Every power or process piping industry uses several pumps in each unit. The analysis of pump connected piping systems is considered as very critical. In this article, I will try to elaborate on the method followed for the stress analysis of a centrifugal pump piping system. The stress system consists of typical discharge lines of two centrifugal pump (Pump A and Pump B). Fluid from these two pumps is pumped into a heat exchanger. As per P&ID, only one pump will operate at a time, other pumps will be a stand by pump. I will explain the stress analysis methodology in three parts:
- a) Modeling of Pump
- b) Preparation of analysis Load cases and
- c) Analyzing the output results.
Modeling of Pump in Caesar II
For modeling the pump we require vendor general arrangement drawing or outline drawing. All rotary equipments are modeled as a weightless rigid body in Caesar II. From the outline drawing, we need to take the dimensions to some fixed point. Let us take the example of the outline drawing shown in figure 2.
From the above drawing, we can get the dimensions for elements 10-5000 as 8.5 inches and element 5000-5020 as 6.19 inches. At node 5020 we will provide fixed anchor. During modeling of the above elements, we need to use line size and thickness as the diameter and thickness of the equipment. Line temperature and pressures as equipment properties. We have to provide anchor (with a CNode) at node 10 for checking nozzle loads which we will compare with the allowable value as provided in Fig. 3 below:
In absence of allowable load value, the Pump design code (API 610 for API pumps, ANSI HI 9.6.2 for non-API pumps) can be followed for the same.
After the pump is modeled as rigid body the piping modeling need to be done from pump-piping interconnection flanges.
Preparation of Analysis Load Cases:
Along with normal load cases, two additional load cases need to be prepared. Normally in the refinery and petrochemical industry, one pump operates and the other pump acts as a stand by pump. So we have prepared load cases as follows:
1. Hydrostatic case (WW+HP HYD)
2. Operating case with both pump operating (W+T1+P1 OPE)
3. Operating case with the total system in maximum design temperature ( W+T2+P1 OPE)
4. Operating case with pump A operating and pump B Stand by (W+T3+P1 OPE)
5. Operating case with pump B operating and pump A Stand by (W+T4+P1 OPE)
6. Operating case with the total system in minimum design temperature ( W+T5+P1 OPE)
Next all normal load cases like static seismic, static wind, etc are to be built as per stress analysis or flexibility specification.
When pump A is in operating condition and pump B stand by, the normal pipe operating temperature has to be inserted till Tee connection for pump A and ambient temperature will be the input for pump B as shown in Fig. 4. Similarly, reverse the input when pump B is operating.
After equipment is modeled completely start modeling the piping following dimensions from piping isometric drawings. Try to make a closed system. Normally pump lines are connected to some vessel, tank or heat exchangers. So it will create a close system. Then run the analysis to check stresses, displacements and loads.
Analyzing the output Result:
Once Caesar completes its iteration process we can see the output results in output window. At nozzles (the nodes which we anchored with a CNode) we can check the force values. These values we have to compare with the allowable values. If the actual values are less than the allowable values then the nozzle is safe. Otherwise, we have the make changes in supporting or routing to bring the nozzle load values within allowable. A sample output restraint is provided in Fig. 5 for your reference.
As can be seen from the above figure that we are checking nozzle loads in load cases 2, 4, 5 and 6. For rotary equipment, normally nozzle qualification in design or upset temperature is not required.
Special Consideration for Rotary Equipments
Now we have to make one separate caesar file and we have to check sustained displacement at the nozzle at WNC (weight no content) case. This checking will ensure proper alignment of piping flange and equipment nozzle flange. For detailed alignment check analysis steps, follow this link: https://www.whatispiping.com/alignment-check-methodology
Always remember to provide first piping support from pump nozzle as adjustable support (or a spring hanger support) to aid in alignment.
In the case of 3 pump systems, normally two pumps will be operating and one pump will be stand by. So input and prepare load cases accordingly. If you have any confusion or want to add more please write in the comments section.
Few more pump-related articles for you..
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NPSH for Pumps: Explanation and Effect
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Mechanical Seals for Rotary Pumps
Pumps & Pumping Systems: A basic presentation
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Considerable points while Commissioning and starting-up a Process Pump
Considerable points while installing centrifugal pumps at site to reduce vibration