Design Optimization with Pipe Stress Analysis

This pump suction line cycles between 42C cold and 165C when a pump is running. The temperature changes will cause thermal expansion that are expected to create stresses in the pipe and loads on the pump inlet flanges. The pump manufacturer has rated the inlet flanges to API 610 limits. The piping system must be designed to keep the loads below the specified limits.

Initial Design

Fig 1 – P&ID of the piping system running from the tank to the pump suction inlets. Equipment, temperature and pressure are listed. Of interest to this article, the pipe is 12″ diameter, but is reduced to 8″ at the inlets.

Fig 2 – Piping 3D arrangement and initial support locations are taken from the pipe spool drawings and entered into Caesar II. Black arrows are guides. Pink are flanges and valves. The pump inlets are set as anchors, but the loads must not exceed the pumps limits.

Allowable Loads and Setup

Fig 3 – The pumps maximum inlet loads and moments are defined by the API 610 standard.

Table 1 – API 610 nozzle load and moment limits from Fig 3.

Fig 4 – Four different operating cases are run. Left “OPE 1” – both pumps are running – all lines are hot (purple color). Center OPE 2 – Pump A is running and its inlet is hot, pump B and its inlet piping is cold (red color). Right “OPE 3” – Pump B is running and hot, Pump A is cold. The final case “SUS” – both pumps are off and cold is not shown.

Run 1 – Fails

Caesar runs the four load cases – “OPE 1”, “OPE 2”, “OP 3” and “SUS”. The pipe stresses are acceptable for this run and all other runs in this article. The loads on the pump inlet flanges are of interest, and in this case fail for Pump B.

Table 2 – Pump A loads – all are acceptable. Pump B loads – red numbers are over allowables.

The initial design creates inlet nozzle loads that exceed the pumps limits. A redesign is required.

Run 2 – Fails

Run 1 shows that the loads are acceptable for Pump A and fail for Pump B. It is logical to move the inlet location on the header to increase the flexibility of the inlet line on Pump B at the expense of Pump A.

Fig 5 – First redesign – moving the pipe will change the loads on the two pumps. Flexibility for Pump B is expected to increase at the expense of Pump A.

Table 3 – Pump B is still overloaded, but not as badly. Pump A is now overloaded.

This redesign fails. Now both pumps are overloaded.

Run 3 – Acceptable

Another solution is tried. The inlet line is returned to the center of the header and elbows are added to the pump inlets to increase flexibility.

Fig 6 – elbows (circled) are added to the inlet piping to increase the pipe flexibility. The line from the tank is returned to the center of the header.

Table 4 – Acceptable loads on both Pump A and Pump B

Adding the elbows as shown in Fig-6 has increased the flexibility enough to lower the inlet loads to the allowable range for both pumps. This design is acceptable.

Run 4 – Acceptable

Although Run 3 has acceptable results, it increases the material cost by adding elbows and fabrication costs by adding welds. Another alternative is tried.

Fig 7 – Moving the header towards the tank (green arrows) makes room for a longer run of smaller diameter, more flexible, pipe on the pump inlets (red arrows).

Table 5 – Second passing option – all loads are acceptable.

This alternative also is acceptable, although it increases the fabrication cost by adding small diameter pipe welds. It has lower fabrication cost than Figure 6.

Pipe stress provides 2 acceptable design alternatives. The best design for the application can be used.

Pipe Stress Analysis at PVEng

We offer pipe stress analysis services. 

  • Caesar II thermal, flexibility and dynamic pipe stress analysis
  • Seismic analysis for British Columbia CRN registration
  • Water hammer, flow induced vibration & integrity review
  • Fitness For Service using API 579
  • Layout design, hanger, guide, anchor and expansion joint location and specification
  • P.E. / P. Eng. stamping

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