Delivering Results When The Pressure’s On

Recently, Kelair Pumps supplied a dual variable speed pressure system to a major university which sought to improve water efficiency by using re-use rainwater in toilets over eight floors of a new university block.

The pump system supplied consisted of Lowara model 33SV04 stainless steel vertical multi-stage centrifugal pumps, fitted with 11kw Hydrovar microprocessor controllers with integral frequency inverters. The drives for this system were set to maintain a constant pressure of 800kpa.

Each toilet in the block was fitted with a flush valve (as opposed to a cistern), which requires a minimum operating pressure to function. When activated, the solenoid instantly passes a high volume of water (as opposed to a cistern flush), creating a rapid pressure drop in the main.

Typically, this type of flush system is fed either by a local main or a flusherette tank on the building's roof. In these cases, a reasonably constant pressure is maintained in the system. However, in the case of the university block, the system's pressure relied completely on the pump system, which operated under a suction lift condition. The pump system was required to 'suck' the water from the large rainwater tank located 50m away and below the pump's level.

The system's single 100mm suction line rose 3m for a large part of the run, before returning back down to ground level and into the pumps. This resulted in the possibility of an air lock. During the design stage of the project, Kelair suggested a number of modifications to overcome potential priming problems. These included a 20mm town main feed with RPZ valve connected to the suction line so that the mains pressure could be used to purge any trapped air. A valve was also installed at the highest point of the suction line to facilitate the air's evacuation.

The unit was then commissioned and appeared to operate satisfactorily during the construction phase. However, once the building was handed over and in full operation, an unexpected problem arose. When students finished attending lectures, a large number would use the toilets at once, creating a sudden rise in demand on the pump system. This would cause the discharge pressure to rapidly drop from 800kpa to 400kpa. Before the pump system could react one of the following problems would occur. Either:

  • The drives would shut down on low discharge pressure alarm because they could not reach the necessary pressure within the allowable time.
  • The discharge pipe on the upper floors would empty due to the ramp up time of the system.
  • Once the pumps were up to speed and supplying the required volume the system would refill and would then over-pressurize and create water hammer prior to the pumps shutting down.

Upon inspection, a mismatch was found in some sections of the suction line. This was causing a restriction. This occurred because the dual pump system came as standard with a common 80mm suction manifold with 65mm branches into each pump. However, this did not match the manifolds of the rest of the suction line, which were 100mm. In order to fix this, the mismatched manifolds were changed to 100mm with 80mm branches on the pump inlets. This improved the system's operation but didn't eliminate the problem with the water hammer.

delivering resultsAfter consultation with Kelair's Technical Manager, Jeff Greely, it was determined that the system required additional storage volume to cater for the initial draw off spike. The pumps also needed to be able to accelerate to full speed in under two seconds. Because of the high current draw in accelerating an 11 kW motor from 0 to 50 Hz in under two seconds, the existing 11kw Hydrovar controllers were upgraded to 22kw. The issues with the initial injection volume were overcome by installing 10 x 16 litre capacity hydro-pneumatic accumulators into the discharge line. The reason for using multiple smaller accumulators instead of one or two larger ones was due to the outlet size: all the accumulators have the same 25mm discharge. This application required injecting as much liquid as possible in the shortest possible time. In this case you don't want the 25mm outlet acting as an orifice plate and restricting the flow.

The Kelair Service Technician and site plumber worked over the weekend to fit the new equipment. When the students returned the next week, these changes allowed the pumps to successfully maintain sufficient pressure under high demand with only a 50kpa pressure drop.

Overall, the success of this application demonstrated Kelair's ability to meet the challenges provided by a unique set of circumstances and unusually high demand requirements. Such a result was only possible due to the technical expertise and service capabilities of Kelair Pumps, allied to an excellent working relationship with the plumber and the builder.

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