Mechanical Research & Design, Inc. | Custom Sealing Solutions | 30″ Lower Level Outlet (LLO) Gate Valve Replacement Plug

05 Nov 30″ Lower Level Outlet (LLO) Gate Valve Replacement Plug

Posted at 00:34h in Hydropower by MRD

THE PROBLEM:

Our client, an underwater construction dive firm, was tasked with inspecting a gate valve positioned at the lower level outlet discharge line for a lake community. The gate had previously been cycled about once per decade and the primary issue facing our client was that if they were able to open the valve, they may not be able to close it again, allowing the popular lake to drain until it was empty. To get a closer look, the client sent divers to the outside of the valve as well as several hundred feet upstream through the outlet to inspect both sides. Unable to gain a further clear idea as to the operability of the gate valve, our client came to us. After discussion, the best course of action was to design a sealing bulkhead, or plug, to seal behind the gate valve to catch the flow of water, in the event the gate could not be closed again. Inspection of the lower level outlet discharge piping confirmed it was in poor structural condition to handle the load (up to 100′ of water head on the sealing bulkhead)… the design warranted a plug that could distribute the load onto the lower profile valve thimble and thrust block elbow immediately behind the valve. Furthermore, the installation of the 30″ sealing bulkhead rated for 43 psi would have to be installed several hundred feet up the incline of the LLO, in a very urgent timeline.

WHAT IS A DAM OUTLET WORKS, LOWER LEVEL OUTLET (LLO)?

THE PROBLEM AND THE SOLUTION

THE SOLUTION:

Mechanical Research & Design project and design engineers leapt into action, working with our client’s divers directly, to design a functional, safe product, to work within all of the environmental limitations. Our engineers designed a product, within weeks, to seal and provide primary restraint on the geometrically short thimble, as well as provide a secondary back-up restraint system that worked through the elbow of the system. The system was to be installed in pieces for weight management and have a built in lifting system to aid the single diver in the ultimate assembly and deployment of the several hundred pound plug. With the client’s approval, MRD fabricated and assembled the product for delivery in weeks, allowing the client to immediately install and fully inspect the function of the valve. The plug eventually became a longer-term isolation plug while rehabilitation occurred on the gate and allowed a diver to crawl several hundred feet up a confined space to safely remove the plug afterwards.

The plug solution utilized a single SEALFAST® active compression mechanical seal for maximum sealing safety over time as well as a primary integral restraint dog design to fully support the plug against pressure, with a secondary restraint pad restraint system around the elbow. The product was successfully installed and removed in 2025.