POWER March 2013 - 19

2. Pressure points. Locations of drain line pressures referenced
in the article are shown. Courtesy: Bechtel Power Corp.
two-phase nonchoked, or two-phase choked flow. The critical
pressure at the exit is required to determine if the flow is choked
or nonchoked.
Technical literature provides several methods for calculating
this critical pressure. One useful methodology is based on the
homogeneous equilibrium model (HEM) for two-phase flow and
involves simultaneous solution of energy and momentum conservation
equations. That is the approach used for calculating
critical pressure in this article.
This critical pressure at the exit (Pce
) corresponds to choked
) of the fluid at the
the correct pressure at the exit.
Once the pressure at the exit is determined, the pressure
gradient is calculated working backwards from the drain piping
exit at the downstream heater toward the level control
valve outlet, considering single-phase flow or two-phase flow
(using HEM), as applicable.
There are three scenarios to be considered when determining
the end receiver inlet pressure:
■ If Pce
> Psat, then there can be no steam flash, and the entire
■ If Pce
section of drain piping downstream of the level control valve
must be treated as having single-phase flow. In this case, Psat
is considered as the end receiver pressure and the single-phase
pressure drop worked backwards to establish the control valve
outlet pressure P2
< Psat and Pce
.
> P2dh, then steam flash is occurring (under
choked flow conditions) and the section of drain piping
downstream of the level control valve must be considered as
having two-phase flow. In this case, Pce
must be used as the
end receiver pressure instead of P2dh. The two-phase pressure
■ If Pce
drop is then worked backwards to establish the control valve
outlet pressure P2
< Psat and Pce
.
< P2dh, then steam flash is occurring (under
nonchoked flow conditions) and the section of drain piping
downstream of the control valve must be considered as having
two-phase flow. However, in this case P2dh
can be used in the
flow calculation as the end receiver pressure. The two-phase
pressure drop is then worked backwards to establish the control
valve outlet pressure P2
.
Calculate Control Valve Outlet Pressure
If there is no condensate flash, the section of drain piping
downstream of the level control valve (up to the downstream
heater) must be treated as single phase. In this case, the
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19
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flow at the downstream heater inlet based on pipe diameter,
flow rate, and saturation pressure (Psat
temperature it leaves the upstream heater. As discussed below
and shown in Figure 2, Pce
and against the downstream heater pressure (P2dh
can then be compared against Psat
) to select
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