Combination Soft Starter Eliminates Piping Cavitation and Reduces Energy Consumption in Steel Mill Spray Nozzle Pumps

Intended Audience: Facility Managers, Mill Engineers

Objective: Reduce the maintenance costs and energy consumption of a 150 kW (200hp) hood spray pump.

Description of System: The subject pump is a hood spray pump installed in a steel mill that operates 7200 h/year. This 140 kW (200 hp) rotodynamic pump supplies pressure to the hood spray nozzle, which is only required to spray 40% of the time. Pressure is controlled with a bypass valve and orifice, as shown in Figure 1. 

Figure 1: System Schematic

When the spray is not required, all flow discharges through the bypass line. Under this condition, operation is at 44% of the 180 m3/h (800gpm) flow rate, as shown in Figure 2. When the hood spray nozzle was open, the pump consumed ~150 kW (200 hp), and when the spray nozzle was closed, the pump consumed ~123 kW (165 hp). 

Figure 2: Pump and system curves

When the pump is not delivering flow through the spray nozzle, it is significantly oversized. This represents approximately 60% of its operating time.  

Operation in this mode results in excessive pressure loss across the flow-restricting orifice, which both wastes energy and causes significant audible cavitation within the piping system. This cavitation has resulted in erosion damage to the piping downstream of the orifice, requiring regular maintenance (see Figure 3). Additionally, the average power consumption, including motor inefficiency, is 145 kW (194 hp). This costs the end user $62,500 at a rate of $0.06/kWh.  

In addition to the routine operation and maintenance costs of $100,000 per year, the end user spends an estimated $50,000 annually to repair pump damage caused by low flow rate operation and bypass piping damage caused by cavitation.  

Figure 3: System schematic with cavitation damage 

There were several red flags in this system that indicated it should be assessed for optimization: 

• High maintenance costs   

• Relatively high energy (150 kW (200 hp))  

• Pump running 100% of the time in batch process system 

• Audible cavitation  

• Bypass line continually open  

Description of Intervention:

The system was reviewed by an assessment engineer who considered the following elements¹:  

•  Capital costs of solution   

• Installation and commissioning  

• Energy usage   

• Operation and maintenance  

Solution 1: Replace Motor with More Efficient One  

The current 150 kW (200 hp) motor is older and less efficient than newer models. It could be replaced by a new, premium motor with 95% efficiency.  

This option would cost $10,000 for the motor and $3000 for installation. A total of 4 kW (5 hp) of power for 7200 h/year would be saved, resulting in 28,800 kWh savings annually. At $0.06/kWh, this results in annual energy costs of $60,800, representing a savings of $1700.  

While this option reduces total energy costs, it does not eliminate the low flow-related pump and piping maintenance costs.  

Solution 2: VFD Implementation  

Implementing a variable frequency drive (VFD) would allow the pump to operate closer to its best efficiency point when not discharging through the spray nozzle. For this installation, variable speed could be implemented to operate the pump at 70% speed (its minimum allowable), which would draw 80 kW (110 hp) less, as shown in Figure 3. 

Figure 4: Variable speed pumping option 

Upgrading the motor to inverter duty would cost $13,000. Purchase and installation of a 150 kW (200 hp) VFD would cost $20,000.  

For this solution, a total of 80 kW (110 hp) of power for 4000 h/year would be saved, resulting in 320,000 kWh savings annually. At $0.06/kWh, this would result in annual energy costs of $43,500, representing a savings of $19,000. Furthermore, the additional $50,000 repair maintenance costs would be eliminated.  

Solution 3: 200-hp Combination Soft Starter  

Installing a 150 kW (200 hp) combination soft starter would allow the pump to be turned off during the 60% of the time that the nozzles are not spraying, which would limit the stress on the motor and pump caused by cyclic operation. Historical data shows that the pump cycles approximately five times per hour.  

This option requires the purchase and installation of a soft starter and controls. The cost of purchase and installation would be $10,000.  

This solution would eliminate 4320 h of 123 kW (165 hp) of power, resulting in 530,000 kWh savings annually. At $0.06/kWh, this would result in annual energy costs of $30,700, representing a savings of $31,800. Furthermore, the additional $50,000 repair maintenance costs would be eliminated. 

Summary of Results: A lifecycle cost (LCC) analysis was performed to determine the best solution. A summary is provided in Table 1: 

LCC elements	Original	Solution 1	Solution 2	Solution 3
Project capital cost	N/A	$13,000	$33,000	$10,000
Annual energy cost	$62,500	$60,800	$43,500	$30,700
Annual O&M cost	$150,000	$150,000	$100,000	$100,000
Total annual cost	$212,500	$210,800	$143,500	$130,700
Annual savings	N/A	$1700	$69,000	$81,800
Simple payback	N/A	7.5 years	0.5 years	0.12 years
Project life	10 years	10 years	10 years	10 years
Discount rate		4%	4%	4%
IRR		5%	209%	824%
NPV		$790	$525,000	$660,000

Table 1: LCC Analysis 

Recognizing the low LCC, the steel mill decided to implement Solution 3. This solution eliminates the bypass line as part of the system and adds controls to start and stop the pump when spraying is required. The payback summary is reported in Table 2 

Total annual savings	$81,800
Project capital cost	$10,000
Project life	10 years
Discount rate	4%
NPV	$660,000
Simple payback	0.12 years

Table 2: Simple payback summary of Solution 3 

Conclusion: Installing a 150 kW (200 hp) combination soft starter allows the pump to be turned off when not needed, making it an ideal solution for a process with frequent cycling. In this case, implementation of this solution saved the end user significant cost in both energy usage and expected maintenance, resulting in a simple payback period of 0.12 years. 

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Written by: Members of the Committee, 1^st Edition   
Published In: Pump Systems Optimization: A Guide for Improved Energy Efficiency, Reliability, and Profitability  
Year of Publication: 2018  

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