For the most part, treatment plants use concrete tanks for unit processes. The interface between the concrete and the equipment is critical as there may be very little adjustment of the equipment possible to account for variations in the concrete.
Most equipment supply work for treatment plants involves replacement of equipment in existing concrete tanks where both equipment suppliers and the installing contractor must work to a fixed project completion date. However, prior to bid, the only choice the equipment supplier has is to base the price and schedule for the equipment on the limited dimensional information for the existing tanks that is available with the project bid documents. This presents an interface issue that frequently impacts the project schedule and cost of the new equipment. The installing contractor is not responsible for the existing tanks that may have dimensions or configurations that vary from general information indicated in the bid documents. The uncertainty of existing tank dimensions does not allow the equipment supplier to account for additional costs and schedule impact if there are configuration and dimensional variations within or among several tanks. This is like requesting bids for foundation work without first making subsurface information, including log and soil sample test data, available to the bidders.
Certainly, all parties involved in the project have an interest in making sure the equipment is sized for the existing tankage prior to fabrication, because, once fabricated, there is very limited adjustment to the overall dimensions of the equipment. Project specifications may state that the contractor must provide equipment suppliers with verified tank dimensions prior to the submittal. However, since the contractor is not known at the time of bid, the verification process cannot take place until after the prices and schedules have been committed to, and without consideration for potential costly revisions to the equipment if problems are found with dimensional variations in the existing concrete.
The process of obtaining verified tank dimensions after a contract is awarded is a common source of delay in the schedule for both equipment suppliers and the installing contractor. The usual problem is restricted access to the tanks that must be surveyed. Restricted access creates a situation where the equipment supplier and the contractor are committed to a schedule but neither can proceed because the critical dimensions of the existing tanks cannot be verified in a timely manner. It is possible that the equipment supplier could proceed to fabricate equipment based on the general dimensions stated in project specifications, however there is a question of responsibility when, if later, it is found that there are flat areas of tank walls, or floor slopes and other elevations are not what they should be, all resulting in equipment that does not fit with the existing structure.
The problem is further complicated when new mechanisms are specified for multiple tanks that are described in bid documents with identical general dimensions (diameter, length, width, depth, floor slope etc.). The real cost and schedule impact to the supplier and installing contractor is not realized until there is verification of the actual tank dimensions.. Encountering these problems is always a costly disappointment to all involved in the project.
When new equipment is to be installed into existing concrete tanks, verification of the tank dimensions should be completed prior to bid and included in bid documents so that costs and schedules can be more accurately established by both suppliers and installers.
About the Author: Michelle McNish is President of Walker Process Equipment, a Division of McNish Corporation. She is also a Board Member of the Water and Wastewater Equipment Manufacturers Association (WWEMA), a non-profit trade association that has been representing water and wastewater technology and service providers since 1908. WWEMA’s members supply the most sophisticated leading-edge technologies and services, offering solutions to every water-related environmental problem and need facing today’s society. For more information about WWEMA go to www.wwema.org.