Valve curve

Figure 7-8. The new inlet valve position can be determined from the valve curve.

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 air-flow control, the system may contain a control valve or damper that automatically or manually modulates system pressure drop. The dotted curves in Figure 4a on each side of the system resistance curve might represent operating extremes of the system resistance as the control valve is varied from maximum to minimum opening. These curves also intersect the fan curve at desirable operating portions of its range both for efficiency and flow control. 

System Curves In addition to the pump design, the operational performance of a pump depends upon factors such as the downstream load characteristics, pipe friction, and valve performance. Typically, head and flow follow the following relationship ... 

FIG. 10-78 Approximate performance curves for a rotary positive-displacement compressor. The safety valve in discharge line or bypass must he set to operate at a safe value determined hy construction. 

Curve 4 Sieve-valve plate,, 38-mm weir, 49 valves, 140 holes... 

The rated discharge is at a static head of /7j and a motor h.p. P. In the process of controlling the discharge from Q to and Qy, the valve is throttled, which increases the head loss of the system (or system resistance) from A/ji to and H,yy respectively. The operating point on the curve now shifts from point A[ to /It... 

Consider Figure 6.42 with typical Q-Hj curves at different speeds and different system resistances, introduced by the throttle. Point A refers to the rated discharge at rated speed and head when the throttle valve is fully open. Lets us consider the condition when the discharge is to be reduced to say, 0.67 0. ... 

Caution should be taken to prevent excessive acceleration. One solution is to provide for opening of the compressor discharge vent valve to increase the compressor flow and increase blower horsepower. This is effective with a centrifugal compressor, however, not with an axial compressor unless the compressor is provided with adjustable stator vanes that are reliable. This can be seen from the performance curve. The head versus flow curve for a given vane setting is extremely steep and opening the vent valve is ineffective. However, if the vanes operate fully open and the vent valve opens, the combined effect is satisfactory. The vent valve must not be oversized. 

Figure 4-143 represents a computer-generated plot that demonstrates the effectiveness of eddy current brakes in preventing overspeed of the string. The lower curve assumes the butterfly valve characteristic is linear from 60° open to the closed position. The rate of closure is 3.25 sec. (Butterfly valves are normally used to throttle the expander inlet gas.)... 

The top curve of overspeed versus time demonstrates that the string will accelerate to 22% overspeed due to expansion of the gas trapped between the valve and expander nozzle ring. However, if an eddy current brake were part of the string, the acceleration of the string would be reduced to 11% overspeed. This would provide an important margin of safety. 

Step 8 Calculate step on the bypass valve. Now that the Ac bypass of the bypass valve is known, this must he translated into a new valve position. A similar process as described in Step 6 is used. However, in this case there is not an absolute value of c, but instead a Ac. Therefore, first determine the actual bypass bet- This is accomplished by taking the actual bypass valve position and using tlie manufacturer s curve (Figure 7-9). From this point, the method to determine the new valve position is the same as described in Step 6. 

Fig ure 7-9. The bypass valve position can also be determined from the valve s curve. 

The pumps can be either centrifugal or positive displacement types. The centrifugal pumps should have a head curve continuously rising toward the shut-off point. The standby pump should be piped into the system in a manner that permits checking of the pump while the main pump is in operation. To achieve this, a restriction orifice is required with a test bleeder valve piped to the return oil line or the reservoir. 

Figure 1. Enter on the abscissa at the water vapor pressure at the valve inlet. Proceed vertically to intersect the curve. Move horizontally to the left to read rc on the ordinate (Reference 1).

Figure 3. These are characteristic curves of common valves (Reference 2).

The equation gives results within about 7% of the aforementioned pump curves. This means within 7% of the curve valve, not 7% absolute, i.e., if the curve valve is 50%, the equation will be within the range 50 + 3.5%. 

Figure 1. Bhp per million curve mechanical efficiency—95% gas velocity through valve—3,000ft/min (API equation).

When a pump has a motor drive, the process engineer must verify that the motor will not overload from extreme process changes. The horsepower for a centrifugal pump increases with flow. If the control valve in the discharge line fully opens or an operator opens the control valve bypass, the pump will tend to run out on its curve, giving more flow and requiring more... 

To assist the engineer in making estimates, the curve in Figure 3-6 gives values of efficiency plotted against pressure ratios. The values on the curve include a 95% mechanical efficiency and a valve velocity of 3,000 feet per minute. Table 3-1 and Table 3-2 are included to permit a correction to be made to the compressor horsepower for specific gravity and low inlet pressure. They are included to help illustrate the influence of these factors to the power required. The application of these factors to... 

For wide ranges of operation, multiple valves help keep the efficicncic high (see Figure 7-15). As power output is increased, the lower curve flow... 

Characteristic curves. 3 Charles Law, 15 Check valves, 472 Chord, 226 Clearance... 

Figure 14. Capacity correction factors due to overpressure for relief and safety relief valves in liquid service. The curve shows that up to and including 25 percent overpressure, capacity if affected by the change in lift, the change in oriflce discharge coefficient and the change in overpressure. Above 25 percent, the valve is at fidl lift and capacity is affected only by overpressure.

Figure 17. Variable or constant back pressure sizing factor, Kw for 25% overpressure on balanced bellows safety relief valves (liquids only). The curve represents conqiromise of the valves reconunended by a number of relief valve manufacturers. This curve may be used wiien the make of dw valve is not known. When the make is known, the manufacturer should be consulted for the correction factor.

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