Page 111 - Industrial Plants

• A consequence of the clamping contact is that the
stator is a wear part, to be exchanged at regular
intervals that depend entirely on the application
and are hard to predict. Non-stop operation over
three years as required by API 676 (para 6.1) is not
something a PCP can guarantee.

4. There is the eccentric kinematic of the rotor
and its impact on vibration:
• For a given capacity, vibration levels of a PCP are
necessarily higher than other API 676 pump types
based on concentric motion. A standard PCP may
exceed the limit of 3.8 mm/s (para 6.11). cross-reference other API standards or US
• This low frequency vibration is inherent in the manufacturing standards.
design and not a sign of trouble, thus vibration
monitoring makes little sense. 6. Gearbox and bearings
• Vibration is a critical issue and limiting factor only • API 676 references AGMA and ABMA standards
for vertical semi-submersed PC pumps, where the and requires gearboxes > 18 kW (25 HP) to
shaft can become several meters long. conform with AGMA 6010, or even API 677.
• API 677 gears are generally not available for the
5. PCPs typically operate at slow speed 100 - typical power ratings and drive configuration of
300 rpm, about 10-20% of a centrifugal pump. PCP. Please refrain from specifying API 677; pump
Thus: vendors could not comply even if they wanted to.
• There is no motive for precise alignment or • Outside the Americas the AGMA and ABMA
balancing. PCP baseplates are typically not standards are not used. European gear
designed with precision machined pads and manufacturers will design and calculate according
jacking screws that facilitate alignment. to DIN 3990 or ISO 6336. American and European
• Shaft bearings are grease lubricated and typically gear standards are not compatible because they
sealed / lubricated for life. Elaborate oil lubrication are based on different physical models and basic
systems with associated monitoring and cooling assumptions. It is also not possible for the vendor
function are not needed but nevertheless specified to demonstrate that a gearbox designed to DIN/
by API 676 / API 614 and present in the mind of ISO is ‘equal or better’ to an AGMA design, which
project engineers familiar with other pump types. is a frequent requirement of project engineers
• It is common practice for PC pump design to omit trying to circumvent the issue. Sometimes this
dedicated shaft bearings altogether and use the leads to a hot debate of almost religious
gearbox bearings to support the shaft – this is dimensions without practical result. Comparative
called “block pump” and it can omit also the papers by gear specialists can be found on the
coupling. web. However, it is absolutely safe to assume that
• Shaft seals do not generate much heat by friction, a gearbox following European standards will
therefore, seal cooling is rarely needed and perform just as well.
standard seal plans offered by PCP vendors may
minimize or omit the cooler, as a deviation from 7. IEC/IEEE
API 682. API 676 requires motors to follow IEEE 841.
• However, most international projects outside the
Finally, there are those exceptions that are related to Americas prefer IEC motor standard. IEEE 841
sourcing and US standards. API standards usually motors are only produced in the Americas; import
by European pump vendors raises the price and
lead time.

8. Welding
• API 676 references ASME BPVC IX for welding of
pressurized parts and AWS D1.1 for structural
welding.
• While ASME IX is customary in Oil&Gas
applications, most European workshops are
basing their fabrication on ISO standards which

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