Procedure for Pump Line Analysis

Procedure for Pump Line Analysis

               The analysis of pump-connected piping systems is considered very critical. In this article, I will try to elaborate on the method followed for the stress analysis of a centrifugal pump piping system

REQUIRED DOCUMENTS

Following Listed basic documents are required to perform stress analysis of Pump Connected System.

1. Pump line Isometrics, 
2. Pump vendor drawing, 
3. Allowable nozzle Load, 
4. P& ID,. 

     In the absence of allowable load in vendor drawing, twice of API-610 values are used. CAESAR axis system should match with vendor drawing or API-610 for comparing nozzle allowable loads and moments.

PUMP MODELING IN CAESAR-II

Most commonly used centrifugal pumps are:

1. End suction – Top discharge
2. Top suction – Top discharge
3. Side suction – Side discharge

For above mentioned pumps detail modeling procedure in CAESAR-II is discussed as below

1.  End suction – Top discharge

Referring to Fig.1, node marking 1-2 shows nozzle flange with given flange rating as “rigid” element. Node 2-3 shows nozzle standby with pipe element (Diameter equal to pump impeller diameter and thickness equal to pipe wall thickness) as rigid element with zero weight from flange welding joint to pump impeller center line.

Restraints –

1. The pipe flange to be get anchored as C- Node with pump flange to get nozzle load due to piping reaction. This load value should be less than allowable values.
2. Rigid element at Impeller C/L to be get anchored i.e. anchor at node3.

Fig.1

Pump with End Suction & Top Discharge

2.  Top suction – Top discharge & 3. Side suction – Side discharge

Referring to Fig.2 & 3, node marking 1-2 shows nozzle flange with given flange rating as “rigid” element. Node 2-3 shows nozzle standby with pipe element (Diameter equal to pump impeller diameter and thickness equal to pipe wall thickness) as rigid element with zero weight from flange welding joint to pump impeller center line. Node 3-4 shows rigid element with zero weight up to center line of the pump.

Restraints –

1. The pipe flange to be get anchored as C- Node with pump flange to get nozzle load due to piping reaction. This load value should be less than allowable values.
2. Rigid element at pump C/L to be get anchored i.e. anchor at node

Fig.2 : Top suction – Top discharge

Fig.3 : Side suction – Side discharge

Pump with Top Suction & Top Discharge

 

Pump with Side Suction & Side Discharge

 

LOAD CASES FOR PUMP LINE STRESS ANALYSIS

Load cases for the pump connected lines are discussed below.

1.  Hot Cold Load case:

When two or more pumps are used in a system then from stress analysis point of view the different load cases are made. Like,

Ø  Both pump operating.

Ø  One pump pump operating & one stand by.(This point to be get discussed with process, what will be the requirement, whether “at any moment only one pump will operate, for more than two pump case”)

Ø  Hot Cold analysis to be done at design and operating temperature. 

Ø  For the pump, which is not operating, ambient temperature should be considered from Tee point

Ø  When two pumps are present in the system hot cold cases can be made as below (Refer Fig. 4).

CASE 1 : When both pumps are in operating condition, provide design and operating temperature for node 10-20-30-40-A & 20-50-60-B

CASE 2 : When pump “A” is operating and pump “B” is cold, provide design and operating temperature for node 10-20-30-40-A and ambient temperature for 20-50-60- B.

CASE 3 : When pump “B” is operating and pump “A” is cold, provide design and operating temperature for node 10-20-50-60-B and ambient temperature for 20-30-40- A.

Ø When three pumps are present in the system hot cold cases can be made as below (Refer Fig. 5).

 

Fig. 5 : Pipe connected to three pump

Case 1: When three pumps are in operating condition, provide design and operating temperature for whole system.

Case 2: When pump “A” is cold and pump “B & C” are operating, provide design and operating temperature to node 10-20-30-70-B & 20-50-60-C and ambient temperature for 30-40-80-A.

Case 3: When pump “B” is cold and pump “A & C” are operating, provide design and operating temperature to node 10-20-30-40-80-A & 20-50-60-C and ambient temperature for 30-70-B

Case 4: When pump “C” is cold and pump “A & B” are operating, provide design and operating temperature to node 10-20-30-40-80-A & 30-70-B and ambient temperature for 20-50-60-C.

2.   WNC (Weight No Content) Load case:

The basic philosophy for “WNC” is by what magnitude the piping flange displaced at the mating point while cold alignment. For “WNC” case, make the pump flange anchor node free and replace the spring support to rigid support. Go in load case spread sheet, drag the “WNC” from side menu to load case row and select “Sustain case”. Run the case and get the displacement at the pump nozzles, this displacement should be less than 1mm (or consult mechanical department to get allowable deflection).

3.   Load Cases in CAESAR-II for two pump connected system

Load cases for stress analysis of two pump connected piping system are discussed below. Same philosophy is applicable for more than two pump connected system.

Abbreviation:   W = Weight; T= Temperature; HP = Hydro Pressure; WW = Water Filled Weight; WNC = Weight No Contents; P = Design Pressure. U1 = Uniform load (NS Seismic); U2 = Uniform load (NS Seismic);

A.  Load Cases for Static Analysis (Two Pumps A & B) 

CASE1: WW+HP (HYD)

CASE2: W+P1+T1 (OPE) ………Both pumps at design temperature.

CASE3: W+P1+T2 (OPE) ………Both pumps at operating temperature.

CASE4: W+P1+T3 (OPE)   ………Pump “A” at design temp and Pump “B” standby.

CASE5: W+P1+T4 (OPE)   ………Pump “B” at design temp and Pump “A” standby.

CASE6: W+P1+T5 (OPE) ………Pump “A” at operating temp & Pump “B” standby.

CASE7: W+P1+T6 (OPE) ………Pump “B” at operating temp & Pump “A” standby.

CASE8: W+P1 (SUS)

CASE9: WNC (SUS)  

CASE10: L2-L8 (EXP)

CASE11: L3-L8 (EXP)

CASE12: L4-L8 (EXP)

CASE13: L5-L8 (EXP)

CASE14: L6-L8 (EXP)

CASE15: L7-L8 (EXP)

1. For load case no 2, 3, 4, 5, 6,7and 8 pump nozzle load is checked and it should be within acceptable limit as specified by vendor or API 610.
2. For load case no 1, 8, 10, 11,12,13,14 and 15 stress value is checked and it should be less than allowable stress as specified by relevant code.
3. For load case no 9 (WNC case) deflection is checked at pipe flange for cold alignment case as discussed in earlier topic of WNC load case.

B. Load Cases for Static Analysis (Three Pumps A, B & C) 

CASE1: WW+HP (HYD)

CASE2: W+P1+T1 (OPE) …Three pumps at design temperature.

CASE3: W+P1+T2 (OPE) …Three pumps at operating temperature.

CASE4: W+P1+T3 (OPE) ….Pump “B&C” at design temp and Pump “A” standby.

CASE5: W+P1+T4 (OPE) …..Pump “A&C” at design temp and Pump “B” standby.

CASE6: W+P1+T5 (OPE) …..Pump “A&B” at design temp and Pump “C” standby.

CASE7: W+P1+T6 (OPE) …..Pump “B&C” at operating temp and Pump “A” standby.

CASE8: W+P1+T7 (OPE) …..Pump “A&C” at operating temp and Pump “B” standby.

CASE9: W+P1+T8 (OPE) …..Pump “A&B” at operating temp and Pump “C” standby.

CASE10: W+P1 (SUS)

CASE11: WNC (SUS)

CASE12: L2-L10 (EXP)

CASE13: L3-L10 (EXP)

CASE14: L4-L10 (EXP)

CASE15: L5-L10 (EXP)

CASE16: L6-L10 (EXP)

CASE17: L7-L10 (EXP)

CASE18: L8-L10 (EXP)

CASE19: L9-L10 (EXP)

1. For load case no 2, 3, 4, 5, 6, 7, 8, 9 and 10, pump nozzle load is checked and it should be within acceptable limit as specified by vendor or API 610.
2. For load case no 1, 10, 12, 13, 14, 15, 16, 17, 18 and 19 stress value is checked and it should be less than allowable stress as specified by relevant code.
3.  For load case no 11 (WNC case) deflection is checked at pipe flange for cold alignment case as discussed in earlier topic of WNC load case.

C.  Load Cases for Seismic Analysis:

           For seismic stress analysis make separate file as “FILE NAME_sei”. In this file remove design and operating temperature and enter ambient temperature as T1.

Replace all “+Y” restraint by “Y” restraint. Remove all friction and guide gap at restraints. Click on uniform load in CAESAR spread sheet and enter 0.34 value in UX  and UZ direction. 

Activate uniform load in G’s option in Kaux – special execution parameters. Make load cases as below:

CASE1: W+P1 (SUS) 
CASE2: U1 (OCC) 
CASE3: U2 (OCC) 
CASE4: L1+L2 (OCC)
CASE5: L1+L3 (OCC)

For load case no 4 and 5 stress value is checked and it should be less than allowable stress as specified by relevant code.

IMPORTANT POINTS TO BE CONSIDERED:

1. Strainer at suction line should be modeled with actual weight.

2. First support from the pump nozzle should be of adjustable type support. If first support is spring then it should be locked and adjustable for alignment of pump.

3. Closed drain piping to be modeled for even 2” or below near pump nozzle.

4. Frictions to be given properly on all point from pump to header at least.

5. All rigid element weight to be considered while modeling e.g. valves, flanges etc.

Links for Reference articles: 

Procedure for Pump Modeling

Stress Analysis Of Column Piping System - 01

Stress Analysis Of Column Piping System - 02

Skirt Temperature/Expansion Calculation