By Marc Robertson, Victaulic
When an 8,000 MW coal plant upgraded its scrubbers, a coupling developed especially for use in wet flue gas desulfurization (FGD) systems was used for the nozzle-to-header pipe connections, instead of the typical pipe flange. Installation and maintenance benefits accrued.—Nancy Spring
In March 2005, the Environmental Protection Agency (EPA) issued the Clean Air Interstate Rule (CAIR), capping emissions of sulfur dioxide (SO2) and nitrogen oxides (NOX) in the Eastern U.S. To comply with this ruling, coal-fired power plants have installed flue gas desulfurization (FGD) units, or scrubbers, to remove emissions of SO2 from exhaust stack gases.
In a typical wet scrubber installation, a limestone slurry mixture discharged from a nozzle absorbs SO2 and other pollutants from stack gases that are forced up and through the mixture, producing cleaner emissions.
To reduce its SO2 emissions, an 8,000 MW, seven-boiler coal-fired power plant located on the Ohio River contracted a major FGD technology provider and manufacturer to construct wet scrubbers for its plant upgrade. The plans called for two scrubbers, containing a total of nearly 3,000 ceramic nozzles.
In the wet FGD units, the pipe connecting the header pipe to the nozzles was 4-inch grooved 317 LMN alloy. Traditionally, nozzle-to-header pipe connections are made with flanges but installation sensitivities and over-tightening of the flange bolts can lead to cracked and fractured nozzles, resulting in increased installation and maintenance time and costs, as well as leaks and uneven spray patterns.
The technology provider worked with the mechanical piping system company Victaulic to evaluate the performance and costs of joining the nozzle to the alloy pipe using flanged connections versus the alternative, flue gas nozzle couplings. The plant owner and FGD manufacturer ultimately selected Victaulic Style 220 flue gas nozzle couplings for use in the FGD vessels based on ease and speed of installation and maintenance, performance and reduced total installed cost.
The coupling chosen allowed for easier installation than a flanged system. This particular coupling is joined using a single alloy nut and bolt. The housing has a hex recess on one side, permitting simple one-wrench tightening. Proper assembly is confirmed by visually inspecting the pad-to-pad engagement of the coupling housings, preventing over-tightening and nozzle breakage. A flanged system requires four larger alloy nuts and bolts that must be tightened in the proper sequence and at the proper torque stages. Multiple fractured nozzles could result, increasing installation time and requiring product replacement.
The torque requirements and precise tightening sequence required to install flanged joints are critical to achieving proper sealing loads on the flange gasket. A key part of this process is a flat sealing surface, but flanged ceramic nozzles have variances that can result in uneven surfaces unsuitable for proper installation. Workers may have to reinstall the nozzles using caulk sealants along with the flange gasket to minimize leakages, a process that can lead to over-tightening the flange bolts, which can fracture the nozzles. If not installed properly, unseen fractures within the flanged nozzles will cause leaks and, potentially, change the spray pattern, leading to unstable joints when the system is pressurized and sub-prime emission absorption.
The Victaulic coupling can also be disassembled quickly for routine maintenance work. In an FGD system, dried limestone deposits may become encrusted around the bolt and have to be cut apart. This coupling is designed so that the single bolt can be cut with a hacksaw or grinding wheel. Once the bolt is cleared, it is easy to disassemble the two housings and gain entry to the system for routine maintenance or product replacement.
The couplings use a gasket specifically designed to provide a triple seal between the pipe and shouldered ceramic nozzle. The ethylene propylene diene monomer (EDPM) gasket is suitable for the limestone slurry pumped through the nozzle and provides a cushioning effect for the shouldered ceramic nozzle. The cushioning, combined with the shouldered nozzle design, reduces stress on the ceramic nozzle compared to the bolt point loading created by the flanged type. The ceramic shoulder is loaded uniformly around its entire circumference by the coupling housing on one side and the EPDM gasket on the other. The housings meet pad to pad and cannot overload the nozzle no matter how tight the bolt is.
The Vitaulic nozzle coupling provides 360-degree nozzle orientation that allowed the customer to adjust nozzle orientation based on actual, not theoretical, spray patterns. A four-bolt flanged connection permits 90-degree nozzle indexing. When properly installed, the coupling holds the nozzle in the desired orientation and provides more than 50 lb./ft. of rotational resistance against nozzle rotational slippage—well above the rotational torque generated by typical spray nozzle weight and discharge thrust. It also holds the nozzle concentric with the pipe.
The coupling housings are made using the same or similar material as the pipe, providing comparable corrosion resistance. The coupling housings are thicker than Schedule 40 alloy or fiberglass reinforced pipe, providing additional strength and external erosion allowance.
The reduced material costs of the single-bolt coupling versus the four-bolt flange, along with the reduced surface area of nearly 3,000 shouldered-end nozzles, afforded the technology provider initial cost savings of approximately 20 percent.
Author: Marc Robertson is an industrial business development representative for Victaulic. He received a bachelor’s degree in marketing and logistics from The Ohio State University.