Print this article

Comment

01 February 2003

Sailing to the islands

Islands of automation among goals in budget-strapped upgrades.

By Prabhakar Keskar

Sludge and wastewater are not glamorous. They are not in short supply-ever-and they keep coming through the pipe whether you're ready or not.

During the past few years, this organization has replaced, retrofitted, and upgraded existing obsolete chemical feed systems for water and wastewater plants, specifically the polymer feed systems for wastewater sludge handling facilities and lime feed systems for water treatment units.

Some of the problems associated with the existing chemical feed systems have been clogging in the long chemical feed lines, lack of sufficient pressure at the feed end, and obsolescence of existing control system hardware and software.

We consistently find that replacement of existing lime and polymer systems with a longer recirculation loop, very short feed lines, and simple control strategies has worked well to improve the performance of existing chemical feed systems.

Upgrade of the control system hardware and software is another major issue. Due to budgetary constraints, it is normally not possible to undertake the upgrade of an entire plantwide automation and control system while implementing a relatively small chemical feed system upgrade.

A more practical and prudent tactic is a modular approach whereby one creates several "islands of automation" for the plant and assigns one of these several islands to the chemical systems area.

The chemical feed system upgrade project can then deal with the smaller chemical area's island of automation, keeping in mind that this island module should gracefully and elegantly integrate into the long-term plantwide automation upgrade, which will include a number of islands of automation.

EXISTING SYSTEM LOOPS

In our example, concentrated (neat) liquid polymer was stored in polymer storage tanks and pumped into two polymer use tanks via polymer dilution units. A level-based control strategy was used to maintain a preset level in each polymer use tank.

An ultrasonic level transmitter was used to provide the level signals for controlling polymer transfer pumps and maintaining level inside the polymer use tanks.

All control loops were resident in an existing distributed control system (DCS). The basic concept was to use air pressure to push liquid polymer through the dip tube into the polymer feed network via a long polymer feed line originating at each tank and terminating at the polymer feed network.

Polymer diluted by mixing with a preset water flow to the two major polymer feed lines before the lines joined the polymer feed network. Flow and pressure control loops (loops 1-3, 1-4, 2-3, 2-4) were used to maintain pressure and flow in the water lines.

The polymer feed network consisted of seven parallel lines. Each of the seven lines branched into three subfeeds to feed three centrifuges. The polymer feed network fed polymer to a total of 21 centrifuges.

Control loops for all centrifuges were the same. The control strategy for feeding polymer was simple. Measure sludge flow to the centrifuge, and the polymer flow should be a fixed ratio of the sludge flow.

The set point for the polymer flow controller was a preset ratio of the sludge flow. The flow controller output modulated the polymer flow control valve to maintain the preset polymer flow set point. Polymer then injected into the centrifuge and the incoming sludge line.

The operating philosophy of the polymer feed system was to use one of the two polymer tanks for feeding polymer to the centrifuges, while the other tank was in the tank filling and standby mode, such that at any given time, one tank is feeding polymer to the centrifuges while the other tank is being filled or is already full.

WORKING BUT PROBLEMATIC

As good as the system looked on the paper, in practice it was plagued with frequent operational and maintenance problems.

The hydropneumatic polymer delivery system simply did not work well. During high demand periods, the system could not maintain adequate pressure and flow at the distantly located centrifuges. The long polymer lines clogged during low flows, and the inactive branches of the network also clogged frequently, requiring maintenance and repairs.

The existing DCS had lived beyond its useful life and needed upgrade or replacement to improve the overall performance.

MODIFICATION OF DICTATIONS

Brainstorming produced many solutions. One idea was to replace the entire system with 21 small polymer blending and feed units, each dedicated to one of the 21 centrifuges.

This idea dropped out because the cost of the new system was high due to the fact that it necessitated demolishing all existing equipment, piping, and valves.

Circumstances dictated that the new system would allow existing equipment to be reused and salvaged rather than outright demolished. The winning proposal for modification embraced several concepts and actions:

• Convert existing pressurized tanks to atmospheric pressure tanks by venting each tank.
• Reuse existing concentrated polymer tanks but install new blending units for blending and transferring polymer to the two polymer use tanks (converted to atmospheric pressure).
• Reuse existing level transmitters, but replace the existing polymer filling line with new piping and a valve on the fill line at the point where it connects to the tank.
• Provide a pair of variable speed polymer feed pumps-connected in parallel-with suction lines connected to polymer use tank #1 and discharge connected to one of the two existing polymer feed lines.
• Also provide a single variable speed polymer feed pump with suction connected to second polymer use tank and discharge connected to the second existing polymer feed line as shown.
• Add a polymer recirculation line, returning polymer back to the use tanks. Add a flow meter, a pressure transmitter, and a back pressure control valve in the polymer recirculating line.
• Reuse the entire existing piping/valve network for all 21 centrifuges, including associated instrumentation. As in the existing system, only one polymer use tank feeds polymer to centrifuges. The other tank is in fill and standby mode.

As to the control strategy, it remains uncomplicated. The polymer feed system starts with a single lead pump pushing polymer through the system.

DEMAND REACHES THE POINT

First, one selects one of the two use tanks for polymer feed operation.

A flow controller modulates the pump speed to maintain the recirculation flow.

As the number of centrifuges in operation increases, the polymer demand increases and the pump speed is ramped up to maintain the recirculation flow set point.

When the demand reaches the point where the lead pump reaches full speed, the lag pump cuts in. The lead pump ramps down, and the lag pump ramps up, until both pumps ramp up and down in unison in response to polymer demand.

The third pump is a standby. The second pump cuts off if the speed of the pair drops to 60% of the flow set point. The usual features of lead-and-lag pump selection and alternation are in force.

Finally, a back pressure control loop main tains a preset back pressure in the recirculation line.

Replace the polymer feed and dewatering portion of the existing proprietary DCS with a programmable logic controller (PLC)-based open architecture control system with a dedicated computer system as a human-machine interface (HMI), and we have a self-contained module-an island of automation.

PLC PARTIALLY REPLACES DCS

For each location-dewatering station-we removed the existing DCS controllers and replaced them with a single PLC. The PLC and the I/O racks mounted in existing DCS cabinets in place of the DCS control lers and I/O modules. We disconnected all field signals from DCS I/O modules and reconnected to the PLC I/O modules.

Each PLC programmed to reflect the same logic resident in the existing DCS controllers. The new polymer system PLC included new logic for polymer feed control. Each PLC had a dedicated operator interface to allow local control of the related process.

Each PLC used an existing 24-VDC power supply system (batteries/chargers). The existing DCS transmitter cabinets remained in place, and these signals reconnected to the PLC I/O modules.

Each PLC got an Ethernet card and communicates with a dedicated computer/HMI system via a fiber-optic, self-healing Ethernet ring using TCP/IP.

The computer system include a main computer and a hot backup computer with continuously updating databases and com mercially available HMI software packages for supervisory control and data acquisition (SCADA).

Each of the 21 existing centrifuges has a dedicated PLC. These PLCs have a peer-to-peer high-speed data link for inter-PLC communication-a welcome departure from the DCS, which installed before such options were available.

For this DCS replacement project, we added an Ethernet interface to the existing PLC peer-to-peer data link to allow direct high-speed data exchange between the new computer system and the existing centrifuge PLCs, eliminating the need for hard-wired I/O point duplication.

In summary, the DCS replacement project replaced several existing DCS controllers and associated I/O points in different locations with individual PLCs and I/O racks. A total of 1,277 DCS I/O points converted to the PLC system.

DEDICATING A FIBER-OPTIC RING

Partial conversion of the obsolete DCS system to a dedicated PLC-based system for polymer feed and sludge dewatering was an added benefit and also improved the performance of the polymer feed system.

However, the PLC upgrade had to take place in a way that allowed efficient integration of the new PLC system into the overall phased implementation of the plantwide PLC upgrade that would occur over the next few years.

The islands-of-automation concept was used as the basis for the phased implementation. Several major subsystems in the sludge handling facility were candidates for each phase of PLC conversion.

Each subsystem handled the SCADA requirement for one or more subsystems. Each subsystem would have a dedicated Ethernet fiber-optic ring communicating with dedicated HMI or computer workstations.

Each Ethernet ring would then communicate with the central control room with multiple workstations. The five islands of automation include the liquid area, thickener, and digesters; dewatering and polymer feed; sludge dryers; sludge storage, material handling, and railroad loading; O2 plant; and miscellaneous applications.

Each island includes a dedicated fiber-optic Ethernet communication link for the PLCs. All five islands link to the central computer system/HMI via Ethernet.

For the smaller upgrade projects that do not have budgets for plantwide upgrade of control systems, a phased implementation of plantwide control system upgrade by creating islands of automation works well.

For plantwide automation in water/ wastewater plants, the concept of an open architecture distributed PLC system, commercially available SCADA software packages, and Ethernet TCP/IP communication links offers more flexibility, ease of implementation, and expandability than proprietary DCSs. IT

Behind the byline

Dr. P. Y. Keskar is a senior electrical, instrumentation, and control systems engineer with CH2M HILL in Gainesville, Fla. He has 35 years' experience in the design and implementation of control and SCADA systems for the water and wastewater, phar maceutical, power, and pulp and paper industries. He is a senior ISA member and a licensed P.E. in several states. Write him at pkeskar@CH2M.com.