GLOBE VALVES—MODULATING/THROTTLING FUNCTION
6.2.9 Flow Characteristics
Flow characteristics, the relationship between flow coefficient and valve stroke, depend on the shape of the disc/plug as well as the valve’s internal geometry. The three most common types of flow characteristics are equal percentage, linear, and quick opening [5.1, 5.2]. Figure 6-12 shows the ideal inherent characteristic curve for each of the flow characteristics. These characteristics can be approximated by contouring the plug.
However, the real curves often deviate considerably from these ideal characteristics because there are body effects and other uncontrollable factors, in addition to the need for maximizing the flow capacity for a particular valve.
Figure 6-12
Inherent Flow Curves for Various Valve Plugs with Constant Delta P Across the Valve
A brief synopsis of each of the three flow characteristics is given below:
• Equal Percentage. Equal percentage is the characteristic most commonly used in process control. The change in flow per unit of valve stroke is directly proportional to the flow occurring just before the change is made. While the flow characteristic of the valve itself may be equal percentage, most control loops will produce an
installed characteristic approaching linear when the overall system pressure drop is large relative to that across the valve.
• Linear. An inherently linear characteristic produces equal changes in flow per unit of valve stroke, regardless of plug position. Linear plugs are used on those systems where the valve pressure drop is a major portion of the total system drop.
• Quick Open. Quick open plugs are used for on-off applications designed to produce maximum flow quickly.
Inherent versus Installed Characteristics: When a constant pressure drop is maintained across the valve, the characteristic of the valve alone controls the flow; this
characteristic is referred to as the inherent flow characteristic. Installed characteristics include both the valve and pipeline effects. The difference can best be understood by examining an entire system.
When placed into service in actual systems and the pump characteristics and piping loss are accounted for, equal percentage, linear, and quick open inherent flow
characteristics change significantly to what is referred to as installed characteristics. The deviation of the installed characteristics from the inherent characteristics depends on the system flow resistance and system head source. In systems with very small flow resistance and constant head (such as between constant pressure upstream and downstream reservoirs), the difference between installed characteristics and inherent characteristics is small.
A typical example is containment isolation valves where the containment represents an infinite reservoir. In systems with high flow resistance and variable head source (such as a centrifugal pump), the difference between installed characteristics and inherent characteristics can be very significant (see Figure 6-13 for a typical example). In Figure 6-13, the inherent equal percentage trim exhibits a nearly linear installed characteristic, while the inherent linear trim appears to be almost quick opening when installed.
Figure 6-13 contrasts inherent characteristics with installed characteristics. The curves in Figure 6-13 show, from the standpoint of proportional band, that in the low flow operating region, for a given flow change, a very small change in lift is required for the linear trim, compared with the equal percentage trim. Thus, the flow rate is sensitive to valve opening in the low flow rate region.
Figure 6-13
Comparison of Installed Characteristics versus Inherent Characteristics
Operating in the higher flow region, the opposite is true; that is, a larger change in lift is required for the same change in flow for the equal percent trim, while the linear trim requires an even higher change in lift. Consequently, overall sensitivity will be
decreased for both trims. The equal percentage trim would exhibit an almost constant sensitivity over the entire operating range, thus requiring only one proportional band setting in the controller. Because the linear trim does exhibit a nonlinear change in flow, as a function of lift, it would require several proportional bands.
In deciding whether an inherent linear characteristic or an inherent equal percentage characteristic should be chosen, the general rule is that if the valve is the primary pressure loss mechanism and the inlet pressure is constant, the linear characteristic should be chosen. However, such a system (having very little system pressure loss and/or constant inlet pressure) is not typical. On the other hand, if pipe and fitting resistance are major factors in the system, equal percentage would be the appropriate choice (which is the case in the majority of applications).
In actual practice, control instruments can be adjusted to handle normally anticipated flow changes without having to be readjusted. It is difficult to determine from control performance whether the valve has linear or equal percentage trim, unless manual control is required, then there will be a tremendous difference.
To illustrate the above flow characteristics, assume that a centrifugal pump supplies water to a system in which a control valve is used to maintain the downstream pressure at 80 psig. The pump characteristics and system flow schematic for this set of
conditions are given in Figures 6-14 and 6-15, respectively. Assuming a maximum flow rate of 200 gpm with a pump discharge pressure (P1) of 100 psig and that pipe friction losses are negligible, the flow coefficient (Cv) can be determined to be 45, using the ISA liquid sizing formula (see Section 24). A 2-inch valve would provide this flow capacity.
Figure 6-14
Typical Pump Characteristics
Figure 6-15
Flow Schematic without Piping Losses
To determine the plug valve characteristics that should be specified, analyze the installed flow characteristic of equal percentage and linear trim for this 2-inch valve.
Based on the typical pump characteristic in Figure 6-14, Table 6-1 shows several values of flow, the required valve Cv and the percent of maximum Cv that the valve must have to control flow.
The inherent percentage of total valve lift for equal percentage and linear plugs can be determined using Figure 6-12. The installed characteristic, plotted as valve lift versus flow in gpm, is shown in Figure 6-16. A study of Figure 6-16 shows that either installed characteristic would provide good control for this situation.
Table 6-1
Valve Cv and Pressure as a Function of Flow Rate without Line Losses
Q Flow Rate (gpm)
P1 Pump Discharge Pressure (psig)
∆P Across Valve (psid)
Cv
Required
Percent of Required Maximum Valve Cv
200 100 20 45* 100
150 125 45 22 49
100 150 70 12 27
50 170 90 5.2 11
* Cv = 45 is assumed to be maximum Cv
Figure 6-16
Installed Characteristics without Piping Losses
In the previous idealized example, the downstream pressure was held constant and pressure drop variations were due to the pump only. A more realistic installation exists where the delivered pressure must be held constant after passing through the valve with some line restriction (R) in series with the valve. This installation is shown schematically in Figure 6-17.
Figure 6-17
Flow Schematic with Piping Losses
To find the installed characteristic of equal percentage and linear trim in a suitably sized valve, a pressure drop distribution must be determined. The pressure drop across the control valve, ∆Pv, is given by:
P1 = Pump discharge head, psig
∆PR = Pressure drop across the restriction, R, psi
=
CR = Flow coefficient of the restriction, R, gpm psi
Let CR = 50 gpm psi
At maximum pump flow rate of 200 gpm, the control valve pressure drop is given by:
psi
The corresponding valve flow coefficient is given by:
The control valve can then be sized for the maximum required Cv of 100 gpm/ psi . A 3-inch control valve would be chosen to handle these maximum flow conditions.
Since the pressure drop across the restriction will vary with flow in accordance with the square root law Q=CR ∆Ρ, the available pressure drop across the valve at various flowing quantities can be determined, keeping in mind the pump characteristics. This is shown in Table 6-2. As before, the percent of maximum Cv that the valve must have to control flow is calculated, and the installed characteristic is plotted, as shown in Figure 6-18.
Table 6-2
Valve Cv and Pressure as a Function of Flow Rate with Line Losses
Q
Figure 6-18
Installed Characteristics with Piping Losses
6.2.10 Rangeability
The Instrument Society of America (ISA), in Standard S75.05, “Control Valve Terminology” (6.39), defines inherent rangeability as the ratio of the largest flow
coefficient (Cv) to the smallest flow coefficient (Cv) within which the deviation from the specified inherent flow characteristic does not exceed the stated limits.
Permissible deviation values between actual and manufacturer-specified inherent flow characteristics for globe and butterfly valve specimens are published in ISA S85.11,
“Inherent Flow Characteristics and Rangeability of Control Valves.” These deviations (or acceptable limits) vary from approximately ±10% at 100% Cv to ±18% at 10% Cv.
A quick opening plug has a fairly linear characteristic over the first 80% of its flow range (Figure 6-12), and the linear characteristic is maintained down to a point close to its seat. Rangeability is generally in excess of 100 to 1, with higher values observed on the larger sizes with plugs having low seating angles.
Linear and equal percentage plugs follow their intended characteristic down to a plug position, at which the flow is a function of the proximity of the seating surfaces rather than of the plug contour. This point generally occurs at around 5% flow in the smaller plugs and drops to about 1% in the larger sizes, giving rangeabilities from 20:1 to as high as 100:1.
This inherent rangeability should not be confused with the range of loads over which it will operate satisfactorily in service. If, for example, a linear plug is selected with a rangeability of 100:1 for a liquid pressure control application, a narrow range of loads would be available over which optimum control could be obtained within the
capability of the controller. An equal percentage plug, even with a lower rangeability than the quick opening plug, would perform well over a wide range of loads. On the other hand, a liquid level loop might operate satisfactorily over a wider range of loads with a quick opening plug than a high rangeability equal percentage plug. Only where the valve characteristic is well matched to the application will the valve rangeability correspond to the range of loads (with constant relative stability) observed in service.
6.2.11 Stability
Valve stability must be given consideration while sizing a control valve actuator. When stability criteria for actuator sizing (discussed in Appendix D1 in Reference 1.2) are not fulfilled, certain valve/actuator combinations can lead to unstable operation. Unstable operation is characterized by oscillations of the stem, sometimes at a very high
frequency, around the desired travel position. In addition to causing poor control (or loss of control), rapid stem cycling can cause quick degradation of the stem packing, actuator rubber diaphragm failure due to fatigue, or damage to the plug and seat areas.
Valve stability is achieved when the actuator rate of change of forces exceeds the rate of change of forces acting on the valve plug. Figure 6-19 shows a typical control valve force balance diagram.
Figure 6-19
Force Balance Diagram for Control Valves
Criteria for stability have been well established for different types of valve internal designs and actuators. In general, increasing the actuator spring stiffness to well above the force gradients, due to fluid forces across the plug, eliminates instability problems.
See Appendix D1 in Reference 1.2 for more detailed quantitative criteria specific to the valve and actuator combination of interest.
6.3 Installation Practices
Valve sizing coefficients are usually determined by the manufacturer from tests with the valve mounted in a straight run of pipe that is the same diameter as the valve body.
If the installed process piping configurations are different from the standard test manifold, the valve capacity is changed.
Control valves are often smaller than the line size in which they are installed, and care should be exercised to ensure proper alignment in the pipeline to avoid overstressing the valve.
Care should be used when laying out piping adjacent to control valves to avoid interference between the control valve operator and the piping.
The valve should be installed with the stem vertical and up. With the stem in other than the vertical orientation, uneven and unpredictable wear can occur on the guides,
guiding surfaces, and seats. The stem packing life will also be shortened. In addition, maintenance becomes more difficult with the stem shifted from the vertical.
Long air lines leading to the air-operated actuator may result in poor control and response.
Changes from the installation design should not be made without first ascertaining that the change will not have an adverse impact on valve operation or seismic integrity, where applicable.
6.4 Operation Practices and Precautions
Unlike most isolation valve operations, control valve operation is automatic and requires no special instructions to the operator. Theoretically, all of the operating parameters are addressed at the outset and are incorporated into the selection and specification so that the final installation will function in a satisfactory manner with no additional operator intervention required.
Unfortunately, there are occasions when, due to improper or incomplete specification, control valves cannot meet the actual system requirements and must be manually operated until such time as replacement parts or a replacement valve can be
substituted. Under these conditions, concerns should be for proper system operation, with the understanding that there is no automatic compensation for process deviations in the control process parameter.
6.5 Common Problems
Common problems encountered with improperly sized and/or specified control valves include:
• Erosion resulting from excessive flow velocities and cavitation.
• Wire drawing caused by operating the plug close to the seat over extended periods.
This most often is the result of oversized trim in the valve.
• Broken, worn parts resulting from excessive vibration.
• Malfunctioning positioners.
• Instability, which may result in poor control, high packing wear, and actuator component wear. See Appendix D1 in Reference 1.2 for a detailed discussion of valve stability.
• Chattering and seat damage when throttling near the seat.
All of the above problems can be the result of operation of the valve beyond the
conditions for which it was designed. This may be due to changes made to the system, incomplete specification data, poor communication between designers and suppliers, or a combination of the above.
6.6 Maintenance Methods
For a general discussion of good maintenance practices, see Sections 17, 18, and 19.
Most control valve manufacturers have highly skilled service engineers available for maintenance and repair of their valves and actuators. Many recurring valve problems are the result of improper maintenance and/or the use of substandard or counterfeit parts. It is recommended that all service and maintenance work be performed by qualified personnel, using authorized parts furnished by the control valve
manufacturer. Training programs are available through the manufacturer to train personnel in the operation, maintenance, and service of the equipment. When ordering replacement parts, always include the model and serial number of the valve being repaired. Periodic inspections should be made to ensure that biasing springs have not vibrated out of adjustment.