Trying to Insure Against Uncertainty During Design Can Lead to Inflated Requirements, Over-Sized Pumps and Operational Headaches
By Ray Hardee
Oversizing pumps is insurance for early-stage design uncertainties.
Creating specifications for a pump system usually takes a long time, and pump selection has a major impact on subsequent design calculations. Specification development happens early in the design process, and it requires engineers to make many assumptions that drive sizing calculations and pump specs.
In engineering, it’s often said that good designs are based on experience, and you gain experience by making mistakes. But no one wants to make a mistake when sizing a pump for a process system, so there’s a tendency, “for safety’s sake,” for the pump’s specs to exceed what’s actually needed.
Oversizing pumps is insurance for early-stage design uncertainties. It’s natural to factor in life’s uncertainties; that’s why we purchase insurance. I’d never recommend going without insurance, but I don’t want to pay for more coverage than I need. So, in pump selection, we must respect the need for insurance, but be realistic about our assumptions, and document the results.
Another thing about engineers is that we like to come up with acronyms to describe existing processes. Back in the day, the first step in any design was called preliminary engineering; now it’s called FEED, for Front-End Engineering Design.
This month’s Pumps & Systems article discusses the FEED process for pump selection, how it can lead to oversizing pumps, and how these plus-size pumps affect system operations. In a future article, we’ll cover how to obtain a realistic view of pump requirements, and suggest ways to reduce the costs associated with oversized pumps.
All piping systems consists of pump elements, process elements and control elements, all working together to meet the intended system design. The pump element is sized to pass the design flow rate through the process and control elements to make the product or provide the service.
In the example I’m about to describe, a manufacturing company needs to expand its plant to add a new product to its line. After completing a market study, it estimates that it requires a system capacity of 600 gallons per minute (gpm) for the foreseeable future. Because of the facility’s long life-expectancy and, seeking to leverage the expense of a new system, the company decides to double its designed capacity to 1,200 gpm. It selects an engineering firm for system design, specifies this desired output, and provides the process requirements.
Then the FEED process begins. The first step is estimating the locations and elevations of the system equipment within the facility.
Image 1. Flow diagram showing the location and elevations of all the equipment along with the details needed for pump sizing.
The engineering firm estimates design requirements for the process and control elements based on the firm’s sizing criteria. For example:
Table 1 summarizes the system losses needed for calculating the pump’s total developed head. It equals the sum of the calculated head losses for the process and control elements at the design flow rate. The minimal pump head requirement of 210.4 feet of fluid is needed at the design flow rate of 1,200 gpm.
Head Loss Needed for Pump Selection
|Equipment||Head Loss for Design Operation|
|Static Head||84.6 feet|
|Pipeline Head Loss||56.5 feet|
|Process Element HX||34.6 feet|
|Control Element CV||34.6 feet|
|Pump Total Head||210.3 feet|
Table 1. The method of determining the pump total head sizing requirements during the FEED process.
At this point, for the sake of insurance, the firm adjusts head loss estimates upward to account for unforeseen conditions. These are “safety factors” that, in this example, may reflect uncertainties associated with estimated pipe length, the count of valves and fittings, losses associated with heat exchanger fouling, control valve operation, variations in tank levels and pressures, and any other concerns expressed by members of the review team.
Often, Engineer A may add a safety factor to the calculated value before passing the spec on to the next person, Engineer B, involved in the review process. Engineer B may add their own safety factor, and so on until the team has reviewed and recommended a final design. In this example, a 30% safety factor is added to the calculated head losses, resulting in a pump total head requirement of 274 feet of head at 1,200 gpm.
The engineering firm completes the pump specification using the design point of 1,200 gpm and 275 feet of head and sends it to the pump suppliers. The winning supplier recommends an 8×6-20 end suction process pump operating at 1,750 rpm. The selected pump needs a 16.5-inch diameter impeller with a pump efficiency of 70% requiring 119 horsepower.
The pump’s Best Efficiency Point (BEP) of 73.4% occurs at a flow rate of 1,550 gpm. The pump is selected because the design flow was left of the BEP so at the system capacity increases the pump’s efficiency would improve.
The person selecting the pump increases the impeller diameter to 17 inches to take advantage of the 200 hp motor supplied with the pump. This resulted in a pump head of 296 feet of fluid at the design flow rate of 1,200 gpm, and is shown on Image 2 below.
Image 2. Pump curve for the recommended pump, along with the Best Efficiency Point and the recommended range of 80% to 110% of BEP flow.
The selected pump is purchased, and the pump details are used in various stages in the engineering process. The pump is shipped to the job site, and installed.
The Pump in an Operating Environment
After the system is commissioned and turned over to operations, the current usage only requires a capacity of 400 gpm. The operations group wants to know the effects of operating the system at this lower capacity.
A System Energy Balance is developed using the displayed pump curve and heat exchanger operational data supplied by the manufacturers. The pipeline head loss is based on the installed piping, and the static head is calculated by the energy difference between the tanks. The control element is determined by subtracting the energy of the process elements from the pump elements. (See Table 2 below).
System Energy Balance
|Equipment||Current Operation400 gpm||Expected Operation600 gpm||Design Operation1200 gpm|
|Pump Element||305.6 feet||306.2 feet||296.4 feet|
|Static Head||84.6 feet||84.6 feet||84.6 feet|
|Pipeline Head Loss||5.9 feet||13.1 feet||51.6 feet|
|Process Element HX||3.7 feet||8.3 feet||33.2 feet|
|Control Element||211.4 feet||200.2 feet||127.0 feet|
Table 2. The System Energy Balance showing how the energy is used within the system.
The System Energy Balance shows how the energy is used within the system. All the energy supplied by the pump elements equals the sum of the energy consumed by the process and control elements.
Next, the operations group compares how the system equipment is actually performing to industry norms. Table 3 shows key equipment information under the actual operations of 400 gpm, the expected operations of 600 gpm, and the design operations of 1,200 gpm based on future conditions.
Operation of System Equipment
|Equipment||Current Operation400 gpm||Expected Operation600 gpm||Design Operation1200 gpm|
|Percent of BEP Flow||26%||39%||74%|
|Pump efficiency (%)||42%||53%||70%|
|Pump power (HP)||68||86||127|
|HX Pressure drop (psid)||1.6||3.6||14.4|
|Pipeline fluid velocity (ft/sec)||2.6||3.8||7.7|
|Valve open position (%)||13%||20%||53%|
|Differential pressure (psid)||92||87||55|
Table 3. Comparing the operation of the system elements to industry guidelines.
Looking at the supplied pump curve, notice the Best Efficiency Point is 80% occurring at 1,550 gpm. The “Rotodynamic Pumps’ Guideline for Operating Regions (ANSI/HI 9.6.3)” standard recommends operating the pump at a flow rate between 80% to 110% of the BEP flow rate as shown on the pump curve. The ratio of the actual pump flow to the pump’s BEP flow results in the percentage of BEP flow.
We can see the selected pump will not operate within the recommended range at any time during pump’s operation. This will have a major impact on the pump’s operation and the system’s reliability for the foreseeable future.
The differential pressure across the heat exchanger is much lower than the maximum value stated in the specification for the expected operation. This lower pressure drop across the heat exchanger reduces pumping requirements, but comes at a greater capital cost.
Fluid velocity within a pipe has a major effect on head loss. Common industry practice recommends a pipe’s fluid velocity between 4 to 10 feet per second. We can see the fluid velocity for expected operations is outside this range. The lower pipeline head loss reduces pumping requirements but again comes at a greater capital cost.
In looking at the control valve, notice the valve positions range between 13% to 53% open with a resulting differential pressure from 92 to 55 psid. The fact that the control valve position is 20% or less at flow less than 600 gpm can cause major operational problems. Notice the differential pressure at the design condition is much greater than the 15 psi differential pressure stated in the control valve specification. The higher differential pressure does not improve the valve operation.
The use of “safety factors” reflects the team members’ experiences when dealing with system unknowns early in the design process.
In the process we’ve described for selecting pumps in piping systems, each step was based on sound engineering principals. The use of “safety factors” reflects the team members’ experiences when dealing with system unknowns early in the design process.
But, as we saw in this example, when the piping system was placed in operation, both the pump and process elements were oversized. This, in turn, resulted in a differential pressure across the control valve that’s much greater than expected. These factors, in my judgment, increased the system’s capital costs, operational costs, and maintenance costs.
In our next Pumps & Systems article, we will look for ways to improve the pump selection process, while still maintaining operational flexibility to efficiently run the system through its lifetime.