Selecting the Proper Drain for the Application Part 2 – Trench Drainage
August 1, 2016
Last week we discussed selection criteria for general purpose drains. One type of drain that is popular due to it’s effectiveness in collecting surface runoff is linear (or trench) drainage. Trench drains are extremely advantageous due to their ability to be installed in long runs, avoiding pooling and making them extremely suitable when runoff from a wide surface area is necessary.
Similar to area drainage selection, project application and installation location are extremely important factors in determining the proper trench drain to specify for your project. The majority of trench drains consist of 4 types of material:
Plastic – Can be effective in a wide variety of applications from pedestrian to high class load rating (providing a heavy duty frame and rail is installed). Common plastics are HDPE, Polypropylene and Fiberglass. Each of these plastics have positive and negative characteristics and careful application consideration must be taken into account before selecting these drains.
Stainless Steel – While expensive, stainless steel is the material of choice in high purity and food service applications due its non-porous nature. In addition, the temperature ratings of this material, typically make it the only suitable material for extremely high temperature applications. Stainless is also preferable in areas where tighter radiuses or complex shapes are required as it is a fabricated product that can be shaped to size.
Polymer – Polymer or Polymer Concrete is a concrete composite substance that adds tensile strength to the drain. While these drains are much heavier and can be more fragile than their plastic or metallic counterparts, they are extremely effective in heavy duty applications due to the increased strength of the material.
Precast / Poured In Place – While more labor intensive, this is commonly used to create custom or shorter trench lengths and widths. These frame and grate systems are boxed out in the field and two concrete pours are utilized to form a trench. The frames and grates can typically handle a wide variety of load class ratings, but are typically difficult (if not impossible) to lock down.
There are multiple factors that must be taken into consideration when determining which material to use or trench to specify these include the following:
Dimensions –Dimensional selection is more important than you think. Wider and deeper aren’t necessarily better than shallow and narrow. Most all of the above materials can be purchased in a variety of widths and depths, but there are several factors that must be taken into consideration when selecting these dimensions:
Is the application a retrofit or are their obstructions or invert issues? If so, a shallow trench may be necessary to avoid any jobsite installation complications.
How much water will you flow? Did you know that 12-inch-wide is RARELY required to handle a job’s flow demand? Most 12 inch drain applications originated from poured in place systems that were installed with a (12 inch) shovel, making that width “standard.” With the advent of manufactured trench drain systems, this width can now often be reduced to 6 inch (or even smaller) depending on the application. A 6 inch drain (which typically has a 4 inch inside throat width) is capable of flowing around 300 gallons per minute through a four inch outlet. Even on 8, 10 or 12 inch trench systems, a 4 inch outlet will restrict the flow, so specifying a larger than necessary trench for an application could unnecessarily add cost to your job. It is important, however to do a complete hydraulic calculation to determine surface runoff rates and thus, pipe discharge size and trench width. If needed, many manufacturers or good factory representatives can help you with these calculations in house.
How will the trench be cleaned? This may be the only time when upsizing may be preferable. Most shovel heads are 12 inch in diameter, making a 12 inch wide trench easier to clean. However, some manufactures offer narrower shovel heads to solve this issue.
Liquid Temperature – Stainless Steel wins handily in this market. With ratings of over 200 degrees Fahrenheit, stainless steel is the material of choice in high temperature applications including breweries, boiler blow down or condensate, and more. Fiberglass also is decent in this area with ratings up to 180 degrees, followed by polymer concrete and other plastics with ratings of around 140 degrees.
Chemical Resistance – Stainless Steel has the most chemical resistance, followed by Polypropylene and Fiberglass. Polymer Concrete also is decent in this arena. Most manufacturers will provide chemical resistant charts upon request which should be referenced if chemicals are suspected to be introduced into the drain.
Load Ratings – Polymer concrete is the material of choice in extra heavy duty applications. It has a much higher compressive, flexural and tensile strength than HDPE or Polypropylene making it ideal for the heaviest traffic areas. While in real life applications, plastic trench has been able to compensate for many of these deficiencies by adding a much heavier frame and grate for the most demanding load class cases, the material properties of polymer concrete make it ideal for the most heavy-duty applications.
UV Resistance – Polymer Concrete and Stainless Steel win here. When using plastic trench, it is important to store it inside before installation and is preferable to install it only after the area has been covered.
Ease of Installation – In general, both plastic and stainless steel are the easiest to install. Polymer concrete is much heavier and has the potential to break in transit or on site if not packaged and handled with care. Poured in place trench can also be labor intensive, almost always requiring two pours to complete.
Grate (and Frame) Selection – Grate Selection is as important as it often relates to the traffic and application. Heavy duty areas will require ductile iron frames and grates, while pedestrian areas may be more decorative and have heel-proof and/or ADA requirements. Is the drain being installed in an area that is exposed to the elements or will it be in contact with salt or salt water? If so, it is highly recommended to choose a galvanized or stainless frame and grate option to avoid rust or corrosion. Grate types often come in a wide variety of materials and styles ranging from stainless to bronze to cast and ductile iron and can incorporate designs, custom logos or be plain slotted.
Material Cost – The least expensive is poured in place as a trench body will not be required. The most expensive is stainless steel, followed by fiberglass, with polymer concrete and other plastics being among the least expensive manufactured options.
Catch Basin – While not required, it is ALWAYS recommended to install an inline catch basin (providing there is not a depth restriction). Catch basins add only a small cost to the initial installation, but save the owner a lot of money and headaches in the long term. They catch debris, help prevent line clogs, give a single point for cleaning, and allow for a much more efficient system.
While there are many considerations that must be taken into account when choosing the correct trench drain solution, a good manufacturer or local representative will be able to assist you in doing a complete system analysis to help choose the correct material, lengths and dimensions to best achieve your drainage goals and provide the longest system life possible.