Cooling Water Circulating Pump Low throughput Resolved

Root Cause Analysis (RCA): Cooling water circulating Pump Low throughput resolved & proposed Rs 20 million pump add averted.

Article Type:	Root Cause Analysis (RCA)
Category:	Mechanical
Equipment Type:	Miscellaneous Rotating Machine Problems
Author:	S. Raghava Chari

Note: This root cause analysis (RCA) is from real-time scenarios that happened in industries during the tenure of two or three decades ago. These articles will help you to improve your troubleshooting skills and knowledge.

Cooling Water Circulating Pump Low throughput

The process engineers attributing inadequate pump capacity recommended adding one more pump. They had had no answer for the Author’s query,

“What about System Resistance (SR)?”

He explained SR and took them to the pump. The discharge header pressure gauge read 324 kPa against the Process Flow Diagram (PFD) 320. The process engineers triumphantly said, “We are right Sir. SR has not increased much.”

To double-check, the author zeroed the PG, and found it still showing 324 – obviously stuck gauge!  A new gauge showed 370 kPa.

This suggests the below given possible root causes for the low throughput problem:

1. Way high 370 kPa system resistance
2. Consequent, far below the required 10% header relief valve margin
3. Maybe the too low margin lets the relief valve pass heavily  
4. The passing flow, piped back to the cooling water basin is not visible 
5. Fouled CW circuits of various plant heat exchangers; Crew found a small shut down heat exchanger water circuit so fouled that one wondered how water ever flowed in it! All other heat exchangers CW circuit similar severe fouling is most likely.
6. Under the above circumstances even the proposed pump addition would serve no purpose           

Author Solution

On the Author’s suggestion, the plant advanced the turnaround by two months; by then a hydro-jet contractor’s cleaning set up was ready. He hydro‑jet cleaned the shell and tube circuits of all heat exchangers metal clean.  

With all heat exchangers, clean pump discharge pressure was only 280 kPa. Operators throttled many heat exchangers’ water valves to get correct exit streams temperatures. 

Finally, the pump discharge header PG settled at 310 kPa, and pump delivery matched with the Characteristic Curves data. 

Benefits of Author RCA solution

1. The various heat exchangers’ exit streams’ temperatures and surface condensers’ vacuums attained normal values, and production exceeded rated capacities.
2. Near 20 million Rs. Pump purchase and installations costs avoided
3. Space constraints posed problems additional installation costs avoided
4. Even with the new pump the problem would have persisted; the consequent severe criticism, even ridicule avoided

Author: S. Raghava Chari

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