The Piping Analyst

Several major elevation requirements are Pump NPSH which sets the fractionator tower elevations based on the bottoms line condition going to the pump. 

Reboiler circuits may set tower elevations. A thermo-syphon reboiler requires enough liquid static head to provide a driving force so that the reboiler will work properly. This head determines the circulation ratio and the amount of vapor returned to the tower, thereby setting the entire tower gradient. Reboiler circuits must be considered with pump NPSH considerations as they set the tower elevation. 

A flashing liquid must have enough static head to offset the line friction loss and the total loss through an orifice. When the process includes flashing liquids, the vessel from which the liquid is being drawn and the orifice flange itself must be given a definite elevation or a relative elevation. 

Gravity flow determines the relative elevations of related equipment and probably will determine the exact elevation of the related equipment itself. i 

To do this, the piping analyst decides on the valves required to meet the process and specification requirements including all the instruments shown on the process control diagrams. He then 

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The piping designer needs three essential source documents engineering flow diagrams, nomenclature, and equipment elevations. These documents are usually furnished by the piping analyst. 

The specification group provides the piping analyst with detailed design requirements for the piping, valves, and other piping related equipment. 

The project engineering department furnishes the piping analyst special requirements for all utility and auxiliary systems which are not shown on the process flow diagram. 

Some analysts use more detailed procedures. They suggest that 90% of all pipe failures result in a hole size less than 50% of the pipe area. The following approach is suggested ... 

The following T6 Aluminum properties have been used for the piping analyses from reference 9- 96. The elastic properties and geometry weren t changed for the T6 Aluminum cases, because the structure stiffness requirements will be independent of material. The only difference for the structure properties is the CTB used for the T6 Al analysts cases is actually that of T6 Aluminum. The CTE of Aluminum is higher than any other primary support structure material possibilities. 

Understanding the positions of sample and other measurement locations within the equipment is also important. The presence or absence of isolation valves needs to be identified. While isolation valves may be too large for effective sampling, their absence will require that pipe fitters add them such that sample valves can be connected. This must be done in advance of any test. If analysts assume that samples are from a liquid stream when they are vapor or that temperature measurements are within a bed instead of outside it, interpretation of results could be corrupted. Analysts should also develop an understanding of control transmitters and stations. The connection between these two may be difficult to identify at this level in fully computer-controlled units. 

Unit layout as installed is the next step of preparation. This may take some effort if analysts have not been involvea with the unit prior to the plant-performance analysis. The equipment in the plant should correspond to that shown on the PFDs and P IDs. Wmere differences are found, analysts must seek explanations. While a hne-by-line trace is not required, details of the equipment installation and condition must be understood. It is particularly useful to correlate the sample and measurement locations and the bypasses shown on the P IDs to those ac tuaUy piped in the unit. Gas vents and liquid (particularly water-phase) discharges may have been added to the unit based on operating experience out not shown on the P IDs. While these flows may ultimately be small within the context of plant-performance an ysis, they may have sufficient impact to alter conclusions regarding trace component flows, particularly those that have a tendency to build in a process. 

Intent Plant personnel, supplies, and budget are reqiiired to successfully complete a unit test. Piping modifications, sample collection, altered operating conditions, and operation during the test require advance planning and scheduling. Analysts must ensure that these are accomplished prior to the actual test. Some or all of the following may be necessary for a successful unit test. 

Appendix III contains failure rate estimates for various genetic types of mechanical and electrical equipment. Included ate listings of failure rates with range estimates for specified component failure modes, demand probabilities, and times to maintain repair. It also contains some discussion on such special topics as human errors, aircraft crash probabilities, loss of electric power, and pipe breaks. Appendix III contains a great deal of general information of use to analysts on the methodology of data assessment for PRA. 

The topics of consequence and likelihood analysis are fraught with issues that bring into question the accuracy and usefiilness of the results because so many assumptions have to be made, and because the analysts opinions (many of which are implicit) play such an important role. For example, many fires and toxic gas releases start with a leak from a piping system. Yet the size of the leak could vary from a pinhole to a partially failed gasket all the way to a complete guillotine break... 

Another unit in the same chemical complex supplied the hydrochloric acid via pipe-hne. The acid was a by-product of a process that manufactured large quantities of flashing flaimnable liquids. Supervision directed the crew to promptly collect acid samples and dehver them to the Main Lab to analyze for flammables. Lab analysts did not detect the presence of flammable chemicals. 

Leak Duratioa The Department of Transporation (1980) LNG Federal Safety Standards specified a 10-min leak duration other studies (Rijnmond Public Authority, 1982) have used 3 min if there is a leak detection system combined with remotely actuated isolation valves. Other analysts use a shorter duration. Actual release duration may depend on the detection and reaction time for automatic isolation devices and response time of the operators for manual isolation. The rate of valve closure in longer pipes can influence the response time. Due to the water hammer effect, designers may limit the rate of closure in liquid pipelines. 

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