previously identified point of highest pressure. We then must calculate the pressure
losses attributable to the hydrodynamic gradients in each of the pipe segments be-
tween the previously identified point of highest pressure and the point in question.
5. If we maintain a running total of pressure losses computed as we complete the
above step, we can stop the calculation procedure as soon as this total pressure loss
exceeds the hydrostatic pressure difference computed above. Otherwise, we must
continue the calculation, in which case we will have identified a new maximum
pressure location.
6. Steps 2 through 5 are repeated until we reach the end of the piping network. At
branching points we will need to proceed out each branch following the procedure
as outlined.
MINIMUM ABSOLUTE PRESSURE CONSTRAINTS
Minimum allowable pressure constraints arise from three distinct considera-
tions.
1. Net Positive Suction Head (NPSH) requirements of the pump.
2. Minimum pressure over atmospheric necessary to preclude the infusion of air
into the system.
3. Pressure necessary to prevent flashing of the liquid.
The constraint resulting from minimum NPSH requirements necessary to pre-
clude pump cavitation only needs to be satisfied at the inlet to the pump. This
pressure requirement will be a function of the saturation pressure and hence the
temperature of the liquid at that point. The NPSH requirement is usually specified
by the manufacturer of the pump. Thus, this constraint is simply
Php,r ≥ PNPSH
(4-18)
where Php,r is the pressure in the return line at the inlet to pump (N/m2) and PNPSH
is the minimum allowable pressure at the pump inlet from NPSH requirements
(N/m2).
The amount of pressure over atmospheric necessary to prevent infusion of air into
the system will be another area where engineering judgment will be required. This
will be an issue primarily in portions of the system that are operating at tempera-
tures below 100C, since the saturation pressure constraint (eq 4-20) will dominate
it at higher temperatures, given equal safety margins. If, as we assumed earlier, no
intermediate pumping is being used, then the minimum pressure level will be at the
inlet to the pump for a system that is at or below the level of the heating plant at all
points. For other systems, we must check for dominance of this constraint or the
saturation pressure constraint derived below, and then constraint satisfaction must
be verified at all points within the system. At the heating plant, this constraint can
be written as
Php,r ≥ Pa + Pasa
(4-19)
where Pa is atmospheric pressure (≈ 105 N/m2) and Pasa is the minimum safety
margin above atmospheric pressure (N/m2).
The second constraint on minimum allowable absolute pressure results from the
requirement that the fluid must be maintained above its saturation pressure some
finite amount to preclude flashing to the vapor phase. The amount of excess pressure
above the saturation pressure of the fluid is a matter of engineering judgment.
Because localized areas of pressure lower than the "bulk" pressure of the fluid may
occur because of hydrodynamic effects, a safety margin above the saturation
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