Centrifugal pumps are common at many plants — Figure 1 shows a typical centrifugal pump and Figure 2 a complex multi-casing one for a high-pressure service. Unfortunately, plants often suffer unnecessary pump downtime. Operating conditions not mentioned in the pump order document and not considered in pump design and manufacturing account for more than 75% of all unscheduled shutdowns. Alternative operating points and transient operating conditions are particular culprits. So, it's important to accurately indicate flow rates, required heads, liquid details and net positive suction head (NPSH) available for all possible operating scenarios for a pump.
In a poor pump selection, the specified rated point is located either in the overload region — which can indicate too small a pump has been chosen — or relatively near the shutoff point. The rated operating point should be close to the best efficiency point (BEP). It's usually difficult to give a general guideline because the limits depend upon the pump details, the pump curve and the application. However, as a rough rule, the rated capacity preferably should fall in the range of 65% to 112% of the BEP flow. The overload region generally poses more concern as far as potential under-sizing of the pump — 112% is a firm limit. In some situations, 55% of the BEP flow may be acceptable. Most damage and reliability issues, especially with bearings, seals and the like, stem from pump operation at point(s) relatively far from the BEP point.
A flat pump curve can cause operational problems. A pump with a flat curve can experience rapid flow changes/fluctuations and instability as a result of a small head change downstream. Any pump should have a minimum 8% rise from the rated point to the shutoff head. Some process pumps, including critical ones, should have a 12% minimum head rise to the shutoff. High-pressure, high-speed and integrally geared pumps each usually have relatively flat curves. So, their selection demands special care.
Avoid double suction pumps. They usually provide poor reliability and performance and prompt many operational problems.
Both the American National Standards Institute (ANSI) and the American Petroleum Institute (API) provide pump standards. Pumps meeting the ANSI standard often are chosen for less-demanding chemical and utility services. The API-610 standard is more stringent — and usually is considered the minimum specification for pumps that handle hazardous, flammable, toxic and explosive liquids because any reliability issue or even a small incident in the pump could result in a disaster. API-610 pumps also are very popular in extreme temperature services (even in some critical water services such as boiler feed-water pumps) or low temperature applications. In some cases, firewater and critical cooling pumps are specified to comply with the API-610 to achieve high reliability.
ANSI and API pumps differ in some important ways. For example, horizontal ANSI pumps usually are foot-mounted while horizontal API pumps often are centerline-mounted. The thermal expansion/contraction of the shaft centerline of a foot-mounted horizontal ANSI pump can be two-to-three times greater than that of a comparable centerline-mounted horizontal API pump. This could directly affect pump train alignment and pump reliability.
API-610 pumps are superior for process liquids, hydrocarbons, petrochemicals, extreme temperature services and critical applications.
There's always some discussion about whether or not to use the API-610 standard for a service. The decision should depend upon the application (the liquid, the plant, etc.), the pump head, the service temperature, the power rating, the capacity, the pump speed and the expected reliability. The main variable is the service (the liquid, the plant and the desired reliability). For a differential pressure of more than 20 bar, API-610 usually is specified, even for some critical water and utility services. When the pumped liquid temperature exceeds 140oC or is very low, API-610 always is preferred. Using the pump power rating to decide whether to use API-610 is a bit tricky because there are many non-API and manufacturer-standard pumps available with successful references for high power ranges in wide variety of services. For process pumps, consider API-610 for applications above 400 kW. (For some services, this limit could drop to 100 kW.) Non-API high-speed pumps usually aren't popular. For a service requiring a pump speed above 4,000 rpm, an API pump could be a good option.
Use a between-bearing pump design when size, power rating and power density exceed a certain level. As a rough indication, this limit could be >450 kW for a high-speed pump but could be lower, e.g., >200 kW, for some critical service pumps.
FACTORS IN RELIABILITY
Lubrication and sealing systems should play an important role in pump selection, design, manufacturing and purchase because they significantly affect pump operation and reliability. Often, repairing a pump doesn't solve the underlying problem and the failure recurs. In these cases, a truly effective solution requires correct root-cause analysis — this demands sufficient knowledge of the lubrication and sealing systems.