ANCILLARY EQUIPMENT
EACH TYPE OF SCREEN OR FILTER NEEDS VARIOUS ITEMS OF ANCILLARY EQUIPMENT. THERE ARE A NUMBER OF SUCH DEVICES.
This section describes the various ancillaries BEAUDREY can provide : spray-water systems - trash baskets - debris filter trash collection systems - differential level meamsuring systems - electrical control cabinets - cathodic protection systems - by-pass valves - sluice gate guide scrapers - condenser tube-cleaning balls
SPRAY-WATER SYSTEMS
This feeds the backwash spray-water pipes of the rotating screens. Various layouts are possible:
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Each screen is fed by a dedicated pump
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All screens are fed from a common header fed by one or more pumps
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The screens are fed from the existing pressurized water source; no extra pump is needed if the pressure is sufficient (2.5 bars, at least).
In all cases, all debris in the water must be removed so as to avoid clogging the spray nozzles. BEAUDREY screens have a built-in, self-cleaning strainer. All other screens require strainers in the pipework.
BEAUDREY proposes various types of pumps:
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Horizontal, centrifugal pumps fed from an existing, low-pressure pipe (cooling water pump manifold). A motorized valve is required on the circuit. It is opened when the spray pump runs.
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Vertical pumps with motor above deck. For the relatively small flow rates required, they are expensive, complex and very sensitive to installation conditions.
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Fully-submerged or submersible pumps. Such pumps are well suited to the screen application. They can be easily lifted out for inspection and are relatively economical while very robust. The single-impeller type is to be preferred.
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Self-priming pumps. Very well suited for plants with a water level up to 3 m (10 ft) below deck. To be used only with the one-screen/one-pump layout as they are not suited for variable flows required with multiple pumps in parallel.
Butterfly valves are well suited for screen plants.
MATERIALS
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Pump casing
For fresh-water applications: Cast iron
For seawater applications: AISI 316L or duplex stainless steel -
Pump impeller
For fresh-water applications: AISI 304L stainless steel
For seawater applications: AISI 316L or duplex stainless steel -
Pump shaft
For fresh-water applications: AISI 304L stainless steel
For seawater applications: AISI 316L or duplex stainless steel -
Pipework
For fresh-water applications: carbon steel
For seawater applications: HDPE or GRP -
Valve casing
In all cases: Cast iron with EPDM lining -
Valve obturator
For fresh-water applications: Cast iron with EPDM lining, AISI 304 or AISI 316 stainless steel
For seawater applications: Cast iron with EPDM lining, AISI 316 or duplex stainless steel
SPECIAL CAUTION
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Pumps should not run at a flow rate of less than 75% of their nominal flow.
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Pumps should not be subjected to run out (maximum flow at no pressure)
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Considering how important the spray-water supply is for the screening plant, it is advisable to install a sufficient back-up capacity (stand-by pump).
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Pumps should never be stopped for periods longer than 72 hours to avoid corrosion and damage to sealing devices.
TRASH BASKETS
The wash-water and debris from the screens run in a deck flume and reach the debris collection zone. Two solutions are possible:
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The water falls into a perforated basket; the debris are retained in the basket and the water flows out to be discharged into the sea or river.
BEAUDREY can supply standard baskets that are emptied either by being tilted or via a door on one side.
The basket design should be decided to accommodate the needs of the contractor in charge of disposing of the trash.
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Another solution consists in installing a mechanical separator that removes the debris from the water and downloads them into a nearby basket. BEAUDREY, in cooperation with reputed specialists of trash disposal systems, can supply such systems.
CAUTIONARY NOTE
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While there are periods with little or no debris in the water, brutal high concentrations can occur thereby filling the basket in a very short time and without warning. The water and debris then overflow and go to the discharge point.
The first precaution is to ensure the basket is large enough.
The second is to make sure the return discharge is located so as to avoid any possible recirculation to the screens.
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The screens arrest all sorts of things present in the incoming water. This may include snakes, crabs, stinging fish, animals, toxic waste etc. The plant must be designed to be safe in such conditions for the operators.
DEBRIS FILTER TRASH COLLECTION
The debris filter backwash water flow is rather high compared to that of rotating screens and the water returns under pressure to the condenser outfall side.
In closed cooling circuit plants (cooling towers) or when the law forbids debris returning discharge to nature, the debris must be removed.
The BEAUDREY online trash concentrator arrests the debris. When the filter backwash has finished, the arrested debris are discharged by emptying the concentrator into a trash basket. The concentrator is then filled and back in service, ready for the next debris filter backwash.
The materials used for manufacturing the concentrator are the same as those used for debris filters.
DIFFERENTIAL LEVEL MEASURING SYSTEMS
Cleaning screens and filters is controlled in three manners:
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Operator:
The operator manually initiates a cleaning cycle. -
Timer:
A timer starts a cleaning cycle periodically. The periodicity is preset and generally adjustable. -
Head-loss:
The head-loss of any screen or filter increases as more debris are arrested and more mesh surface is obstructed.
Safe operation requires that when the head-loss reaches a preset value, the cleaning cycle is started and carries on for a minimum period of time or until the head-loss has dropped back to clean machine values.
There are two main cases:
1 - Gravity-fed screens (trash rakes, travelling band screens, drum screens, WIPs etc.)
The head-loss is measured as the level difference between the water surfaces upstream and downstream.
Various systems are available:
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Float and pneumatic types which are now obsolete.
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Ultrasonic or radar types which have the advantage of not being in contact with the water.
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Piezometric systems for which the sensors are immersed and require regular cleaning. To be used only when the previous systems do not fit.
All these systems require special expertise to manage the interference between neighbouring systems and other obstacles (foam, etc.). BEAUDREY can supply the proper solution.
BEAUDREY has worked with a highly-reputable supplier to produce the sensors specially suited to screen applications. The signals received from the sensors are processed by either:
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A dedicated black box
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The screen control PLC (Programmable Logic Controller) in the control cabinet
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The plant’s DCS system (Distributed Control System)
2 - For pressure-line filters, self-cleaning strainers, debris filters and tube-cleaning systems the head-loss is measured as the difference in pressure between the upstream side and the downstream side of the machine.
In practice, it is always done using a differential pressure membrane pressostat. Two layouts are possible
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Each side of the pressostat is connected directly to each side of the machine shell. The pressure transmission pipes need periodical maintenance if growth or deposit occur.
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The pressostat is connected to silicone-filled capillary tubes, each of them transmitting the pressure applied on a membrane installed on the shell’s corresponding pressure-sensing branch. The solution is more costly but much easier to maintain. Extreme care must be taken in selecting the membrane’s material.
Signal processing is the same as for the gravity-fed screens above.
BEAUDREY ELECTRICAL CONTROL CABINETS FOR AUTOMATIC OPERATION
BEAUDREY supplies and installs the necessary control cabinets for its supply. BEAUDREY can meet any customer-imposed standard for instruments, computers and switchgear.
There are essentially two distinct parts:
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The control and display system
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The motor control center for power
CONTROL AND DISPLAY SYSTEM
Control can be achieved using one of the following systems:
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All relay: this is suited for very simple plants and little available specialist maintenance.
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Local PLC: this is well suited to most cases. Care must be taken to shelter the PLC from high temperatures.
The choice of makes and types of PLC is large and prices soar when sophistication increases. Such sophistication is generally not necessary in the case of screens. BEAUDREY can help you limit your costs.
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DCS control: this avoids using a local PLC in most cases. The cost of the control hardware is reduced but owing to possible misunderstanding between the DCS programmer and our requirements, the duration and cost of commissioning may be high.
THE MOTOR CONTROL CENTER (MCC)
BEAUDREY can either supply control cabinets that included the power control in a separate compartment or the MCC in a separate cabinet.
The size and the cost of the MCC depends very much on the customer’s electrical specification (fixed circuit breakers or withdrawable type, etc.).
The variable frequency drives for the motors are generally part of the MCC.
As for the control part, BEAUDREY can assist in specifying the minimum, necessary equipment.
CAUTIONARY NOTE
Screening plants are opened to wind, rain, sea-spray, burning sun and sand. While the cabinet enclosure can be chosen to resist these conditions when locked, there are many days in the year when it is totally unsafe to work in the cabinet.
Whenever possible, cabinets should be in a safe, climate-controlled building.
CATHODIC PROTECTION
There are two main types of cathodic protection systems BEAUDREY can propose.
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SACRIFICIAL ANODES
The anodes are located so as to spread the protection of the machine evenly; BEAUDREY’s extensive experience is the key to a good design. The anodes, generally made from an aluminium alloy, are sized to protect the immersed area of the machine for a chosen duration (in practice from 6 months to 3 years).
Anodes need to be changed at the end of their life, but otherwise, the system is maintenance-free, very efficient and has no limitations induced by the type of material to be protected.
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IMPRESSED CURRENT CATHODIC PROTECTION
BEAUDREY works in association with a world-wide, reputed specialist.
The screens have some added features to suit the system. Anodes are located after careful study, to induce a homogenous protection.
Voltage control is achieved by reference anodes that feed the information to the control cabinet.
The anodes are chosen to suit each case. Control is fully automatic to accommodate tidal variations, evolution of the machine’s coating and growth without exceeding the voltage limits.
Such a system needs a periodical specialist check-up to make sure there are no voltage excesses that may create serious corrosion rather than prevent it.
Marine growth also interferes with the efficiency of the system.
Great care must be taken to avoid stray current interference. When well designed and maintained, it gives remarkable results.
SLUICE GATE GUIDE SCRAPERS
As shells and marine life tend to grow in the grouted stoplog guides, BEAUDREY can supply a heavy guide scraper frame which is lowered into the guides. Its blades scrape off the shells and growth which have adhered to the surface. After having used the scraper, gate lowering and sealing capacity are much improved.
BY-PASS VALVES
By-pass valves are an optional feature for drum screens and all types of travelling band screens (except for the thru-flow type).
They are actuated by the difference in water levels between the upstream and the downstream faces and open when head-loss reaches a set level, 30 cm or 1 ft, generally speaking. The water flows through the valve.
They are used when it is essential to have a water supply even when the rotating screens are obstructed (power failure, lack of wash-water, etc.).
They are absolutely safe with no mechanical items, no power requirements nor automatic controls. If required, a signal can tell the operator if it opens.
By-pass valves may appear simple but require calculations and laboratory testing. The BEAUDREY by-pass valve is the best technology available.
CONDENSER TUBE-CLEANING BALLS
The efficiency of a condenser tube-cleaning system is dependent on the quality of the cleaning balls injected to clean the tubes. The sponge balls have a diameter that is slightly larger than the ID of the condenser tubes, allowing them to have constant contact with the tube walls. The balls gently scrub and sweep the tubes as they move through them.
These balls are made from open-pore sponge rubber of varying diameters and densities depending on the specific type of fouling encountered in a given site.
BEAUDREY can propose a variety of diameters and hardnesses to meet the needs of each client. They are readily available for after-sales purposes.
