PCI10202

Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for Saudi

Control Loop Elements And

CONTENTS PAGE

EXPLAINING THE GENERAL SELECTION CONSIDERATIONS

FOR PROCESS INSTRUMENTATION ... 1

Selection Considerations as Specified by Saudi Aramco Engineering Standard SAES-J-003, Basic Design Criteria ... 1

Other Selection Considerations... 6

Primary Elements/Transmitters ... 6

IDENTIFYING THE MOST COMMONLY USED TRANSMITTERS... 9

Role of Transmitters ... 9

Transmitter Types... 9

Transmitter Components ... 11

Simplified Transmitter Schematics ... 12

Pneumatic Type ... 12

Electronic Type ... 13

Resonant Wire Type ... 14

DESCRIBING THE MOST COMMON FINAL CONTROL ELEMENTS USED IN PROCESS APPLICATIONS... 18

Control Valve Terminology ... 18

Valve Body Types... 20

Globe Styles... 20

Globe Style Three-Way Valves... 23

Angle Valves ... 25

Cage Valves... 26

Butterfly Valves... 28

Ball Valves ... 30

Signal Converters ... 38

Current to Pressure Signal Converters, I/P ... 38

Pressure to Current Signal Converter, P/I ... 39

Analog to Digital, A/D, or Digital to Analog, D/A ... 40

Volume Booster ... 41

Valve Positioner ... 43

There are various selection considerations involved in the proper choice of instrumentation for a process control loop. The general objective, within some constraints, is to select the best possible instruments for the given application. This should minimize future problems substantially and make the control of the process as easy as possible. The choice of instrument quite often is a compromise or tradeoff between the application requirements and the various attributes and limitations of the hardware available.

Selection Considerations as Specified by Saudi Aramco Engineering Standard SAES-J-003, Basic Design Criteria

There are design requirements for each type of instrument covered by individual Saudi Aramco standards and specifications. SAES-J-003 is the Engineering Standard which, together with its references, specifications, codes, forms and drawings, covers the requirements for the selection, design and application of process instruments and systems.

SAES-J-003 states that the selection of instrumentation should normally take into consideration the following items:

Some of these terms are straightforward and require no further explanation. Others might benefit from some further explanation.

Reliability - Infers the probability that a component, piece of equipment or system will perform its intended function for a specified period of time, usually operating hours, without requiring corrective maintenance.

Accuracy - This is one of the most difficult terms to understand due to various interpretations. It is poorly defined and means different things to different people. The term accuracy actually implies inaccuracy or error.

The following accuracy related information is from ISA S-51.1, "Standard Process Control Terminology."

Accuracy - In process instrumentation, degree of conformity of an indicated value to a recognized, accepted standard value, or ideal value.

Measured accuracy - The maximum positive and negative deviation observed in testing a device under specified conditions and by a specified procedure. See Figure 1.

Note 1: It is usually measured as an inaccuracy and expressed as accuracy.

Note 2: It is typically expressed in terms of the measured variable, percent of span, percent of upper range value, percent of scale length, or percent of actual output reading.

Accuracy rating - In process instrumentation, a number or quantity that defines a limit that errors will not exceed when a device is used under specified operating conditions. See Figure 1.

Note 3: Accuracy rating includes the combined effects of conformity, hysteresis, dead band and repeatability errors. Refer to the glossary for definition of these terms. The units being used are to be stated explicitly. It is preferred that a + sign precede the number or quantity. The absence of a sign indicates a + and - sign.

MAXIMUM ACTUAL POSITIVE DEVIATION ACTUAL DOWNSCALE CALIBRATION CURVE HIGH OR POSITIVE PERMISSIBLE LIMIT OF ERROR SPECIFIED CHARACTERISTIC CURVE MEASURED ACCURACY ACTUAL UPSCALE CALIBRATION CURVE INPUT 100% SPAN LOW OR NEGATIVE PERMISSABLE LIMIT OF ERROR 0 OUTPUT MAXIMUM ACTUAL NEGATIVE DEVIATION ACCURACY RATING ACCURACY RATING FIGURE 1

Accuracy rating can be expressed in a number of forms. The following five examples are typical:

It may help our understanding of accuracy if we look at the term from a process instrumentation and control point of view. What we call accuracy is actually error and can be broken into its components of precision and bias errors. Thus we can describe the accuracy of a device at a point as follows:

Accuracy (Error) = Precision Error + Bias Error

where,

Precision relates to the ability of a measuring device to give or repeat the same reading or output for the same input. Precision is a characteristic of the particular instrument And can not be improved for a given device. If not satisfactory, the only alternative is a more precise instrument, usually at an additional cost.

To illustrate these terms let us look at the following example. Assume in this example we throw darts at a target with the following results:

HIGHLY ACCURATE DEVICE, GOOD PRECISION

WITH MINIMUM BIAS

PRECISE WITH BIAS ERROR

POOR PRECISION SMALL BIAS ERROR

A B C

Looking at the results and relating the example to process instrumentation it is obvious that device (A) is the best to use and device (C) is the worst to use. In process work most of the time we deal with type (B) devices. These are precise or repeatable devices with some bias error. Operators learn to make good product without the need to know the absolute accuracy. They can deal quite well with repeatable information produced by type (B) devices. The only exception is in custody transfer applications (buying and selling) where additional effort is put in to eliminate the bias errors and improve the overall accuracy.

Other Selection Considerations Primary Elements/Transmitters

Range. The region between the limits within which a quantity is measured is the range

of that measurement. It is expressed by stating the lower and upper range values. The selected primary element must have sufficient range to measure the controlled variable throughout its operating range.

Span. The measurement span is the algebraic difference between the upper and

lower range values. The selected primary element must have the required span to cover the entire measurement.

Minimum Span . The minimum span of measurement that the primary element can be

used to measure within its accuracy rating. The selected primary element must satisfy the minimum span requirement of the controlled variable.

Maximum Span . The maximum span of measurement that the primary element can

be used to measure within its accuracy rating. The selected primary element must satisfy the maximum span requirement of the controlled variable.

Rangeability (Turndown) . In flow applications, rangeability is the ratio of the

maximum flow rate to the minimum flow rate within the stated accuracy rating. The selected primary element must have sufficient rangeability or turndown to satisfy the rangeability needs of the controlled variable, i.e. this particular specification helps you to determine at what point during a startup the information of the primary element is accurate enough to switch a loop into automatic operation.

Zero Elevation and Suppression . The range at which the zero value of the

measured variable is not at the lower range value. (Check Glossary.) The selected primary element has to satisfy the zero value of the measured variable.

Response Time - An output expressed as a function of time, resulting from the

application of a specified input (step) under specific operating conditions. The response time, in seconds or minutes, gives us information regarding how quickly the primary element responds to a specific input. A good primary element should have a fast (short) response time. An accurate primary element is useless in an industrial application if it takes too long to respond to a process input.

Time Constant . This is a specific measure of a response time. It is the time required

for a first order system to reach 63.2% of the total change when forced by a step.

t = 0

τ

τ

τ

τ

τ

It takes the output of the device approximately five time constants (5τ) to reach its final steady state value. The time constant τ is a function of the resistance (R) and capacitance (C) associated with the measuring device. τ = RC =Time Constant.

Note: For the output of a first-order system forced by a step or an impulse, is the time required to complete 63.2% of the final steady state value for a step input. In higher order systems, there is a time constant for each of the first-order components.

Characteristic Curve (Input-Output Relationship) . A curve that shows the ideal

value of an input-output relationship at steady state. This curve shows the output variable of an element or device as a function of an input variable. The selected primary element forms one of the elements in a loop and its input-output relationship will dictate the overall loop linearity, i.e. if the input-output relationship of the element is linear, then the gain of the element is constant and does not contribute to loop gain non-linearity. On the other hand, if the primary element's input-output relationship is not linear, then its gain is not constant with consequences on overall loop performance. PCI 102.05, Steady State Gains, addresses these issues.

Reproducibility . The selected primary elements should be highly reproducible. This

means that there should be a closeness of agreement among repeated measurements of the output for the same value of the input made under the same operating conditions over a period of time, approaching from both directions.

Reproducibility includes the effects of hysteresis, dead band, drift and repeatability.

Noise. In process instrumentation noise is an unwanted component of a signal or a

process variable. Some measurements are inherently noisy, subjecting the primary elements to undesirable levels of noise. Noisy process measurements are not desirable and in certain control applications not acceptable. Noise should be eliminated or minimized to acceptable levels, either through noise filtering techniques or better instrument applications techniques. In addition, the measurement span should be selected in such a way that the signal noise to signal span ratio is at an acceptable level, i.e. in boiler drum level applications the span of level measurement is typically 30 in. of water. With this span the signal noise to signal span ratio is usually in a more acceptable level. If the span was 10 in. of water the effects of noise would be three times as much for this span making the noise more difficult to handle.

Role of Transmitters

Sensing a process variable is only the first step in process plant control. The information obtained must be used to make necessary process changes to adjust for external forces, such as increased throughput demands.

It is impractical to locate all control instruments in a plant near the process. One operator cannot monitor information scattered around the plant to make wise operating decisions. And, placing an operator at each controller would be an inefficient use of valuable manpower.

To complicate matters, it is not easy to bring most measurements to a certain location in their original form.

The process fluids could be at high pressure or of dangerous chemicals which would create a hazardous condition in case of rupture or leakage.

Another difficulty is regarding the response time of an element with several hundred feet of capillary tubing. The additional dead time or lag produced by this system will not allow effective control.

These difficulties are overcome with the introduction of signal transmission systems. This allows the centralization of control operations since signals can be sent longer distances and are of a uniform standard value.

Transmitter Types

Transmitters as purchased from vendors usually consist of two major components. The body of the transmitter (bottom works) contains a measuring element (transducer) that converts process conditions or parameters into motion, force or some other parameter. The transmitter head (top works) contains pneumatic, electronic, or microprocessor-based components to convert the transducer signal into a standard signal suitable for transmission to other locations (centralized) in the plant. These conversions allow the process parameters to directly relate in a known function (linear or square root, for example) to the output signal.

Smart transmitters are essentially the same as conventional transmitters, except that the microprocessor-based transmitter head has certain additional capabilities. This may include remote calibration checks, self diagnostics, configuration and reranging with a hand-held communicator (at the transmitter, in a field junction box, or from the control room).

In process instrumentation most transmission systems are either pneumatic or electronic devices working with analog (continuous) signals. Recent introduction of digital transmitters provide an alternative and might change the picture in the future.

The schematic below shows the use of a differential pressure transmitter using an orifice as a primary device to determine flow in a pipeline. The transmitter converts the d/p into a usable signal and transmits it to a remote controller. The controller manipulates a control valve in the pipeline to adjust the flow rate.

FT FIC 1 I P PROCESS TRANSMITTER PRIMARY ELEMENT FLOW INDICATING CONTROLLER SIGNAL TRANSMISSION LINES r

Transmitter Components

Transmitter top works consist of three basic components. These are:

1. Detector - The function of the detector is to detect the process signal input or primary element output.

2. Amplifier - The function of the amplifier is to amplify the detected signal.

3. Negative Feedback - The function of negative feedback is to balance (stabilize) the mechanism allowing it to accept changes in the primary element's output.

These three components are shown below as part of a simplified pneumatic transmitter. AIR SUPPLY PROCESS SIGNAL INPUT (PRESS,TEMP, FLOW, LEVEL ) OUTPUT ( 3 - 15 PSI ) INPUT TO CONTROLLER AIR SUPPLY TO AMPLIFIER AND DETECTOR TYPICALLY 20 PSI AMPLIFIER OR RELAY RESTRICTOR FEEDBACK BELLOWS ( STABILIZER ) FLAPPER

d =

Simplified Transmitter Schematics Pneumatic Type

The actual force-balance pneumatic transmitter's simplified schematic may look like this:

Electronic Type

The simplified scheme of an electronic force-balance differential pressure transmitter may look like this:

Resonant Wire Type

The resonant frequency of the oscillator circuit changes as a function of wire tension. The wire in this circuit is represented by the resistance.

VIBRATING WIRE PRESSURE TRANSDUCER

OSCILLATOR DIAPHRAGM BACKING PLATE SI OIL FILL HIGH PRESSURE SIDE WIRE ATTACHED TO DIAPHRAGM HERE WIRE ATTACHED TO BACKING PLATE HERE LOW PRESSURE

It is impossible for manufacturers to make transmitters customized to meet exactly the user process conditions regarding range and span. This is not practicable and the price most likely not acceptable to the user. Instead, manufacturers make instruments to serve a broad range of conditions and applications.

The user or manufacturer, with appropriate information regarding the span and range of measurements, must make adjustments to make sure the instrument meets the process requirements. This act of ascertaining outputs of the transmitter corresponding to a series of values of the quantity which it has to transmit is called calibrating the instrument.

In ordering the transmitter the user, with manufacturer's assistance, must make sure the particular device has the correct specifications to meet the job requirements. Accuracy, rangeability, area classification, minimum and maximum spans, and process static pressures are some of the specifications of interest.

The calibration procedure usually involves the following adjustments:

Zero Adjustment - Adjust a zero screw so that at 0% measurement input the output is 0% (usually 4 mA DC or 3 psi).

Span Adjustment - Adjust the span of the instrument so that at 100% measurement input, the output is 100% (usually 20 mA DC or 15 psi).

In newer transmitters this could be the end of the calibration procedure. In some older transmitters, however, the zero and span adjustments interact, so both adjustments have to be checked iteratively until making a change in one adjustment does not affect the other. A second concern while calibrating is to check the transmitter's linearity by putting a 50% input to see if the output goes to 50%. A non-linear transmitter requires either further calibration, in the case of older, motion-balance type pneumatic transmitters (angularity adjustment), or requires troubleshooting, in the case of force-balance pneumatic transmitters which are inherently linear. Most electronic transmitters are either inherently linear or have built-in linearization functions.

TYPICAL TRANSMITTER CALIBRATION CURVE 0% 25% 50% 75% 100% 0 PSI 0°F 100 PSI 500°F INPUT 200 PSI 1000°F 20 16 12 8 4 OUTPUT mA dc NOTE: 4 mA = " LIVE " ZERO An Actual Transmitter Curve Might Look Like This

" Ideal " Transmitter curve

Transmitters can be calibrated to be either direct acting or reverse acting. The preferred choice in most applications is to have a direct acting transmitter. This is much more comfortable for the operator since a change of output directly relates to a change of input. If the transmitter output increases with an increase in the measured process parameter, the transmitter is direct acting. If the output decreases with an increasing process parameter, the transmitter is reverse acting. Keep in mind that the transmitter, as one of the elements of the loop, contributes to achieving a negative feedback loop requirement of an odd number of reverse acting elements.

The gain of an element as you recall is a vector made up of steady state and dynamic components.

G _{T = KT GT∠∅T}

Transmitters are fast acting devices with not much of a dynamic gain concern, that is, GT ∠∅T ≈ 1 ∠0°. This implies that the transmitter gain is essentially a steady state

concern. PCI 102.06 gives a thorough treatment of this subject. The steady state gain of the transmitter was defined as the change of output divided by the change in input at a steady state. The gain of the transmitter could have dimensions and is the slope of the input-output relationship as shown below.

0% OUTPUT 100% 0 INPUT 200°F 20 mA 4 mA

T = SLOPE OF CURVE = CONSTANT

= ² OUT ( 20 - 4 ) mA ² IN ( 200 - 0 ) °F = 16 mA 200°F T = 100% INPUT SPAN 100% 200°F 1% 2°F = = OR = K K

If the transmitter is recalibrated to a new span of measurement, i.e. 0 to 100°F range and a span of 100°F. The gain will change as follows:

KT = _{INPUT SPAN =} 100% _{100°F =} 100% 1%_°F

This gain is twice as much as the previous gain and will double the loop gain with the potential of making the loop unstable.

The controller gain must be backed-off to accommodate the additional transmitter gain and maintain loop stability.

Remember that the open loop gain is the product of all the elements' gains forming the loop and it dictates the loop response.

There are various selection considerations regarding the choice of the final controlling element used in regulating the supply to the process. Most control loop problems evolve from and are somehow related to the final controlling element selected and sized for the particular application. On some processes the final controlling element could be a damper or the speed of a motor but by far the most common final controlling element is the control valve.

The control valve is by far the weakest element in the loop. There have been tremendous advances through the introduction of microprocessors to improve the performance of the transmitters and process computers. These innovations have had little effect on the the valve performance.

There are various considerations involved in the selection of the valve for the particular application. Before we get into the selection considerations let us look at the common control valve types along with the terminology.

Control Valve Terminology

A control valve is an engineered variable flow restriction, by means of which flow rate, pressure, temperature, liquid level, and composition are maintained at desired values in the process. The input signal to the control valve is the output signal from a controller. The control valve is constructed such that the stem lift (plug position) is related to the input signal. The relationship between stem position and the area open for flow is called the valve characteristic. This relationship is extremely important in determining the suitability of a given valve for a given service, and therefore receives much attention from control engineers and valve manufacturers.

The valve, as seen, can be divided into two major portions. The upper portion forms what is referred to as the valve actuator, motor, or operator. It is usually a spring loaded diaphragm actuated by the controller output either directly in pneumatic applications, or through current to pneumatic transducers (I/P) in electronic or digital applications.

The bottom portion of the valve (the valve body) contains a valve plug which is positioned by the diaphragm actuator. The valve body-plug combination is essentially a variable restriction or orifice. The valve characteristic depends on the shape of the opening between the valve body and plug and the pressure drop variations caused by the flow.

Valve Body Types

The two general categories of control valves are linear actuated and rotary actuated. Globe, angle and cage valves are linear actuated, while butterfly and eccentric plug and ball valves are of the rotary type.

Globe Styles

The most common control valve body style is the conventional globe type, although it is declining in popularity in favor of the cage and rotary valves. New installations use fewer globe styles because of economic and performance advantages of these other control valves. The globe control valve can be either single or double-seated.

Single-Seated - Single-seated valves are usually employed when tight shut-off is

required, or in sizes of 1 inch and smaller where the unbalanced forces acting on the valve stem is unimportant as a factor in actuator selection. Tight shutoff in this case usually means that the maximum expected leakage is less than 0.01 percent of the maximum valve C_v when subjected to an air test with 50 psi upstream and 0 psig downstream pressure. Single-seated valves may have a top or top-and-bottom guided construction; that is, the valve stem is guided within the lower portion of the valve bonnet, or top closure, and on the bottom of the body. Single-seat design also allows a somewhat higher flow capacity than top-and-bottom guided valves for a given orifice size. Saudi Aramco Engineering Standard SAES-J-700 specifies that single-seated globe valves shall be used in shut-off service and gas compressor recycle service.

Double-seated - Double-seated valve is generally top and bottom guided. Here

practical leakage approaches 0.5 percent of the rated Cv because it is nearly impossible to close the two ports simultaneously; particularly, if thermal expansion causes additional distortion after the valve is installed. However, the advantage of double-seated construction lies in the reduction of required actuator forces, because the hydrostatic effects of the fluid pressure acting on each of the two seats, tend to cancel out opposite forces. Double-seated valves have upper and lower ports of different diameters (to allow withdrawal of the smaller plug through the larger port). This contributes to an unbalanced force condition which must be corrected by the actuator. Complete cancellation of these forces is not possible because of the hydrodynamic effects of the fluid that passes the plug contour at high velocity. Fluid passing the lower seat (which tends to close the plug) has a further tendency to "suck" the plug into the seat, thereby creating a dynamic imbalance between this force and the differential pressure acting across the upper-plug area.

Globe Style Three-Way Valves

Three-way valves are a design extension of a typical double-seated globe valve. Here a distinction is made between a valve used for diverting service and one used for mixing or combining service. A typical three-way valve, as shown, has a modified double-ported body with the lower plug seating opposite from the normal shut-off position. Either a direct or a reverse actuator is used, again depending upon the desired fail-safe action. Note that the plug has additional rib guiding in the orifice to compensate for the omission of the lower guide post. The body has an internal bridge to separate the right hand and the lower outlet. A diverting valve might be used for a heat exchanger bypass where the heating medium enters Port "C." Part of the fluid leaves Port "U" to bypass the exchanger. The remaining portion of the fluid then goes to the heat exchanger, through Port "L," to heat an independent process stream and then rejoins the bypass fluid stream from Port "U."

The drawing below illustrates a three-way valve used for combining or mixing service. Here, two separate fluids entering Port "L" and Port "U," respectively, are combined in a desired ratio and leave through the common Port "C." The ratio between the amount of fluid coming through either Ports "U" or "L" is determined by the plug position. An upward movement of the plug decreases the flow passing Port "L" and at the same time increases the flow area for Port "U." Note again that skirt guiding is employed to provide additional guiding in line with the upper guide post.

Saudi Aramco Engineering Standard SAES-J-700 specifies that the flange for the common port shall be marked "common" or "open".

Angle Valves

Angle valves generally are single-seated valves with special body configurations. This allows them to additionally satisfy the need of an elbow to suit specific piping designs. The streamlined interior passage of angle valves tends to prevent an accumulation of solids on the body wall. This type of valve has been used for coking hydrocarbons and high pressure-drop service. The underlying thought was to keep turbulence that is created by the throttling process away from the valve's internal parts. However, in doing this, the energy conversion takes place in the downstream pipe, which may lead to severe pipe vibration and noise. Another characteristic is the relatively high pressure recovery obtained with the streamlined flow pattern. High pressure recovery means a low cavitation index and occurrence of cavitation under low or moderate pressure drops with liquid media. Angle valves may also be used in cases where the piping layout does not allow the installation of a globe valve and may be used for handling certain erosive fluids, such as abrasive catalyst material. In the latter, a discharge into the outlet pipe (with flow direction tending to close the plug) prevents erosion of the inside of the valve housing. If cavitation or flashing is inevitable, an angle valve may be arranged to discharge directly into a vessel or other enlarged fluid volume, thus avoiding damage to internal valve parts.

An additional advantage of the angle valve is the self-draining feature which is of value when handling certain dangerous fluids such as corrosive or radioactive liquids. This allows for the process fluids to be totally drained between process steps or prior to start of the next batch.

Saudi Aramco Engineering Standard SAES-J-700 specifies the use of only single-seated angle valves.

Cage Valves

Cage valves are a variation of the single-seated globe valve and the most often specified process control valve due to substantial advantages in typical globe valve applications. An important economic advantage is lower cost over the globe valve for the same flow capacity. So called "top entry" or cage valves have the advantage of trim removal. The drawing below shows a typical top entry valve with balanced, single-seated trim. Valves of this type usually have streamlined body passage to permit increased flow capacity. The inner valve parts, often referred to as "quick change trim," can easily be removed after removing the bonnet because of the absence of internal threads.

The cage and seat ring are sometimes designed as one piece. The valve plug then assumes the form of a flat disk, The valve flow characteristic is achieved by shaped windows usually cast in the circumference of the cage,

The valve plug can also be made as a piston having a hollow internal passage that permits the fluid pressure to communicate to both sides of the plug. A sliding seal, located in a groove near the top of the piston seals the upper plug area against the outlet portion of the valve. This balanced design, as shown below, tends to cancel out the hydrostatic forces acting on the plug and leads to a great reduction in the required actuator forces. However, as in double-seated valves, a substantial increase in seat leakage must be tolerated.

As mentioned previously, the biggest advantage of the top entry valve is the ease of maintenance. The valve trim can be replaced, if necessary, without removing the valve body from the line. This is one of the reasons why this particular valve type has gained wide acceptance since its recent introduction.

CAGE VALVE SINGLE SEAT BALANCED

The inherent flow characteristics of the valve can be changed by using cages with different openings. This allows inherent flow characteristics such as linear, equal percentage and quick opening.

Butterfly Valves

The most common type of rotary valve used for control is the butterfly valve shown below. The typical application range is in sizes from 3 inches up to 72 inches, for low or moderate pressures, or on unusual applications involving large flows at high static pressures but with limited pressure drop.

A typical butterfly valve, in throttling service, is normally limited to a 60° opening. In all metal construction, the vane-to-body clearance in the closed position gives a leakage flow that is generally equivalent to the leakage of double-seated valves (0.5 percent of the rated C_v). Many design modifications are available which minimize leakage, including angle-seating and the use of piston rings. Because of the simple body design, elastomer inserts can be adapted to give a tighter seal within the temperature limits of the insert material. Typical linear materials are Neoprene, Buna-N, or for high temperature and special chemical services, Viton A. Actuator force requirements are dictated by a combination of two factors: the friction load imposed by side loading on the shaft due to the differential pressure, and the dynamic torque induced by the flow around the vane. The dynamic torque acts in the closing direction and, in the conventional design, reaches a maximum at about 70 degrees open.

In designs recently introduced, special shaped vanes (Fisher-Fishtail and Masoneilan Mini-Torque) are used to reduce the dynamic torque and to permit 90 degrees operation for increased capacity on certain installations. Another design modification involves an eccentrically mounted vane to permit, for example, tight closure on cryogenic fluids using a seal of Teflon or Kel-F, which may be mounted either within the body or on the circumference of the vane.

The most common body design is the flangeless "wafer" type for bolting between line flanges. ANSI body ratings are used but the valves are also rated for maximum pressure drop in the closed position and in the 60° open position. This valve does not have standard ISA or API face-to-face dimensions.

Saudi Aramco Engineering Standard SAES-J-700 specifies that butterfly valves in hydrocarbon service shall be flange or lug-type design. Wafer-type design butterfly

Design of the rotary shaft and its supporting shaft bearings are important in assuring the success of butterfly valves in control service. Bearing materials range from reinforced Teflon for very low to moderate temperatures and pressures, up to combinations of the hard-facing alloys that permit use at temperatures well above 1000°F. For many years, outboard bearings were specified almost exclusively. These are located outside the rotary packing box and were not in contact with the process fluid. This simplified the bearing design, but resulted in eccentric loading on the packing. The recent trend has been to use inboard bearings because of the availability of wear resistant and corrosion-resistant materials. A variety of bearing materials are available compatible with different process fluids.

Properly selected, the butterfly valve, in sizes 3 inches and above, generally offers the advantages of simplicity, low cost, light weight, and space saving, in combination with good flow control characteristics. For moderate temperatures and pressures, the elastomer-lined valve includes the possibility of tight shutoff. In selecting the valve, consideration must be given to the possibility of cavitation on liquid flow due to the high pressure recovery coefficient; the possibility of damage due to water hammer with fast closing in liquid service; and compensation for the effect of pipe reducers in computing the valve capacity because of the high basic Cv rating.

Ball Valves

Another common rotary shaft valve is the ball valve. Ball valves use a full sphere or a portion of a spherical plug that controls the flow of fluid through the valve body. Although initially designed for manual operation requiring tight shut off, their durability along with their ability to handle high-capacity flows has made them a good candidate for certain process applications.

Ball valves are supplied in two basic types, throttling ball valve supported entirely by seats and the characterized ball valve (hollowed out spherical segment) that turns on two short shafts similar in design to those of a butterfly valve. The inherent flow characteristic of the full ball valve is equal percentage while the characterized ball is normally linear. Throttling ball valves are rated for high pressure drops to 3,000 psi and characterized ball valves are rated considerably lower to 300 psi. Ball valves have a high flow capability for a given size. Consequently, ball valves are often about one-half the size of the pipe they are installed in. While reduced size is an economic advantage, it should be noted that this valve reaches choked flow sooner and is prone to cavitation problems at lower delta p's since it is a high recovery valve. Ball valves of larger size than required by flow calculations may be needed to avoid cavitation due to the inherent high recovery of the valve.

The major problem area with ball valves concerns the seating surface of the ball or segment and the surface of the ball itself. Particles entrained in the fluid stream can become lodged between the ball and its seating surface causing rapid wear or erosion of special coatings applied to the ball to prevent fluid corrosion. A practical method of limiting ball surface erosion is to use chrome plating on the throttling ball or contoured ball. This also improves its tight shutoff capability.

Eccentric Plug-Type Ball Valves SHAFT CENTER CLOSED 50° OPEN PLUG CENTER SEAT

ECCENTRIC PLUG - TYPE CONTROL VALVE

The ball valve can be characterized by using a segment of the ball. The plug (ball segment) operates on an eccentric path as shown above. The popularity of this valve has increased substantially over the last few years. A typical version of this valve manufactured by Masoneilan, Inc. is shown above. It is supplied without flanges and is mounted between pipe flanges. Face-to-face dimensions are non-standard and should be handled for mounting purposes like the butterfly valve. The normal rotary stroke is 50 degrees and an essentially linear flow characteristic is obtained with a C_v rating similar to a high capacity cage valve. Operating torque is low due to the eccentric motion of the spherical face. The valve is suitable for flow in either direction. Seating action is positive and sliding seal problems associated with conventional ball valves is eliminated. The rotating plug is constructed of silicon carbide and is unaffected by erosive materials that might be encountered in most services. One disadvantage is that the valve must be removed from the piping to replace the seat.

The low cost of this valve type for a given flow capacity, when compared with more conventional designs, is undoubtedly responsible for its widespread use. An extra advantage of high rangeability adds to the incentive for eccentric plug valve application.

Control Valve Application Considerations

Saudi Aramco Engineering Standard SAES-J-700 states the following regarding the design and application of control valves.

Control valves shall not be used as emergency shutdown (ESD) valves nor as emergency isolation valves (EIV). Control valve positioning may be included in ESD or EIV logic. (e.g. for venting or draining equipment) when failure of the control valve would not increase the severity of the emergency situation.

Selection of control valve design shall be based on application, operating conditions, installation requirements, and economic considerations. Refer to SAES-L-008, Selection of Valves, for general valve selection criteria.

The following valve designs may be considered: globe valves (2- and 3-way), angle valves, ball valves, butterfly valves, axial flow valves and rotary plug valves.

Design guidance may be obtained from the Saudi Aramco Design Practices Manual and the references listed therein. Mandatory design requirements are listed in the following sections.

The following is a list of some of the selection considerations regarding the choice of the valve. Most of the relevant terms are thoroughly discussed in PCI 103. Our interest is to look at a few select items that affect loop performance.

Valve Body Material

End Connections and Ratings Valve Trim

Valve Trim Material

Flow Capacity Required: Minimum and Maximum Flows Sizing, Cvmax, Cvmin

Pressure Information on the Valve Maximum Outlet Pressure Minimum Outlet Pressure Maximum Inlet Pressure Minimum Inlet Pressure

Pmin and Pmax Across the Valve Pressure Drop at Normal Flow Pressure Drop at Tight Shutoff Valve Shelf (Inherent) Characteristics Installed Valve Characteristic

Rangeability (Shelf and Installed) Dynamic Considerations

Cavitation Flashing

Fluid Properties: Temperature, Viscosity, Specific Gravity, Type of Fluid Valve Auxiliaries

Valve Hysteresis Valve Positioners Power Loss

Let us investigate some of these items as they relate to control loop concerns.

Control Valve - A final controlling element, through which a fluid passes, which adjusts the size of flow passage as directed by a signal from a controller to modify the rate of flow of the fluid.

Valve Action - The valve is one of the elements in the loop and its action direct or reverse affects the choice of controller action for negative feedback.

Pmin / Pmax Ratio - This pressure ratio across the valve varies due to the maximum and minimum flow rate across the valve. To calculate this ratio it is necessary to know (calculate) the maximum and minimum inlet and outlet pressures. This ratio along with the selected shelf (inherent) valve characteristic provides the installed valve characteristic.

Flow Characteristic - Relationship between flow through the valve and percent rated travel as the latter is varied from 0 to 100 percent. This is a special term. It should always be designated as either inherent flow characteristic or installed flow characteristic.

Flow Coefficient, CV - Is a capacity coefficient which is defined as the number of U.S gpm of 60°F water which will flow through a wide-open valve with a constant pressure drop of 1 psi across the valve.

The flow coefficient is experimentally determined for each style and size of valve by the valve manufacturer. It relates the actual flow to CV in a liquid flow application as

follows:

Q = CV ∆P G

where:

Inherent Flow Characteristic - Flow characteristic when constant pressure drop is maintained across the valve, or (∆Pmin / ∆Pmax) = 1.

INHERENT FLOW CHARACTERISTIC CURVES 0 20 40 60 80 100 100 80 60 40 20

PERCENT OF STEM POSITION

PERCENT FLOW QUICK OPENING LINEAR EQUAL PERCENTAGE

Inherent Linear Flow Characteristic - An inherent flow characteristic which can be represented ideally by a straight line on a rectangular plot of flow versus percent rated travel. (Equal increments of travel yield equal increments of flow at a constant pressure level.)

Inherent Equal Percentage Flow Characteristic - An inherent flow characteristic which for equal increments of rated travel, will ideally give equal percentage changes of the existing flow.

Installed Valve Characteristic - This is the actual input-output characteristic of the valve. The slope of this curve dictates the gain of the valve at that particular operating point and loop performance depends on this actual characteristic. The curves change shape as a function of the valve plug characteristic and the ∆Pmin / ∆Pmax ratio across

the valve. The installed valve characteristic of an inherent linear characteristic valve shifts towards a quick opening installed characteristic as ∆Pmin / ∆Pmax << 1.

0 100 100 0 AS ² PMIN ² PMAX<< 1 ² PMIN ² PMAX = 1 INHERENT CHARACTERISTIC STEM POSITION, % FLOW, %

The installed valve characteristic of an inherent equal percentage characteristic valve shifts towards a linear installed characteristic as ∆Pmin / ∆Pmax << 1.

100 AS ² PMIN ² PMAX<< 1 ² PMIN ² PMAX = 1 FLOW, %

Rangeability or Turndown - This is the ratio of maximum controllable flow to minimum controllable flow. This information allows us to determine when to safely put a loop in automatic. The installed rangeability usually is less than shelf rangeability as a function ∆Pmin / ∆Pmax .

RINSTALLED = RSHELF _Pmax Pmin

Dynamic Considerations - In certain applications the speed with which the valve will respond is an important consideration. The speed of response depends mainly on the actuator used and its dynamic characteristics. Valves usually are limited in the speed with which they can move. Stroking speed or stroking time for full stroke may be an important consideration in assessing how fast a valve moves, i.e. the bypass valve in a compressor control application.

Power Loss - Maximum power loss through a valve occurs typically at 50-65% flow. It may be beneficial in some cases to minimize this power loss by running the loop at 85-95% flow rate. Controlling at the higher flow rate lowers the power loss through the valve and provides the additional benefit of a higher production rate.

Valve Fail-Safe Consideration - This is not a control consideration but is an important safety issue. We would like to choose the failure mode of the valve so that if for any reason we lose our air supply, i.e. the air compressor fails. The fail-safe characteristic of the actuator will cause the valve plug, ball or disc to either fully close, or fully open, or remain in a fixed position, usually the last position before the air supply failure.

Besides the major loop elements discussed so far, such as the transmitter, controller, process and final actuator control strategies might dictate the use of various other elements in the loop.

Signal Converters

Current to Pressure Signal Converters, I/P

Most loop applications today, whether pneumatic, electronic or digital, use pneumatic, air-operated valve actuators. If the loop is electronic an I/P is used to convert the electronic signal into a pneumatic signal at the valve. The I/P should be mounted as close to the valve as possible to minimize the transmission lag produced from pneumatic tubing. PROCESS I / P C T r I / P AIR

SUPPLY 20 PSI 35 PSI etc. CURRENT SIGNAL FROM CONTROLLER PNEUMATIC SIGNAL TO VALVE ACTUATOR 3 - 15 PSI 6 - 30 PSI 4 - 20 mA DC 3 - 15 PSI

Other electrical signals such as voltage can be converted to pneumatic signals with a voltage to pneumatic converter, E/P.

Pressure to Current Signal Converter, P/I

Pneumatic signals are converted to electronic signals for a variety of reasons such as longer transmission distances or to eliminate (minimize) lag in tough to control pneumatic loops. Additionally conversion might be required to prepare the signal for inputting electronic recorders, indicators, computers, etc. The most common conversion is the 3-15 psi input for a 4-20 mA DC output. Notice that the conversion attempts to maximize the electronic transmission distances while minimizing pneumatic transmission distances. FT I / P FC LONG TRANSMISSION DISTANCE 4 - 20 mA DC 3 - 15 PSI 4 - 20 mA DC 3 - 15 PSI FLOW PROCESS CONTROL ROOM P / I

Analog to Digital, A/D, or Digital to Analog, D/A

These are devices used to convert analog information to approximate corresponding digital information and vice versa in digital or computer applications.

3 - 15 PSI COMPUTER DIGITAL VOLTAGE D/A I/ P PROCESS OUTPUT CONDITIONER PID INPUT CONDITIONER ENGINEERING UNITS PRODUCT A /D T DIGITAL VOLTAGE

DIRECT DIGITAL COMPUTER CONTROL

4 - 20 mA DC DIGITAL TO ANALOG CONVERTOR FINAL ACTUATOR 4 - 20 mA DC ANALOG TO DIGITAL CONVERTER PULSE RATE OR FREQUENCY

Computer applications as in Direct Digital Control (DDC) usually will handle several loops, each loop requiring its own A/D and D/A converter.

Volume Booster

This is a pneumatic device that acts as an isolating amplifier increasing the volume of air, or the pressure of air, or both. It is used whenever a larger volume or a higher air pressure is required. VB A /S 3 - 15 PSI 3 - 15 PSI or 6 - 30 PSI WITH 2 /1 AMPLIFICATION

AIR SIGNAL OUT AIR SIGNAL IN

In most applications the volume booster is a repeater. It repeats the input pressure as output but with a larger volume (capacity) of air.

Volume boosters are pneumatic devices used in pneumatic applications primarily for the following purposes:

1. To allow an increase in pneumatic signal transmission distances. Pneumatic transmission distances typically are limited to between 200 and 400 ft. depending on size of tubing. A volume booster placed in series between pneumatic tubing can increase the transmission distance as shown.

VB

A /S

2. In addition to longer transmission distances a volume booster helps to isolate a controller from a large capacitive load of the actuator.

t = 0 t = 0 LAG t = 0 f CONTROLLER C = VOLUME OF VALVE ACTUATOR C m C R∝ d4

In the pneumatic application shown above the output of the controller has to charge a large volume, the valve motor, through a small and possibly long tube. The result is a long time constant, τ = RC, where C is the capacity (volume) of the valve motor and R is the resistance of the tubing and is directly related to the diameter of the tube (d4_).

The controller output will have a difficult time in moving the final actuator due to the long lag time.

t = 0 t = 0 t = 0 f CONTROLLER m C 3 - 15 PSI 3 - 15 PSI AIR SUPPLY 20 PSI VOLUME BOOSTER

Valve Positioner

A valve positioner is a proportional-only controller whose main function is to eliminate or minimize valve hysteresis as shown.

VALVE WITHOUT A VALVE POSITIONER

f m

VALVE WITH POSITIONER INSTALLED f ( m ) m ( r ) ( c ) VP

Hysteresis is caused by valve packing, plug seals, bushings, seal rings, etc. It usually deteriorates and becomes a greater problem in time.

HYSTERESIS WITH POSITIONER HYSTERESIS MINIMIZED STEM POSITION ( FLOW ) INPUT SIGNAL, m

WITHOUT POSITIONER ACTUAL STEM POSITION ANYWHERE WITHIN HYSTERSIS BAND

Since a valve positioner is a proportional-only controller its application in a control loop effectively makes the control strategy into cascade. PCI 102.07 takes a further look at this application when cascade control is discussed. Beyond this main function, however, the valve positioner can perform a variety of other tasks as listed below.

a) Can be used to provide tighter shutoff through additional actuator loading pressure driving the pressure acting on the diaphragm to full supply pressure.

b) Can be used as a volume booster to improve the dynamic response of a valve with a large actuator achieving a faster stroking speed. This application excludes a fast loop like flow or liquid pressure where the implementation of a valve positioner may create other problems.

c) Can be used as an I/P converter. Changing a 4-20 mA DC signal to a pneumatic 3-15 psi signal to the valve.

d) Can be used as a characterizer. Some manufacturers allow output characterization through cams or other means, thus effectively changing the valve characteristics.

e) Can be used to change the valve action from direct to reverse and vice versa.

f) Can be used in valve sequencing (split ranging) for additional rangeability or turndown.

Valve Sequencing Examples . Despite some manufacturers' claims of high rangeabilities, the practical rangeability of a control valve is limited to approximately 100/1 with most valves falling below 50/1. These rangeability values are sufficient for most control applications. In some applications however, such as pH, the rangeability required may exceed 1000/1 and the control scheme must be designed to satisfy this requirement in order to achieve good control. These additional rangeability concerns come up whenever a single control valve or other control element is not available to provide the required rangeability or turndown. In these situations a strategy known as valve sequencing or split ranging may be implemented. In split ranged or sequenced strategies, the controller's output actuates more than one valve, typically two valves. This allows the individual valves to operate in the specific areas of their range, but together as a team provide the larger rangeability required by the particular application. In the typical application as shown the valves are installed in parallel and their flow rates are additive. The valves usually are of different size with a small overlap between the maximum flow of the small valve and the minimum flow of the large valve and only one valve open at any time. The size difference should not be such that larger valves leakage rate affects the smaller valves operation.

FLOW OUT FLOW IN

m ( Controller Output ) VP2 VP1

Calibrate VP1 so that valve operates from 3-9 psi (fully open at 9 psi). Calibrate VP2 so that valve operates from 9-15 psi (fully open at 15 psi). If the rangeability of each valve is

10

1 the overall rangeability would be 10 1 × 10 1 = 100 1 .

A second application using two equal-percentage valves of different size and having a 50₁ rangeability will produce an overall rangeability of (50)2 _{or 2500.}

To maintain the overall equal-percentage characteristic only one valve should be open at any given time. Logic must be provided in this scheme so that if the output exceeds the

GLOSSARY

absolute pressure Pressure referenced to zero absolute pressure (vacuum, the absence of pressure). The unit is psia.

accuracy In process instrumentation, degree of conformity of an

indicated value to a recognized, accepted standard value, or ideal value.

conformity (of a curve) The closeness to which it approximates a specified curve (e.g. logarithmic, parabolic, etc.)

control valve A final controlling element, through which a fluid passes, which adjusts the size of flow passage as directed by a signal from a controller to modify the rate of flow of the fluid.

converter A device that receives information in the form of an

instrument signal, alters the form of the information, and sends out a resultant output signal. A converter is a special form of relay sometimes referred to as a transducer.

dead band In process instrumentation, the range through which an

input signal may be varied, upon reversal of direction, without initiating an observable change in output signal. differential gauge A gauge having two connections and a means to measure

and indicate differential pressure. differential pressure Difference between two pressures.

fail-open A condition wherein the valve port remains open should the actuating power fail.

fail-safe A characteristic of a particular type of actuator which, upon loss of power supply, will cause the valve plug, ball, or disc to fully close, fully open, or remain in fixed position.

final controlling element The forward controlling element which directly changes the value of the manipulated variable.

gauge pressure Pressure referenced to atmospheric pressure (14.696 psi at sea level). The unit is psig.

hysteresis That property of an element evidenced by the dependence

of the value of the output, for a given excursion of the input, upon the history of prior excursions and the direction of the current traverse.

impulse line the connection between the instrument and the process. instrumentation A collection of instruments or their application for the

intrinsically safe

equipment and wiring Equipment and wiring which are incapable of releasingsufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration.

pressure gauge Instrument to measure and indicate pressure relative to atmospheric pressure.

primary element The system element that quantitatively converts the

measured variable energy into a form suitable for measurement.

range The region between the limits within which a quantity is

measured, received, or transmitted, expressed by stating the lower and upper range-values.

Note 1: For example: a. 0 to 150°F b. -20 to +200°F c. 20 to 150°C

range, elevated-zero A range in which the zero value of the measured variable, measured signal, etc., is greater than the lower range-value.

For example: -10 to +15 in H2O

range, suppressed-zero A range in which the zero value of the measured variable is less than the lower range value. (Zero does not appear

reliability (MTBF) The probability that a device will perform its objective adequately for, the period of time specified, under the operating conditions specified.

repeatability The closeness of agreement among a number of

consecutive measurements of the output for the same value of the input under the same operating conditions, approaching from the same direction, for full range traverse.

reproducibility In process instrumentation, the closeness of agreement among repeated measurements of the output for the same value of input made under the same operating conditions over a period of time, approaching from both directions.

span The algebraic difference between the upper and lower range-values.

Note 1: For example:

a. Range 0 to 150°F, Span 150°F b. Range -20 to 200°F, Span 220°F c. Range 20 to 150°C, Span 130°C speed response

(stroking speed) In control valve operation, this term describes the rate oftravel of the actuator.

trim The internal parts of a valve which are in flowing contact

with the controlled fluid. (In a globe valve body, trim would typically include valve plug, seat ring, cage, stem and stem pin.)

trim, soft-seated Globe valve trim with an elastomer, plastic, or other readily deformable material used as an insert, either in the valve plug or seat ring, to provide very tight shutoff with minimal actuator force.

valve positioner A control valve accessory which transmits a loading pressure to an actuator to position a valve plug stem exactly as dictated by the instrument pressure signal from an automatic controller.

valve trim The internal parts of a valve which are in flowing contact with the controlled fluid. The primary function of the valve trim is to proportion the valve orifice area in such a manner that a prescribed relationship exists between flow capacity and valve plug lift.