stem travel valve

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Control Valves - Flow Characteristics

Flow capacities vs. stem opening..

The relationship between control valve capacity and valve stem travel is known as

• the Flow Characteristic of the Control Valve

Trim design of the valve affects how the control valve capacity changes as the valve moves through its complete travel. Because of the variation in trim design, many valves are not linear in nature. Valve trims are instead designed, or characterized, in order to meet the large variety of control application needs. Many control loops have inherent non linearity's, which may be possible to compensate selecting the control valve trim .

Inherent Control Valve Flow Characteristics

The most common characteristics are shown in the figure above. The percent of flow through the valve is plotted against valve stem position. The curves shown are typical of those available from valve manufacturers. These curves are based on constant pressure drop across the valve and are called inherent flow characteristics .

• Linear - flow capacity increases linearly with valve travel.
• Equal percentage - flow capacity increases exponentially with valve trim travel. Equal increments of valve travel produce equal percentage changes in the existing C v .
• A modified parabolic characteristic is approximately midway between linear and equal-percentage characteristics. It provides fine throttling at low flow capacity and approximately linear characteristics at higher flow capacity.
• Quick opening provides large changes in flow for very small changes in lift. It usually has too high a valve gain for use in modulating control. So it is limited to on-off service, such as sequential operation in either batch or semi-continuous processes.
• Square Root

The majority of control applications are with valves with linear, equal-percentage, or modified-flow characteristics.

Installed Control Valve Flow Characteristics

When valves are installed with pumps, piping and fittings, and other process equipment, the pressure drop across the valve will vary as the plug moves through its travel.

When the actual flow in a system is plotted against valve opening, the curve is called the Installed Flow Characteristic .

In most applications, when the valve opens, and the resistance due to fluids flow decreases the pressure drop across the valve. This moves the inherent characteristic:

• A linear inherent curve will in general resemble a quick opening characteristic
• An equal percentage curve will in general resemble a linear curve

Related Topics

Control valve sizing, related documents, ball valves - flow coefficients cv, control valves - leakage classification, process controllers - p, pi & pid, steam control valves - calculate flow factor kv, straight through diaphragm valves - flow coefficients, valve authority, valve selection guide, valve type classifications, valves - specific services, valves - typical operating temperatures, weir diaphragm valves - flow coefficients and flow factors.

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What is Travel Stop in Control Valve?

A travel stop in the control valve is used as a mechanical limiter for stem travel . In fact, these travel stops are used to limit the stem travel at certain % travel whether limited to travel in the up direction or down direction. This restriction usually comes because of the capacity limitation in the downstream of the control valve.

Travel Stop in Control Valve

For example, we have a pressure control valve that serves as the over-pressure control of a slug catcher separator. The downstream of this valve is a flare header.

The flare itself has a certain maximum flow rate capacity. If the flow rate that can go through the control valve at 100% open exceeds the flare capacity then we need to put some travel stops.

Let’s say flare capacity is 300 mmscfd, after selecting our control valve, we get the result that for 100% opening it will have 350 mmscfd flow rate. The flow rate of 300 mmscfd is around 80% opening. So, we will put travel stops at 80% opening so that the full opening of the valve will only 80% with a flow rate below 300 mmscfd.

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• Compare BDV and SDV Valves
• Control Valve Calibration Procedure
• Self Actuated Control Valve Principle
• Inspection of Control Valves

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Flow Characteristics

The Flow Characteristic of a Control Valve is the Relationship between the Control Valve Capacity and valve stem travel.

Inherent Characteristics represents this relationship with constant pressure drop across the valve.

Installed Characteristics represents the same relationship but with varying pressure drop due to losses in the pipeline and any equipment installed in series.

Equal Percentage

Equal Percentage is considered to be the most common characteristic used in process control. The inherent characteristic of a equal percentage plug allows for flow capacity to increase exponentially with valve stroke. i.e. equal increments in valve stroke result in equal percentage change in the existing Cv. When Installed, an equal percentage plug will tend to behave similarly to a Linear Characteristic valve in Inherent conditions.

The Linear characteristic creates equal changes in Cv per unit of valve stroke, when the pressure drop is constant. Linear plugs are frequently used in systems where the differential pressure through the valve corresponds to the major part of the total differential pressure of the system. When installed, and due to the changes in pressure drop across the valve, a Linear trim will tend to behave similarly to Quick-Open trim.

Quick-open plugs are used in on-off services and are designed to create large increments of flow rate, even from small opening percentages.

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Control Valves

• Control Valve Capacity
• Control Valve Sizing for Water Systems
• Control Valve Sizing For Steam Systems
• Control Valve Characteristics
• Control Valve Actuators and Positioners
• Controllers and Sensors

Back to Learn about steam

This tutorial briefly describes the basic components of different types of linear and rotary action control valves available for use in steam and water systems.

Block 6 of The Steam and Condensate Loop considers the practical aspects of control, putting the basic control theory discussed in Block 5 into practice.

A basic control system would normally consist of the following components:

• Control valves
• Controllers.

All of these terms are generic and each can include many variations and characteristics. With the advance of technology, the dividing line between individual items of equipment and their definitions are becoming less clear. For example, the positioner, which traditionally adjusted the valve to a particular position within its range of travel, can now:

• Take input directly from a sensor and provide a control function.
• Interface with a computer to alter the control functions, and perform diagnostic routines.
• Modify the valve movements to alter the characteristics of the control valve.
• Interface with plant digital communication systems.

However, for the sake of clarity at this point, each item of equipment will be considered separately.

Whilst a wide variety of valve types exist, this document will concentrate on those which are most widely used in the automatic control of steam and other industrial fluids. These include:

• Valve types which have linear and rotary spindle movement.
• Linear types include globe valves and slide valves.
• Rotary types include ball valves, butterfly valves, plug valves and their variants.

The first choice to be made is between two-port and three-port valves.

• Two-port valves ‘throttle’ (restrict) the fluid passing through them.
• Three-port valves can be used to ‘mix’ or ‘divert’ liquid passing through them.

Two-port valves

Globe valves

Globe valves are frequently used for control applications because of their suitability for throttling flow and the ease with which they can be given a specific ‘characteristic’, relating valve opening to flow.

Two typical globe valve types are shown in Figure 6.1.1. An actuator coupled to the valve spindle would provide valve movement.

The major constituent parts of globe valves are:

• The bonnet.
• The valve seat and valve plug, or trim.
• The valve spindle (which connects to the actuator).
• The sealing arrangement between the valve stem and the bonnet.

Figure 6.1.2 is a diagrammatic representation of a single seat two-port globe valve. In this case the fluid flow is pushing against the valve plug and tending to keep the plug off the valve seat.

The difference in pressure upstream (P1) and downstream (P2) of the valve, against which the valve must close, is known as the differential pressure (ΔP). The maximum differential pressure against which a valve can close will depend upon the size and type of valve and the actuator operating it.

In broad terms, the force required from the actuator may be determined using Equation 6.1.1.

In a steam system, the maximum differential pressure is usually assumed to be the same as the upstream absolute pressure. This allows for possible vacuum conditions downstream of the valve when the valve closes. The differential pressure in a closed water system is the maximum pump differential head.

If a larger valve, having a larger orifice, is used to pass greater volumes of the medium, then the force that the actuator must develop in order to close the valve will also increase. Where very large capacities must be passed using large valves, or where very high differential pressures exist, the point will be reached where it becomes impractical to provide sufficient force to close a conventional single seat valve. In such circumstances, the traditional solution to this problem is the double seat two-port valve.

As the name implies, the double seat valve has two valve plugs on a common spindle, with two valve seats. Not only can the valve seats be kept smaller (since there are two of them) but also, as can be seen in Figure 6.1.3, the forces are partially balanced. This means that although the differential pressure is trying to keep the top valve plug off its seat (as with a single seat valve) it is also trying to push down and close the lower valve plug.

However, a potential problem exists with any double seat valve. Because of manufacturing tolerances and differing coefficients of expansion, few double seat valves can be guaranteed to give good shut-off tightness.

Shut-off tightness

Control valve leakage is classified with respect to how much the valve will leak when fully closed. The leakage rate across a standard double seat valve is at best Class III, (a leakage of 0.1% of full flow) which may be too much to make it suitable for certain applications. Consequently, because the flow paths through the two-ports are different, the forces may not remain in balance when the valve opens.

Various international standards exist that formalise leakage rates in control valves. The following leakage rates are taken from the British Standard BS 5793 Part 4 (IEC 60534-4). For an unbalanced standard single seat valve, the leakage rate will normally be Class IV, (0.01% of full flow), although it is possible to obtain Class V, (1.8 x 105 x differential pressure (bar) x seat diameter (mm). Generally, the lower the leakage rate the more the cost.

Balanced single seat valves

Because of the leakage problem associated with double seat valves, when a tight shut-off is required a single seat valve should be specified. The forces required to shut a single seat globe valve increase considerably with valve size. Some valves are designed with a balancing mechanism to reduce the closing force necessary, especially on valves operating with large differential pressures. In a piston-balanced valve, some of the upstream fluid pressure is transmitted via internal pathways into a space above the valve plug, which acts as a pressure balancing chamber. The pressure contained in this chamber provides a downforce on the valve plug as shown in Figure 6.1.4, balancing the upstream pressure and assisting the normal force exerted by the actuator, to close the valve.

Slide valves, spindle operated

Slide valves tend to come in two different designs; wedge gate type and parallel slide type. Both types are well suited for isolating fluid flow, as they give a tight shut-off and, when open, the pressure drop across them is very small. Both types are used as manually operated valves, but if automatic actuation is required, the parallel slide valve is usually chosen, whether for isolation or control. Typical valves are shown in Figure 6.1.5.

The parallel slide valve closes by means of two spring loaded sliding disks (springs not shown), which pass across the flow-path of the fluid, the fluid pressure ensuring a tight joint between the downstream disk and its seat. Large size parallel slide valves are used in main steam and feedlines in the power and process industries to isolate sections of the plant. Small-bore parallel slides are also used for the control of ancillary steam and water services although, mainly due to cost, these tasks are often carried out using actuated ball valves and piston type valves.

Rotary type valves

Rotary type valves, often called quarter-turn valves, include plug valves, ball valves and butterfly valves. All require a rotary motion to open and close, and can easily be fitted with actuators.

Eccentric plug valves

Figure 6.1.6 shows a typical eccentric plug valve. These valves are normally installed with the plug spindle horizontal as shown, and the attached actuator situated alongside the valve.

Plug valves may include linkages between the plug and actuator to improve the leverage and closing force, and special positioners that modify the inherent valve characteristic to a more useful equal percentage characteristic (valve characteristics are discussed in Module 6.5).

Ball valves

Figure 6.1.7 shows a ball valve consisting of a spherical ball located between two sealing rings in a simple body form. The ball has a hole allowing fluid to pass through. When aligned with the pipe ends, this gives either full bore or nearly full bore flow with very little pressure drop. Rotating the ball through 90° opens and closes the flow passage. Ball valves designed specifically for control purposes will have characterized balls or seats, to give a predictable flow pattern.

Ball valves are an economic means of providing control with tight shut-off for many fluids including steam at temperatures up to 250°C (38 bar g, saturated steam). Above this temperature, special seat materials or metal-to-metal seatings are necessary, which can be expensive. Ball valves are easily actuated and often used for remote isolation and control. For critical control applications, segmented balls and balls with specially shaped holes are available to provide different flow characteristics.

Butterfly valves

Figure 6.1.8 is a simple schematic diagram of a butterfly valve, which consists of a disc rotating in trunnion bearings. In the open position the disc is parallel to the pipe wall, allowing full flow through the valve. In the closed position it is rotated against a seat, and perpendicular to the pipe wall.

Traditionally, butterfly valves were limited to low pressures and temperatures, due to the inherent limitations of the soft seats used. Currently, valves with higher temperature seats or high quality and specially machined metal-to-metal seats are available to overcome these drawbacks. Standard butterfly valves are now used in simple control applications, particularly in larger sizes and where limited turndown is required.

Special butterfly valves are available for more demanding duties.

A fluid flowing through a butterfly valve creates a low pressure drop, in that the valve presents little resistance to flow when open. In general however, their differential pressure limits are lower than those for globe valves. Ball valves are similar except that, due to their different sealing arrangements, they can operate against higher differential pressures than equivalent butterfly valves.

There are always a number of options to consider when choosing a control valve. For globe valves, these include a choice of spindle gland packing material and gland packing configurations, which are designed to make the valve suitable for use on higher temperatures or for different fluids. Some examples of these can be seen in the simple schematic diagrams in Figure 6.1.9. It is worth noting that certain types of gland packing produce a greater friction with the valve spindle than others. For example, the traditional stuffing box type of packing will create greater friction than the PTFE spring-loaded chevron type or bellows sealed type. Greater friction requires a higher actuator force and will have an increased propensity for haphazard movement.

Spring-loaded packing re-adjusts itself as it wears. This reduces the need for regular manual maintenance. Bellows sealed valves are the most expensive of these three types, but provide minimal friction with the best stem sealing mechanism. As can be seen in Figure 6.1.9, bellows sealed valves usually have another set of traditional packing at the top of the valve spindle housing. This will act as a final defence against any chance of leaking through the spindle to atmosphere.

Valves also have different ways of guiding the valve plug inside the body. One common guidance method, as depicted in Figure 6.1.10, is the ‘double guided’ method, where the spindle is guided at both the top and the bottom of its length. Another type is the ‘guided plug’ method where the plug may be guided by a cage or a frame. Some valves can employ perforated plugs, which combine plug guidance and noise reduction.

Summary of two-port valves used for automatic control

By far the most widely used valve type for the automatic control of steam processes and applications is the globe valve. It is relatively easy to actuate, it is versatile, and has inherent characteristics well suited to the automatic control needs of steam.

It should also be said that two-port automatic control valves are also used within liquid systems, such as low, medium and high temperature hot water systems, and thermal oil systems. Liquid systems carry an inherent need to be balanced with regard to mass flows. In many instances, systems are designed where two-port valves can be used without destroying the balance of distribution networks.

However, when two-port valves cannot be used on a liquid system, three-port valves are installed, which inherently maintain a balance across the distribution system, by acting in a diverting or mixing fashion.

Three-port valves

Three-port valves can be used for either mixing or diverting service depending upon the plug and seat arrangement inside the valve. A simple definition of each function is shown in Figure 6.1.11.

There are three basic types of three-port valve:

• • Piston valve type
• • Globe plug type
• • Rotating shoe type

Piston valves

This type of valve has a hollow piston, (Figure 6.1.12), which is moved up and down by the actuator, covering and correspondingly uncovering the two-ports A and B. Port A and port B have the same overall fluid transit area and, at any time, the cumulative cross-sectional area of both is always equal. For instance, if port A is 30% open, port B is 70% open, and vice versa. This type of valve is inherently balanced and is powered by a self-acting control system. Note: The porting configuration may differ between manufacturers.

Globe type three-port valves (also called ‘lift and lay’)

Here, the actuator pushes a disc or pair of valve plugs between two seats (Figure 6.1.13), increasing or decreasing the flow through ports A and B in a corresponding manner.

Note: A linear characteristic is achieved by profiling the plug skirt (see Figure 6.1.14).

Rotating shoe three-port valve

This type of valve employs a rotating shoe, which shuttles across the port faces. The schematic arrangement in Figure 6.1.15 illustrates a mixing application with approximately 80% flowing through port A and 20% through port B, 100% to exit through port AB.

Using three-port valves

Not all types can be used for both mixing and diverting service. Figure 6.1.16 shows the incorrect application of a globe valve manufactured as a mixing valve but used as a diverting valve.

The flow entering the valve through port AB can leave from either of the two outlet ports A or B, or a proportion may leave from each. With port A open and port B closed, the differential pressure of the system will be applied to one side of the plug.

When port A is closed, port B is open, and differential pressure will be applied across the other side of the plug. At some intermediate plug position, the differential pressure will reverse. This reversal of pressure can cause the plug to move out of position, giving poor control and possible noise as the plug ‘chatters’ against its seat.

To overcome this problem on a plug type valve designed for diverting, a different seat configuration is used, as shown in Fig. 6.1.17. Here, the differential pressure is equally applied to the same sides of both valve plugs at all times.

In closed circuits, it is possible to use mixing valves or diverting valves, depending upon the system design, as depicted in Figures 6.1.18 and 6.1.19.

In Figure 6.1.18, the valve is designed as a mixing valve as it has two inlets and one outlet. However, when placed in the return pipework from the load, it actually performs a diverting function, as it diverts hot water away from the heat exchanger.

Consider the mixing valve used in Figure 6.1.18, when the heat exchanger is calling for maximum heat, perhaps at start-up, port A will be fully open, and port B fully closed. The whole of the water passing from the boiler is passed through the heat exchanger and passes through the valve via ports AB and A. When the heat load is satisfied, port A will be fully closed and port B fully open, and the whole of the water passing from the boiler bypasses the load and passes through the valve via ports AB and B. In this sense, the water is being diverted from the heat exchanger in relation to the requirements of the heat load.

The same effect can be achieved by installing a diverting valve in the flow pipework, as depicted by Figure 6.1.19.

• Mechatronics and Motion Control

Positioner Guidelines

G uidelines are never a substitute for good engineering practices and experience but they are better than proceeding down a trial-and-error learning path.

Guidelines for all positioner types

Positioners can reduce control valve deadband caused by friction. Most control valves without positioners, even those using ‘low friction’ packing material, may demonstrate a 5% deadband. Deadband greater than 1% can produce loop controllability problems. Use of a positioner can reduce deadband caused by friction to less than 1%.

Positioners can reduce the effects of frictional stick/slip. Control valves with significant stick/slip have poor performance as witnessed by a limit cycle graph (see CE, Oct. ’99 ). Limit cycle occurs when a valve first sticks and then jerks (slips) to a new position. Frequently the stick/slip action causes process variables to overshoot the setpoint. Control action reverses the signal to the control valve and the stick/slip action is repeated in the opposite direction. A positioner’s stem feedback can reduce stick/slip effects and maintain the process variable closer to the setpoint.

Split ranged control elements normally require positioners. Processes requiring extended flow rangeability may use two control valves. The first valve operates over the first half of the range. When the first valve is nearly 100% open, the control action begins to open the second valve. This is called control valve split ranging. When split ranging is performed on the pneumatic signal to the valve actuators, positioners on each valve are used to achieve full valve travel over the reduced input range. Most applications need a predictable overlap region in the middle of the signal to avoid a zone of no control. Positioners provide accuracy to ensure the correct overlap exists.

Positioners can be used to increase seating force and improve shutoff. A positioner will drive the output pressure to either zero or full supply pressure whenever the valve reaches a physical travel stop. An air-to-close actuator can use the full supply pressure to provide greater valve seat loading. Completely removing the air signal from air-to-open actuators allows the full force of the spring to load the valve seat.

Double acting actuators must have a positioner. All double acting piston actuators must have a positioner because the pressure on both sides of the piston must be precisely controlled. This can only be accomplished with the stem feedback in a positioner.

Positioners with feedback cams can be used to linearize control valve and process characteristics. Care should be used when applying cam linearization, generally cam linearization works only for slow (i.e., temperature) reacting process loops.

Two-stage pneumatic analog positioners have superior performance over single-stage pneumatic analog positioners. Positioners using a nozzle flapper and relay are called two-stage pneumatic analog positioners, and positioners using a spool valve are single stage pneumatic analog positioners. Single stage positioners offer the advantage of being very simple in mechanical design, but will not provide the performance of two-stage positioners. The nozzle flapper in two-stage positioners provides very high gain for accuracy, and the relay ensures high airflow for fast response. Fast response is especially important for fast processes such a liquid flow and pressure.

Controller tuning can be modified to stabilize a ‘nervous’ fast process with a positioner. Older guidelines stated that control valves in fast processes should not have positioners because the loops become too nervous and hard to control. That was true of pneumatic controllers because of their limited adjustment range.. Today’s microprocessor controllers provide broad tuning adjustment ranges. The controller gain can be set much lower than historical guidelines to stabilize fast processes and controller integral (reset) can remain fast to match the fast process.

Guidelines for digital positioners Intelligent digital positioners can extend capabilities and benefits of traditional analog positioners. Using digital communications, such as HART and FOUNDATION Fieldbus, digital positioners provide the features listed above, but can also provide the following additional benefits.

Calibration can be performed automatically and remotely. Digital positioners can perform the same zero and span calibration in a few minutes. A task that can take a few hours with non-digital positioners.

Characterization is provided on the output signal. The output signal can be characterized to match the system to achieve a linear process with constant gain. Digital positioner’s characterization benefit, over cam linearization described above, is the linearization performed on the output signal, not the feedback from the valve stem.

Digital noise filters can be applied. Filters should be applied carefully, but where appropriate a digital filter time constant can be applied to minimize the effects of excessive process noise. Users should remember filters add to process response times. Applying a filter will likely require retuning the control loop.

Positioners can generate alarms to the operator interface. Users can assign positioner-based alarms such as valve travel deviation from the input signal, travel beyond a certain point, and others. These alarms can be displayed on operator graphics.

Maintenance related data could be provided. Digital positioners can track valve reversal and total stem travel data that can be correlated with time and actual maintenance events to improve predictive maintenance forecasting.

Valve stroke speeds can be slowed. Applications where hydraulic ‘hammering’ might occur can use a digital positioner to slow the valve stroke.

Valve travel limits can be applied. Applications where a valve should never reach full close can have travel limits applied in digital positioners.

Installed valve performance testing can be automated. Digital positioners communicate well with control valve performance software. Following a pre-defined test, data curves and calculated results can be compared with previous tests to help determine if a control valve requires maintenance. Being able to compare control valve performance can save time and money during planned outages by focusing maintenance activities on the control valves needing maintenance.

Digital positioners are less susceptible to vibration influences. The solid state electronics in digital positioners provides a device with few moving parts and helps maintain positioner performance in high vibration installations.

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26 Feb, 2019

Control Valve Selection

A previous blog-post discussed the importance of control valve sizing and energy optimization opportunities. This blog-post will focus more on the topic of control valve selection although, both topics shouldn’t be considered to be mutually exclusive. 

When selecting a control valve for process plant, there are many things to be considered. These can include the valve flow characteristic, size, valve body and trim materials, noise, potential for damage from cavitation or flashing, actuator type and size, dynamic response to changes in control signal etc. This summarises the typical considerations when making a control valve selection. 

Selecting an improperly sized control valve can have serious consequences on safety, operation and productivity. The following list outlines some of the things to consider when making a control valve selection:  

• Give careful consideration to selecting the correct materials of construction. Take into consideration the components of the valve that come in to contact with the process fluid such as the valve body, the valve seat or any other valve components exposed to the process fluid. 
• Consider the operating temperature and pressure the control valve will be exposed to. Consider the local ambient atmosphere and any corrosives that can occur which may affect the exterior of the valve. 
• Consider the degree of control you require and ensure the selected valve is mechanically capable of achieving the desired operating conditions. 
• Consider the inherent flow characteristic of the control valve you are selecting. Different valve types have different flow characteristics. The flow characteristic can be generally thought of as the change in rate of flow in relationship to a change in valve position. This item is discussed in a little more detail later. 
• Aim for optimal valve travel. When a valve is sized correctly, the range of operation will correspond well to the control range of the valve. Some industry literature recommends travel at normal flow should fall within 50 to 70 percent opening angle. Travel at maximum flow should fall below 90 percent whereas travel for minimum flows should be above 20 percent open to avoid erosion of the trim. When modelling a control valve in FluidFlow, the software enunciates a warning message if the valve position falls outside of an optimal operating range. Users can also adjust the desired settings for minimum and maximum valve positions. This helps prompt the designer into considering valve position for the given design operating conditions. Ultimately, it also helps the designer make a better and more appropriate valve selection for the application in hand. 
• Avoid oversizing a control valve. If the control valve is too large for the required application, only a small percentage of travel is required. This is due to only a small change in valve position having a large effect on flow which in turn makes the valve hunt. This can cause excessive wear. Some published literature sources recommend sizing a control valve at about 70% to 90% of travel. 

This list represents just some of the criteria to be considered when selecting a control valve. It is generally recommended that the final valve selection is discussed with an appropriate and experienced supplier or manufacturer before making your final selection. 

Control Valve Flow Characteristic

The flow characteristic of a valve represents the inherent relationship between the valve opening and flow rate. As a valve gradually opens, the flow characteristic allows a certain amount of flow though the valve at a particular opening percentage. This permits predictable flow regulation through the valve. The most common flow characteristics are linear, quick opening and equal percentage. 

Linear Flow Characteristic

This flow characteristic exhibits a linear relationship between valve position and flow rate. The flow through the valve varies directly with valve stem position. 

Linear Flow Characteristic – FluidFlow.

Quick Opening Flow Characteristic

The flow characteristic of a quick opening valve is such that for a relatively small initial change in valve stem travel, a large increase in flow occurs. The noticeable characteristic of this valve type is that maximum flow is achieved at a relatively low percentage of the valve stem range. 

Quick Opening Flow Characteristic – FluidFlow.

Equal Percentage Flow Characteristic

The flow characteristic of an equal percentage valve produces equal percentage changes in the existing flow for equal increments of valve travel. The change in flow rate is proportional to the flow rate just before the change in position is made. 

The above summarises the most common valve flow characteristics.  

Control valves actually have two characteristics, inherent and installed characteristic. The inherent characteristic is that published by a valve manufacturer based on tests conducted on a system where care is taken to ensure the pressure drop across the valve is held constant at all valve opening positions and flow rates. The inherent characteristic therefore represents the valve flow capacity and valve opening position when there are no system effects involved.

The installed characteristic is the relationship between the valve position and flow in the system taking into account any changes in the pressure differential available to the control valve due to the flow squared relationship between flow and piping pressure losses and/or the behaviour of a centrifugal pump’s head curve. 

The performance of control valves in a process system can have a dramatic effect on the plant efficiency, asset life cycle costs and overall profitability. It therefore goes without saying that the cost-effective operation of any plant, industrial or otherwise, requires considered design and careful control valve sizing and selection. A correctly sized control valve can provide significant savings as well as increase process availability, reduce process variability and reduce maintenance costs. Correctly sized control valves also last longer than unmatched or incorrectly sized valves. 

Oversized valves have a higher capital cost and tend to cause instability in the operation of the system whereas undersized valves simply won’t pass the required flow of fluid in the line. 

As designers, it is therefore worth giving careful consideration to both the sizing and selection of the control valve to affect efficient and effective operation of a process plant whilst optimising operating costs. 

Selecting the Right Device

The essential steps required to optimize control valve performance as well as prevent erosion problems include proper valve sizing and selection of valve body and trim materials. They could mean the difference between continued operation and unplanned shutdowns. There are of course other decisions involved in selecting the right valve solution. Many companies choose globe type valves for their proven performance and life cycle advantages. When compared to other available valve designs, this valve offers:

• Better control performance.
• Better low or partial load performance. 
• High differential pressure across the valve. 
• Smaller physical profile than a comparable ball valve. 
• Use for steam, water or water/glycol fluids. 

In general, a globe valve modulates flow through movement of a valve plug in relation to the ports located within the valve body. The plug is connected to the valve stem which in turn (no pun intended !!!) is connected to the actuator. 

Importance of Trim Material

Proper control valve selection can result in a high level of performance but how can this be maintained? Like other piping components, control valves can wear over time which can produce continued deterioration of the initial control valve performance. Left unchecked, this progressive deterioration can eventually lead to failure, shutdowns we well as the associated repair costs and financial impact of equipment shutdowns. 

Trim refers to the internal elements of a control valve and these elements are a crucial consideration in the process of valve selection. Trim typically includes the valve seat, disc and stem as well as the sleeves within the valve which are required to guide the stem. The interface between the disc and seat along with the relation of the disc position to the seat normally determines the performance of the control valve. 

A control valve’s trim may be selected to create a variety of passage shapes that control the flow in specific ways. The gap within the valve opens by moving the plug, disc or valve away from the seat. The length of the valve stroke determines the opening size and how much fluid passes the seat. Changing the size of the internal gap can increase, decrease or maintain the flow though the valve. Whenever the process parameter or variable being controlled does not equal the design requirement, the control valve operates and alters the opening to achieve the setpoint conditions. 

Manufacturing plants can encounter significant problems from erosion or weakening of valve bodies or trim components from severe process conditions. Typical damage can include seal rings and gasket loss, stem, body and trim retainer wear on the seat ledge, plug, seat ring and cage wear and packing leakage. 

There are several common causes for premature trim wear in control valves. One example would be where flashing occurs, i.e., when the pressure of the flowing fluid falls below its vapor pressure and changes the fluid phase-state from a liquid to a vapor. Small vapor cavities are formed under these conditions which cause wear at the outlet of the valve and its trim components. 

Cavitation is similar to flashing except the fluid pressure recovers to a level above its vapor pressure at flowing conditions. This causes the vapor cavities to implode producing impinging jets with the potential cause severe erosive damage. Outgassing occurs when the pressure of a fluid drops below the saturation pressure of a dissolved gas. When this point is reached, the gas separates from the fluid or solution and produces a high velocity erosive vapor droplets. The simplest way of appreciating this occurrence is to think of an unopened can of soda/soft drink/fizzy pop.  Once we open the can, which of course is under pressure, the sudden pressure drop causes some of the carbon dioxide to escape from the solution as a gas. When the outgassing condition arises in a flow system, in addition to the wear from vapor droplets, it can lead to vibration and eventually the trim can no longer shutoff the flow or maintain the desired flow stability. 

Benefits to plant operators 

Demanding business or manufacturing environments require the most accurate and reliable control of production processes possible. The failure to meet and achieve specific operating standards can produce an inherently inefficient plant, can lead to serious consequences for quality and safety and can significantly affect the financial margins for the final product. Optimum control valve performance is therefore vital in preventing such scenarios. 

Industrial organisations can benefit greatly from working closely with their manufacturer representatives or instrumentation suppliers to initially specify appropriate measurement and control devices. This collaboration can achieve important performance criteria including:

• Precise flow and pressure control. This produces stable and consistent production results. 
• Efficient energy usage.
• Reduced operating costs.
• Fewer unplanned and undesirable plant shutdowns.
• Increased plant availability.
• Lower maintenance and repair costs resulting in longer valve trim life. 

Control valves are required to withstand the erosive effects of the flowing fluid while maintaining an accurate position to control the process. In order to successfully perform these tasks, control valves need to be sized accurately and correctly for the application as well as being designed, built and selected such that it is appropriate for the process operating conditions. 

References:

• Processing Magazine.

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About On/Off Valves

Snap-shut threaded on/off valves.

For applications that require intermittent operation, such as spraying and washing, push or pull the lever on these valves to start flow. They spring closed when the lever is released.

Zinc valves are good for low-pressure applications. Use them as a control valve for tools such as blow guns and tire-changing machines.

Brass valves are stronger and less brittle than zinc valves for use in higher-pressure applications.

Push - lever valves are commonly used in car wash systems. However, you can’t use them as blow guns. Those with a Viton® fluoroelastomer rubber seal stand up to long-term use with harsh solutions, such as wheel and tire cleaners. Valves with a TFE plastic seat are more durable than those with a Viton® fluoroelastomer rubber seat. For even more durability, choose those with a thick spring — their spring is stronger and lasts longer than standard springs. You can install valves with a mounting stem and nut in a fixed position. Valves with offset ports come with a wand bracket to attach an upholstery cleaning wand, so they’re best for floor and upholstery cleaning equipment. Connect the bracket to the top of the offset ports.

Replace worn components on 30° elbow valves with a repair kit (sold separately).

Flow coefficient (Cv) is the amount of water (in gallons per minute) at 60° F that will flow through a fully open valve with a difference of 1 psi between the inlet and the outlet.

Fast-Acting Compact Threaded On/Off Valves

How Do Control Valves Work?

The control valve contains two major components. One is the valve body that actually controls the flow of fluid. Actuator is another major part that moves valve body either manually or by using electric, pneumatic or a hydraulic signal.

The control valve opens and closes to regulate the flow of fluid through a pipe. Actuator receives signal from a controller such as flow meter, pressure sensor or temperature sensor and controls the valve body according to received signal from a controller. This process continues until the process variable reaches desired set point or comes within the desired operating range.

Check how different types of control valves work in the below given posts.

• How temperature control valve works?
• How level control valve works?
• How flow control valve works?
• How pressure control valve works?
• How control valve actuators works?

Control Valve Characteristics

The relationship between the control valve opening (also known as ‘valve travel’) and the flow through valve is known as the flow characteristic of that valve. An inherent flow characteristic is the relation between valve opening and flow under constant pressure conditions.

The gain of a valve is defined as the change in flow per unit % change in the valve opening.

• The linear flow characteristic has a constant slope, meaning that valves of this type have constant gain through complete range of flows. These valves are often used for liquid level control and certain flow control operations requiring constant gain.
• Equal percentage valves are known by that name because whenever the valve opening is changed, the percentage change in flow is equal to percentage change in the valve opening. This means the change in flow proportional to the flow just before the incremental valve opening is performed. This can also observed in the following figure. This type of valves is commonly used for pressure control applications. They can be considered for applications where high variations in pressure drop are expected.
• Quick opening type of valves does not have a specific mathematical definition. These valves give a large increment in flow for relatively smaller valve opening, as can be observed in the following figure. These valves usually find use for on-off service applications.
• Modified parabolic valves lie somewhere between the linear valves and equal percentage valves. As can be observed in the following figure, they can be used for throttling at low flow levels and have almost linear characteristics at higher flows.

Types of Valves

Valves can be broadly categorized based on the type of stem movement – linear or rotary type.

Globe valve

Globe valves are most commonly used liner stem motion type control valves. The flow control for this type of valves is achieved by motion of a plug as shown in the following figure. The shape and type of the plus also determines the valve flow characteristics.

Gate valves use linear type of stem motion for opening and closing of valve. These valves use discs as closure member, as can be seen in the following figure. The faces of this disc can be either parallel or the disc can be wedge shaped.

Butterfly Valve

Butterfly valves are known for their compact size and low initial costs, which is primarily due to the small wafer and body size of these valves and the simplicity of this design. This valve belongs to the rotary stem motion type of valves.

These valves use a spherical shaped closure member with a cylindrical bore through the member for passage of flow. This spherical closure member has to be rotated by 90 degrees to bring the valve from fully closed position to fully open position. This type gets the name from the shape of the closure member. If the diameter of the cylindrical bore is same as that of the connecting pipe, the valve is known as full bore valve. If this diameter is less than that of the connecting pipe, the valve is known venturi style valve. These valves rotary stem motion.

Click the button and check the given guide for details on types of control valves . Also study the use of different type of valves as control valves based on application as well as services and caution where not to use it.

Sizing of Control Valves

The orifices of control valves can be adjusted to control the flow through them. Control valve sizing and selection is based on a combination of theory and empirical data. The capacity, characteristic, rangeability and recovery are four important elements for selection of a control valve . C v is known as flow coefficient or orifice coefficient of a valve. This coefficient C v is related to the flow and pressure conditions by the following basic liquid equation.

The required C v for a valve can thus be calculated based on flow and pressure conditions. This C v should then be matched to that of the available valves and a suitable valve should be selected so that the required C v falls between 70% to 90% of the selected valves C v capability. The possibility of maximum and minimum process flows has also to be taken into consideration while selecting the valve.

Many flow cases may often fall outside the range of the basic liquid equation mentioned above for C v calculation. For these cases a modified liquid equation for C v calculation is given as,

The basic equation mentioned above for liquids is given below in the modified form for gases.

The capacity of a control valve is represented as,

C d = C v / d 2 (d being the diameter of the valve)

Valve characteristic is the relation between valve opening (valve travel) and flow through the valve.

Rangeability of valve can be defined as the ratio of maximum to minimum flow over which good control can be achieved by using the valve.

Recovery is refers to the pressure recovery from the low pressure at vena contracta to the valve outlet. Pressure recovery is high for well streamlined valves.

Control valve sizing calculation is crucial for maintaining efficient and reliable industrial processes. By following the proper sizing process and selecting the appropriate valve type, engineers and operators can ensure the optimal performance of their industrial processes. This post contains a sample control valve sizing calculation which demonstrates steps to be followed when solving control valve sizing problem for flow control.

Parts of Control Valves

Control valves are typically composed of several key components that work together to control the flow of fluid in a system. Check the given post for detailed study on different parts of control valves.

Control Valves Applications

In the Process industry whether it is oil and gas, refinery, petrochemical, chemicals, specialty chemicals, pharmaceutical, power etc., to control the process or process related utilities some control elements is required. Control valves play a crucial role in maintaining desired process conditions such as pressure, temperature, flow rate, and level in process industries. Check detailed post on control valve applications.

Selection of Valves

Different structures of different types of valves make them suitable for a wide range of applications. For each application some types of valves are more suitable compared to others. Check this post that lists some criteria for selection of valves that are most suitable for a given application or service.

Control Valve Troubleshooting Guidelines

Valve p&id symbols.

Study here various commonly used valve P&ID symbols ( Piping and Instrumentation Diagram symbols) for manual valves such as gate valve , ball valve , plug valve, butterfly valve , 3 way valve, globe valve , check valve etc.

Control Valves P&ID Arrangement

Take a quick look at typical P&ID arrangement for control valves along with basic elements used in designing and operating control valves.

Typical Control Valve Datasheet

In order to be able to order the best control valve suited for the application, the user has to provide the valve manufacturer with all the necessary information to select and design a suitable control valve.

A typical control valve datasheet captures important parameters for its design and selection - like operating conditions, design conditions, line size, valve type, actuator type etc. This information help engineers and professionals make informed decisions.

Types of Valve Actuators

Control valve actuators are devices that are used to control the opening and closing of control valves in various industrial processes. These actuators are responsible for converting the control signal into mechanical motion, which then positions the valve accordingly. Check different types of valve actuators in this post.

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Tire Valve Stem Core Quickly Replacement Tool Kit,with Stem Removal Tool Set,Perfect Tool for Replacing Leaky Valves in Cars,Trucks,Motorcycles, etc.with 4PCS TR413 Valve Stems

1SET Tool & 4PCS TR413 Valve Stems

2 Set Tools

Purchase options and add-ons

• ★【Essential Super Tools】Very simple and quick valve replacement tool, even when the tire can not be simply removed for modification and repair, this set of tools can come in handy.And the stem-repair-tool-set that comes with it is easy to remove and install all types of tires, compatible with all vehicle tire valves.This set suitable for cars, light trucks, small trailers, unicycles, lawn mowers, ATVs and motorcycles.
• ★【Useful Toolkit】Each tool in the kit will perform its full function. The T-hook will quickly pull out the damaged valve stem, the tapered positioning tube will provide positioning when the new valve stem is installed, and the push rod will push the new valve stem into the valve hole and hold it in place.
• ★【201 Stainless Steel】All metal parts in the kit are made of 201 stainless steel, which can keep well even in a wet and oil-filled workshop, rust and corrosion resistant. But there is no doubt that all tools need to be maintained with care.
• ★【Universal But Attention】This kit is for .453 valve stems only and will not work with 5/8" valve stems and will not work with tires with Tire Pressure Monitoring Systems(TPMS). The kit comes with 4PCS TR413 valve stems, so if that's what you need, you don't have to repurchase the stems.
• ★【How to Use】 1-Please deflate the tire first, then insert the T-hook into the gap between the hub and the valve. 2-After the T-hook is inserted, rotate the T-hook so that it hooks over the valve and pull it outward. 3-Screw the new valve onto the pushrod.Apply lubricant to the bottom of the new valve. 4-Align the positioning tube with the valve hole and fully advance the push rod through the positioning tube. 5-Pull the pushrod outward until the valve is installed and unscrew it.

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Product Description

★ Essential Super Tools ★ ★ Features: ——Very simple and quick valve replacement tool, even when the tire can not be simply removed for modification and repair, this set of tools can come in handy. ——Suitable for cars, light trucks, small trailers, unicycles, lawn mowers,ATVs,motorcycles and etc.. ——The T-hook will quickly pull out the damaged valve stem, the tapered positioning tube will provide positioning when the new valve stem is installed, and the push rod will push the new valve stem into the valve hole and hold it in place. ——All metal parts in the kit are made of 201 stainless steel, which can keep well even in a wet and oil-filled workshop, rust and corrosion resistant. ——This kit is for .453 valve stems only and will not work with 5/8" valve stems and will not work with tires with Tire Pressure Monitoring Systems(TPMS). ★ How to Use★ 1-Please deflate the tire first, then insert the T-hook into the gap between the hub and the air valve. 2-After the T-hook is successfully inserted, rotate the T-hook so that it hooks over the valve and pull it outward. 3-Screw the new valve onto the pushrod. It is recommended to apply lubricant to the bottom of the new valve. 4-Align the positioning tube with the valve hole and fully advance the push rod through the positioning tube. 5-Pull the push rod outward until the valve is installed and unscrew it to complete the replacement. ★ Attention: 1.Due to the different monitor and light effect, the actual color of the item might be slightly different from the color showed on the pictures. 2.Please allow little measuring deviation due to manual measurement. ★ Package Include: A set of valve stem quick change tool with 4PCS TR413 Valve Stems & Stem Removal Tool Set(Spool Remover x2, Four-Way Valve Tool x1, Spool x10, Stem Cover x10).

Product information

Technical details, additional information, warranty & support, looking for specific info, customer reviews.

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Customers say

Customers have negative opinions about the hook size and ease of removal of the auto accessory. They mention that the hook is too large to fit in the hole to pull the valve stem out. They also have issues with durability. Customers also have mixed opinions on ease of use, performance, value, and quality.

AI-generated from the text of customer reviews

Customers are mixed about the performance of the tire inflator. Some mention that it works well, while others say that it didn't work like it should, and was useless. Some say that the tool didn''t pass the valve to pull out the tire.

"... Ended up not working at all .As others ave mentions, the hook to pull out the old stem is total junk. It broke before pulling the old stem out...." Read more

"...I would definitely buy this again because of the build quality, functionality and storability...." Read more

" Didn't work with alloy rims as I couldn't get the tool pass the valve to pull out the valve stem...." Read more

" This kit works great . I did a valve stem repair on the wheel of my Jeep and the whole process took less than 10 minutes...." Read more

Customers have mixed opinions about the ease of use of the auto accessory. Some mention that the install of the new valve stem was easy, while others say that it's not as easy as the videos or directions showed. Some customers also mention that using the forcing cone was a little difficult.

"...spray silicone down the valve stem insert tube, the valve stem goes into the wheel easily ...." Read more

"...The new valve stem slid in super easy .I added air to the tire and checked it for leaks and it was perfect...." Read more

"...stem through the hole in the rim and mysteriously, the instructions don't give much detail on how to position the "funnel" (go on Youtube)...." Read more

" Valve stem installer worked well . It saved me the cost of removing the tire from the wheel and rebalancing charge." Read more

Customers are mixed about the value of the auto accessory. Some mention it's money well spent, a cool kit, and cheaper than taking the tire in. Others say it'd be a total waste of money.

"... Well worth it ." Read more

" This kit is awesome . You can change out the valve while the tire is still on the rim. Make sure all the air pressure is out the tire...." Read more

"...It worked and Ill keep it but is way overpriced . Glad I didn't get the $45 version." Read more

"...But this worked just fine, and was cheaper than taking the tire in ." Read more

Customers are mixed about the quality of the auto accessory. Some mention it's a great tool, while others say it has poor quality and a very poor design.

"...It is also very weak and bent when I tried to push through. Not great quality ." Read more

"...Landed perfectly on the first try. No leaks and the tool is good as new for next time...." Read more

" Kinda of cheaply made ." Read more

"...But let me tell you this tool did a great job ...." Read more

Customers are dissatisfied with the durability of the auto accessory. They mention that the stems are flimsy and break easily. The puller tool gets stuck and breaks after the first use. The tapered funnel breaks and the tube cracks in half almost the first try to insert.

"...The old stem broke off and a portion remains in the tire. On an Offroad vehicle like mine it’s ok...." Read more

"...Second, they must put the bend in it with the metal cold. The hook busted off first use ...." Read more

"...The shaft of the hook bends easily , could be made of stronger materials...." Read more

"...It broke before pulling the old stem out ...." Read more

Customers are dissatisfied with the hook size of the auto accessory. They mention that the tool is both flimsy and the hook is too large. They also say that the insert tool wasn't small enough to fit in the valve hole.

"To start with, the hook on the end is so big that it barely fits through the hole that the valve stem is in in. That’s problem enough...." Read more

"...The product had to be modified when I got it. The hook itself was too wide to enter the valve step hole so I had to narrow it with a hammer...." Read more

"...The problem is the inside diameter of the rim was too small to allow the tool to line up correctly, even after we cut some of the components down...." Read more

"...The T-shaped removal tool was cheap and weak! It struggled to get into the hole where the valve stem was and then it couldn't get it out...." Read more

Customers find the removal of the old stem difficult with the tool. They mention that the hook is too large, the extractor tool would not come out, and the tool gets stuck in the hole. Some say that the tool is useless with no hook on the end.

"...It was a bit tricky to get the stem through the hole in the rim and mysteriously, the instructions don't give much detail on how to position the "..." Read more

"...to get into the hole where the valve stem was and then it couldn't get it out ...." Read more

"...being said, after modifying the hook, the product worked great removing the old valve stem and replacing with a new one." Read more

"...bent easily, although the valve came out, the tool bent and was hard to remove ...." Read more

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How To Fix a Leaky Street Valve on Your Water Line

Time: 10 minutes

Complexity: Beginner

Cost: $0 - $20

Repack a Stem Seal on a Valve

If you're dealing with a leaky street-side valve at your water meter, you don't have to pay the utility to come out and shut off the water so you can fix it. You can repack the stem with the water on.

You will get some water spray once you loosen the packing nut, however. So before starting, be sure to move anything you care about away from the meter area. It would be a shame to have to deal with water damage  after such an easy fix.

Clear the valve

Most old valves have rust and mineral deposits in the receiving groove. That crud can jam the gate and damage the valve.

Gently close the valve first. Don't force it shut; you don't want to cause more damage. Then open the cold faucet at your laundry tub.

Next, open the street-side valve slightly. The rush of water will flush out the crud. Shut off the laundry faucet, then the street-side valve (in that order).

Wind on special packing rope

Buy Teflon packing rope at any hardware store or home center. With the valve in the off position, loosen the packing nut and slide it toward the handle. Then repack the stem .

Wrap one wind of stem packing rope around the stem. Tighten the packing nut and open the valve. If it still leaks , back off the packing nut and add a few more winds of packing rope.

And there you have it, a quick and easy fix for a leaky street valve. You can solve this problem in no time!