Fluid_Flow_Rules_of_Thumb_for_Chemical

12

Rules of Thumb for Chemical Engineers

Compressible Flow--Short (Plant) Lines

For compressible fluid flow in plant piping, one can use Mak's ~ Isothermal flow chart (Figure 1). Mak's chart was provided originally for relief valve manifold design and adopted by API. 2 The relief valve manifold design method, and its derivation, is discussed in Section 20, "Safety." Mak's methods can be applied to other common plant compressible flow situations. Since Mak's Isothermal flow chart is intended for relief manifold design, it supports calculations starting with Pz, the outlet pressure, that is atmospheric at the flare tip, and back-calculates each lateral's inlet pressure, P1. These inlet pressures are the individual relief valves' back pres- sures. The chart parameter is M2, the Mach number at the pipe outlet. Having M2 is very useful in monitoring prox- imity to sonic velocity, a common problem in compress- ible flow. For individual plant lines the fol lowing cases are easily solved with Figure 1 and the tabulated steps.

2. Determine fL/D. 3. Obtain Z. Figures 5, 6, and 7 are provided for convenience. 4. Calculate Pz/P~.

2. Same. 3. Same.

4. Calculate M2.

See Equation 3. If M2 > 1 flow is choked, so set M2 at 1 and determine a reduced W. Read at the reset value o f M2 = 1 i f applicable. Note: This case (given P2 and W) is the same as an individual lateral in relief manifold design.

5. Get M2 from Figure 1. If below the

5. Get Pz]P1 from Figure 1.

critical flow line, use M2 = 1. 6. Calculate W. See Equation 3.

6. Calculate P~.

Given: P2 and P~ Find: W Steps: 1. Get f from GPSA graph (Figure 4).

P2 and W P~ 1. Same.

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