This month's Sealing Sense was contributed by FSA Members Marty Rogin and Jim Richter
The March 2009 Sealing Sense concluded that pipe strain can cause failure of the packing or mechanical seal since they are both sensitive to misalignments introduced by pipe strain. A further study in the same article found that up to 70 percent of pump failures could be attributed to excessive strain on pump casings. Unacceptable strain was defined in the study as any forces from the pipe that will cause equipment deformation of more than 0.002 in.
This article will examine the stresses and forces induced by thermal strain on pump systems and provide guidance on relieving these potentially damaging loads.
Assembly Procedure-Then & Now
Long ago, it was normal for pipefitters to assemble pump and pipe manifolds on the jobsite. First, the pump would be set in place. Then, the pipe and fittings would organically "grow" from the pump as pieces were added. Everything around the pump fit together, and free bolting was common. (Free bolting is where flange bolts slide into place with no interference, in accordance with ANSI/ASME B31.3)
Today, to save time and money, a great deal of piping is prefabricated offsite. The assembled piping is installed at the pump, and any misalignment is accepted. Unfortunately, this means that most fitters' toolboxes contain a set of drift pins and the ever popular come-a-long. When bearing or seal failures occur more often than expected, it can be an indication that these tools were used.
Thermal Effects of Connected Pipe
The effects of misalignment are compounded if the system is subjected to a change in temperature without accommodating for pipe movement. The consequent stresses and bending moments can easily exceed pump manufacturers' allowable limits.
Consider the simple piping arrangement in Figure 1. The pump is directly connected to straight, anchored pipe sections. Straight pipe sections can generate tremendous axial reaction forces when subjected to temperature changes. Pipe anchors can be designed to accommodate these loads, but pump components, while functioning as anchors, cannot handle this loading.
Figure 1. Problem Piping Arrangement
Table 1 summarizes the stresses and resulting reaction forces in a straight piece of schedule 40 carbon steel pipe subjected to a 100 deg F temperature rise. The pipe is assumed to be 10 ft long.
Table 1. Pipe Reaction Forces
Note there are only axial reaction forces at the pipe ends. No reaction moments are generated due to the geometry of the pipe layout. There are also reactions due to the weight of the pipe and fluid. These forces, while not negligible, are normally much less than the forces due to thermal expansion and are ignored in this example.
If a flange connection leaks after a few on-off cycles, these reaction loads could be crushing the gasket and causing the leak.
Rubber Expansion Joints to the Rescue
The terms rubber expansion joints and flexible pipe connectors are often used interchangeably. While they are both effectively flexible connectors, there is a distinction between the two devices. Expansion joints are usually designed with arches to allow for compression, extension, offset and angular movement. A flexible pipe connector is a straight rubber pipe primarily designed to absorb noise and vibration in a piping system.
Although expansion joints introduce another set of design considerations, mostly a pressure thrust component, the expansion joint will absorb almost all of the reaction force.
If expansion joints are added to the piping, the pipes are allowed to expand with little resistance. The forces on the pump casing are now due to the pressure across the largest area of the connector (pressure thrust) and the spring force of the elastic expansion joint.
The spring force is normally much less than the pressure force.
Table 2 summarizes the reaction forces generated by an expansion joint in the same piping arrangement shown in Figure 1.
For a given pipe size, the reaction force generated by a rubber expansion joint is significantly less than a direct-piped installation. Isolating the pump from the piping with a rubber expansion joint prevents damage from thermal movement as well as absorbs noise and vibration.
Application of rubber expansion joints also presents a challenge. When the joint is pressurized, it wants to expand in diameter. Fabric coupled with metal reinforcing when required minimizes this phenomenon. It also wants to elongate. If the joint is not restrained in the axial plane, control rods should be added to prevent this expansion.
Expansion joints should be on both the suction and discharge sides of the pump to isolate the pump properly. Expansion joint manufacturers can provide guidance on selection and installation.