Head Off Centrifugal Pump Problems

Table 1. API 610 [1] considers these tolerances acceptable.

After the purchase order is awarded and the pump is built, conducting a certified performance test is sensible. The certified head at the rated flow and pump shutoff must meet the specified tolerances of the predicted performance described in the bid. Table 1 shows the allowable tolerances given in API-610.

Potential Problems

Let’s now look at two scenarios where the allowable rated and shutoff head tolerances could create unexpected rework and impact project schedule/costs. Brownfield/revamp work can be particularly susceptible to risks because the pump must be integrated into existing systems and the flexibility to modify those designs may be limited.

Scenario 1: Positive tolerance at pump shutoff exceeds system design pressure. The pump shutoff head typically is selected such that it won’t exceed design pressures of downstream systems. In certain revamp scenarios to avoid changing piping classes, rerating the piping/vessels or adding a pressure safety valve/high-integrity pressure protection system may be necessary to avoid exceeding design pressure. The proposed pump may be acceptable per the predicted shutoff head — but once the API-610 tolerances are applied, the actual shutoff head could be 5 to 10% higher. Figure 1 illustrates how the allowable positive tolerance at shutoff can cause a certified performance curve to exceed the design limits of an existing system.

In this scenario, the pump impeller would need to be retrimmed and the pump retested to ensure the system design limits aren’t exceeded. This also may impact the rated performance and feasibility of the selected pump.

Figure 1. Positive tolerance at shutoff may lead to head that exceeds system design.

Scenario 2: Negative tolerance at rated point results in an underperforming pump. Without considering the allowable tolerances of the rated head, the performance of the preliminary curve may appear acceptable. However, a negative deviation of the rated head may lead to a pump that underperforms. For example, a high-head cavern injection pump may require a rated head of around 2,500 m. API-610 allows a tolerance of ±3% of the rated head. If the certified pump has rated head 3% less than the predicted curve, then the pump could lose up to 75 m of developed head. In typical liquefied-petroleum-gas service, this can result in a loss of up to 370 kPa of developed head, which may be significant. The process engineer should consider the potential for reduced head and determine if the system has sufficient hydraulic capacity to absorb deviations between the predicted and certified performance. Figure 2 illustrates how the allowable negative tolerance at the rated point can cause a certified performance curve to fail to meet pressure requirements.

If the system lacks sufficient hydraulic capacity, the pump impeller either would need to be replaced and retested, or a reduced flow may need to be accepted. However, depending upon the design limitations of the system, replacing the impeller to gain head could lead to exceeding the system’s design pressure at pump shutoff.

Figure 2. Negative tolerance at rated flow may mean pump doesn’t provide sufficient head.

In either scenario, unexpected modifications to impellers and retesting can create additional costs and impact schedule. Pump disassembly and impeller trimming or recasting could be a lengthy process depending upon the size and style of pump. For example, large high-head multistage pumps would take longer to modify. The pump purchaser will bear the costs associated with the required impeller modifications and retesting along with any schedule delays if restrictions on API-610 tolerances weren’t specified and agreed upon earlier in the procurement process.

In the worst case, an impeller modification may not allow the selected pump to meet the required conditions; this either would result in accepting a derated performance or switching to a different pump. If a different pump is necessary, the new procurement process would further delay the project.

Heed This Heads-Up

Understanding the tolerances allowed under API-610 with regard to pump shutoff head and rated differential head can avoid costly rework and schedule delays. When specifying the required performance of a pump, the process engineer should identify any potential issues with API-610 allowable tolerances on pump performance and include a note on the datasheet that restricts the tolerances. By determining early on in the procurement cycle that full API tolerances aren’t acceptable, the process engineer can help minimize risks to schedule and cost.

JONATHAN R. WEBBER, P.E., is a process engineer for Fluor Canada Ltd., Calgary, AB. DUNCAN J. BLAIKIE, P.E., and THERESA R. WINSLOW, P.E., are process engineers for Fluor Canada Ltd., Saint John, NB. Email them at [email protected], [email protected] and [email protected].

ACKNOWLEDGMENTS
The authors thank Jeff McKay, lead mechanical engineer for Fluor at Irving Oil, for reviewing our draft. It was developed under the Fluor Calgary office Professional Publications & Presentations Program (P4), a mentorship program that encourages and assists first-time authors interested in developing publications.

REFERENCE
1. “Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries,” API Standard 610, 11th Ed., Amer. Petroleum Inst., Washington, DC (2010).