(FREQUENTLY MADE MISTAKES)
In my 34 years of quoting engineered wastewater pumping systems, I found that some mistakes were often repeated. This is understandable, since wastewater systems only appear on a limited number of projects and the specifying engineers may not be familiar with the components of a system and how these components interact. I will list and expand on some of the situations I’ve encountered in a series of FMM postings.
SEWAGE BASINS
A great number of the mistakes I have seen come with the selection of a basin for the project. Because the basin is installed in the earliest phase of a project, a mistake at this stage can cause a complete change of other components or a costly fix. Here are the most common mistakes I witnessed over the years.
1. THE SPECIFIED PUMPS WILL NOT FIT INTO THE SPECIFIED BASIN. Most pump manufacturers publish drawings that list the minimum basin size for a specific pump model. This can happen in new installations, but most often we see it with pump replacements.
The mistake can happen if the dimension of the existing basin is measured at the outside of the cover, which is, by necessity, 4 to 6 inches larger than the actual basin ID.
If the pump models are changed during the design phase and the basin size is not updated to a properly sized basin. (I will admit to making this mistake myself).
When converting an existing column pumping system to a submersible system. A suspended column pump has a much smaller footprint than a submersible pump.
2. THE SPCIFIED BASIN WILL NOT FIT INTO AN EXISTING BUILDING. This, of course, is seen in remodel and retrofit projects.
This mistake comes from a misunderstanding of the actual measurements of the basin. For example, a 36” ID basin has a flange diameter of 42.5”. A 36” door will not allow for the basin to be transferred into the building. If the basin is to be transferred to its final location using an elevator, the size of the elevator must also be considered. (This size restriction can be overcome by constructing the basin in sections, with a base and extensions that fit through a narrow door or into an elevator. The segments are then assembled in the basement.)
Narrow hallways and stairs with sharp corners that must be navigated should also be considered when selecting a properly sized basin.
3. LOCATING THE BASIN TOO CLOSE TO A FOUNDATIONAL WALL.· This is a drawing of the manufacturer's recommended installation. The manufacturer requires a distance of 5 ft from the footing and then a final depth of the basin bottom that is measured from the ground grade to the basin bottom at a 45-degree angle. If the selected basin is very deep, the final correct placement could be in the middle of the basement floor. The solution is to use a basin with a larger diameter and a smaller depth.
· These requirements are due to the pressures applied to the fiberglass structure. Another solution would be to pour a concrete basin.
4. FAILURE TO ACCOUNT FOR A RAIL REMOVAL SYSTEM. The use of a rail removal system is highly recommended when the pumps are large, the basin is deep or the overhead space is limited. The rail system will make future maintenance much easier and we highly recommend using them. However, their use requires some additional considerations.
The rail system will require a substantially larger diameter basin.
The discharge orientation ( vertical or side discharge) must be specified.
A rail removal system requires a hatch door cover to accommodate the upper guiderail bracket. A solid cover can be used, but internal cross supports must be used to accommodate the upper guiderail bracket.
A standard simplex or duplex cover will not work with a rail removal system.
·The pump mounting studs for the specific system must be installed at the time of construction. This will increase the manufacturing time. It is possible to use a stud plate that can be lowered into the basin after the basin has been installed, but this will increase the cost of the installation, and may extend the lead time.
5. THE WRONG COVER IS SPECIFIED
·We already discussed the use of a hatch door cover when using a rail system.
Be sure to specify a simplex or duplex cover to match the number of pumps in the installation.
Traffic rated covers. Some manufactures offer a H-20 traffic rated cover. (16,000 lbs. wheel load and 32,000 lbs. axel rating. While the covers have this rating, the fiberglass basins cannot stand any weight close to this rating. The covers are installed so that the load is distributed to a concrete support ring and not to the basin itself. We recommend placing basins and covers out of the traffic flow as much as possible.
6. THE BASIN IS UNDERSIZED FOR THE SYSTEM PERFORMANCE
This topic was discussed in my blog on basin sizing and selection, but it is worthwhile to repeat the requirements here as a final check on sizing.
The basin should have enough "working area" to allow the pump to run a minimum of one minute. ( The distance between the "start" and "stop" points should have enough storage to all for 1 minute run time based on the GPM of the pump). If the working area is too small, the pump will have excessive starts and could over-heat causing premature pump failure.
When pumping sewage, it is a good practice to pump a volume ( in total gallons) that is equal to the volume of wastewater in the discharge line. By changing the wastewater in the discharge line every cycle you reduce the possibility of build-up of solids the the line,.
These are the situations we most frequently see. If you have any questions or cases you have seen, let me know at gwilliams@geofwilliams.com or at williamsgeoffrey06@gmail.com .
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