Pressure Sciences has performed many interesting design projects, several of which are described below. We can also provide formal, independent design review, and PE review and certification services for our client's in-house projects.

DESIGN OF A TANKER FOR HYPERGOLIC FUEL

Pressure Sciences worked with Process Engineering, Inc., (Plaistow, NH) who was awarded a contract by EG&G Florida, Inc. for the design of a semi-trailer for the transport of the highly hazardous and toxic hypergolic fuel components, nitrogen tetroxide and mono-methyl hydrazine, for NASA. The semi-trailer, jointly designed by Process Engineering and Pressure Sciences, consists of an inner vessel of Type 304L stainless steel and an outer vessel of Type 304 stainless steel. Stainless steel structural elements are used to support the double-vessel on a standard set of bogies and wheels, and a standard landing-gear and king-post. Crushable, energy-absorbing material is positioned at the front of the trailer, and the sides are strengthened with reinforcing members to protect against breaching of the inner vessel under impact.

Early in the program, the design criteria for normal operation and accident conditions were defined, in conjunction with Process Engineering, EG&G, NASA, and the US Department of Transportation. Next, an overall beam finite element model, and a number of models of the manway and manway flange, the support cone joining the inner and outer vessels, the rollover protection system, and other major subassemblies were created to support the design for both normal and accident loading.

Most of the analyses to support the normal operating conditions were performed using the finite element models. Then, hand calculations were performed to demonstrate detailed compliance with MC-338, the USDOT regulation. Dynamic analysis performed by imposing a known over-the-road vibration spectrum on the beam model was used to obtain the structural response of the tanker. Then critical components were then evaluated for their capability to withstand the resulting fatigue loading.

The accident analysis to demonstrate capability to withstand a lateral, or "T-Bone", impact was done by a combination of hand calculations relating to impact and dynamic finite element analyses using the beam model.

The tanker was demonstrated to be capable of withstanding a 55 mph frontal impact with a rigid bridge abutment without rupture, and a 55 mph lateral impact by a tractor-trailer combination of the maximum allowable over-the-road weight of 80,000 pounds, also without rupture. It was also shown to be capable of withstanding immersion in a fire with a flame temperature of 1800° F produced by burning diesel fuel until the fuel from the tanks of two colliding tractors is consumed.

Twenty-three of these tankers have now been built: the first two for the Kennedy Space Center and the others for Kelly Air Force Base.

DESIGN OF SPECIAL PRESSURE VESSEL FOR SUPERCRITICAL CO2 APPLICATION

For a silicon chip manufacturer, Pressure Sciences undertook to design and build a small high-pressure aluminum vessel to contain supercritical CO2. The supercritical gas was to be used to clean nine-inch silicon wafers as part of a microchip manufacturing process. The vessel was designed for a maximum allowable working pressure of 1800 psig at 95°F.

Because the silicon wafer to be cleaned was large in diameter, but quite thin, the vessel had a 9.25 inch ID, but an internal working length of only 0.25 inch. It consisted of two heavy, flat, circular aluminum plates bolted together at their periphery. The bottom plate was bored for the CO2 passages, and for heating elements to rapidly heat the highly conductive vessel material. The top plate was bored and fitted for internal water-cooling passages, and the two were sealed together during operation with a special O-ring.

Pressure Sciences designed the vessel and its heating and cooling systems, performed the mechanical and thermal analyses, and selected the special O-rings. We then had the vessel constructed, hydrostatically tested, and stamped as an ASME Section VIII, Division 1 vessel by a Pittsburgh manufacturer.

DESIGN RULES FOR HIGH PRESSURE RESEARCH VESSELS

Arco Oil and Gas Company's Plano Research Center is engaged in devising enhanced recovery methods for Arco's North Slopes Operation in Alaska. It tests recovery methods under simulated "down-hole" conditions and uses hundreds of small-diameter, very high pressure vessels for simulating these conditions on a small scale. As with many laboratory applications, Arco's population of such small vessels "just growed."

When Arco needed a method for determining whether all these vessels were safely designed and suitable for continued service, it called on Pressure Sciences. Our first task was to develop a specification for the procurement of new vessels and for the evaluation of existing ones. The vessels have small diameters, so they are outside the scope of the ASME Boiler and Pressure Vessel Code. In addition, the ASME Code does not contain rules written explicitly for vessels subject to very high pressure service. We first developed design rules based on the best current thinking for the design, construction, inspection and testing of high pressure vessels. Next, we recommended acceptable materials of construction for containing samples of a variety of chemistries and at a variety of temperatures. Finally, we evaluated some existing vessels to verify the practicality of the new rules.

An important issue with many of the existing vessels is that they were constructed of Type 304 stainless steel. The down-hole environment contains brines, and brines can subject sensitized austenitic stainless steels to intergranular stress corrosion cracking. Even unsensitized material is susceptible to transgranular cracking at very modest temperatures. Therefore, as part of the job, an envelope of safe environmental and temperature conditions was established for the use of these vessels.

We then developed a computer program written in Visual Basic that checks a vessel against the specification. This program helps Arco to evaluate vessel safety quickly and simply to help protect lab personnel from possible injury resulting from the use of these hundreds of aging vessels.

SILO DESIGNS

Pressure Sciences was requested to design a silo that was susceptible to dust explosions with a frangible roof, using the techniques of API Standard 620, and to design a silo cone for external pressure due to a dust-explosion between the cone and the support-skirt.

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