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 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-

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

(4-18)

where *P*hp,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

(4-19)

where *P*a is atmospheric pressure (≈ 105 N/m2) and *P*asa 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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