Engineers at Union Carbide’s plant in Texas City, have conducted studies to improve the operation of their storage facilities.
There were more than 150 tanks, containing an assortment of organic products. Many of the tanks had nitrogen blanketing in the vapour space to control pollution while protecting against damage caused by pulling a vacuum.
Some of the tanks had pressure relief valves that relieved to vapour-recovery systems. Others vented to atmosphere, depending on what chemical was stored within. High nitrogen consumption was a common problem.
Two approaches to improvement were undertaken simultaneously. First, a test programme was developed to test the relief valves for possible leakage. This revealed that considerable amounts of nitrogen were being lost because of leakage through the seats of these devices. Leaking seats were reconditioned and returned to service.
The supply side of the nitrogen blanket was the other focal point of the study. Plant personnel had experienced continual mechanical problems with pressure regulators that fed nitrogen to the tanks. Frequently, these regulators would become stuck in the open position or exhibit other mechanical problems, permitting high nitrogen loss and requiring a great deal of maintenance.
Control System Engineers developed a test programme to evaluate a new type of nitrogen blanket valve. The valve they selected was specifically designed for tank-blanketing service, rather than simply a pressure regulator.
The valve can be mounted directly on a tank nozzle. The sensing port can be located in the same nozzle if no outlet block valves are installed. The valve has a stainless-steel body, a carbon steel diaphragm housing and soft seats to ensure tight shutoff.
Approximately 150 tanks have now been retrofitted with these blanketing valves. Essentially, no maintenance has been required.
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