Piping Design: Carefully Consider Nozzle Loads

Figure 1. This layout typifies piping design for horizontal pumps.

Let's look specifically at nozzle loads for pumps and compressors.Pump nozzle loads. These are specified in the pump's codes and standards (for example, API 610). The API 610 standard covers nozzle loads for horizontal pumps, vertical in-line pumps and vertically suspended pumps for nozzle sizes up to 16 in. (400 mm). For larger pump nozzles, come to an agreement with the vendor about nozzle loads before placing the order. Figure 1 depicts a piping design for horizontal pumps. Figure 2 shows real piping of an electric-motor-driven pump in a process plant. Generally, small pumps not anchored to their foundations can tolerate higher nozzle loads than anchored ones. Allowable nozzle loads for vertical in-line pumps with supports not anchored to the foundation could be twice those of anchored pumps.Compressor nozzle loads. For centrifugal compressors, the API 617 standard specifies the nozzle load limits. However, many purchasers usually ask for two times the API 617 nozzle loads (2×API 617) to make piping design easier. While some machinery engineers and vendors may consider the API nozzle-load values optimum, in many situations piping engineers and stress analysis specialists can't achieve these values. A higher value (particularly 2×API 617) is a good solution to allow piping without expansion joints at nozzles or to avoid very complex piping systems. For special applications with very large differences between operating and ambient temperatures and very large nozzle sizes, it may make sense to specify a nozzle load three times the API 617 values (3×API 617). Figure 3 shows an example of a large compressor piping.Assign relatively low nozzle loads to integrally geared centrifugal compressors or rotating machines primarily designed for low pressures (such as some axial compressors, low-pressure overhung compressors, and machinery with open impellers) that rely upon close radial and axial clearances of rotating components (impeller or rotor assemblies) to machinery casings. For these compressors, allowable nozzle loads above conventional values in the API 617 standard aren't feasible. While the nozzle loads in AP 617 usually can be achieved, the nozzles loads often instead are limited to 0.9×API 617. In any event, come to an agreement with the vendor on suitable nozzle loads before ordering such machines.For screw compressors, the API 619 standard recommends nozzle loads. The nozzle loads of reciprocating compressors are left for the purchaser and vendor to jointly set. Generally, screw or reciprocating compressors come as packages; the purchaser and vendor should agree upon allowable nozzle loads at vendor interfaces.

Figure 2. Electric-motor-driven pumps such as this are common at process plants.

WHY THE FUSS?To achieve maximum reliability, a machinery engineer's goal usually is to keep nozzle loads as low as practical. However, piping designers and stress analysis engineers generally design piping systems based on allowable (i.e., maximum) or even sometimes slightly higher than allowable nozzle loads to avoid very complex and expensive piping systems. The piping should have optimum flexibility to prevent distortion of machinery alignment or component damage.So, it's important to carefully consider two effects of nozzle loads:1. Internal alignment problems, i.e., distortion of a machine's casing and internals. Misalignment of internal machine components will create accelerated wear, rubbing or even early failure.2. External alignment problems, i.e., misalignment of various shafts in a machine's train — for example, between a driven shaft and a driver shaft. The effects of external misalignment might not be as obvious as those from internal misalignment. However, external misalignment in time will take a toll. Vibration levels increase as couplings become misaligned. A high vibration trip could result in an unscheduled outage. Extended operation at high levels of misalignment could cause coupling failure, possibly bearing damage or even catastrophic failure.To minimize misalignment of various shafts in a machine's train because of piping load effects, it's crucial to ensure that train casings, casing supports and baseplate(s) have sufficient structural stiffness to limit displacements of casings and shafts. Differences in thermal growth as well as errors in piping fabrication, and alignment all contribute to actual deflection values and final nozzle loads achieved in the field.

Figure 3. Large centrifugal compressors often are piped in this way.

Avoid to the maximum extent possible the use of expansion joints — they are expensive and maintenance-prone. Instead, put in more bends or loops to accommodate expansion. Also, avoid conservative stress analysis. One modern approach is to have the vendor model the entire system (including the piping and the machinery) at the same time. Concurrent modeling can reduce inherent conservatism and could allow the thermal movements to be accommodated correctly by both systems. This may result in a more-flexible combined system and can allow better optimization. Elimination of the expansion joint often can pay for the engineering time needed to remodel, re-evaluate and redesign the entire system. Ideally, include such an optimization-simulation in the vendor scope before the order.

AMIN ALMASI is a rotating equipment consultant based in Brisbane, Australia. E-mail him at [email protected]