Triplex Lift Station Controls

Why three pumps

A duplex station has one pump sized for peak flow and one spare. When peak flow exceeds what one pump can move, or when the utility wants a spare while two pumps run, the station gets a third. Two philosophies exist and they produce different control sequences. In the first, two pumps carry the design peak and the third is standby: the station never needs all three, and losing one changes nothing about capacity. In the second, all three are needed at peak, and losing one means the station is short at the worst moment. The second is cheaper and is common at stations that grew into their flow; the control narrative should say which the station is, because the alarm priorities and the operator response depend on it.

Roles and rotation

With three pumps the sequence has three roles. The lead starts first at lead-on. The lag starts at lag-on if level keeps rising. The standby, or second lag, starts at a third setpoint above that, or on a run-time or rate-of-rise call as for a duplex station. Rotation advances all three roles one position at each all-off event, so that over three cycles each pump has been lead, lag, and standby once.

In dry weather the standby pump never runs under this scheme, because the third setpoint is never reached. That is the reason run-hour balancing is more attractive on a triplex station: choosing the lead by lowest run hours pulls the third pump into rotation as lead every third cycle, and the standby role is only idle for the cycle it holds it. A weekly exercise run of whichever pump has not run is the simpler alternative.

Availability

The availability rule from a duplex station generalizes. Build the ordered list of pumps that are in AUTO, not faulted, not locked out, and not failed to prove. Assign lead, lag, and standby down that list from the current rotation point. With two available, the station runs as a duplex and the loss of redundancy is alarmed. With one available, that pump is lead every cycle, and the alarm is higher priority because a second failure is an overflow. A pump that fails to prove while holding a role is dropped and the roles below it move up immediately, not at the next cycle.

(* Roles from availability and rotation. Rotation advances 0,1,2 at each all-off. *)
Avail_Count := 0;
FOR i := 1 TO 3 DO
    IF Pump[i].Available THEN
        Avail_Count := Avail_Count + 1;
        Avail_List[Avail_Count] := i;
    END_IF;
END_FOR;

(* Rotate the available list by the rotation pointer, then assign roles in order. *)
FOR r := 1 TO Avail_Count DO
    idx := ((r - 1 + Rotation) MOD Avail_Count) + 1;
    Role[Avail_List[idx]] := r;       (* 1 = lead, 2 = lag, 3 = standby *)
END_FOR;

Redundancy_Lost := Avail_Count < 3;
Station_Critical := Avail_Count < 2;

The third pump on the system curve

Three pumps discharging into one force main each see the head that the combined flow produces. The friction loss rises with the square of flow, so each added pump slides further up its curve and delivers less. A representative station might see 1,000 gpm with one pump, 1,600 with two, and 1,950 with three: the third pump adds about a third of what the first one did. On a long force main it can be less. That has two consequences. The station wet weather capacity comes from the system curve, not from three nameplates, and the third stage setpoint should be placed with the understanding that it buys less time than the second did. Where the third pump was bought for capacity rather than redundancy, the system curve is the check on whether it delivers it.

Electrical and panel

• Starting: the three motors start in sequence with a delay between them, never together. On a generator the delay lets the set recover voltage and frequency between starts, and the generator is sized for two motors running when the third starts across the line, or for the reduced inrush of soft starters or drives.
• Service: the service and the main are sized for all three running, since on the day it matters they will be.
• Panel: three starters or drives, three HOAs, three sets of seal-leak and thermal monitoring, and a controller with the I/O for all of it, in an enclosure whose heat calculation includes three drives if drives are used.
• Backup: the float backup path calls pumps through relay logic as for a duplex station, usually calling two on the backup-start float and the third on the high-level float, subject to the redundant-off float.
• Force main: air release at high points and a check valve per pump; a common discharge header with isolation valves so any pump can be removed with the other two in service.

What to trend and alarm