Following an approach from a major customer within the Waste Water sector, Pulsar Process Measurement have developed a new pump station control device based on proven non-contacting ultrasonic measurement techniques. The new unit, named Quantum, features a control set that meets the needs of an organisation controlling multi-pump installations over a wide geographical area, minimising the number of visits by maintenance staff and automating certain functions. This paper is based on the installation of around 200 Quantum units.

The immediate result has been a reduction in ‘nuisance’ call-outs to reset pumps of approximately 40%, benefitting both staff efficiency and reducing their business Carbon footprint; there is also a financial benefit of approximately £28,000 per month on the current installed base. For context, typical equipment cost of a Quantum unit with an associated transducer is approximately £1,200.

Non-contacting ultrasonic level, flow measurement, and control is a well-established technique throughout the worldwide water and waste water industries, with many tens of thousands of installations in place over several decades. The equipment typically consists of a transducer in place above the water/waste water that emits and receives a high frequency (approx 50kHz) sound pulse, and a remote controller unit that together use the speed of sound to determine the distance to the surface. The latest equipment makes use of extremely sophisticated digital echo processing techniques such as Pulsar’s DATEM to disregard competing return signals from items like ladders or flanges, in an often narrow and cluttered environment and provide accurate, robust measurement.

Pulsar were approached to develop a control system that would fulfil the following criteria:

Provide an alarm or warning on ‘time to spill’, allowing the customer to prioritise response in a more
sophisticated way than simply having a ‘high level’ alarm
Monitor for burst mains rising from a wet well
Reduce maintenance visits with an automatic pump reset function

The first two points are related, both having regard to the need to minimise or avoid the risk of environmental pollution from either the wet well spilling over or a rising main bursting.

Time to spill

The likelihood of ‘a spill’ depends both on the rate of change of the level and the operating efficiency of the pumps, which can be affected by failure, blockage or underperformance. If a pump has failed, the rate of change or a ‘high level alarm’ may not be important, because the level may well creep up slowly to reach a dangerous level. More important is to know how much time remains before a critical high level, or an overspill level, is reached.

The Quantum unit continually measures level and so is able to continually calculate the rate of flow into the well, and when the pumps are operating is also able to derive a flow rate out of the well. In this way, an alarm may be set and a warning given when a well is in danger of overflowing so that the emergency teams can be alerted and remedial action taken.

To achieve the time to spill control, the operator uses the menu-driven software system to set a single alarm relay with four setpoints and two functions. It operates in the same way as a high-level alarm, with the usual ‘on’ and ‘off’ points set into the software. In addition, the same relay is programmed with the level that would be considered a ‘spill’ condition and the target ‘time to spill’ parameter. So, for example, the Quantum unit might be programmed with a 45-minute time to spill, and an alarm would be initiated at the point at which, at the current rate of change of level, there are 45 minutes remaining until the spill level is reached. This alarm remains activated until the ‘time to spill’ goes below 45 minutes AND the level is below the ‘off’ point of the high level alarm.

Burst main/blocked pump

Two of the more critical factors that affect the accidental discharge of waste water from a wet well are the problem of a burst in the rising main from the wet well and the inefficiency of a blocked pump. Part of the project brief was to develop a response to these issues. Quantum can be calibrated with the pumping rates of each pump under ideal conditions which can be related, via a volume measurement of the wet well, to an optimum rate of change of liquid level. An alarm can then be set:

i) If the rate of change is much greater than the expected rate, then downstream pressure may have
been reduced, indicating a likely burst rising main.
ii) If the rate of change is much less than expected, then this can indicate a blocked pump.

An additional benefit is that Quantum can be calibrated to the volume of the wet well, so that a flow rate can be measured in litres per second, in many cases removing the need to install Magnetic flow meters, again significantly reducing costs.

Automatic pump reset

Quantum includes ten relays (digital outputs) and seven digital inputs. The digital inputs are used to detect the status of the pumps, and some of the digital outputs are dedicated to the trip circuit of the pumps. Quantum uses the digital outputs to restart tripped pumps, a major saving in time for the maintenance staff who often have to attend site simply to restart tripped equipment. An example of the control routine might be: Quantum will make three attempts to restart a tripped pump, then wait ten minutes before trying again, then a further ten minutes before making three final efforts to restart the pumps. If Quantum has not been able to restart the pump after half an hour, or if the pump trips six times within a twenty-four hour period, then an alarm will warn site management of a possible problem. All time parameters are site adjustable.

In more detail: In a trip event, the trip relay in the control panel closes a digital input on the Quantum unit. The Quantum immediately de-energises the pump relay and brings the back-up pump on line. At that point a ‘reset interval’ timer starts, typically for ten minutes, at the end of which time a reset pulse is provided. If this is successful, then the failure is logged and operation carries on as normal. If the pump remains tripped, the ‘reset interval’ timer restarts. The user can configure the number of consecutive pump resets to attempt before a pump is considered to have failed, and the total number of trips allowed in a rolling 24 hour period before the pump is considered to have failed, even if the pump has been healthy in between. All information is stored for maintenance information.

In addition to these special features, Quantum is also a fully-featured pump controller. Quantum also includes Pulsar’s Tariffguard software, designed to protect the water company from the worst of the high electricity prices. Most electricity suppliers will have occasional TRIAD periods, where electricity costs rocket for a period of hours, and it is clearly an advantage to minimise energy usage during these periods. The Tariffguard software built into Pulsar’s Quantum (and Zenith) units monitors inflow and outflow rates and optionally rain gauges, overriding the normal on/off levels of the control unit to keep pump usage to an absolute minimum during the TRIAD period, for example pumping the well as dry as possible just before the start of the period and allowing the level to rise above the normal start point of the pumps wherever safe to do so.

The Quantum controller also features RS485 digital communications (Modbus & optional Profibus DPV 0 & DPV 1) allowing the status to be monitored and the unit to be programmed remotely. Additionally it has a large on board data-logging ability as standard.