Caesar II:input basics 4

Caesar II:input basics 5

Caesar II:input basics 6

Caesar II:input basics 7

Caesar II:input basics 8

Caesar II:input basics 9

Buried Piping/Pipelines Stress Analysis with PASS/Start-Prof Tutorial

Controlling Piping System Vibration ⬇️⬇️⬇️⬇️

The method used to solve a particular vibration problem depends on the type of vibration and the type of piping system. For example, adding restraints to a piping system will not solve a vibration problem caused by high acoustic energy.

It is not always possible to eliminate or isolate the source of vibration in many piping systems. Therefore, other means must be used to control vibration. Some common methods for doing this are briefly described below.

Bracing ⬇️

One way to control piping vibration is by adding bracing (i.e., restraints) to the system. This increases the mechanical natural frequency of the piping, thus ensuring that resonance due to low frequency excitation will not occur. It also limits large deflections that could be caused by slug flow, water hammer, etc.

When bracing addition is being considered, it must be confirmed that this will not adversely affect the thermal flexibility of the piping system. Adding restraints increases the system stiffness and can cause higher pipe thermal stresses and end-point reaction loads. New thermal flexibility calculations may be required to confirm that the design is acceptable after adding the restraints.

Snubbers ⬇️⬇️

Vibration amplitudes can be decreased by installing hydraulic or mechanical snubbers. These devices (e.g., dashpots or other frictional devices) increase the system damping by resisting rapid displacements, such as that resulting from vibration. However, they permit movement resulting from slow displacements, such as those from thermal movement. Thus, snubbers may be used in situations where bracing must be added to reduce or prevent vibration movements, but rigid restraints would cause unacceptable thermal displacement stresses or loads.

Surge Suppressers ⬇️⬇️⬇️

Surge suppressers may be used to control surge or pulsation-induced vibration. A typical surge suppresser consists of a pilot operated valve which quickly opens after a power failure through the loss of power to a solenoid, or by a sudden large pressure reduction or increase at the surge suppresser. The open valve releases liquid from the line being protected, thus smoothing and reducing the pressure fluctuation. The valve is closed at a slower rate by using a dashpot in order to limit the pressure rise as the liquid flow is shut off.

Accumulators ⬇️⬇️⬇️⬇️

An accumulator is a pressure vessel that is partially or completely filled with a gas (usually inert). This vessel is then connected by pipe to the main line being protected. In the simplest case, the liquid in the pipe is in direct contact with the gas. In some cases, an elastomer membrane separates the liquid from the gas but transmits pressure between them. In other cases, a rupture disk forms a more rigid barrier between the gas in the accumulator and the liquid in the pipe. For the first two configurations, the device acts instantaneously to a rise in pressure at the gas/liquid interface. In the last configuration, the rupture disk delays the reaction time from 0.2 to 2.0 milliseconds since it must rupture before pressure is transmitted between the two fluids.

Temporary Restraints ⬇️⬇️⬇️⬇️⬇️

Cables or chains can be used to temporarily control large deflections caused by vibration in piping systems. The cable or chain is attached to the pipe, connected back to nearby structure, and tightened to stop the pipe movement. Quite often, even blocks of wood or scrap steel are used as wedges between the pipe or its supports and nearby structure to stop pipe movement. This approach is useful in stopping large amplitude vibration before it can damage the pipe, and to determine the best locations to place permanent bracing. This technique is not a permanent solution to a piping vibration problem.

Since the temporary restraints are installed while the system is in operation (and hot), their presence could restrict pipe thermal movement when the system is shut down. This should be considered when locating the restraints to determine if they would cause excessive thermal stresses or loads when the system is shut down. In extreme cases, it might be necessary to remove the restraints before or as the system is shutting down in order to permit free thermal movement.

Whats new in Revised ISO 14692: 2017 Edition

⬇️⬇️⬇️⬇️

In the year of 2017, a renewed edition of the ISO-14692, the governing standard for fiber reinforced plastic piping, was officially released. There are many significant changes with respect to the earlier edition. For example there are changes in the regression gradient for qualification, the maximum pressure rating terminology, the stress intensification, stress envelope definition, the scaling rules for qualified components, buried pipe assessment, fatigue method, test methods for flanges etc. All these changes can be studied in details from the latest code. This article will try to explain few of the important points in brief.

The objective of ISO 14692 is to provide the oil and gas industry, as well as the supporting engineering and manufacturing industry, with mutually agreed specifications and recommended practices for the purchase, qualification, manufacturing, design, handling, storage, installation, commissioning and operation of GRP piping systems.

The previous official release for the code was in the year 2002. Since then the experience with FRP-piping has increased significantly, much of this experience is now included in this latest revision of the standard. Issue 2017 of the standard now contains more background to design requirements and provides more clearly that defines a step-by-step set of guidelines. The standard offers a questionnaire to be filled-out by the end-user in the bidding stage. This questionnaire aids the end user to provide the right information to the manufacturer and designer. Information such as pressure and temperature requirements as well as other information crucial to select the appropriate FRP piping component composition as pressure class. By providing clearer requirements for the provision of information, there is a much higher chance that all selection and design of the FRP system occurs, conform the standard requirements.

Contrary to the last official release of the standard in the year 2002, the new edition addresses buried GRP piping. A pipe buried in soil with a certain cover depth will experience a vertical deflection due to the weight of the soil and additional soil loads. Previously, to assess such deflection, the engineer would need to divert to the rules and guidelines of the AWWA M45 standard. Since this assessment is already standard practice by many engineers in the design of buried piping, the AWWA M45 vertical deflection assessment was brought into the ISO14692, thus making this a mandatory component of the assessment of buried piping when following the ISO 14692.

One aspect of the standard that did not match industry common practices has been the application of the stress intensification factors (SIF’s) and flexibility factors. Many of the original research on these values was done in the context of stress analysis of components made of anisotropic materials. The design of anisotropic components generally differs considerably from that of isotropic materials. The 2002 edition of the standard provided SIF’s for fiberglass fittings which were based on the SIF’s in the BS7159. A study performed by SINTEF has shown that the BS7159 underpredicts the stiffness of GRP fittings substantially, from which it can be concluded that the SIF’s from the BS7159 are not applicable to GRP fittings. Therefore one common practice in the industry was to use a SIF of 2.3 in combination with modelling the true reinforced wall thickness of the applicable fitting. This modelling approach was based on experience of the industry rather than the ISO14692 philosophy as presented in the 2002 edition of the standard.

The 2017 edition of the standard provides a new modelling approach to be used for pipe stress and flexibility analyses which are based on using a standard SIF of 1.5 in combination with an equivalent fitting thickness. The latest revision also provides a standard for qualification of the SIF. Thereby a manufacturer also has the possibility of taking credit for a potentially lower SIF than 1.5 for a specific elbow design. Other important features of the ISO14692 have undergone very significant changes as well, such features are: the qualification of GRP piping components, the standard on GRP flanges, the design stress envelope, fatigue in GRP and static electricity.

10th edition of TEMA
Houmaan ashrafi