Autoclave Upgrade for Corrosion Fatigue Testing

Expand the University of Idaho’s current fatigue testing capabilities to include an autoclave load frame that supports testing of metals as well as operating conditions experienced in nuclear power plants. =Problem Definition=

Background


The objective of this project is to expand the University of Idaho’s current fatigue testing capabilities to include an autoclave load frame for use in fatigue testing of materials with a focus in metals to be used in nuclear power plants. Currently, no system is on hand to accomplish the fatigue testing environment desired by Dr. Stephens. Currently, there are two load frames with furnaces capable of reaching temperatures up to 700°F, but neither of them have an airtight seal installed. The proposed new system will be able to tolerate temperatures of up to 750°F and 3000 psi while delivering 10,000 lbs of force.

Currently, the lab’s load frames can do fatigue creep testing in an air environment. Fatigue tests slowly apply a tensile load which tears apart a specifically shaped sample over an extended period. The growth rate of the crack is of interest because it is used to predict the lifespan of the material in other applications. The proposed system will perform the same style of tests but will be capable of testing the response of the material to corrosive and pressurized environments.

Deliverables
The scope of this project includes designing/purchasing all of the components for the fatigue testing apparatus, assembling it, and testing it for proper usability as defined in the following deliverables.

Initial Client Meeting & Expectations The initial client meeting was conducted on June 14th, 2018. Dr. Stephens and Nicholas Schaber prepared the following list of Primary Objectives for the project.
 * Load Frame Upgrades
 * Loading system (motor/drive/gearbox, rated to &asymp;10 kips)
 * Load cell (coinciding with applied loads)
 * Load cell calibrated in line with seal
 * Load train displacement measurement (LVDT)
 * Extension rates up to 0.05 mm/s
 * Lid lifting/holding device
 * Load Control and Measurement
 * Feedback control loop (load controlled)
 * User interface for load frame
 * Programmable loading waveforms including:
 * Variable strain rate
 * Fatigue (cyclic loading, variable frequencies)
 * Creep-fatigue (variable hold time and set loading)
 * Constant load
 * Crack measurement/monitoring (DCPD)
 * User interface over/under warnings (temp, pressure, load)
 * Autoclave Upgrades
 * High temperature/pressure dynamic seal
 * Lid modifications
 * Loading rod
 * Internal load support
 * Isolated through-lid fittings for DCPD wires
 * Operating temperature of 400&deg;C (&asymp;750&deg;F)
 * Operating pressure of 20MPa (&asymp;3000psi)

Specifications Utilizing the above list of deliverables from the client, a refined list of specifications has been produced. The full document is available in the "Additional Documentation" section.

The following is a shortened list of our most important specifications and deliverables.


 * Functionality
 * UI should emulate that of other load frame currently in use by U of I
 * Emergency stop function digitally via LabView UI and mechanically via physical “button”
 * Extension rates up to 0.05mm/s during testing
 * The loading system (motor/drive/gearbox) design shall be able to deliver a maximum load of at least 10,000 pounds.
 * Design shall be able to accommodate standard sized test specimens
 * All components shall have 99% reliability. A DFMEA will take this into account
 * Programmable loading waveforms including:
 * Variable strain rate
 * Fatigue (cyclic loading, variable frequencies)
 * Creep fatigue (variable hold time and set loading)
 * Constant load
 * Crack measurement/monitoring via DCPD


 * Evironmental Requirements
 * All internal autoclave components shall be designed to withstand temperatures of at least 300° C
 * All internal autoclave components and seals shall be designed to withstand pressures of at least 28MPa (4000 psi)
 * All internal autoclave components and seals shall be designed to withstand the same corrosives as the 316H stainless steel that the autoclave is made of

Acronyms used in this text
 * UI:  User Interface
 * LVDT: Linear Variable Differential Transformer
 * DCPD: Direct Current Potential Drop

=Design Considerations=



=Project Learning= The autoclave load frame system being developed has many components, but the two requiring the most research and consideration are the Dynamic Seal and the Actuation System.

Dynamic Seal
With the design shown in Figure 1, a rod is moved linearly through the top of the autoclave pressure chamber. This requires a strong, Dynamic Seal to handle the movement of the rod while containing the required maximum temperature, pressure, and corrosive environment.

One of the seals considered is a Stuffing Box. According to Wikipedia, a stuffing box is an assembly which is used to house a gland seal. It is used to prevent leakage of fluid, such as water or steam, between sliding or turning parts of machine elements.
 * Advantages
 * Inexpensive
 * Easy Maintenance
 * Simple Design
 * Disadvantages
 * Short lifespan of packing material (frequent maintenance required)
 * High friction on load arm
 * Potential Source and Cost
 * MSC (Manhatten Supply Company) sells the packing material at $10/ft.

Another option considered is a spring seal, which is a shaft seal with a Garter Spring contained within the seal to provide compression around the shaft and prevent leakage.
 * Advantages
 * Quickly responds to changes in pressure or temperature
 * Proven system for this application
 * Disadvantages
 * Requires external cooling
 * Easy to damage
 * Potential Source and Cost
 * We sourced this from Cortest at $75.

The next seal considered is a Metal Bellows. Wikipedia defines metal bellows as elastic vessels that can be compressed when pressure is applied to the outside of the vessel, or extended under vacuum. When the pressure or vacuum is released, the bellows will return to its original shape (provided the material has not been stressed past its yield strength).
 * Advantages
 * Handles conditions without modification
 * Ease of integration with current system
 * Disadvantages
 * Requires additional safeguards in case of failure
 * Limited range of motion
 * Potential Source and Cost

Actuation System
Referring again to Figure 1, an Actuator System is needed to raise and lower the gripper arm attached to the test specimen. This system needs to be robust enough to provide the test loads required for fatigue testing of the specimen.

Hydraulic

 * Advantages
 * Capable of delivering faster load frequencies (15-60 Hz)
 * Higher max speed ramp up rate and turnaround rate
 * Significantly less expensive
 * More commonly available components
 * Disadvantages
 * Requires an external pump and cooling
 * Larger amount of individual components

Electric

 * Advantages
 * Simpler system (less individual components)
 * Smaller footprint in lab
 * Disadvantages
 * More expensive overall
 * Slower load frequency capabilities (~1 Hz)
 * Lower Lifetime

Decision Matrix
In order to make an informed decision on the type of actuation system we needed to purchase, a decision matrix was created. The categories, weights, and values of the matrix were all discussed and agreed upon by the team as a whole. Using this decision matrix, we were able to easily settle on a hydraulic actuation system as our best candidate for this project.

=Prototyping=

Prototype Seal Design
After a significant amount of discussion and consideration of the benefits of each of the seals described in the Project Learning section above, it was decided to take advantage of both the high-temperature/ high-pressure qualities of the stuffing box seal, and the low friction of the spring seal.



For our design, the load rod will pass the dynamic through a stuffing box gland seal. This seal will be stuffed with a special extreme temperature and pressure stuffing. This stuffing is both impregnated with graphite and has Inconel wire embedded into it. This seal will take the brunt of the high pressure and temperature from the autoclave.

Behind the stuffing box is a water jacket. This water jacket will cool the load rod and prevent heat creep from affecting the load cell above. This water jacket will be sealed above by a spring seal, which does not need to have nearly as high of a temperature rating as the stuffing box seal.

In the event of the stuffing box seal being breached, the high temperature/pressure fluid escaping from the autoclave will be caught in the water jacket and sent out of the return line. The spring seal is rated to hold the pressure that it would experience in this event.

=Validation= Load Rod Seal Testing A test was developed to verify the integrity and safety of our manufactured autoclave lid seal. A test was necessary as the seal is needed to hold back a maximum pressure of &asymp; 4000psi and a temperature of 300 °C, as stated in the product requirements.

A hydraulic hand pump was emptied of hydraulic fluid and then refilled with water. It was then connected to the autoclave lid. The lid was secured with all 12 retaining bolts and any plugs were tightened to assure a good seal. The built-in autoclave pressure sensor was used to check the pressure. The autoclave lid seal packing was inserted into the lid and the seal body firmly tightened down. The packing used was inferior to the type that will be used in the final product. A 3/8" bolt was used to stand in for the load rod, which travels through the lid seal. The autoclave was not heated for this experiment.
 * Method

The autoclave was pressurized to &asymp; 1000 psi for this test as the validity of the pressure sensor reading has not yet been verified. The seal held, even with the inferior packing used. After a couple of small leaks were stopped coming from the plugs on the lid, the chamber lost only about .01 MPa every minute or so. This may have been from a number of sources, including other plugs on the lid, the internal lid seal, the load rod seal, or the hydraulic hand pump. No water was detected coming through the load rod seal.
 * Results & Discussion

Overall, this test was a success. We verified the pressure integrity of our packing box style seal for the load rod at 1000 psi. An independent pressure gauge will be utilized in the next test to verify the accuracy of the built-in autoclave pressure reading. This, combined with the proper type of packing material that will be purchased in the near future, will allow us to properly conduct a full test of the autoclave's maximum pressure rating of 28 MPa (&asymp; 4000 psi).

An interesting observation of this test was the movement of the bolt used in place of the load rod. At &asymp; 4.5 MPa the bolt overcame the static friction of the stuffing box seal and began to move upward until the head of the bolt (which was upside down to prevent it from shooting out) contacted the lid and it ceased moving. By simply multiplying the pressure by the cross-sectional area of the bolt, we find that the static friction of the stuffing box is somewhere around 70 lbf. We plan to verify this result later with our load cell, but it is helpful to know that the friction in the stuffing box can easily be overcome by the hydraulic actuator.

=Final Design=

=Team Members=

=Additional Documentation= Presentations File:2018 AutoclaveExperts DesignReviewPresentation.pdf File:2018_AutoclaveExperts_Snapshot2Storyboard.pdf

Documents

Project Schedule

File:2018_AutoclaveExperts_GanttChart8-1-18R4.pdf

Meeting Minutes

File:2018 AutoclaveExperts MeetingMinutes.pdf

Product Requirements

File:2018_AutoclaveExperts_ProductRequirementsR3.pdf

Client Interview

File:2018_AutoclaveExperts_QuestionsForClient.pdf

Team Contract

File:2018_AutoclaveExperts_TeamContract.pdf