Developing a Pump System Assessment with Energy Cost Balance Sheets

In the April edition of the ESI News Brief, we talked about gaining a clear understanding of pump system operation by developing an Energy Cost Balance Sheet.  This month we’ll put those techniques to work by working through the assessment process. 

The first step in the assessment involves developing an Energy Cost Balance Sheet for the way the system is currently operating.  We can then identify system improvement alternatives and develop an Energy Cost Balance Sheet for each proposed alternative.  These results provide the plant’s financial management team with the data needed to develop a course of action based on the potential risk/ reward. 

The Current System

We’ll demonstrate by performing an assessment on a typical process system (figure 1) consisting of a supply tank, centrifugal pump, process component, destination tank, and two control loops.  Downstream of the process pump some of the flow is returned to the supply tank to maintain a 100 psi pressure.  The remainder of the system flow goes through the process component, flow element, and onto the destination tank.  The level in the destination tank is maintained at five feet by a level control valve.

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Figure 1 – Piping schematic of the example process piping system, with current operating parameters.

The operating data of the plant’s installed instrumentation is displayed in Figure 1, notice the flow rate into the destination tank averages 2,500 gpm to maintain the tank level in the system.  This system is currently running 8,000 hrs. per year to meet the plant’s production needs.  The system has been operating this way since it was commissioned five years ago. 

A system assessment was performed due to the system’s continuous operation, the position of the level control valve, and the fact that by-pass control is used to maintain the pump’s downstream pressure. 

The way the system currently operates consists of two circuits, a process circuit, and the by-pass circuit.  Both circuits are supplied by a common pump, so the energy (or head) consumed by each circuit equals the head produced by the pump.  The Energy Cost Balance Sheet is presented in Table 1.  A key element of the analysis is the calculation of the pump operating cost, the method to calculate this important value is demonstrated in the following paragraphs. 

The key to understanding pump operation is the manufacturer’s pump curve (Figure 2).  The flow element shows the flow rate to the process tank needed to maintain level and does not include the by-pass flow.  The pump head of the process pump will be used to determine the flow rate through the pump.  The installed pressure gage on the pump discharge will be used.  With no suction pressure gage installed, the level in the Supply Tank, elevation of the pump suction, and the estimated head loss in the suction pipeline will be used to calculate the pump’s suction pressure.

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Figure 2 – Pump curve showing the location of original flow, the flow of alternate 1, and flow and head of alternate 2.

A calculated pump head of 235 corresponds to a flow rate of 4000 gpm, and resulting efficiency of 83% read from the pump curve.  A fluid density of 62 lb/ft3 was obtained from the process engineer, and the average power cost of $0.05/kWh was established by the energy assessment team.

The annual pump operating cost is calculated using Equation 1 at $92,421 per year.

\[AOC = \frac{(0.746)QHp}{(247,000)}(hrs)($/kWh)=\frac{(0.746)4000 x 235 x 62}{(247,000) x .83 x .94}\ (8,000)(.05)\]

Since the process pump is the sole source of fluid energy this cost represents the system’s energy cost.  The Energy Cost Balance Sheet for the system under its current operation is provided in Table 1. 

Total Head (ft) Flow Rate (gpm) Energy Cost
Process Pump Element
Total Pump 235 4000 $92,421
Process Circuits
Static Head (PE) 95 2500 $23,351
Process Equipment (PE) 23.3 2500 $5,727
Pipelines (PE) 22.4 2500 $5,506
LVC (CE) 94.3 2500 $23,179
Subtotal Process Circuit 235 2500 $57,763
Static Head (PE) 25 1500 $3,687
Pipelines (PE) 3 1500 $442
BPV (CE) 207 1500 $30,528
Subtotal By-Pass Circuit 235 1500 $34,658

Table 1 – The Energy Cost Balance Sheet for the system as currently operating. Notice the total of the pump energy and power cost is the same as the sum of the process circuits.

The associated power cost and head consumed by each element in the system were calculated using the operating data from the installed plant instrumentation, pump curve, process equipment data sheets, and control valve data sheets.  The details of these calculations will be the subjects of future articles.

Looking at the Energy Cost Balance Sheet, we can see that approximately one-third of the total energy is used by the by-pass circuit.  With these revelations it appears the system is an excellent candidate for ways to reduce operating cost and improve plant profitability.

Considering Alternatives

We can now develop system improvement opportunities.  Our primary focus will be the elimination of the by-pass circuit.  Four alternatives are presented, two of which can be made without purchasing any new equipment, and two involve purchasing and installing new equipment.

The proposed system alternatives are:

  1. Changing the plant’s operation procedure to eliminate the need to maintain a continual by-pass for pressure regulation.
  2. Eliminate the by-pass control and reduce excessive head across the Level Control Valve
  3. Install a Variable Speed Drive to establish level control in the Destination Tank by varying the pump speed. 
  4. Purchase a new pump to meet current system’s needs.

It must be stressed that none of these alternatives should be made quickly; their effects on the system operation should be well understood by all parties involved.  However, the visibility of the cost of operation helps encourage communication between the various plant groups.

Alternative 1 Eliminating By-Pass Control

After reviewing the design documents, it was discovered the by-pass was originally installed to ensure a minimum flow through the process pump during plant startup and shutdown.  Over time, it was decided that the by-pass should be operated regardless of plant load to eliminate the possibility of low flow rate through the pump during large changes in plant load. 

Plant operating records indicate the flow rate through the system has never been below the manufacturer's minimum flow value for the pump.  After a discussion with the operations and instrument departments and the pump supplier, it was determined the need for continual by-pass was not required.  As a result, a system assessment of the proposed changes was conducted.

With the by-pass closed, the pump delivers 2,500 gpm to the Destination Tank.  The pump’s performance, read from the pump curve, was at 2,500 gpm, a head of 271 ft, and an efficiency of 73%.  Using this revised pump and control valve operating data, an Energy Cost Balance Sheet was developed for the “Close By-Pass” alternative and presented in Table 2.  

Total Head (ft) Flow Rate (gpm) Energy Cost
Pump Elements
Total Pump 271 2500 $75,737
Process Circuit
Static Head (PE) 95 2500 $26,550
Process Equipment 23.3 2500 $6,512
Pipelines (PE) 21 2500 $5,869
LVC (CE) 131.7 2500 $36,806
Total Process Circuit 271 2500 $75,737

Table 2: The Energy Cost Balance Sheet for the option of eliminating the by-pass control on the process system.

Looking at the results we can see there is an estimated annual cost savings of $16,684 simply by changing the plant operating procedure to isolating the by-pass control on the process pump.

Trimming Pump Impeller

Reducing the flow rate through the pump to 2500 gpm caused the Process Pump head to increase to 271 ft.  This increase in pump head resulted in a differential pressure across the Level Control Valve (LCV) of 57 psi.  With this new information, a second alternative is now being considered of isolating the By-Pass Valve (BPV) and trimming the impeller of the process pump to reduce the differential pressure across the LCV. 

After discussions with the plant staff and control valve supplier, it was determined proper level control could be maintained if the dp across the LCV was reduced to 20 psid.  The pump supplier was contacted, and it was determined an impeller trim of 13.875 inches resulted in a pump head of 186 ft at 2500 gpm with a 20 psi drop across the level control valve.  Trimming the impeller also resulted in a pump efficiency of 79%.  These new efficiency and pump values were used to calculate the pumping cost for the “Isolation of BPV and Pump Trim” option.  A completed Energy Cost Balance Sheet is presented in Table 3.

Total Head (ft) Flow Rate (gpm) Energy Cost
Pump Elements
Total Pump 186.3 2500 $48,111
Process Circuit
Static Head (PE) 95 2500 $24,533
Process Equipment (PE) 23.3 2500 $6,017
Pipelines (PE) 21 2500 $5,423
LVC (CE) 47 2500 $12,138
Total Process Circuit 186.3 2500 $48,111

Table 3: Energy Cost Balance for the system alternative of isolating the By-Pass valve and trimming the Process Pump Impeller.

Installing a Variable Speed Drive

Another way to reduce the pump head is to install a variable speed drive (VSD) on the pump/motor combination.  Installing a variable speed drive requires the purchase of additional equipment, but it also could provide for greater savings and increased operational flexibility.

The electrical and controls engineers evaluated installing a VSD with the manufacturer’s rep.  The pump manufacturer was consulted to determine if there would be any difficulties installing a VSD on the pump.  Once it was determined that installing a VSD was a viable option, an Energy Cost Balance Sheet was developed.    

With a VSD installed and set to maintain the level in the Destination Tank Level Control Loop, a pump speed of 1416 rpm maintained the desired level in the destination tank.  The plant decided to leave the existing control valve installed and fully open to minimize construction downtime.

Total Head (ft) Flow Rate (gpm) Energy Cost
Pump Elements
Total Pump 163.8 2500 $41,772
Process Circuit
Static Head (PE) 95 2500 $24,227
Process Equipment (PE) 23.3 2500 $5,942
Pipelines (PE) 21 2500 $5,355
LVC (CE) 24.5 2500 $6,248
Total Process Circuit 163.8 2500 $41,772

Table 4: Energy Cost Balance Sheet for installing VSD

Purchasing New Pump

With the changes in system operation, an alternative of purchasing a new pump to better meet the system’s existing needs was evaluated.  The pump manufacturer was contacted and made a recommendation for a pump that was readily available, could be installed in the existing system with a minimum of modifications and an 86% efficiency rating at 2500 gpm.  An Energy Cost Balance Sheet completed showing the potential operating costs. 

Replacement Pump
Total Head (ft) Flow Rate (gpm) Energy Cost
Pump Elements
Total Pump 187.6 2500 $45,027
Process Circuit
Static Head (PE) 95 2500 $22,802
Process Equipment (PE) 23.3 2500 $5,592
Pipelines (PE) 21 2500 $5,040
LVC (CE) 48.3 2500 $11,593
Total Process Circuit 187.6 2500 $45,027

Table 5: Energy Cost Balance Sheet for replacing existing pump with a new higher efficiency pump.

Conclusion

In conclusion, successful implementation of a pump system improvement program starts with everyone involved having a clear understanding of how the pump system operates. Once everyone has a clear understanding, the next step is to develop an Energy Cost Balance sheet for each element in order to start the pump assessment process. As you can see, when it comes to optimizing a pump system several options are available from leaving the system as is to purchasing and installing new equipment. 

Be sure to come back next month for the next article in this series discussing how to complete a pump system assessment on a system with multiple loads.