Spring-loaded gas regulators primarily are designed to maintain the downstream pressure of a flowing gas. They tend to overshoot before tight shutoff and are not responsive to small pressure drops. Pressure must drop significantly for the regulator to be wide open. Therefore, you must employ some sort of a modified pilot-loading system to get the accuracy and speed of response needed.
Fig. 3 shows a typical nitrogen-blanketed expansion tank. Ideally, at least two or three openings are provided on the top of the tank for introducing the inert gas, monitoring and relieving pressure, and relieving vacuum.
Figure 3. Nitrogen-Blanketed Expansion Tank
A. Shutoff ball valve designed for handling gas, including nitrogen.
B. Supply pressure regulator with built-in relief valve for reducing and regulating a high inlet pressure to a lower outlet pressure and holding the reduced pressure within limits.
C. Pressure gauge to provide positive indication of the pressure prior to the main regulator.
D. Self- or pilot-operated regulator. This valve is the heart of the nitrogen regulation system for the tank. The pressure relief should be set for pressure not to exceed 1 psig. The inlet pressure can range from 25 psi to 5 psi.
E. Pressure gauge to provide positive indication of the pressure at the tank.
F. Check valve to ensure flow in one direction into the expansion tank.
The expansion tank generally is located at the highest point of a closed-loop heating system.
In addition to the components noted in the figure, you might want to consider a rupture disc and a vacuum breaker, installed separately on the expansion tank independent of the nitrogen system. These are good options for existing tanks with questionable pressure-handling capabilities. They can also help in situations requiring added safety to prevent over-pressurization or implosion of the expansion tank. In such cases, temperature swings during heating and cooling of the system's fluid can create a vacuum condition.
Select materials of construction suitable for the temperature of and around the expansion tank. In addition, choose elastomers and springs that are compatible with the temperature and the vapors of the heat-transfer fluid to be used. Install a heat sink between the nitrogen system and the expansion tank to ensure a cooler environment within and around the nitrogen system. Commonly used materials of construction include stainless steel, carbon steel, bronze and aluminum, as well as Teflon or Viton elastomers.
To maintain low heat-transfer fluid vapor pressures at elevated operating temperatures, ensure that the expansion tank is designed for the given temperature and pressure, and is built to the American Society of Mechanical Engineers (ASME), Section VIII, Div. I standards.
* Zak Shums was president of Liquid Process Systems Inc., Indian Trail, N.C. When this article was released in 2003. He is now deceased. The company provides heat-recovery and filtration systems for hot oils or heat-transfer fluid systems. Doug Vargas of Invensys Systems, Melinda Schnell of Tyco Valves and Claude Strick of HVAC & industrial Controls also contributed to this article. For more information, call 704-821-1115 or e-mail firstname.lastname@example.org.