Control valve

Introduction

Control valves are elements used in process control loops to adjust process variables such as flow, liquid levels, pressure, temperature etc. Control valves essentially consist of a valve and an actuator; more valve control elements may also be present in certain cases.

The operation of a control valve involves positioning its movable part (the plug, ball or vane) relative to the stationary seat of the valve. The purpose of the valve actuator is to accurately locate the valve plug in a position dictated by the control signal.

There are several ways of providing this actuation, below list of the most used ones:

• Pneumatic
• Electric
• self-adjustment

In most cases, an actuator is driven by a pneumatic positioner by an electric motor, or even self-controlled (self-regulator).

Internal parts of the control valve

These valves achieve the desired controlling effect essentially by throttling the flow. Figure 1 below shows details

Figure 1

Undoubtedly the most used is the globe valve, as in Figure 1 above, and the other one has an internal cage drilled and a trim which flows inside this cage and closes or opens the holes according to the required flow rate, For more details see Figure 2 below.

Figure 2

There are many other types (we will see them later). Figures 1 and 2 are the most widely used.

Control valve characteristic

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

Figure 3 below shows the different curves with each other.

Figure 3

• 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.
• The 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 services, such as sequential operation in either batch or semi-continuous processes.
• Hyperbolic
• Square Root

Most control applications are with valves with linear, equal-percentage, or modified-flow characteristics.

Applications

It is essential to correctly size a control valve for the application so that the process works effectively and efficiently. A control valve should be selected according to the application flow requirement, not the line size it is installed in.

General rules: How do you decide which valve control to use? Here are some rules of thumb

Linear Characteristics:

• Used in liquid level or flow loops.
• They are used in systems where the pressure drop across the valve is expected to remain fairly constant (i.e. steady-state systems).
• Used when the pressure drop across the valve is a large proportion of the total pressure drop.

Equal Percentage Characteristics:

• They are used in processes where large changes in pressure drop are expected.
• They are used in processes where the valve permits a small percentage of the total pressure drop.
• They are used in temperature and pressure control loops.
• When an equal percentage valve is installed in a system where, due to pressure losses, not only is the shape of the relationship between valve travel and flow changed but the fully open valve flow capacity is significantly reduced, this can occur due to pressure losses in system piping and other pressure-consuming components such as elbows, isolations valves, heat exchangers, centrifugal pumps, etc.

Quick Opening Characteristics:

• Used for frequent on-off service.
• Used for processes where “instantly” large flow is needed (i.e. safety systems or cooling water systems).
• The Quick-Opening characteristic control valve has a flat disk instead of a contoured valve plug. Its flow (or Cv) increase rapidly to its maximum flow with minimum initial valve opening. At the initial or lower portion of the travel position, the valve gain Kv is too high for use in modulating control. Thereafter, the slope is flattish, where the flow rate hardly increases with valve opening m. Therefore, such control valve is limited to ON-OFF service and application or in the specific application which requires fast initial release or discharge of flow.

Two rules of thumb for choosing the right flow characteristic:

• If most of the pressure drop is taken through the valve and the upstream pressure is constant, a linear characteristic will provide better control.
• If the piping and downstream equipment cause significant resistance to the system, an equal percentage will provide better control.

 Globe Control valve

Comparing linear and equal percentage valves, a linear valve might have a 25% valve opening for a certain pressure drop and flow rate, whilst an equal percentage valve might have a 65% valve opening for exactly the same conditions. The orifice pass areas will be the same.

The physical shape of the plug and seat arrangement, sometimes referred to as the valve ‘trim’, causes the difference in valve opening between these valves. Typical trim shapes for spindle-operated globe valves are shown in Figure 4

 Figure 4 The shape of the Trim determines the valve characteristic

In this Module, the term ‘valve lift’ is used to define valve opening, whether the valve is a globe valve (up and down movement of the plug relative to the seat) or a rotary valve (lateral movement of the plug relative to the seat).

Rotary valves (for example, ball and butterfly) each have a basic characteristic curve, but altering the details of the ball or butterfly plug may modify this. The inherent flow characteristics of typical globe valves and rotary valves are compared in Figure 5

Figure 5

Anti Vibration-Cavitation control valve

 Figure 6

Control valves equipped with noise-abatement trim, such as a drilled-hole cage or a tortuous path stack, are typically oriented in the flow-to-open direction. This orientation maximizes noise attenuation by allowing the expanding gases to exit the valve trim and continue downstream without re-converging through the valve’s seat ring, as would happen if the flow direction were reversed

 Control valve vibration may be caused by many factors, some generated internally by the process stream and others generated externally. Some of the different forms of flow-induced vibration are explained below.

• Process Stream Energy Release. A flowing process contains considerable energy, a portion of which is released during its journey through a valve. The released energy takes the form of fluid turbulence, viscous drag, generated noise flow instabilities, and other forms of released energy. The released energy also interacts with the valve trim and may cause noticeable vibration.
• Upstream Equipment Pulsations. Compressors, pumps, and other upstream equipment generate mechanical vibration and pressure pulsations that interact with other process equipment and piping in the system. This vibration energy is efficiently carried downstream by the piping system. In addition, pipe configurations such as elbows and tees upstream of the valve can create swirling flows and introduce turbulence, which may affect the valve.
• Pipeline Resonance. Pipelines with long runs between supports can resonate at the first or second bending mode of the system, vibrating up and down in a “U” or “S” shape. Higher-order pipe natural modes (radial and circumferential) may also be excited. These natural modes can be excited by other vibration sources, causing vibration levels to build up and affect process equipment and the supporting structure.
• Support Structure Vibration. Some form of vibration is always present in a running plant, but normally, it is not severe enough to be problematic. However, if the support floor resonates in tune with this vibration, the vibration amplitude can increase to the point where it may become destructive.

The last generation of Cage control valves is equipped with a labyrinth cage to avoid residual vibration. For more details, see Figure 7 below

Figure 7

The cage is built up with many disks superimposed with each other; of course, the diameter and number of the disks depend on the CV valve.

Created this type of structure to avoid eroding and vibrations to a minimum possible. Regarding the vibration and eroding, in addition to the four points mentioned above, there is another problem that is sometimes not considered, the valve working out of the permitted limit due to the increase in production

Most corrosion and vibration problems are caused by the internal CV valve, which is incorrect. Unfortunately, this happens when the valve works on the limit of pressure and flow. The relationship of DP between the inlet and outlet of the control valve is most important. Figure 8 shows a curve of the control valve from the manufacture which always must be followed and never exit from this curve, this is the reason why the datasheet of the control valve is mandatory.

CV calculation

The Kv coefficient defines the water flow (between 5 ° and 40 °), resulting in m3 / h, which passes through a valve with differential pressure (pressure drop) of 1 bar.

Valves are tested by running water through it before publishing the Cv value. It can be assumed that the tests are done with water (unless otherwise noted), using a specific gravity of 1.0, 1.2 centistoke viscosity, and a standard temperature of 60 degrees Fahrenheit, below the

Cv = Q * √ (SG/ΔP) (units in US GPM, psi)

where:

• Q = Flow in US GPM
• SG = Specific Gravity
• ΔP = Differential Pressure (psi)

All control valves have a rated flow capacity expressed as the Cv rating. The valve flow coefficient (Cv) is the number of US gallons per minute of 60 degrees Fahrenheit water that will pass through a fully open valve with a 1 psi pressure drop. For example, a Hi-Flow™ valve with a Cv rating of 10.75 will pass 10.75 gallons per minute of water with a pressure drop of 1 psi across the valve

Flow factor (Kv) is the metric system equivalent of the flow coefficient (Cv). They are also referred to as the International System of Units (abbreviated as SI system).

It is defined as the flow of water in cubic meters per hour (m³/hr) at a pressure drop of one bar with a temperature ranging from 5 to 30 degrees C.

Kv = Q * √ (SG/ΔP) (units in m3/hr, bar)

where:

• Q = Flow in m3/ hr
• SG = Specific Gravity of water ( for water=1)
• ΔP = Differential Pressure (bar)

The Flow Factor (Kv) should not be misunderstood with the Discharge Coefficient (k). Discharge Coefficient (k) is a characteristic non-dimensional factor of a valve used to calculate flow that discharges from a tank to the environment.

To select the proper valve for the application, it is necessary to calculate the needed flow capacity (Cv). The necessary Cv will be dependent upon the pressure drop across the valve. The greater the pressure drop taken across the valve, the greater the flow through the valve. The amount of pressure drop which should be taken depends on the specific application and the pressure available. The required application Cv is calculated with different formulas according to the medium type (e.g., liquid, gas, or steam).

For Gas, use the equation shown with Cv dependent upon upstream pressure, downstream pressure, pressure drop taken, volumetric flow rate needed, temperature, and specific gravity.

The relationship between CV and KV is:

Cv = 1.156 * Kv  or  Kv = 0.864 * Cv

The flow rate for a liquid Q = Cv * Sq. Root of ( ΔP/SG)

• Q = Flow in US GPM.
• Cv = Flow Coefficient of control valve

Unit conversions from Imperial to Metric (vice versa):

• 1 US GPM = 0.227125 m3/hr. 1 m3/hr = 4.402868 US GPM
• 1 psi = 0.06894757 bar. 1 bar = 14.503773773 psi.
• 60 deg F = 15.5 deg C.

Conversion of units of Cv to Kv:

• Cv = 1US GPM * √ (1/1psi)
• Cv = 0.227125 m3/hr * √ (1/0.06894757 bar)
• Cv = 0.2271*3.818* m3/ hr * bar
• Cv = 0.864 m3/ hr * bar
• Cv = 0.864 Kv

Conversion of units of Kv to Cv:

• Kv = 1 m3/hr * √ (1/1bar)
• Kv = 4.4028812454 US GPM * 0.2625786985 *1/psi
• Kv = 4.4028812454 US GPM * √ (1/14.503773773 psi)
• Kv = 1.1561 US GPM / psi
• Kv = 1.156 Cv

To save time from cursed calculations, below there are automatic table calculations for CV and KV, including conversion flow and pressure conversions

CV and KV calculator

The coefficient of Cv

The coefficient of Cv is used by Engineers to size valves. In layman’s terms, you need a big enough hole for the liquid to go through. If the hole is too small, you can get a pressure drop that makes your liquid drop below its vapour pressure. If your liquid drops below its vapour pressure there will get implosions that damage the valve trims, causing wear. It is called Cavitation.

In effect, the actual flow rate begins to deviate from what is predicted using the flow coefficient equation for sizing control valves. This is because the vapour bubbles occupy more volume as the mass of liquid expands during the phase change, creating an additional resistance to flow. The vapour bubble formation in the restriction prevents the flow from increasing any further. The effect is called Choked flow.

Conclusion

Our recommendations are never to install a valve smaller than the flow rate, but not too big either. The calculation must be done with extreme precision according to production requirements.

Installing a valve with a smaller CV than the flow rate could be a hazard. Figure 8 below shows what happens when the CV of the valve is smaller than the flow rate

Figure 8

 As you can see from the Figure above, the internal trim of the valve is fully destroyed

We suggest never to use a control valve that works above 50% but below. Above 50% should be temporary, practically to get the liquid down quickly from a tank or other. Figure 9 below shows the internal cage of the control valve

Figure 9

As you can see from the Figure above, the holes in the basket get bigger and bigger as the valve opens. Therefore a perfect flow rate is when the valve works with small holes.

Figure 10 shows the traditional control valve

Figure10

As you can see from the figure above, the red line shows how a regulating valve that reaches 50% or 60% no longer has any difference in the internal orifice, perhaps from linear becomes a quick opening. This is why a regulator valve must always work below 50% and never above

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