Pipe Stress Engineering
9 Interface with Rotating Equipment
Rotating machineries are delicate equipment that depend on perfect shaft alignment, balanced rotating parts, and proper clearance for smooth operation. Excessive piping loads and stresses imposed on the equipment can deform machine parts to the point that they may considerably affect the reliability of the equipment. These loads, either from the expansion of piping or weight of the system, can cause shaft misalignment and casing deformation that interferes with the internal moving parts. Therefore, it is important to design the piping system to impose as little load as possible on the equipment. Ideally, it is preferred to have no piping load imposed on the equipment, but this is not possible. The common practice is for the manufacturer to specify a reasonable allowable piping load to which the piping engineers design the piping system to meet. There is no problem with this procedure, except that the allowable loads given by the manufacturers, generally through the standards of their own associations, are too small to be practical. This low allowable piping load is given partially due to the actual design of the machine and partially to protect the manufacturer's interests. The actual load that can be taken could be many times higher for most machines. Nevertheless, most equipment purchasing contracts have a clause stating that the manufacturer will guarantee the machine's performance and integrity only when the piping load is within the allowable limit.
Although the low allowable piping load may have improved the reliability of the machine on paper, it has actually created many unsuspected problems caused by the unusual piping layouts required for reducing the piping loads. To reduce the piping load to the allowable limit, many laborious piping layouts and ingenious restraint schemes are used. These unusual arrangements may show on paper that the allowable load is met, but the reliability of the system is actually compromised. Some of these pitfalls will be discussed later in this chapter.