It will orbit the ringed planet 74 times during its four year mission and execute 45 flybys of Titan, some of them only hundreds of kilometers from the moon’s surface. To keep the probe on its intended course and ensure safe close encounters with Titan and the planet’s other icy satellites, engineers at NASA and the Jet Propulsion Lab devised positioning thrusters that execute minute navigational control of the craft. On the way to Saturn, thrusters provided minor interplanetary trajectory corrections. During orbits they handle orbit trim maneuvers, control spin, and maintain three-axis stability of the probe.
A static-flow-control device feeds the mission-critical thrusters a discrete volume of compressed monopropellant hydrazine at a precise flow rate and time. This so-called flow restrictor is made from a precision sintered porous metal from Mott Corp., Farmington, Conn. In contrast to other dynamic mass-flow controllers and micrometering devices it has no moving or adjustable components that could fail during Cassini’s 10-yr mission.
“There are other static-flow restrictors available such as single orifices and capillary tubes,” says Mott Corp. Vice President and Chief Technology Officer Kenneth Rubow. “But they have limitations in accuracy if precise diameters required for a given flow rate are not met. This can be especially tough for low gas flow rates that need holes as small as 0.01 in., or less. Single orifices and capillary tubes are also sensitive to the presence of particulate matter in the gas stream that could deposit in the orifice and adversely alter the gas flow rate versus pressure drop.
“In contrast, porous metal-flow restrictors contain hundreds of small pores that create a vast array of flow pathways,” explains Rubow. “The large pore count along with operating at high differential pressure helps limit the amount of particulate matter that can deposit, so we see negligible effect on the overall gas flow rate versus pressure drop.”
In operation, the porous metal element sits inside the bore of an industrial gas line fitting or application-specific customized hardware.
“It is neither a pressure control nor a differential pressure controlling device,” says Rubow, “but rather a flow-control device for given pressure conditions. It meters fluid flow, with high accuracy and repeatability, as prescribed by up and downstream pressures. It can also limit gas flow, if there’s a catastrophic device failure or when there’s an inadvertent venting or opening of a critical process gas line.”
Fluid-flow rate versus pressure- drop curves monotonically rise with increasing flow rate and differential pressure drop across a flow restrictor. “This flow rate versus pressure drop curve,” says Rubow, “depends on a number of fluid properties including gas composition and temperature, as well as up and downstream pressures. Properties of the porous metal element including its diameter, thickness, porosity, and pore size along with the tortuous paths of the interconnected pores affect fluid-flow control.”
“For a given restrictor application where these parameters are known and controllable,” says Rubow, “the gas-flow rate versus pressure drop will provide a consistent gas-flow rate for a given pressure drop.”
A characteristic flow curve is distinct for each gas composition and set of system operating conditions. “Differences primarily result from variations in gas viscosity and molecular weight,” Rubow says. “Gas compressibility factors and slip flow effects can also alter the curves. The flow-curve shape, from a fluid-mechanics viewpoint, depends on the system pressures and flow rates, the gas-flow regimes, and gas compressibility.”
“We can design and predict the performance of our flow restrictors for a particular operating condition,” says Rubow. “The restrictor- flow predictive model accounts for the nonlinear relationship between pressure drop and flow rate by incorporating the basic fundamental equations to account for laminar, turbulent, and slip gas-flow regimes, gas compressibility, gas properties such as viscosity, molecular weight, and compressibility factors. The model also accounts for complex size and shape of pores in porous media.”
Sintered-metal elements such as those used in the flow restrictors for the Cassini probe come in a range of alloys including 316L stainless steel; Hastelloy B, C-22, C276, N, and X; Inconel 600, 625, and 690; Monel 400; nickel 200; alloy 20 and titanium. They have flow pores ranging in size from 0.1 to 100 μm. The combination of powder size and shape, pressing pressure, and sintering conditions defines the pore size distribution, strength, and permeability of the porous media.
As part of the manufacturing process, each restrictor is individually calibrated — using NIST-traceable instrumentation — for flow rate and pressure drop based on anticipated conditions using the application gas or an equivalent.
Porous-metal restrictors can manage gas-flow rates from less than 1 sccm to greater than 40,000 sccm at pressure drops ranging from inches of water to 1,000’s psi and system gas pressure conditions ranging from full vacuum to 1,000’s psi. Gas temperatures are only limited by application-imposed limitations. Flow restrictors are suitable for a wide variety of gases ranging from inert to corrosive gases, where material capability is the limiting factor.
Mott Corp., (800) 289-6688, mottcorp.com