Using PIPE-FLO® Professional to Evaluate Choked Flow
PIPE-FLO® Professional will display a warning when the conditions for choked flow are met. Consider the piping system modeled in PIPE-FLO in Figure 4. Cold water at 60 °F is pumped to a heat exchanger and heated to 180 °F. The flow rate is controlled by a globe valve type of flow control valve located at 30 ft elevation.
With this design, the control valve would operate at about 77% open at 750 gpm with a pressure drop of about 27.7 psi. Since the operating flow rate and dP are below the Choked Flow Rate (813.9 gpm) and Choked dP (32.61 psi) values, the valve will not be choked and this system should operate as designed.
Figure 4. Control valve at 30 ft elevation.
Higher Elevations Increase Susceptibility to Choked Flow
Suppose a last minute design modification was made after all the equipment was purchased and required the heat exchanger and control valve to be located at 45 ft elevation instead of 30 ft. Figure 5 shows that this design change would result in choked flow at the control valve. What caused it to choke?
Because of the higher elevation, the inlet pressure of the control valve is lower (pressure head is converted to elevation head per the Bernoulli Theorum). This reduces the Choked dP and Choked Flow Rate according to Equations 2 and 3. Since the required dP across the control valve (27.49 psi) is greater than the Choked dP (27.44 psi), the valve would operate under choked flow conditions at 750 gpm.
Figure 5. Heat exchanger and control valve located at 45 ft elevation. Choked flow is indicated.
Higher Temperatures Increase Susceptibility to Choked Flow
To evaluate the effect of fluid temperature on choked flow in a control valve, consider the same system in Figure 5 with choked flow at the valve at the higher elevation and higher fluid temperature. If the required fluid temperature was 150 °F instead of 180 °F, as shown in Figure 6, the valve would not be choked and would deliver the fluid to the Hot Water Tank at the designed flow rate of 750 gpm. Why isn't the valve choked even when installed at the higher elevation?
Again the answer can be seen by evaluating the equations above, but it also requires understanding how the fluid properties change with temperature, specifically the vapor pressure of the liquid. Figure 6 shows that the lower temperature liquid results in a higher Choked dP (29.4 psi) and higher Choked Flow Rate (790.4 gpm). Since the actual dP is less than Choked dP, the valve will not be choked with 150 °F water flowing through it, but it will be choked with 180 °F water.
Figure 6. Colder water does not result in choked flow, even with the valve at the higher elevation.
Valve Type Influences Susceptibility to Choked Flow
Different types of valves have different values of the Liquid Pressure Recovery Factor (FL) based on the internal flow passages of the valve. The original, non-choked system in Figure 4 was based on selecting a globe valve design which has a higher FL in the range of 0.9. If a different valve with a lower FL (around 0.83) is selected as shown in Figure 7, choked flow conditions are flagged in PIPE-FLO. The lower FL reduces the values of the Choked dP and Choked Flow Rate, causing the valve to be choked at the designed flow rate.
Figure 7. A different control valve with a lower FL selected for the original system (valve at lower elevation with 180°F) results in choked flow.