The previous article in this series discussed setting the basis to be used for allowable loads imposed on rotating equipment nozzles. For additional information on that article, please contact Carmagen Engineering at 973-627-4455 as it is offline. This article discusses one important piping design consideration, temperature. Future articles will discuss other design considerations.
The piping system will have a design temperature associated with it, and this determines its stress and end point reaction loads. What's special about rotating equipment piping systems regarding temperature?
In the simple case of a single pump, the piping system design temperature is used to make the flexibility calculations. The pump is either on or off, and all the pipe is either at its design temperature or its ambient temperature. However, what about the more complicated but common case of a spared parallel pump piping system with either two or three pumps? These installations permit a pump to be out of service while the system remains in operation. Thus the piping to the operating pump will be at design temperature while the pipe to the idle pump will typically be assumed at ambient temperature.
The common suction and discharge headers between the two pumps will be at design temperature. Another situation involving spared pumps is when warm-up lines are used to provide a small flow of process fluid through the idle pump(s) in a spared pump configuration.
Warm-up lines are typically used for either of two reasons:
In either case, the piping to the idle pump will typically be at 50-75% of the normal design temperature. This higher than ambient temperature should be used in the flexibility analysis for the idle portions of the piping system.
For the spared pump configurations discussed, flexibility calculations should be made for all possible combinations of spared/idle pump in order to find the operating case that governs the design. Thus for a two pump system, three cases are typically required: both pumps operating to cover the startup/shutdown of the idle pump, “A” pump operating/“B” pump idle, and vice versa. Some pipe stress engineers try to determine the design governing case without making a computer run, and thus save some computer time. However, except for the simplest and most obvious systems, it's not worth making a “best guess” when today's computer programs make it simple to make the needed temperature changes and check the exact conditions.
The last temperature consideration we'll cover is heat traced piping systems, in particular for centrifugal compressors. Heat tracing is used in this application to ensure that no condensation occurs which could damage the compressor internals. The heat tracing temperature will typically be about 300-350°F, and this may be higher than the normal piping system design temperature.
The pipe wall thickness is determined based on the normal system design temperature. However, if the heat tracing temperature is higher than the design temperature, the flexibility analysis should be made using the steam tracing temperature. This analysis considers the real case of the process flow being off while the heat tracing is left on. In such a shutdown case, the loads on the compressor nozzle may be high enough to affect coupling alignment, and cause problems when the compressor is restarted.
Heat tracing can also be used in pump piping systems to make the liquid less viscous. In such a case for a spared pump system, the temperature of the pipe to the idle pump should be assumed to be at the heat tracing temperature rather than at ambient. This completes the discussion of temperature design considerations for rotating equipment piping systems. Later articles in this series will discuss other design considerations.