage because of the restricted opening.
Even though “science” lends a hand to the correct sizing and selection of a control valve, the methodology thus assumed has taken a “rules of thumb” approach that is generally agreed upon by many.
One of the primary considerations of selection is to define the system for which the control valve would be used for. Essentially, one has to know the medium that the valve will control, whether it is water, air, steam or some chemical compound. This is important because the medium’s specific gravity and viscosity may affect the size of the valve.
This leads to the maximum capacity that the valve could handle but other questions that may be relevantat this stage would include the inlet pressure and temperature at maximum load demand, the pressure differentialthat will exist across the valve under maximum load demand,and the maximum pressure differential that the valve must closeagainst.
The last couple of considerations are important because there is a trade off, in that larger pressure drops increase the operating cost, in terms of pumps, while smaller pressure drops increase the capital cost because a larger valve is required. However, the pressure differential across a control valve in operation will be the difference between the total available head and that required to maintain the desired flow through the valve.
It is, therefore, determined by the system characteristics, but in the interest of economy, the control valve pressure drop is kept as low as possible. But it must be emphasized that as the pressure drop across the valve is reduced, its ability to further increase flow can cease to exist.
Once this is known, the control valve can be selected based on the required flowing quantity of the process. However, the control valve must be selected to operate under several different conditions. Usually the normal flow and maximum flow are used in size calculations, but they should be based on actual operating conditions, whenever possible, without any factors having been applied to them.
Then the valve flow coefficient, or Cv, is calculated. Cv is a constant related to the geometry of a valve, for a given travel, that can be used to establish flow capacity. It is the number of gallons per minute of water at a temperature of 60 degrees Fahrenheit that will flow through a valve, with a one pound per square inch pressure drop. In general, the valve size should not exceed the line size, and if it is properly sized, it should preferably be one to two sizes smaller than the pipe size.
The Cv value is then used as a guide in the valve selection. But before a valve can be selected, the type of valve that will be used needs to be determined.
Flow characteristics
The flow capacity of a valve is related to the type of valve through its “inherent characteristics”. This is the term which is used to explain the relationship between the valve flow capacity and the valve travel when the differential pressure drop across the valve is held constant. It provides an elegant and systematic way of comparing one valve characteristic design to another.
Typical valve characteristics conducted in this manner are named Linear, Equal-Percentage, and Quick Opening. Linear has a flow capacity that increases linearly with the valve travel; flow capacity in Equal Percentage increases exponentially with valve trim travel – equal increments of valve travel would produce equal percentage changes in the existing Cv; and Quick Opening provides large changes in flow for very small changes in lift.
Therefore, when conditions of a constant pressure drop exist, the fluid flow through the valve then becomes only a function of the valve travel and the design of the valve trim. This implies that the trim design of the valve affects how the control valve capacity changes as the valve moves through its complete travel.
Hence, valve trims can be designed to meet a large variety of control application needs. Because many control loops are inherently non-linear, it is possible to compensate for this by selecting the appropriate control valve trim.
However, when valves are installed with pumps, piping and fittings, and other process equipment, the pressure drop across the valve will vary as the valve plug moves through its travel. In most applications, when the valve opens, the resistance due to the fluid flow decreases the pressure drop across the valve. Therefore, for purposes of process optimization, the installed flow characteristic of the entire process, including the valve and all other equipment in the loop, must be taken into consideration.
This installed flow characteristic is defined as the relationship between the flow through the valve and the valve assembly input, when the valve is installed in a specific system, and the pressure drop across the valve is allowed to change naturally, rather than being held constant.
In most cases, the installed flow characteristics of a valve would impact its inherent flow characteristics. For example, a linear inherent curve will, in general, resemble a quick opening characteristic, while an equal percentage curve will resemble a linear curve.
However, many valve designs, particularly rotary ball valves, butterfly valves, and eccentric plug valves, have inherent characteristics which cannot be easily changed. But most globe valves have a selection of valve cages or plugs that can be interchanged to modify the inherent flow characteristic.
The decision finally comes down to as to which valve to use for an application. However, there will be occasional exceptions but it is only possible, by means of a complete dynamic analysis to correctly determine the correctvalve that may be needed.
Valve types
Equal percentage flow characteristics are the most commonly used valve control. They are generally used on pressure control applications and on other applications where a large percentage of the pressure drop is normally absorbed by the system itself, with only a relatively small percentage available at the control valve. Valves with such characteristic should also be considered where highly varying pressure drop conditions are to be expected.
The valves best suited for this type of control are globe valves and butterfly valves, but for different types of applications. Globe valves provide for efficient throttling and accurate flow control. Buttheir disadvantages are that they can be relatively expensive, and are prone to high pressure drops.
By comparison, butterfly valves are inexpensive, with low pressure drops, and can handle large capacity flows with good control. However, they require a high torque for control, and are prone to cavitations at low flows. For these reasons, they are generally used for fully open or closed operations, where minimal trapping of the fluid is required in the line.
Both the globe and butterfly valves are also suitable when a linear flow characteristic is required. Such a characteristic is used in steady state systems where the pressure drop across the valve is expected to remain fairly constant.
Ball valves are also suitable for linear flow characteristic but they have poor throttling characteristics and are quite prone to cavitation. Therefore, they are recommended for fully open and closed operations, and where limited throttling is required. Their low cost, high capacity and good sealing properties with very low torque makes them ideal for high temperature liquids and slurries.
Because of the latter property, ball valves are also used in situations where quick opening flow characteristic is required. But if there is a situation where the valve is infrequently used, gate valves would be recommended. But gate valves cannot be used for throttling; they are subject to poor control and at very low pressure drops, they give rise to cavitation. However, they are used in a wide range of applications such as for oil, gas, air, slurries, heavy liquids, steam, non-condensing gases, and corrosive liquids, where a tight shut-off, high capacity and little resistance to flow is required.
Once the type of valve has been decided upon, it may be prudent to refer back to the valve charts and check the Cv, and the percentage stroke at the minimum flow. The point of reference would be whether the valve is more likely to operate closer at the maximum flow rates quite often or whether it is more likely to operate near the minimum flow rate for extended periods of time.
It must be emphasized it is quite difficult to find the “perfect” valve, and judgment plays a very important role in most cases. One of those judgments is applied to a concept known as gain. In general, gain is defined as the ratio of the magnitude of the output change of a given system or device to the magnitude of the input change that caused the output change, and for a valve, it is the change in flow to the change in stroke or travel.
Typically, the gain of the unit being controlled changes with flow. But a process loop would have been tuned for optimum performance at some set point flow condition. Therefore, if the flow changes the gain would change in the unit being controlled. But in theory, the inherent valve characteristic must compensate for this changing gain. If it does not, there will be a variation in the process loop gain, which might cause process instability, and makes process optimization that much more difficult.
Performance criteria
The goal of any business is to deliver a quality product at an acceptable price. And this can only be achieved by reducing process variability. Dead band, which is defined as the range through which an input signal can be varied, upon reversal of direction, without initiating an observable change in the output signal, is a major cause of process variability.
It is imperative that a valve reach a specific position as quickly as possible, for control to be effective. For example, when a load disturbance occurs in a process loop, the process variable deviates from the set point, and sets off a corrective action through the controller and back through the process.
However, an initial change in controller output that produces no corresponding corrective change in the process variable till some time has passed can drastically affect loop performance.
This time interval, to progress through the dead-band, in which no response of the system is detected following a controller input, is known as dead time. This implies that a quick response to controller signal changes is one of the most important factors in providing effective process control, because by correlation, if a control valve assembly can quickly respond to these controller changes, process variability will be improved.
By inverting this reasoning, one can also surmise that the time measured from the start of the controller input signal change, to when the output reaches 63 percent (by definition) of the corresponding change, as the valve response time.
However, by conjecture, this would also include the dead-time, and the time it takes the control valve actuator to start moving. In most cases, dead time is known as static time while the latter is generally referred to as dynamic time.
This categorization is important because it then becomes critical to measure dynamic time or performance of a valve under fluid flow conditions. Thus, this enables the change in process variable to be compared to the change in valve assembly input signal.
But it must be remembered that control valve assemblies can be a primary source of dead band in an instrumentation loop. There can be a variety of causes for this, such as friction, backlash, or valve shaft wind-up, amongst others.
Therefore, it must be emphasized that the final control element, which is the valve, actuator, and positioner, must achieve good process control under dynamic conditions. That is, the control valve assembly must be optimized or developed as a unit. Valve components that are not designed as a complete assembly typicallydo not yield the best dynamic performance for control purposes.
Matching components
More than one isolated parameter must be considered to reduce process variability. One such area is the valve actuator and positioner design. Both these two pieces of equipment greatly affect the static performance and the dynamic response of the control valve.
The positioner is a type of air relay, used to increase or decrease the air pressure operating the actuator, until the valve stem reaches the position called for by the instrument controller. It acts to overcome hysteresis, packing box friction, and valve plug unbalance due to the pressure drop across the valve. It, therefore, assures the exact positioning of the valve stem in accordance with the controlleroutput.
However, it is important that the positioner and actuator are carefully matched to minimize the total valve response time. For example, in a pneumatic valve assembly, the positioner must have a high dynamic gain to minimize the dynamic time of the valve assembly. This means that if a quick valve response time is required,the positioner relay must correspondingly have the ability to supply a large volume of air to the actuator.
It can be conventionally rationalized the same effect can be realized by maximizing the positioner’s dynamic gain, and minimizing the actuator volume. However, from a stability point of view, this could be very dangerous.
But when it comes to friction, spring-anddiaphragm actuators contribute less friction to the control valve assembly than piston actuators. The latter, therefore, requires more maintenance, but in general, the type of actuator used has an impact on control valve assembly friction. It must be remembered that friction is a major cause of dead band in control valves. It is generally caused by high seal friction and poor drive train stiffness. Because of this, the valve shaft winds up and does not translate motion to the control element.
On the other hand, if there is discontinuity of motion when the controller changes direction, the resultant slack, or looseness of the mechanical connection, is commonly referred to as backlash. Rack-and-pinion actuators are particularly prone to dead band caused by backlash, and it is a difficult phenomenon to eliminate entirely.
But the point of it all is that valve specifications such as dead band, dead time and response time, amongst others, address the real dynamic performance of the valve, and this is quite critical if true process optimization is to be achieved.
The other important point is that parts of the control loop cannot be evaluated in isolation; the entire loop performance must be coordinated under actual process conditions, to achieve the best dynamic performance from the loop. Because it is only through the consideration of these factors that one can have a dramatic impact on the economic results of an operating plant.
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Tips for Maintenance
Proper attention to maintenance can positively impact control valve performance.Nwaoha Chikezie provides some useful tips.
Control valves, properly used and maintained can improve process efficiency and reduce costs. When making repairs on valve types, use the available manufacturer’s technical manuals. As soon as a leak is detected quickly determine the cause, and then apply the corrective maintenance.
Maintenance may be as simple as tightening a packing nut or gland. A leaking flange joint may need only to have the bolts tightened or to have a new gasket inserted. Scale if allowed to collect, will cause leakage. Loose hangers permit sections of a line to sag, and the weight of the pipe and the fluid in these sagging sections may strain joints to the point of leakage.
There are generally five aspects to control valve maintenance:
1. Understanding Markings & Materials Valves are identified by markings inscribed on the body, rims of the hand wheel, or to the adjacent piping. Examples of such markings are color codes, ratings, arrows, etc. Arrows cast on the body of the valves are used to indicate the flow path through the valve. A code on the operating lever indicates the type of fluid flowing through it, e.g. the color code for hydraulic is orange. Valves are also made of different materials because flowing fluid such as gases and liquids are often corrosives, or highly contaminated. For maximum efficiency and control, valve materials must fall into the same category of the fluid flowing through it. '2. Correct ConnectionProper valve connection procedure should be taken into due consideration during installation on piping systems. In otherwords, the pipe should be properly aligned before the installationof a control valve. The valve must not be relied upon to pullthe piping into alignment. If this is not adhered to, valves willnot operate smoothly, because the misalignment stresses, causestwisting of the valve body. Attention must be paid in particularto the installation and operation of diaphragm valves whenthey have to operate within specified time windows. This is toensure no hang-ups due to valve body twisting caused by pipingmisalignment.
3. Ensure Valve Is Clean In good process operations, prior to installation, control valves must be free from all dirt, and foreign matter. In some cases, the valve and the line should be blown out with clean instrument air, steam, or clean water. This removes grit and dirt that might interfere with valve operation or shorten control valve life. The type of blow out system applied is dependent on the type of fluid flowing through the piping system.
4. Proper Storage Control valves should be kept in a safe place, away from weather, dirt, etc. Valves must be in protective covers and kept in place until the time of installation. In some cases, control valves are shipped with the disc fully open, and others fully closed. In any case, the disc must be left in the shipping position if possible, until installation is completed. This is aimed at protecting the seating surfaces.
5. Inspection upon Receipt Whenever a valve is received, thorough inspection should be carried out, because this is always the critical time in the management of the valve. Upon receipt, the control valves must be inspected for in-transit damage. And inspection should be focused on the valve actuator, valve stem, and valve ends etc. It is also advisable to dismantle the valve when received, for the inspection of its internals. This is done to remove any anti-movement restraints installedfor protection during transit.
Troubleshooting
The tables overleaf provide guidance on troubleshooting three common control valveproblems.
Nwaoha Chikezie has a degree in Petroleum Engineering and is a member of the NigerianGas Association.
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