COLUMN | Focus on Fundamentals
Pump Selection: How to Manage System Curve Uncertainty
Sotirios Christofi | Industrial Machinery Consultant
During the early stages of specifying a new pump, engineers tend to be very conservative with the system curve calculation because the routing of the pipelines is not yet known. In addition, most times a pump with the lowest allowable best efficiency point (BEP) flow rate is subsequently selected, because this is usually the lowest quotation.
The result of these two conditions is that the pump purchased has to operate at a flow rate higher than the BEP flow rate, or even sometimes outside the flow rate range allowed by the codes.
Pump Purchase Procedure
During the industrial project management process, the order of major equipment—including pumps—follows the completion of basic design (BD), which can include, among other things, flow diagrams, piping and instrumentation drawings (P&IDs), equipment lists, data sheets, specifications and basic design drawings.
BD should contain enough detail to enable a reliable (±20%) cost estimate necessary for the investment evaluation to be done next. BD does not contain the detailed routing of equipment, which, along with detailed isometric drawings, is an essential part of the detailed design (DD).
The specification, order, manufacture and installation of major equipment are long-delivery items and therefore usually lie on the critical path of the whole project, so the priority is given to this chain of actions, the specification being the first.
The data sheet is the most important part of the order specification, since it contains all technical data for the construction design and manufacture of the equipment. The pump data sheet requires the purchaser to specify, among other things, the values of rated flow rate and rated differential head (rated discharge pressure - rated suction pressure).
Most of the time these have already been calculated by the flow diagrams of the basic design, along with the necessary pipeline size, with which the system can operate at these values. The bidders will then have to quote a pump with a BEP as close as possible to this operating point.
However, in most cases the data sheet has been prepared without having the isometric drawings of the suction and discharge pipelines. Sometimes the process project engineer does not even know the location of the equipment. As a result, there is uncertainty regarding both the suction and the discharge pressure of the pump.
Consequently, the suction and discharge pressures of the pump are calculated while making assumptions about the routing of the pipelines (location of the pipe racks, avoidance of running too close to existing equipment because of proximity limitations, etc.) as well as the fittings layout and the pressure drop caused by equipment such as filters, strainers, etc.
My experience indicates that process project engineers, most of them young, tend to add a safety margin to their calculations, usually around +15% to +20%, to overcome this uncertainty. As a result, the system curve is calculated to need more head than it actually does—or, in other words, to be steeper than it actually is.
The logic is that if the actual flow rate is further to the right of the pump BEP flow rate, the discharge valve can be throttled to achieve the desired flow rate. Unfortunately, this strategy can be costly.
| IMAGE 1: American Petroleum Institute (API) 610, part of data sheet, operating conditions (Image courtesy of the author)
Calculation of System Curve
Let us now look more deeply at the details of calculating the system curve.
- The operating point of the pump is found at the intersection of the pump performance curve and the system characteristic curve.
- The blue pump curve depends on the geometrical characteristics of the pumps. The pump curve is produced by the pump manufacturer.
- The red system curve depends on the geometry of the pumping system— i.e., the pipelines, fittings, equipment etc.—from the suction point to the discharge point. The system curve is usually produced by the owner’s engineer.
The system curve shows the total system head required over a range of flow rates. The equation below is courtesy of the Hydraulic Institute (Eq. 1.A.7):
| IMAGE 2: Definition of a centrifugal pump’s operating point (Image courtesy of the author)
Elevation difference between destination and supply—usually well calculated by BD
Pressure difference between destination and supply—usually well calculated by BD
Frictional head and head loss due to friction, consisting of a piping length component (f x L/D) and a fittings pressure drop component (∑K).
Piping length and fitting/equipment pressure drop not or roughly calculated by BD
What Happens When the System Curve Is Overcalculated?
In the system in Image 3, the above equation ends up as:
Δhsystem = 265 feet (ft) + 0,000775 x Q2
Δhsystem is system head, feet
Q is flow rate, gallons per minute (gpm)
Equation 1
| IMAGE 3: Worked example (Image courtesy of the Hydraulic Institute)
This equation gives the data in the first two columns of the following table. Let us suppose this equation is correct and accurately represents the situation. I have added two columns:
- The third column (calculated system head) with the data that would result in case the system was calculated to have 20% more friction losses than the actual situation (775 x 1,2 = 930)—a conservative approach, but it happens more often than not.
- The fourth column (pump head): The typical performance curve of a pump suitable for the system has been added.
Putting together the table data on the graph in Image 4, we note that:
- The calculated operating point (COP)—the section of pump curve and calculated system curve—lies at 203 gpm. That was given on the data sheet.
- The actual operating point (AOP)—the section of pump curve and actual system curve—is 220 gpm and lies to the right of the COP.
| IMAGE 4: Actual vs. calculated curves and operating points when the system curve is conservatively calculated (Image courtesy of the author)
An important detail: The AOP will be known only after the new pump startup if someone has installed the necessary manometers and flow meter. However, the bidders quote pumps based on the COP, not the AOP, which at the time of the bidding is still unknown. API 610 (§ 6.1.16) specifies that rated flow be within the region of 80% to 110% of the BEP flow rate. Conversely, the pumps should have a BEP with a flow rate ranging from 91% to 125% of rated flow (COP flow rate).
In our example, that allowable BEP range would be between 184 gpm (91% x 203 gpm) and 254 gpm (125% x 203 gpm).
What Happens When a Pump With the Lowest Allowable BEP Flow Rate Is Selected?
Suppose the pump selected has a BEP flow rate at the low end of the allowable region, i.e., 184 gpm—a very probable outcome, since pumps with lower flow rate BEPs tend to be cheaper. The AOP is at 220 gpm. The ratio AOP flow rate over BEP flow rate is 220:184 = 1,196.
| IMAGE 5: Actual operating point vs. pump BEP and end of curve flow (Image courtesy of the author)
However, API 610 (§ 6.1.16) specifies:
- Pumps shall have a preferred operating region of 70% to 120% of the BEP flow rate of the pump as furnished.
- The pump end of curve flow shall be at 120% of the BEP flow rate.
As a result, the pump will have to operate at or perhaps even above the preferred operating region and beyond the end of curve flow.
When two contingencies are present—(1) selection of a pump within API specifications but with a BEP at the lowest allowable flow rate and (2) conservative calculation of the system curve—this can lead to a pump operating at or beyond its limits, i.e., at too high of a flow rate.
The opposite is also true, such as when both of the following happen:
- Selection of a pump with a BEP at the highest allowable flow rate
- Too liberal a calculation of the curve
The pump can operate at or beyond its limits—at a too-low flow rate—but this almost never happens. The widely used remedy when the actual operating point is far to the right of BEP is discharge valve throttling. This is an effective and practical remedy, but it has its costs. For an example (admittedly a bit of an exaggeration, but very illustrative), see Image 6.
- S1 is the system curve with the discharge valve fully open.
- S2 is the system curve with the discharge valve partly closed.
- The green area is the useful energy.
- The red area (over the green box) is the energy consumed because of valve throttling.
| IMAGE 6: Pump energy consumption with a throttled discharge valve (Image courtesy of the U.S. Department of Energy and Lawrence Berkeley National Laboratory)
In Part 2, find out the consequences of operating a pump too far from the BEP flow rate and how to avoid/manage the issues.
Sotirios Christofi, after a career of 38 years in the petrochemical industry, provides consulting and training services for technical personnel on the design, construction, inspection, selection, condition monitoring and maintenance of industrial machinery. He is a mechanical engineer with a master's degree in business administration from the University of Warwick, U.K.
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