Operational stress

During a hydrotest the additional weight of the water superimposes the normal operational stress by steam pressure. The big advantage of the pneumatic test is now, that it is the best imitation of the normal service condition. 

Materials of Construction and Operational Stress. Before a centrifugal separation device is chosen, the corrosive characteristics of the Hquid and soHds as weU as the cleaning and saniti2ing solutions must be deterrnined. A wide variety of materials may be used. Most centrifuges are austenitic stainless steels however, many are made of ordinary steel, mbber or plastic coated steel. Monel, HasteUoy, titanium, duplex stainless steel, and others. The solvents present and of course the temperature environment must be considered in elastomers and plastics, including composites. 

Although reducing the corrosivity of the external environment wrould reduce cracking, in this case it is judged that cyclic, operational stresses are predominant. Until the source of the stresses is determined and reduced or eliminated, it is doubtful that a permanent solution can be achieved. 

In many applications a pressure vessel may be tested for safety simply by hydraulic testing to a pressure that is higher - typically 1.5 to 2 times higher - than the normal operating pressure. Steam boilers (Fig. 16.7) are tested in this way, usually once a year. If failure does not occur at twice the working pressure, then the normal operating stress is at most one-half that required to produce fast fracture. If failure does occur under... 

Some of the most obvious examples of problems with gas and materials are frequently found in refining or petrochemical applications. One is the presence of hydrogen sulfide. Austenitic stainless steel, normally a premium material, cannot be used if chlorides are present due to intergranular corrosion and subsequent cracking problems. The material choice is influenced by hardness limitations as well as operating stresses that may limit certain perfonnance parameters. 

Check Road Tanker while filling Provide secondary task involving other personnel. Supervisor periodically checks operation Stress importance of regular checks for safety Provide automatic log-in procedure... 

Figure 12-46H. Advanced datum impeller design reduces operating stresses and improves flow velocity. (Used by permission Dresser-Rand Company. All rights reserved.)...

Calculated operating stress pounds per square inch absolute... 

Centrifuges, 22 288. See also Hydrocyclones basket, 11 389-390, 391 filtration equipment, 5 542-548 fixed-bed, 11 389-391 materials of construction and operational stress, 5 522-524 moving-bed, 11 391-392 noise, 5 524-525... 

Sm is the maximum allowable operating stress, calculated as specified minimum yield strength x Hf, where Hf is the material performance factor from Mandatory Appendix IX, Table IX-5A or IX-5B. Material performance factors account for the adverse effects of hydrogen gas on the mechanical properties of carbon steels used in the construction of pipelines. 

Ne Number of cycles of maximum computed operating stress range ... 

N, Number of cycles associated with operating stress range, S, (/ = 1,2,...) ... 

When the computed stress range varies, whether from thermal expansion or other conditions, SE is defined as the greatest computed operating stress range. The value of N in such cases can be calculated by eq. (Bid)... 

After the removal of the autofrettage pressure the part shows the compressive residual stresses required to reduce the operational stresses effectively (see Chapter 1 Introduction Fig. 1.4.10). Successful autofrettage treatment needs tough and sufficiently strong steels which should offer a certain potential for strain-hardening without unacceptable embrittlement. 

It should be understood that the positive effects of autofrettage are always based on two different sources compressive residual stresses, which permit a larger amplitude of the operational stress and, depending on the material, the improved local strength of the material owing to strain-hardening by local plastification. 

Fig. 4.3-6. Reduction of the stresses at the inner bore diameter by autoffettage (schematical) left residual stress right operational stresses after pressurization.

Mechanical stresses exist in the MEA as a combination of built-in stresses, assembly stresses, and operation stresses. The cell/stack fabrication and assembly process result in the built-in and the assembly stresses. During operation, stresses can be induced by the change of water content in the membrane. There are possibly other sources of stresses due to various off design conditions. 

The built-in and operation stresses are the consequences of the large swelling and shrinkage of the ionomer membrane when it uptakes and loses water. This is frequently referred to as dimensional instability in the literature. Water in the PFSA membrane is an essential ingredient of its proton conduction behavior. Water affects the morphology13,14 of the ionic clusters (at nanoscale) which... 

During cell/stack operation, water content in the membrane is affected by the local intensive variables, such as local temperature, water vapor concentration in the gas phase, gas temperature and velocity in the channel, and the properties of the electrode and gas diffusion media. The power fluctuation can result in temperature variation inside the cell/stack, which will subsequently change the local membrane water content. As the water content in the membrane tends to be non-uniform and unsteady, this results in operation stresses. When the membrane uptakes water from a dry state, it tends to expand as there is no space for it to extend in plane and it can wrinkle up as schematically shown in Fig. 4 when the membrane dries out, the wrinkled part may not flatten out, and this ratcheting effect can cause the pile up of wrinkles at regions where membrane can find space to fold. The operation stress is typically cyclic in nature due to startup-shutdown cycles, freeze-thaw cycles, and power output cycles. 

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