ndustrial filter presses are an efficient means for separating solids in sludge and suspensions from liquids. The press filtration process is an efficient way to extract and separate water, chemicals and other processing fluids (filtrate) from minerals, ores and other solids (sludge cake). Regardless of which byproduct is desired, the process is the same: a sludge or suspension is forced at high pressure through a textile that is permeable only to liquids. The textile filter captures the solids and what remains is the sludge cake free of liquid.
Press filtration systems are in service across numerous industries and applied in a variety of ways. From mining, metallurgy and minerals processing, to potable and waste water, chemical, pharmaceutical, and food and beverage processing, filter presses separate the good from the bad, help recycle and reclaim process elements, and create process streams that send the sludge cake in one direction and the filtrate in another.
Netzsch Filtrationstechnik GmbH of Selb, Germany, is a leading developer of fully automatic and quasi-continuous operation membrane-and-chamber filter presses (See Figure 1). The company's designs feature powerful pumps that supply the filter presses with sludge and enable a very high throughput using more than 100 filter chambers and filter plates in sizes to 2x2 m.
Figure 1: High-Efficiency Filter Power
The filter presses manufactured by Netzsch provide high throughput by using more than 100 2X2 m chambers and plates.
To quickly remove sticky sludge cake from the filter membranes, Netzsch equips its filter presses with an automatic scraping mechanism that moves over the length of the system (See Figure 2). Following the filtration process sequence, which can last up to 90 minutes, the filter chambers are separated one after the other, which allows two internally braced scrapers to remove the sludge cake off the membrane on both sides from top to bottom. A conveyor usually is located below the filter press to transport the product for further processing.
Figure 2: Solution to a Sticky Problem
An automatic scraping mechanism moves over the entire length of the filter press to remove the collected sludge on both sides of the filter membrane, from top to bottom.
Clogged filter membranes are cleaned by a washing mechanism that uses high-water pressure of approximately 70 bar or 1,000 psi.
With the goal of lowering the scraping cycle time even further, the engineers at Netzsch equipped the pneumatic scrapers with electronic servo drives. Thanks to the servos, the cycle time for a single scraping operation was reduced from approximately 25 sec. to 15 sec. per filter chamber, improving the entire cycle time by 25%.
Mobile OI Refines Machine Setup
Another time and cost-cutting opportunity that targeted hardware costs and made the setup routine more flexible, convenient and efficient was next on Netzsch's agenda. It involved an innovative, mobile operator interface-based solution for setting up and adjusting the filter presses.
As innovative as this solution was, the original mobile operator interface (OI) system relied on a ad-hoc solution fashioned from a conventional OI panel "made" portable, connected to the appropriate PLC via a Profibus cable, and unable to display information in any other way except text form.
"The big problem was that the setup required two people: one at the main operator panel located by the switchgear cabinet near the filter, and another with the portable panel near the movable scraper mechanism," says Roland Schelter, head of electrical engineering Netzsch systems. "They were forced to communicate via hand signals and shouts, because full HMI functionality for setup and/or adjustment was available only at the main operator panel."
If multiple systems could be set up using only one mobile operating panel, hardware costs would be lower. Considering the length of the filter presses (up to 18 m) Netzsch manufactures, the single mobile-OI concept would save the second person from having to help perform the setup. That would make the entire procedure significantly more efficient and eliminate all the shouting and gesturing between the two operators the old system required.
Schelter recognized the advantages of a new Mobile Panel 170 operator panel from Siemens (See Figure 3). "I ordered one with a stop pushbutton,with safety collar, integrated into the emergency-shutdown circuit of the machine, two acknowledgment pushbuttons with three switching stages in the back-mounted handle [according to EN 60204-1], illuminated pushbutton, key-operated switch and handwheel," says Schelter. "The base device is reduced to a display and membrane keys, it's the second level of controls that adds the stop pushbutton."
Figure 3: Operator Panel Changes Routine
The new portable operator panel includes a stop pushbutton-with safety collar--integrated into the emergency-shutdown circuit of the machine, two acknowledgment pushbuttons with three switching stages in the back-mounted handle, illuminated pushbutton, key-operated switch and handwheel.
Mounted directly to the scraping mechanism is a connection box which supplies the mobile panel with power and data. The mobile OI panel connects via Profibus, (or alternatively via Windows-based MPI protocol) to the system PLC at this point.
"The connection box allows the panel to be connected or disconnected while the filter press is operating, without interrupting the emergency-shutdown circuit and, as a result, production," says Schelter. "In many operations, two or more filter presses run side-by-side, requiring operator actions only during setup, an ideal application for the mobile panel." For less-stringent safety requirements, a "basic" connection box is available, but cannot be integrated into the emergency-shutdown circuit.
Identical User Interfaces
"The big advantage of the new solution is that graphical user interface at the main operator panel in the switchgear cabinet is identical to the mobile panel onsite," says Schelter. "This saves us at least two days of configuration work, since the configuration can be transferred 1 to 1, without having to consider an additional device. The cabinet, as well as the connection technology for this could also be deleted from the parts list." With the mobile panel, the setup engineer at the scraper has access to all the HMI's functions, allowing him to set up the press without assistance.
As in previous projects, Netzsch used ProTool configuration software to develop the 10 HMI screens. With them, the scraper's movement in the horizontal and vertical directions, pump pressures, timers and other process parameters can be conveniently adjusted onsite via 14 function buttons. For time-critical switching and control processes with very fast response times, the membrane keys can also be employed as DP direct keys (I/O periphery).
Profibus diagnostics are available directly at the scraper as well, which further simplifies the optimization of the system. A maximum of five (from a total of 21) online languages can be implemented. "The touchscreen functionality of the display was not used for obvious reasons," adds Schelter. "We wanted to maintain continuity with the proven design of the main operator panel."
"The big problem was that the setup required two people: one at the main operator panel located by the switchgear cabinet near the filter, and another with the portable panel near the movable scraper mechanism."
According to Roland Schelter, the decision to specify the Siemens unit was influenced by its unrestricted industrial suitability. With handles at the bottom for left or right-handers, the disk-shaped device is much easier to handle than the somewhat bulky in-house device the company constructed.
"With a diameter of 245 mm and a depth of 58 mm, the mobile panel is compact and light," says Jurgen Speck, Siemens AG, Automation & Drives Div. "It also is shock and vibration-resistant, withstanding drops from a height of one and a half meters without damage." Speck says this can only be accomplished with an operating system needing no hard-drive such as Windows CE 3.0, which also gives the mobile device 16-color graphics capabilities, including bar graphs, input/output fields, buttons, text and graphics lists, password protection, alarm system, trend displays, recipe management, etc. As storage medium for the recipes or backups, compact flash cards are used. The mobile panel case meets IP65 requirements and offers optimum protection against water in case of leaks or when cleaning the system with a high-pressure washer.
"Of particular importance to the electrical engineers was the stop pushbutton of the mobile panel, with which the press can be shutdown immediately and safely in case of danger" says Schelter. "Complying with safety regulations, the stop and acknowledgment pushbuttons are designed with two circuits and achieve safety category 3 according to EN 954-1. The stop pushbutton is gray to avoid mistaking it for the emergency-shutdown-actuator. This is especially important if the mobile operator device is not connected or connected to a different filter press."
The mobile OI panel will become the standard HMI for all of Netzcsh's filter presses. "It makes setup and changeovers much more convenient, flexible and cheaper," states Schelter. "The illuminated pushbutton, key-operated switch and handwheel weren't used on the first two installations, but would be quite useful in the future. The latter, for example, to position/drive the servos of the scrapers in fine, continuous increments. For larger filter presses, multiple connection boxes are also possible, even different visualizations at presses that are not of the same type would be possible with the mobile panel."
The Netzsch sales department is already thinking out loud how easy it's likely be to convince its global clientele of the advantages of a filter press operated exclusively by a mobile OI panel. A device holder for the stationary use of the Mobile Panel 170 is already available as an option.