GUIDELINES TO DESIGNING WATER SCREENING SYSTEMS
HOW TO DEFINE SCREENING
Screening plants face nature. There is, as a result, a measure of uncertainty as to what they have to handle, and a safety margin is needed.
Screening is a filtration process which separates solids from liquids. It has two purposes:
To clean the liquid
To recover / protect the solid matter
Mechanical water-screening uses self-cleaning devices (static and dynamic) to screen water; this allows clean water to be used industrially while at the same time collects and protects valuable water life.
Unless otherwise specified, solids will be referred to as trash or debris, living creatures in the water as living matter (including eggs, larvae, fish, crustacean, plankton etc.), thru-flow, dual-flow band screens and drum screens as rotating screens.
The location of a screening station determines its debris. There will usually be a combination of natural and man-made elements but it is difficult to estimate the amounts and cycles of the trash. Therefore, conservative or even oversizing should be the rule.
Pit maintenance, when done by divers, can be very costly. Investing in stoplogs saves money and time.
Gathering as much information as possible, visiting neighbouring stations and recognizing existing problems will help to design the equipment you need.
At BEAUDREY, new developments are happening at all times. We have been involved with thousands of plants worldwide. By consulting with us at an early stage, you will benefit from our knowledge and experience as much as we will profit by helping and working with you.
INFORMATION TO BE GATHERED BEFORE STARTING
Always add in a safety margin as water flow tends to increase rather than decrease during the design period. New pumps often do better than expected.
Low water levels:
Ascertain operating levels very carefully. Various losses of head are induced by the intake and other upstream features and have to be taken into account. Transient loss of head that occurs when the pumps start should be carefully assessed.
High water levels:
Do not forget to add in the level increase due to transient surges at the pump accidental or normal trip.
Rise induced by global warming must also be added.
ASSESSMENT OF TRASH CONTENT
This is, by far, the most delicate problem. Operational records from neighbouring stations are the first thing to look up. They are good guidelines. Looking at neighbouring shores is the next thing to do. Local fishermen can also give useful tips. A diver can be sent to find out what the bottom looks like. Renew all these investigations over one or more years if possible. Report all facts in your enquiry even if you cannot put figures to what you have found.
Find out from local and national sources what the regulations are that might interfere with water intake design. These can generally be classified in four categories:
Right of passage along the shore. Intake and building limitations.
Navigation-related rules about location, levels, sizes and velocities. These are always binding.
Environmental issues such as imposed mesh sizes, velocities, seasonal restrictions, fish protection devices, etc.
Trash disposal regulations. Can trash be returned to its source and what to do with it if not?
While the first two categories concern the intake rather than the screens, the last two categories have a direct influence on screen selection. Consider making provisions for future increased stringency of environmental and trash disposal regulations.
BAR SCREEN SPACING
This is not, in most cases, subject to regulation. Generally, 40 to 50 mm (1.5 to 2”) is well suited to protect the downstream rotating screens.
MESH APERTURE SELECTION
The choice might differ according to whether the mesh size is imposed by a regulatory source or is just related to the downstream use of the water.
ROTATING SCREEN MESH SIZING
Fish and larval regulations might call for 20 or 25 gauge mesh aperture (1 or 0.5 mm). While this looks small or fragile, it is safe and maintenance-free if supplied by a manufacturer experienced in fine mesh, like BEAUDREY.
When heat exchanger protection is at stake, a safe rule is to choose the dimension of the mesh aperture as ¼ of the exchanger tube ID. This leads to 5 or 6 mm (1/4”) in most cases. While larger mesh sizes are sometimes satisfactory, 5 mm makes condensers maintenance-free in almost every station.
For plate-type exchangers and most other applications, the finer the mesh the better. Do not expect your screen to stop much sand or silt; if sand is present, either get rid of it upstream (settling basin) or increase velocities all over to keep it from settling. In any case, plants have to live with it, at a cost.
MESH TYPE AND TECHNOLOGY
Bar screens should be fabricated with care as they are subject to corrosion and stress.
Bar spacing that is too fine is not an advantage. It is important to know what type of debris will be arriving in the plant. 40 mm bar spacing should be preferred but in the case of long, solid and thin debris, spacing should be decreased in order to avoid damage on the rotating screens.
In most cases, let the more capable rotating screens do the bulk of the job and keep bar screens as safeguards only.
The mesh aperture coefficient is not as important as one would generally believe; trash is arrested by the mesh and rests on apertures and wires alike.
When weed-type material is expected, NOCLING™ mesh or smaller mesh performs better because fibers tend to kit themselves around the wires of larger meshes.
When jellyfish is expected NOCLING™ should be used.
GENERAL PLANT LAYOUT AND MACHINE SIZING
Generally speaking, it is worthwhile to involve us with your screening plant project from its inception, as most cost reduction stems from a good design concept. BEAUDREY can be of the most help at this stage.
The civil engineering cost of a screening plant compared to that of the equipment it houses is always much higher.
Alterations of the equipment in an screening plant, even if possible, can be extremely costly.
Including 30% more trash handling capacity into the design and construction of a new screening plant can save the enormous expense of later additions to an operating plant.
PLANT LAYOUT GUIDELINES
NUMBER OF MACHINES
This should be kept as low as possible. The civil engineering costs of multiple pits are high and maintenance costs are similar for all sizes of machines. BEAUDREY builds very large units that handle anything the largest pumps can take (up to 600 000 GPM or 36 m3/sec).
PUMP AND SCREEN ARRANGEMENT
One pump fed by one screen (whatever type) is the best and easiest arrangement. Two pumps for one large drum screen can also provide compact layouts. The number of pumps and screens a plant should have depends on operational philosophy, yet the fewer low-velocity zones, the better.
The main disadvantage of low-velocity zones is that they encourage shell-fish and algae growth as well as sand settling, causing an increase in condenser maintenance.
When compactness is sought, utmost cooperation between the pump manufacturer, a hydraulic expert and a BEAUDREY engineer is needed. It might also save you the time and money involved in model testing.
Due to the very low failure rate of BEAUDREY screens, there is no real need for a stand-by screen. However, if operational reasons require one, it is preferable to install a stand-by screen and pump unit than to enlarge the pump bay by adding a basin between the screens and the pumps.
ENSURING EVEN VELOCITIES AND TRANQUIL FLOWS
For the equipment to operate properly, the arrested trash must be kept against the screen surface. The forces that do this are purely hydrodynamical. Any whirl, wave, return flow or other disorder will tend to disrupt this pattern. Reasonable care should be given to the shaping of the walls and location of the equipment. Modelisation is advisable in some cases.
Waves should be dampened down to an amplitude of about 8” (200 mm).
BEAUDREY has seen many shapes and layouts in operation over the years and will gladly exchange ideas with designers to help reach a cost-efficient layout at an early stage.
MACHINE SIZING AND TYPE SELECTION
In all cases, this is done by taking into account the expected quantity and nature of debris. In some cases, approach velocity criteria are set by environmental regulations.
THE TRASH HANDLING CAPACITY OF A ROTATING SCREEN IS A DIRECT FUNCTION OF THE SCREEN WIDTH, THE SCREEN AREA AND THE FREQUENCY OF CLEANING (TRAVEL SPEED).
KNOW WHAT DEBRIS TO EXPECT
OTHER NATURAL DEBRIS
Sizing is no problem. Gates should never be omitted as dry pit maintenance might be required. They should be able to meet three requirements:
Avoid undue turbulence
Keep civil engineering simple
Be easy to handle.
BAR SCREEN SIZING
Generally, the expected water approach velocity is the basis for sizing. According to the anticipated trash load, a raking system is installed. Even if no clogging is expected, it is wise to allow room for future rake installation. Its type depends on the nature of the trash (see BEAUDREY Mechanical Rakes).
Fixed head types are more reliable than travelling types and have a greater trash-handling capacity. They are also fully automatic and maintenance-free. The width and height of the bar screen depend on the plant layout. The wall shapes should be kept simple and correspond to the number and size of the downstream rotating screens.
1.4 FPS (0.4 m/sec) can be taken as the base case for sizing.
ROTATING SCREEN SELECTION AND SIZING
This is the most delicate part of the design. The velocity taken into account is that just ahead of the mesh, not in the mesh aperture.
Expected water velocity might be imposed by regulatory agencies or stem from the mesh size and anticipated trash content. The following graph gives a summary of standard velocities related to mesh aperture sizes; from which the necessary area can be worked out. For machine sizing, please ask BEAUDREY.
This is only a guideline as some plants have needed larger screens where others have used smaller ones. Check with BEAUDREY before finalizing. Required areas are the same for thru-flow, dual-flow band screens and drum screens.
Choice of the type of machine
Again, no definite rule can be applied; each case must be carefully studied but the following guidelines can be helpful.
Fine mesh does not suit thru-flow screens well as some debris can by-pass the screens (carry-over).
Thru-flow screens are costly when large mesh areas are called for.
Dual-flow screens perform better than thru-flow screens and are more economical. They are smaller for the same area of mesh and their quality has allowed them to supersede thru-flow screens in many countries.
Drum screens have many advantages. Finding out when and where a drum screen should be used requires a full investigation in every case. The costs related to excavating and civil engineering vary from site to site and have a great influence on the choice. A few reminders:
Drum screen diameter grows with level variation. However, many cost-conscious people use drum screens with tidal ranges exceeding 40 ft as the most efficient and best economical option.
There are no limits to drum size. It is the only machine that can meet any screening area requirement.
The larger the flow per machine, the more suitable the drum screen.
Drums can travel faster and thus can handle more trash.
The driving power of a drum screen does not increase with the loss of head.
Drums require shallower pits than travelling band screens.
Drum screen reliability is higher.
Drum maintenance is cheaper and quicker (at least three to one).
Plant engineers who have operated them tend to ask for drums again even at an extra cost, as dependability is never assessed at its full value in a project.
The following figures may help you to decide between the screens. The first figure shows when a machine is at its best according to mesh size and the second figure shows how level variation and screening area can make a difference in your screen choice (the chart is based on typical, average cases and can only be used as a rough guideline).
BEAUDREY can provide all the facts and figures you may need but ultimately the final decision will be yours.
Required screen width depends on flow rate, mesh size and water trash content. The rules that link width to screening area and trash content are complex and beyond the scope of this brochure.
Here are some basic guidelines:
When the choice lies between a deeply-immersed, narrow screen and a wider, shallower layout, choose the latter.
Never plan anything that does not please the eye.
To make sure that the bid you get meets your requirements, please state your needs and operating philosophy very clearly.
Velocities around the screen
Generally, ahead of the mesh the water velocity should not exceed 0.70 m/sec (2.4 fps) in the narrowest passage and 1.2 m/sec (4 fps) downstream of the mesh.
Refer to specific chapter on Environmental Protection. For full details, call us in.
Water-life protection is handled in two ways:
Lowering the approach velocity allows swimming, living matter to escape because of its fear of the unknown and its natural tendency to swim against any current.
Catching the living matter on an adequately-sized mesh in order to remove it with minimum stress before sending it back gently to where it came from.
Other systems which use a combination of the above principles can be split into two main types:
Existing mechanical screen designs, modified for water-life protection with no impact on the screen’s basic function.
Equipment designed to specifically meet water-life behavioural concepts. So far, facts and figures about their operation and cost are variable mainly because they are too “fish-orientated” and forget basic screening principles. None of them protect non-swimming elements such as crustacean, eggs, larvae and large plankton.
When assessing the feasibility of such systems, these rules can help:
A pipeline laying on the river or seabed is expensive. In certain cases, however, this is the safest and most direct method of returning water-life to its natural environment.
Large civil structures cost more than any other item in a water intake.
A screening area that cannot be cleaned or which is dependent on manual cleaning, however large it may be, cannot be used indefinitely without eventually clogging up completely.
Fish-friendly mechanical equipment should be in permanent operation or cleaning cycle.
No biological protection device is equally efficient with all species, sizes and in varied external conditions.
It takes much more than one plant, one man and one year to find out all about the biological performance of a new screening system.
What BEAUDREY systems all have in common is that while they save living matter, they do not lose any of their best qualities: being reliable, well-proven water screens. They are generally cheaper and meet the “Best available technology” rule in every way.
SPECIFIED OPERATING HEAD-LOSSES
When the screen is not travelling while in operation, the loss of head grows very slowly and is hardly measurable until the screen is about 90% obstructed. Then, very suddenly, the loss of head starts rising as shown below.
The time left before the loss of head reaches prohibitive values beyond screen resistance is measured in seconds.
If the screen is rotating at its highest speed and the loss of head keeps rising, statistics show that there is no chance that loss of head will stop rising and remain stable at a high value (above 12 cm (5”)).
When loss of head rises, it means that the screen cannot handle as much trash as it receives and that the only way to avoid collapse is to shut off the pumps or throttle their flow rate considerably. The only solution to increase the capability of the screen is higher travel speed and wider panels.
From over 100 years of experience, we advise the following:
Do not call for too high an operating loss of head, 30 to 50 cm (12 to 20 inches) are safe figures. Higher losses of head when the screen is travelling require much larger motors, drives and larger components with no increase in capability. A BEAUDREY high travel speed screen can handle two to six times more trash with no chance of every having to operate under high loss of head.
If static resistance to very high loss of head is required, the machine will be much heavier, require larger mechanical components and cost more. The capital involved would be far better spent in corrosion-free materials, larger machines and better engineered screens such as BEAUDREY’s.
Static resistance to 1.8 m (6 ft) differential is ample for a balanced design.
Typical head-losses to be taken into account for the hydraulic design are summarized in the figure below.