Supportting Pump Lines

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Improve design for pump suction nozzles 05.01.2013 | Choudhury, M. G. , Reliance Industries, Mumbai, Maharashtra, India; Kulkarni, A. , Reliance Industries, Mumbai, Maharashtra, India; Koranga, D. ,

Reliance Industries, Mumbai, Maharashtra, India

Keywords:

Several analysis methods exist to determine the effectiveness of the pump suction piping system’s first adjustable support and how that impacts nozzle loading. Parameters for consideration include how to model the trunnion dummy leg, determining the thermal load for the dummy leg and accurately predicting the first adjustable support base plate’s stiffness.

DIAGNOSING THE PROBLEM

The pump piping system is a common component of most hydrocarbon processing complexes. It is essential that this system is analyzed correctly, so that future problems can be minimized. Within this system, the first support adjacent to the nozzle plays a vital role during analysis. This support is often an adjustable type that helps in maintaining the pump shaft axis in line with the nozzle axis.

A popular analysis program uses the pipe centerline modeling approach. Generally, an ambient temperature pipe trunnion from the pump centerline to the trunnion support base plate is modeled during analysis. This approach is sometimes criticized, as the trunnion pipe is welded at the main pipe bottom and the diametrical growth of the pipe is ignored. To circumvent this criticism, some analysts advocate a more refined analysis using the element from the suction pipe centerline to the bottom of the suction pipe, where the trunnion has been welded as a rigid element, with a temperature the same as that of the suction pipe. Then a trunnion with ambient temperature and without fluid density and pressure is modeled.

A commonly used analysis technique is the centerline analysis method. In this method, the dummy is modeled from the straight pipe neutral or at the mid-node of a three-node elbow, as shown in Fig. 1. The trunnion itself is assumed to be at ambient pressure and temperature. This analysis does not address the issue of pipe diametrical growth at elevated temperatures. The rigid-element analysis addresses this issue.

To address the diametrical pipe growth issue, rigid-element analysis is used. In method, a rigid element is modeled from the pipe centerline to its periphery. Then the dummy with ambient temperature and pressure properties is attached to it (Fig. 2). The rigid element takes care of the radial expansion of the parent pipe. Due to this additional radial expansion, there will be a substantial rise in vertical pipe displacement, which increases nozzle loading.

CASE STUDY

This sample scenario offers an opportunity to see the direct effect of both techniques. Consider a side-suction pump system at a 340°C design temperature, as shown in Fig. 3. This system is then analyzed using a recent edition of popular pipe stress analysis software.

Fig. 1. Normal centerline analysis method.

Fig. 2. Rigid-element analysis method.

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Various cases in the software are then formed:

a) Normal centerline method: For this case, only a dummy is modeled in front of the nozzle with a sliding base support, with Fig. 4 showing µ = 0.1.

b) Rigid element method: in this instance, the rigid element is modeled from the pipe center to the parent pipe’s outer periphery (see Node A to Node B in Fig.

5) while maintaining the same conditions as the parent pipe. Then a dummy is attached at Node B, with the base support still sliding.

Table 1 summarizes the resulting nozzle loads from both analysis methods.

Table 1 demonstrates that the loads increase tremendously during the actual simulation of the pipe’s diametrical expansion. This can be attributed to substantial nozzle displacement in the vertical direction, which increases the vertical loads.

There are many ways to counter this vertical loading; the obvious option is changing the line’s routing. But this and other methods increase the overall cost to the system. However, one other way can decrease the loading on the nozzle.

Fig. 3. A side-suction pump system at a 340°C design temperature.

Fig. 4. Normal centerline method.

Fig. 5. Rigid-element method.



The first adjustable support was closely examined for support stiffness implications on the nozzle loading. Normally in software analysis, default stiffness in the program is used. Fig. 6 shows a typical adjustable support.

As Fig. 2 illustrates, the trunnion support rests on a thin stub-base plate that is attached to the ground with bolts and a series of stainless steel (SS), polytetrafluoroethylene (PTFE) and carbon steel (CS) plates.

The base plate material will be the same as the trunnion material. The PTFE plate inserted between the SS and CS plates will reduce the friction loads as its friction co-efficient is 0.1.

The most common assumption made during this analysis is that the first adjustable support’s stiffness is the stiffness default value. In the case of this software analysis, it is 1.0E12 N/mm. This high value of stiffness corresponds to a rigid base, like solid ground or some concrete structures. Since this scenario features a support that is effectively resting on a plate 10-mm thick, the stiffness will be lower.

STIFFNESS CALCULATION

To predict the effects of the first support stiffness on the nozzle loads, the support stiffness must be calculated accurately. The stub base plate’s vertical displacement must be accurate to calculate the plate’s vertical stiffness.

The simulated vertical displacement results are shown in Fig. 7. For simplification, the scenario assumed that the vertical bolt stiffness is sufficiently high as to assume it to be acting as a vertical restraint. The plate dimensions considered were 250 mm 3 250 mm 3 10 mm, with a 1,000-N external load applied on the trunnion periphery. From the stiffness equation, F = KX, for an applied load of 1,000 N, and vertical displacement of .0031512 mm, the vertical (Y) base plate stiffness is calculated as 31,7339 N/mm.

Fig. 6. The trunnion support rests on a thin stub-base plate, which is attached to the ground with the help of bolts and a series of plates.

Fig. 7. Simulated vertical displacement results.



UTILIZATION

By utilizing the base support stiffness, a more accurate analysis can be performed. Table 2 lists the results of all three analysis techniques.

Table 2 shows that the vertical loads are considerably reduced. This is due to the fact that the vertical displacement was reduced by utilizing the calculated support stiffness.

By predicting the accurate support stiffness, more accurate analysis of the pump suction piping is possible. Even after using the pipe element radial growth, the nozzle loads have not increased. This analysis type approach may be adopted when it is necessary to do a rigorous analysis for a critical pump nozzle load.

FINAL ANALYSIS

For high-temperature lines, diametrical pipe growth plays an important role in assessing the nozzle loading on a pump. When incorporated in a pump system analysis, this thermal loading will greatly increase the loading on the nozzle. To minimize the overall alteration of the system, it is advisable to use the actual support stiffness of the first adjustable support, thus reducing the overall cost to the system. HP

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