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控制阀阀芯 出口的动能和流速  

2012-12-24 22:32:01|  分类: 默认分类 |  标签: |举报 |字号 订阅

  下载LOFTER 我的照片书  |

1.什么是动能的设计标准?

流体的动能=pV2/2g,密度,v:速度,


Crunch the numbers on 470 control valves retrofitted in the field, with new valve trim, and the user sees that new trim controls the fluid exit energy level.

With that, the user is back in control of the process, and that's important.

由于没有达到理想的效果,需要对阀门做改进。The retrofits took place because the valves were not performing their intended control function. The database shows that after retrofit and using trim  that limits the kinetic energy below 70 psi (480 kPa), a valve application results that meets or exceeds the user's expectation.数据显示改进后,当动能限制在70PSI时,阀的应用效果满足或超过了用户的期望。

The population of retrofitted valves covers a wide range of sizes, pressure classes, design types, applications, and original suppliers.改进的阀包括各种尺寸、范围、结构形式,各种应用场合。

The inertia associated with a change of a control valve in the field is quite significant, and a user will need a strong motivation in order to take this step.

The problems and causes that have pushed the user to implement a retrofit of a valve are many. In almost every case, there have been lots of trials in the field with some unique work-a-rounds and fixes for the problems.

A retrofit is not a trivial undertaking. The user has a significant problem he or she has not been able to solve.改进不是通过零碎的工作可以实现的,用户遇到了很大的问题,而没有能力自己解决。

The valves in this study all failed and all resisted any fixes. Retrofitting them with a trim to control the fluid exit kinetic energy turned them into successes.


Kinetic design criterion

The kinetic energy density combines the influence of density with velocity of the jet exiting a valve trim. The term density makes sense because the energy level is per unit volume.

The term density is in the text here whenever kinetic energy is the reference. We define kinetic energy as this:

 

The velocity in this expression is the average trim outlet-jet-fluid velocity and Greek letter rho (ρ) is the density of the fluid at the exit.

The fluid kinetic energy, at the trim exit, criterion for control valve applications in continuous duty applications is 70 psi (480 kPa) or less. Levels of energy are in the same units as pressure, and the equation for kinetic energy density is the expression for dynamic pressure.

The application of the kinetic energy criterion is in addition to the traditional design process. That is to say, all decisions regarding materials, capacity sizing, body, trim and actuator selection, adjustments for erosion, cavitation, and/or noise take place and then a check of the trim design is made to be sure the kinetic energy level meets the design criterion.

The kinetic energy criterion (70psi) is an additional consideration and not a substitute for using all prior control-valve design knowledge and practices. The experience as shown by the results presented below will provide the best assurance available that the resulting control valve application will perform to expectations.动能标准是一个附加的考虑项目,无法代替先前选阀时应用的任一个项目和条件。


Desired tolerances and interfaces

A retrofit is a replacement of a valve's main flow control trim component using the original valve body as installed without alteration. The trim component that is the control component usually goes by the name cage or in the case of a top guided trim set; it is the seat ring and the plug (closure member) parts that form the throttling orifice. The sole purpose of changing the flow control trim component is to minimize the fluid exit kinetic energy. The goal is to reduce the energy level to 70 psi (480 kPa) and lower if space permits.

Other valve internal parts such as the seat ring, plug assembly, guide bushings, and soft goods (seals) change out for the purpose of maintaining desired tolerances and interfacing with the existing valve body. It is an unusual case when body machining takes place. There are occasions when the body is in such bad condition it must be restored by repair and machining. Most of the time sanding and cleaning is enough to assure a proper interface and sealing surface. The actuator may be changed if the needed trim will require a longer stroke and/or to maintain proper seating loads for leakage control.

One of the most important steps in the process is the communication with the customer. It is important to know what problem is causing the need to retrofit the current valve so focus is on the root cause and the original design problem does not recur. This is a design by application, and there can be many questions. There may be as many as 22 conditions of process information needed to make sure an application is properly accommodated (some may be better know by the valve manufacturer than the users). The key design issues will originate from the root cause of the dissatisfaction and failure of the current installation. After this, steps transpire to determine dimensions of the existing valve body either by implication from replacement parts or actual measurements. Some field machining of supplied parts may be necessary to assure a good fit of the energy control trim.

Once the proper process conditions have been determined and agreed to with the user, sizing happens per the ISA/IEC Standard. The required capacity remains constant when selecting the proper number of discreet pressure reduction stages to achieve a 70-psi (480 kPa) fluid kinetic energy density criterion for each condition. If gallery space in the existing valve body permits, additional stages go in to enhance flow control thereby minimizing the trim exit kinetic energy. The trim inlet kinetic energy stays low as well by this focus on the outlet criterion.

In a few cases, the retrofit is a replacement of a top guided or a thin wall cage that does not allow enough pressure drop stages to work. There is a trade off between the outside diameter of the trim and the need to maintain enough space between the trim and the body wall. If this space is not sufficient then a judgment as to whether the energy level is close enough to 70 psi (480 kPa) for the application or a significant enough reduction in the exit energy level has occurred to be assured of a good control valve application takes place. In a number of cases, the retrofit approach is abandoned and a complete valve replacement is used. This judgment comes into play for about 10 to 15% of the retrofit cases and uses the experience gained from designing by application and the excellent performance history of almost 500 retrofitted valves.

In all cases, the energy control trim used is the multi-stage, multi-path tortuous path trim. This trim allows a large number of pressure drop stages to install in a relatively thin radial wall. Each right angle turn acts as a speed bump, reducing the fluid velocity, so that by selecting the correct number of stages, the desired kinetic energy level at the exit of the flow path happens.

Because the flow per channel is less, enough channels work such that the valve capacity transpires. This leads to longer strokes in many cases than would be used for a less tortuous path trim design. Because of the longer stroke, the diaphragm actuators frequently swap out as part of the retrofitting activity with a piston style actuator. The piston actuator also allows higher actuation pressure that provides better resolution, stiffness, and shutoff seating loads.

Energy control trim for retrofit


Root causes for a retrofit

During the communication phase of the retrofit activity, the user provides what he or she feels is the cause of the problem with the original installation. Usually there are multiple causes, and so multiple reasons go to the record book. There is no attempt to edit or narrow the input, as an understanding of the problem is the most important issue.

As an example, a user may say the problems are poor control and cavitation. In the discussion, it is apparent the poor control occurs during the start up of the process. This would suggest erosion due to cavitation is the real root cause. The erosion of the internal parts does not allow sufficient resolution during the initial travel of the plug to give good start up control, and operators notice the lack of control. In this case, the users' input of poor control and cavitation go to record, even though the true problem is erosion of the trim due to cavitation or excessive fluid kinetic energy.

Note: The leading complaint for the liquid valves is controllability followed by erosion, leakage, vibration, and cavitation. All of these relate directly to the kinetic energy levels in the valve. A big surprise is the number of complaints of stem separation or breakage, six instances, or 3% of the complaints, but it affected 41 valves, 7%. Five of the six complaints regarding a stem problem also listed either vibration or cavitation as a cause for the retrofit. Noise for liquid valves is not a frequent complaint.

Of the 155 liquid application complaints, 145 of them are clearly associated with trim fluid design issues such as fluid energy, sizing, and capacity. The 10 complaints not associated with trim design included high maintenance, lack of support from the original vendor, and linkage breakage. Apart from the lack of support, these relate to excessive fluid energy levels. None of the reasons for a lack of vendor support is acceptable. Reasons for a lack of support could include poor communication channels between the user and the vendor's technical experts, a lack of understanding about the problem or the application, and insufficient resources, to name a few.

Note: The leading complaint for the gas valves is noise followed by vibration and leakage. Controllability is a distant fifth, which likely reflects the reduced potential for erosive damage with a gas. Again, stem separation or breakage came up three times as a complaint with an impact on 3% of the valves. Stem problems always went with vibration as an accompanying cause.

Of the 100 gas application complaints, 85 of them are clearly associated with trim fluid design issues such as fluid energy, sizing, and capacity. The remaining 15 complaints could also relate to fluid energy issues, but it is less obvious. Vendor support, again, reflects a frustration by the user with the supplier's inability to solve the problems with the valve application.


Meeting the energy criterion 70 psi

There are a few cases where the original trim met the energy criterion. Still the valves were retrofit. Except for nine designs, these valves had other flow conditions specified for the application in which the energy exceeded 70 psi (480 kPa). In reviewing the nine designs, the applications represent traditionally difficult applications in which there are problems with cavitation (pump recirculation), vibration (nuclear plant service water), control (auxiliary steam supply and deaerator level), and noise (compressor recycle and a gas valve with sonic exit conditions.) The most probable cause for a failure in these cases is the normal practice of designing to flow condition information only and not considering or understanding the application.

Other excuses could be the original supplier was not conservative enough in their design practice and decided to proceed with the risk and/or the user was proceeding with what they thought was the lowest cost option.

There are a number of cases in which the energy level exceeded the criterion even after retrofit. These represent the cases in which the criterion was impossible because of a gallery space limitation in the valve body. However, a review of the applications and the significant reduction in trim exit energy for each retrofitted case resulted in a judgment to proceed. In all of these cases, the reduction in the energy level was more than eight times and frequently much greater. Overall, the average ratio is 21 for the original trim energy level to that of the energy control trim.

The fact that there are a few cases in which the kinetic energy exceeded the criterion of 70 psi (480 kPa) would suggest it is not a hard rule. There is some validity to this as demonstrated by the data; however, during the original valve procurement the incremental cost in achieving the criterion is small in comparison to the risk. The data from "Kinetic energy – all cases" shows 90% of the flow cases (original trim energy greater than the criterion) resulted in field failures when ignoring the criterion.

The maximum energy level supplied with an original trim design was 3280 psi (22.6 MPa), which is almost 50 times the criterion. The average was 480 psi (3.3 MPa) or about seven times the criterion. This is a normal expectation from a supplier when there is no consideration of the trim exit energy. With these high fluid dynamic pressures, it is not surprising that a lot of damage can take place in a control valve piping system.

For the energy control trim, the average energy level was 44 psi (300 kPa) with a maximum of 340 psi (2.3 MPa) for one case. Only 3% of the data exceeded 140 psi (960 kPa), which is twice the criterion. Therefore, there is not a lot of data that would support an increase in the criterion or a push to accept a higher risk by exceeding the criterion.

One question that may arise is whether this database of retrofitted valves is representative of all control valve applications. We divided the valves into three sections representative of the service from "mild" to "robust" to "severe" service conditions.

The "mild" service upper boundary is representative of an ASME Class 300 Pound valve with an inlet pressure on the order of 700 psi (4825 kPa). The "robust" service upper boundary is representative of an ASME Class 600 Pound valve with an inlet pressure on the order of 1400 psi (9.65 MPa). The "severe" service inlet pressure has no limit, however the maximum inlet pressure encountered here was 5840 psi (40.3 MPa).

With these three-market application designations, the number of data points in each segment was 105, 106, and 218 for the "mild," "robust," and "severe" service categories, respectively.

These numbers are a statistically significant representation of the entire application population. They add up to the 429 flow conditions in the total database.

Kinetic energy before and after retrofit


Final telling observations

We have looked at the many causes for a user to take the difficult step of retrofitting a control valve in the field. This step only happens after many trials to fix a problem valve.

The retrofit of almost 500 valves has shown the power of assuring that the design meets a minimum trim exit fluid kinetic energy density criterion. This energy control feature plays after all other known valve design criteria have implemented.

The data presented shows when the kinetic energy level is not reviewed, very significant jet energy results. This energy is available to enforce and amplify any damaging impact of the trim exit jets and feed the turbulence that can negatively influence the piping system.

One of the most significant observations is the very high kinetic energy levels existing in the original valve designs. These designs failed repeatedly.

These failures became successful applications, for a wide range of original suppliers, applications, and conditions that encompass the entire control valve spectrum. The only change was to reduce the fluid energy level exiting the valve trim to acceptable levels.



Fluid Kinetic Energy as a Selection Criteria for Control Valves

 

Abstract

A selection criteria is provided that assures a control valve will perform its control function without the attendant problems of erosion, vibration, noise and short life. The criteria involves limits on the fluid kinetic energy exiting through the valve throttling area. Use of this criteria has resolved existing valve problems as demonstrated by retrofitting of the internals of many valves and vibration measurements before and after the retrofit. The selection criteria is to limit the valve throttling exit fluid kinetic energy to 70 psi (480 KPa) or less.

NOMENCLATURE


Ao Valve Throttling Area, in2 or m2


c Fluid Sonic Velocity, ft/s or m/s


KE Fluid Kinetic Energy, psi or Kpa


M1 Units Conversion Factor, Table 1


M2 Units Conversion Factor, Table 1


Vo Fluid Velocity at Trim Throttling Area, ft/s or m/s


w Mass Flow Rate, lb/h or kg/s


ro Fluid Density at Trim Exit, lb/ft3 or kg/m3


introduction

For many years the traditional method of sizing and selecting control valves has been to select a valve that contains the design pressure and temperature and meets the maximum capacity requirements. In addition to meeting capacity requirements and serving as a pressure boundary vessel, the valve should perform its intended control function, have long internals (trim) life, provide good shut-off and be relatively maintenance free. There is a need for guidelines to evaluate if a valve and its trim will provide this type of service.

This paper presents a criteria for selecting a valve design capable of eliminating problems. The guideline imposes limits on the kinetic energy of the fluid exiting from the valve trim. The criteria applies to all linear motion valve types. Each type of valve is capable of meeting the kinetic energy criteria for many of the flow conditions that have been dictated by tradition and experience.

Butterfly and Ball valves usually meet the proposed criteria for kinetic energy. The pressure drop across these valves is not large enough to accelerate the flow to a high velocity level. Thus, a much lower value of energy is realized.

Examples are given in which measurements have been made on problem valves and the same measurements made after the valve has been retrofitted with a different trim that limits the kinetic energy exiting from the valve trim. The measurements that are reported are vibration measurements that quantify the effect of the change in the valve trim. The vibration of the valve and the piping system is a strong indicator of valve integrity. It is a direct result of the energy levels in the fluid passing through the valve. As such, it is an indicator of the ability of the valve to provide good control with long life and low maintenance costs.

VALVE SELECTION PROCESS

There are numerous texts that cover the selection of control valves. Two of the most frequently used are ISA (1976) and Driskell (1983). The first step in selecting a control valve is to calculate the required flow capacity, Cv, based upon the requirements of flow rate, inlet and outlet pressure, and the fluid properties. Internationally accepted standards for calculating the required capacity are available in ANSI/ISA (1985) and IEC (1976, 1978, 1980) publications. The required Cv is then compared against tables of valve size and designs provided by the valve manufacturers. The valve hardware is selected to provide enough flow for the given conditions. Many different valve designs will satisfy the capacity requirements and so additional selection guidelines are used to make the final decision. Final selection of the valve and trim type is made through experience and/or by looking at one of the following parameters: pressure drop, pressure drop ratio (pressure drop divided by inlet pressure), fluid velocity or as presented here, the fluid kinetic energy.

Valve designs have ranges for the amount of pressure drop (energy) that they can effectively absorb. For example, low pressure drops are handled by butterfly valves. As the pressure drop level increases, a ball valve would be needed. Still larger pressure drops would require the linear motion globe/angle type valves. The globe/angle designs incorporate many different valve trims depending upon the level of pressure drop starting with a simple plug that opens an orifice. The next range of pressure drops would require a ported cage to help guide the plug and contain the energy dissipation. For the largest pressure drops, a tortuous path trim design is needed. For different valve and trim selections, the acceptable pressure drop ranges overlap. In general, the cost of the selected valve increases with the valve's ability to handle higher pressure drops. Manufacturers have developed designs to extend the pressure drop ranges in order to serve the market with the lowest first cost equipment. This extension of ranges usually is achieved by harder materials that may tolerate the resulting cavitation, erosion, vibration and noise levels.

Driskell (1983) in his chapter titled “Velocity, Vibration, and Noise” discusses the reasons why velocity should be controlled. Excessive velocity causes all of the destructive effects that result in a poor valve application. He notes that velocity induced vibration and noise are “...a blessing in disguise in that they are a warning of impending failure.” Driskell does not discuss where in the valve the velocity needs to be controlled. Unfortunately, when velocity guidelines have been translated to control valve selection they have been interpreted as the velocity exiting the valve body. By the time the fluid is ready to exit the valve body, the influence of “high energy” has already been imprinted into the fluid stream. For example, the fluid velocity exiting the trim may have created high velocity, erosive jets, areas of low pressure with resulting flashing and cavitation damage and noisy shock waves. Velocities should be controlled at the trim outlet, not the valve outlet.

The valve industry has in some cases defined velocity through the trim as a design guideline. These are presented in Ho (1995), Kowalski, et al. (1996), Laing, et al. (1995), Miller (1993, 1996), Stratton and Minoofar (1995) and are used as a basis for the presentation of the criteria discussed in this paper. Schafbush (1993) argues for emphasis on the driving force, pressure drop, instead of the results of the driving force (velocity and kinetic energy) as the selection criteria. Just looking at the pressure drop or fluid velocity at the trim exit ignores the density of the fluid, which is an important parameter in accessing potential problems. A guideline based on the kinetic energy exiting the valve trim involves the driving force, pressure drop, the resultant velocity and the fluid density. Many years of experience in applying this criteria have indicated it is a reliable indicator that is not overly conservative and is applicable to all valve designs.

TRIM OUTLET KINETIC ENERGY

For kinetic energy evaluation, the location in the valve of greatest concern is just downstream of where the fluid is throttled or controlled. At this location, the flow area is the smallest and the fluid velocity and kinetic energy are the highest. The parts of the valve responsible for controlling and seating are often located at this point and are therefore subjected to the highest energy fluid.

Figure 1 shows the throttle area for various kinds of valve trim. For a top guided globe valve, the trim outlet flow area is the annulus area between the plug and seat. In a cage guided valve, the trim outlet flow area is the exposed area of the windows in the cage. For a multi-path cage, the trim outlet flow area is the total area of all the exposed flow paths. For multi-stage trims, the flow area from stage to stage must not increase too rapidly or else the throttling will take place across the first stages and the later stages will be ineffective, see Figure 1(e).

In a valve, the disk or plug moves to increase or decrease the area through which flow can pass. For a given set of conditions, a fixed area of the trim is open to flow. Under any significant pressure drop conditions, this area will be considerably less than the inlet or outlet area of the valve. As a result, the fluid passing this point will have much higher velocities and kinetic energy levels than in other valve locations. The only way to increase this flow area without increasing the flow rate, is to increase the resistance of the throttling flow path. The flow conditions define how far the valve is open and the valve’s trim design (flow path resistance) defines how much flow area exists at the trim outlet. Once this area is defined, the continuity equation can be used to calculate the velocity of the fluid at the outlet of the trim.

控制阀阀芯 出口的动能和流速 - 老雷 - 老雷的博客(1)


控制阀阀芯 出口的动能和流速 - 老雷 - 老雷的博客

Figure 1. Throttling exit area (Ao ) examples for typical valve trim designs

The fluid density and velocity are used to establish the fluid’s kinetic energy.

控制阀阀芯 出口的动能和流速 - 老雷 - 老雷的博客 (2)

For gas or steam, the fluid velocity at the trim outlet may be sonic. If it is, the density of the fluid at the trim outlet must be higher than the outlet density, ro, in order to pass the given mass flow rate, w. This higher density can be estimated using Equation 1 by substituting the fluid’s sonic velocity, c, for the outlet velocity, Vo, and solving for density. Then, this density and sonic velocity are used in Equation 2 to find the kinetic energy.

Table 1. Numerical Constants for Velocity and Kinetic Energy Equations


Constant

Units Used in Equations

M

w

ro

Ao

Vo

KE

M1

25

lb/h

lb/ft3

in2

ft/s

-

 

1.0

kg/s

kg/m3

m2

m/s

-

M2

4636.8

-

lb/ft3

-

ft/s

psi

 

1000

-

kg/m3

-

m/s

KPa


VALVE TRIM KINETIC ENERGY CRITERIA

The piping industry has long recognized the need to control the kinetic energy levels in the transport of fluids through a pipe. The industry has created design criteria that limits the fluid velocity in the pipe to acceptable limits. For example, a normal criteria for liquids in pipes is to limit the fluid velocity to a range of 5 to 50 ft/s (1.5 to 15 m/s). Assuming normal water densities, this is equivalent to a kinetic energy of 0.16 to 16 psi (1.1 to 110 KPa). The typical criteria for gases is to keep the fluid Mach number (actual velocity divided by the fluid’s sonic velocity) below 0.15. Assuming saturated steam of 100 to 1000 psi (0.7 to 7 MPa) and a sonic velocity of 1630 ft/s (500 m/s), the kinetic energy is in the range of 1.5 to 15 psi (10 to 100 KPa).

Velocity criteria for liquids are much lower than for gases because liquid densities are much higher, resulting in higher energy levels. While the velocity limits are quite different, the kinetic energy limits are very close to the same.

Table 2 shows criteria for a valve trim’s outlet kinetic energy. The valve trim should be selected to keep the kinetic energy below these levels. The examples that follow support the values shown in the table.

Table 2. Trim Outlet Kinetic Energy Criteria


Service Conditions

Kinetic Energy Criteria

Equivalent Water Velocity

 

psi

KPa

ft/sec

m/s

Continuous Service, Single Phase Fluids

70

480

100

30

Cavitating and Multi-phase Fluid Outlet

40

275

75

23

Vibration Sensitive System

11

75

40

12


For most conditions, an acceptance criteria of 70 psi (480 KPa) for the trim outlet kinetic energy will lead to a trouble free valve. In some applications, where the service is intermittent (the valve is closed more than 95% of the time) and the fluid is clean (no cavitation, flashing or entrained solids), the acceptance criteria can be increased, but should never be higher than 150 psi (1030 KPa).

In flashing service, liquid droplets are carried by their vapor at much higher velocities. To eliminate the risk of erosion, the acceptance criteria for flashing or potentially cavitating service should be lowered to 40 psi (275 KPa). The same criteria exists for liquids carrying entrained solids.

Special applications may require even more stringent kinetic energy criteria. For example, pressure letdown valves used in pump test loops must be vibration free so that proper evaluation of the pump can be made. These valves are designed with trims that reduce the kinetic energy to less than 11 psi (75 KPa). Gas or steam valves with very low noise requirements may also result in extra low trim outlet kinetic energy requirements.

RETROFIT EXAMPLES

Table 3 shows a summary of the service conditions, before and after trim style, and the corresponding kinetic energy levels for four valve designs retrofitted with multi-stage trim. Each of these valves was retrofitted because the original valve trim was not allowing good control or there were limitations in the valve’s use due to excessive vibration. In some cases, the valves would cause the system to trip. After repeated attempts to fix the problems and the plant’s need for working valves, the valves were retrofitted with trim designed to reduce the kinetic energy at the trim outlet. The only change made to the valves was to change the internal valve trim and hence, the trim outlet kinetic energy. No changes were made to the valve bodies. Since the bodies were not changed, the fluid velocities exiting the valve bodies were the same before and after the retrofit. In all cases, significant improvements in valve performance were achieved by retrofitting the trim to meet the suggested kinetic energy design criteria.

Table 3. Attributes of Four Valves, Before and After Retrofit


   

Example Number

 

Units

1

2

3

4

Application

 

Residual Heat Removal

Feedwater Regulator

Core Spray

Steam Dump

Quantity

 

4

2

4

1

Fluid

 

Water

Water

Water

Steam

Flow Rate

MM lb/hr kg/s

4.5 560

4.4 550

2.2 280

1.8 230

Inlet Pressure

psia KPaa

155 1070

1546 10660

295 2030

740 5100

Inlet Temperature

F C

100 38

440 227

110 43

511 266

Outlet Pressure

psia KPaa

35 240

972 6700

15 100

334 2300

Capacity, Cvreq’d/Cvtotal

 

820 / 830

400 / 780

290 / 300

1400 / 1432

Valve Size

 

14” x 14”

12” x 12”

8” x 8”

18” x 18”

Valve Inlet/Outlet Kinetic Energy

psi KPa

3 / 3 20 / 20

5 / 5 34 / 34

5.5 / 5.5 38 / 38

6.4 / 14 44 / 97

Plug Size

 

12”

9.5”

5.5”

14”

Original Trim Type

 

Top Guided Plug

Drilled Hole Cage

Top Guided Plug

3 Concentric Drilled Hole Cages

Original Trim Outlet Kinetic Energy

psi KPa

148 1020

380 2630

290 2020

83 570

New Trim/Cage

 

4 and 2 Stages

10, 6 and 4 Stgs

4 and 2 Stages

8 Stages

New Trim Outlet Kinetic Energy

psi KPa

13 - 24 88 - 168

17 - 61 118 - 420

30 - 57 204 - 390

25 172


For examples 1 and 3, the water outlet pressures were close enough to the water’s vapor pressure to suggest cavitation and two phase flow conditions may exist. Therefore, the acceptance criteria for the trim outlet kinetic energy was the more stringent 40 psi (275 KPa) value for the pressure conditions that could result in vaporization.

EXAMPLE 1, RESIDUAL HEAT REMOVAL, arnold, et al. (1996)

The valves were originally top guided, Figure 1(a) control valves without a cage. The valves were retrofit with a tortuous path trim such as shown in Figure 1(g). The kinetic energy on the top guided version is calculated in the annulus area created between the plug and the seat ring. The kinetic energy for the retrofitted trim is at the outlets of each of the disks forming the cage.

A typical reduction of the vibration velocity is shown on Figure 2. The accelerometer that resulted in this maximum output was located on the actuator and measured a direction that was rotational around the centerline of the pipeline. Vibration velocity for the five accelerometers located on each of four valves showed reductions in value that ranged from 49 to 91 percent with even larger reductions occurring on piping components in the system.

The retrofitted valves were able to pass full design flow rates without the accompanying cavitation. All concerns regarding the potential of piping fatigue as a result of the vibration were eliminated.


控制阀阀芯 出口的动能和流速 - 老雷 - 老雷的博客

Figure 2: Residual heat removal


EXAMPLE 2, FEEDWATER REGULATOR, parker, et al. (1994)

The original trim started out as a cage guided trim and was later changed to a drilled hole cage in one of many attempts to salvage the valve design. The retrofit used a tortuous path trim, Figure 1(g), that absorbs the fluid energy inside the trim and has an acceptable exit kinetic energy. The throttling points in the original trim are the cage orifices, Figure 1(b), and the small holes in the drilled hole cage, Figure 1(c), tried later.

The vibration velocity results are shown in Figure 3. The reductions in the vibration are quite dramatic because the vibration levels are not very high to begin with. The comparison of results is made with the drilled hole cage trim as results did not exist with the original cage trim. The reductions resulted in at least a 40 percent reduction in the velocity at the piping frequency of 10 Hertz and essentially an elimination of the vibration at the higher frequencies. Displacement measurements showed reductions of 53 percent and acceleration measurements showed an 86 percent reduction.

All of the problems of vibration, plant trips, and broken stems were resolved by the lower kinetic energy levels at the trim exit. The plant was started up and power escalated to full load for the first time on automatic control as was intended from the inception of the plant design.

EXAMPLE 3, CORE SPRAY, arnold (1995)

The valves were designed with a top guided plug, Figure 1(a). The trim was retrofitted with the tortuous path trim using the logic of Figure 1(g). The throttling area of the original valve was the annulus area between the plug and the seat ring. The retrofit throttling point was the exit from the disk outlets.

Another change in the system was made in this application. When the retrofitted trim was installed, downstream restricting orifices were removed. Thus, the valve pressure drop was increased to a value equivalent to the original trim and the orifice. This represents a more severe set of conditions for the retrofit trim in controlling any destructive affects due to the higher potential energy that would be converted to kinetic energy across the trim.

The downstream piping system vibration measurements were the most significant changes recorded between the pre and post retrofitted valves. These measurements showed that the pipe displacement dropped from 64 to 92 percent. Pre-retrofit values of displacement were 0.090 inches (2.3 mm) or greater and the largest displacement after the retrofit was 0.020 inches (0.51 mm).

The root cause of the system vibration was cavitation. The post retrofit vibration levels were minor and eliminated any concern regarding the piping system stresses and potential for damage due to fatigue.


控制阀阀芯 出口的动能和流速 - 老雷 - 老雷的博客

Figure 3. Feedwater regulator

控制阀阀芯 出口的动能和流速 - 老雷 - 老雷的博客

Figure 4: Steam dump


EXAMPLE 4, STEAM DUMP, persad (1995)

The valve instrumented was a steam valve with a flow to open trim consisting of three concentric cages with drilled holes in each cage. The cages were tightly touching so that there was no axial flow between the cages. Each cage was

slightly offset to create a tortuous path for the pressure letdown. This type of trim is shown in Figure 1(f). The throttling area is the flow area caused by the restriction of the last two cages. The outlet area of the last cage is not the throttling area because there is little pressure letdown associated with the expansion from the overlap orifice. The expansion is too large to have much influence and the jet from the overlap area is the dominate kinetic energy source exiting the trim.

The values reported in this case were a sum of the vibration velocity peaks in the 0 to 500 Hertz range. The results are shown in Figure 4 where the vibration velocity magnitude is plotted as a function of the valve stroke. Values are not available beyond 65 percent of stroke for the original trim as the valve was not operated in this region because of the severity of the vibration.

The reduced trim exit kinetic energy solved the severe vibration problems associated with this steam system.

OTHER EXAMPLES

All of the examples presented above happen to be installed in nuclear plants. However, these are typical control valve applications and are representative of the many applications in different industries that have been experienced. In the past 20 years, over 150 valves ranging in size from 2" to 24" x 36" have been retrofitted to achieve the kinetic energy criteria identified above.

Table 4. Partial List of Retrofit Applications

Applications

Condensate Recirculation

Atmospheric Steam Dump

Aux Pump Recirculation

Turbine Bypass

Recirculation

Reactor Water Cleanup

DA Level Control

Wellhead Choke

Aux Feedwater Regulator

Compressor Recycle

Feedwater Regulator

Compressor Anti-Surge

Reheat Spray

Water Injection

Steam Letdown

Injection Control

Auxiliary Steam

Moisture Separator Reheater

Condenser Steam Dump

Gas to Flare

Table 4 is a partial list of applications involved. All of these retrofits arose as a result of a problem with the original installation. In all of the cases, the retrofits were successful in resolving the root cause of the valve problem and the only significant change was the limiting of the fluid kinetic energy exiting the valve trim.

CONCLUSIONS

A criteria for the selection of a control valve has been provided which goes beyond the many rules and exceptions being used in the industry. The criteria is a limit on the kinetic energy exiting from the valve trim, defined as the throttling point of the trim. It addresses the energy that contributes to the problems associated with valves. The combination of fluid velocity and density cause:


unstable forces inside the valve,


low local pressures that result in cavitation,


erosion of critical parts,


shock waves that create unwanted noise, and


turbulence that results in vibration.


Using the kinetic energy criteria, which has many years of application experience, will eliminate valve problems. It will provide the user with a means to evaluate the different types of valve designs that look as though they will meet the system needs.

The first step is to select the valves that can meet the capacity requirements. Then the valve types are sorted to select the correct valve by using the kinetic energy levels. This will assure the engineer that the lowest cost system is installed.

ACKNOWLEDGEMENTS

We wish to thank the utilities that shared their vibration measurements so that the retrofitted valve benefits could be quantified. Dr. S. V. Sherikar is also acknowledged for his arranging, collection and analyzing the Example 2 data.


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