Archimedean Screw Pumps
An Archimedean screw pump is represented as a link of zero length, where the discharge is determined by the head of the upstream node. The boundary condition between the link and a node is that of equal water levels. The pump switch-on level determines when the control first comes into operation; the pump will continue running until the upstream water level drops below the lower switch-off level.
Flow Characteristics
The flow characteristics in the positive direction are determined from the pump characteristic curve (with Hc denoting the closed valve head).
The governing model equation assumes a monotonic increase so that the simulation engine may determine a unique discharge for a given head. The equation is as follows:
|
|
where: Q - Discharge (m3s-1) f(h) - User-defined function h - Head across pump (m) |
The head is equal to the upstream water level and is independent of the downstream level.
The link connects two nodes representing the water level upstream and downstream of the Archimedean pump; the discharge will be identical at each on convergence of the iteration. If the upstream level is below the switch-on level (for instance the pump is off), then QARCH = 0.0, DQ1 = ERR, DQ2 = ERR.
The relationship between discharge and head in the positive direction is represented as a data table, QTAB. The table contains up to 15 discharge values at head values equally spaced by the increment HEADD. The first discharge value must be zero. Intermediate valves are linearly interpolated from the table. The discharge corresponding to a head outside the bounds of the table is extrapolated from the table.
The pump status is updated every major time-step. If, at a minor timestep, the pump status is on but the upstream level drops below the switch-off level an intermediate switch-on state is entered; this conserves volume.
