Installed valve characteristic

The installed valve characteristic. This depends upon the ratio of the pressure drop through the valve to the total pressure drop across the whole process line installation including the valve. If a valve of linear sensitivity is handling the entire system pressure drop, then its installed characteristic will also be linear. As the percentage of the pressure drop falls off, the installed characteristic rapidly changes to that of an on-off valve. The increasing-sensitivity valve also loses its characteristic in the same way, but to a far lesser extent. Hence, this type of valve is frequently chosen in preference to the linear type. Peters(U> has discussed the effects of choosing valves with incorrect characteristics. 

FIGURE 15.20 Installed valve characteristics for a typical equal percentage (=%) valve and linear valve. 

Then, from equation (3.3), for a linear installed valve characteristic, for... 

If a square-law flow meter is used, it can be shown that the most desirable installed valve characteristic is also square law. This is to maintain constant stability as flow rate changes. If a linear flow meter is used, the optimum installed valve characteristic is linear. Most instrument engineers choose equal-percentage valves whose installed characrerisdes most often are scwnewhcrc between equal percentage and linear. A linear flow measurement is therefore a better choice. 

Choice of Valve Characteristics, The choice of valve characteristic and its effect on valve sizing deserve some discussion at this point. A valve with linear behavior would appear to be the most desirable. However, the designer s objective is to obtain an installed valve characteristic that is as linear as possible that is, to have the flow through the valve and all connected process units vary linearly with . Because APv usually... 

This design attempt should attempt to minimize pumping costs by keeping the pump capacity (related to P as low as possible. In no case should AP /AP be greater than 0.33 at the nominal flow rate. Show, by means of a plot of the installed valve characteristic q vs. ), just how linear the final design is. 

If the titration curve is relatively fixed and known, it can be used to provide a feedforward signal. Generally this is simply flow feedforward where the reagent flow is ratioed to the influent flow and corrected by a feedback pH controller (Ref. 1.3(. It relies on a secondary reagent flow controller that receives a remote set point that is the flow ratio multiplied by the actual influent flow corrected by the pH controller output. If a flowmeter is not available for either the influent or reagent flow, an inferential flow measurement can be calculated from the installed valve characteristic and pressure drop. 

Recently, adaptive controllers have been developed to rapidly identify a process model and provide process model parameters that can be displayed, trended, and diagnosed. Changes in the model parameters can reveal changes in feed or reagent compositions and installed valve characteristics and a coating of the sensor and plugging of the valve. Tuning rules can be selected based on user preference to match the process and the plant objectives. Furthermore, these controllers remember the results for similar conditions, eliminate repetitious identification, and take the initiative [Ref. 8.1]. 

FIG. 8-82 Installed flow characteristic as a function of percent of total system head allocated to the control valve (assuming constant head pump, no elevation head loss, and an R equal 30 equal-percentage inherent characteristic). 

The relation between the valve stem position and the flow through the valve at constant pressure drop is termed the valve characteristic. Two characteristics must be evaluated for valve selection, the inherent and the installed characteristics 67. ... 

Note To emphasize the difference between overpressure (safety relief valve characteristic) and accu-mulation (code limitation on the pressure vessel), safety relief valves installed for fire cases will have an overpressure of 10% like most safety relief valves, even if the allowed accumulation on the pressure vessel is 21% in the case of ASME VIII. ... 

A flow control valve will be installed in a process system in which the flow may vary from 100 to 20 percent while the pressure drop in the system rises from 5 to 80 percent. What is the required rangeability of the control valve What type of control-valve characteristic should be used Show how the effective characteristic is related to the pressure drop the valve should handle. 

There essentially two major valve types one with a hnear characteristic and one with an exponential characteristic. One should keep in mind, however, that a hnear valve may not behave linearly when installed in a pipeline circuit. The flow rate is not determined solely by the valve characteristics, but also depends on the resistanee of the pipehne and the pump characteristics. It is therefore essential to evaluate the entire eincuitiy in which the valve is installed. Figure 31.14 shows how the pump pressure varies with the flow through the system. At F = 0 the pump head is APp o- The maximrrm flow without valve is F. The range of the valve covers only a part of the total possible range and varies from F ose. to F... 

If the manipulated valve has a linear installed flow characteristic (preferred), and if there is no level self-regulation (if Ap, does not change appreciably with change in level), then the dynamic response of the proportional-onlyt level control system may be defined by a first-order time constant ... 

For pumped systems rapidly increasing energy costs have aroused considerable interest in designing for very low valve-pressure drops. This pushes us toward line-sized valves. If piping and equipment pressure drops are large in comparison with valve-pressure drops, valve turndown sui rs (becomes smaller). The installed flow characteristic also becomes very different ftom the inherent flow characteristic. A linear valve tends toward an installed square-root flow characteristic, while an equal-perccntJ e valve tends toward an installed linear characteristic. At the hi -flow, high-lift end of the plot, however, all curves, r ardless of y alve inherent characteristic, level off at a maximum flow. This is so because the valve has run out of pressure drop. 

The use of small valve pressure drops makes it more difficult to get a desired installed flow characteristic than if larger valve pressure drops were used. On the positive side, small valve AP s mean less tendency to flashing or cavitation. 

Since we will be looking increasingly at column control systems where column pressure is allowed to float, care must be taken to check valve maximum capacity at minimum APp and valve turndown at minimum flow at maximum APp. These flows, in many cases, will be manipulated to control liquid level, which means that a linear installed flow characteristic is desired. Cascading from level control to flow control is recommended. 

If the PI level control system (or proportional-only) is not a cascade level-flow system, then it is desirable to have a control valve with a linear installed flow characteristic, that is, dQ lddc = constant. Then control-loop dynamics would be independent of flow rate and... 

If pvunp and valve curves are available, and if die hydraulic resistance and static heads of the process equipment are known, we can usually calculate the installed flow characteristic. Alternatively, with permission from production supervision, we can experimentally make a plot of valve loading signal versus flow. The slope of this curve at average rate is tlQj,ldQc-... 

For a cascade level-flow system, the control-valve installed-flow characteristic should also be linear. ... 

Figure 7 Installed flow characteristic curve and installed gain curve fora butterfly and a globe control valve. [1] Other factors affecting control valve perfonnafice ...

Another factor that affects the linearity of the response is the characteristic of the valve selected. Three of the most common valve characteristics include quick opening, linear, and equal percentage. If the majority of the system pressure drop is taken across the valve, then a linear valve should be used since its installed characteristic will also be linear, giving the linear response desired. However, if the pressure drop across the valve is a small part of the total line drop and is not constant, then an equal percentage valve can be used since its installed characteristic will be close to linear. Quick-opening valves are most commonly used with on-off controllers where a large flow is needed as soon as the valve begins to open. More information on these valve flow characteristics can be found in Chapter 2. 

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