Flexible Operations Doesn't Always Mean Piping Flexibility

By Doug Stelling

Several piping flexibility analyses were recently made for a large East Coast refinery to help design a number of bypasses. The bypasses were being installed to provide additional operational flexibility. However, this additional flexibility in operations doesn't necessarily increase the piping flexibility of the system. This means that piping flexibility stresses and heat exchanger nozzle stresses may exceed allowable values if the new bypass lines are not properly designed.

One problem that must be overcome is the temperature difference between the bypasses, the onstream piping, and the heat exchangers. In the simplest case, if there are two exchangers, then three thermal flexibility cases must be checked. In one case both exchangers are in service. In the two other cases, either one or the other exchanger is in service. If there are three exchangers then four thermal flexibility cases typically must be checked. In addition, when an exchanger is being bypassed, the design temperature of the piping to a downstream exchanger usually increases.

Another problem is that the bypass piping is usually fairly rigid. This is because bypass piping is typically laid out by the piping designers with a minimum amount of pipe and elbows to avoid extra cost and to permit locating them in congested areas around the exchangers. In addition, since the bypasses allow the heat exchangers to be taken out of service for maintenance, they typically have several block valves to permit completely isolating the exchangers from the process stream.

These valves are much more rigid than straight pieces of pipe, and they greatly decrease the overall flexibility of the piping system.

Nozzle loads as a result of the piping modifications are also a concern. Heat exchanger nozzles are normally designed in accordance with the ASME Code Section VIII, Division 1 to reinforce the opening in the exchanger channel or shell for the circumferential or hoop stresses due to internal pressure. However, designing heat exchanger nozzles for external piping loads was not and still is not a common practice unless the Owner explicitly asks the heat exchanger manufacturer to design the nozzles for these loads. The normal Code reinforcement for internal pressure also provides some minimal reinforcement for external loads. However in many cases, it is found that the Code reinforcement is insufficient for the higher nozzle loads associated with some bypass piping configurations. Since modification of a heat exchanger nozzle is somewhat more involved, the bypass piping is usually redesigned to reduce the imposed piping loads.

What all this adds up to is a lot of engineering work to ensure that the simple bypass that process wanted to increase its operating flexibility doesn't adversely affect the integrity of the piping system or heat exchangers. Luckily, modern piping flexibility analysis computer programs like CAESAR II include auxiliary programs for analyzing nozzle loads per WRC 107 and 297. These bypass piping design problems can be readily solved through the use of these programs.