Technical Paper: API 676 compliance of progressive cavity pumps

Owing to their operating principle and their roots in waste water treatment, Progressive Cavity Pumps (PCP) come with typical deviations against petrochemical standard API 676. Some of these deviations are inevitable, others can be healed. It is a preservative headache and topic of discussion for project engineers to understand and identify which of these deviations are relevant, critical or acceptable for their project.

Progressive cavity pumps are positive displacement pumps that handle fluids gently and with minimal shear or agitation / turbulence / emulsifying. They manage fluids of highest viscosity and highest solids content, even abrasive or large particles. They can generate high pressure, 48 bar and more. They are suitable for high vapor pressure (low NPSH) as well as multiphase fluids. Due to linear performance curve and low pulsation they are suitable for dosing applications. Flow direction can be reversed simply by reversing the motor rotation.

In the oil & gas industry PCP are often applied to convey sludge or oily water, or in situations where light hydrocarbons raise the vapor pressure to a level where other pump types would cavitate. A forte of PC pumps is vertical semi-submersed installation on a drum; open/closed drain drums or flare knock-out drums are a frequent application.

The following table compares some basic requirements of API 676 and petrochemical applications on the one hand, and typical waste water applications on the other hand.

Let us look at the two critical issues that give rise to most discussion and customization.

Casing design

Welded constructions with their associated high effort of engineering, procedure approval, fabrication, non-destructive examination, inspection and documentation are needed to meet such requirements.

Shaft Seal
PCP typically have rotor joints with cardanic articulation to allow for the eccentric rotor movement. A rotor with such joints comes with assembly restrictions that are not compatible with installation of an API 682 mechanical seal which is bolted to the pump casing. PCP design typically clamps the seal housing between the pump casing and a lantern. As a result, the mechanical seal cannot be fully API 682 compliant; it can be a heavily engineered seal in spirit of API 682 at best.
Most PCP vendors use bespoke universal cartridge seals tailored to the needs and dimensional restrictions of the pump type.
Other deviations and exceptions are rooted in the PCP principle itself and cannot – or need not – be avoided. Again we look at reasons and consequences:

1. First and foremost, the progressive cavity principle is based on permanent tight contact (clamping) of a steel rotor and elastomer stator. There is no gap.

• Elastomer can never be fabricated with the same precision as steel; this results in performance tolerance exceeding the limits of API 676 (para 8.6.3) which is +3/-0% of characteristic capacity; a PCP can typically manage +10/-5%; tolerance is even higher for small pumps or high pressure. The only standard that correctly takes into account the characteristics of a PCP is the ancient German VDMA 24284 which is why PCP vendors still refer to it for test tolerance.
• 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.

2. 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).
• This low frequency vibration is inherent in the design and not a sign of trouble, thus vibration monitoring makes little sense.
• Vibration is a critical issue and limiting factor only for vertical semi-submersed PC pumps, where the shaft can become several meters long. [ILLU long BE pump]

3. PCPs typically operate at slow speed 100 – 300 rpm, about 10-20% of a centrifugal pump. Thus:

• There is no motive for precise alignment or balancing. PCP baseplates are typically not designed with precision machined pads and jacking screws that facilitate alignment.
• Shaft bearings are grease lubricated and typically sealed / lubricated for life. Elaborate oil lubrication systems with associated monitoring and cooling function are not needed but nevertheless specified by API 676 / API 614 and present in the mind of project engineers familiar with other pump types.
• It is common practice for PC pump design to omit dedicated shaft bearings altogether and use the gearbox bearings to support the shaft – this is called “block pump” and it can omit also the coupling.
• Shaft seals do not generate much heat by friction, therefore, seal cooling is rarely needed and standard seal plans offered by PCP vendors may minimize or omit the cooler, as a deviation from API 682.

Finally, there are those exceptions that are related to sourcing and US standards. API standards usually cross-reference other API standards or US manufacturing standards.

4. Gearbox and bearings

• API 676 references AGMA and ABMA standards and requires gearboxes > 18 kW (25 HP) to conform with AGMA 6010, or even API 677.
• API 677 gears are generally not available for the typical power ratings and drive configuration of PCP. Please refrain from specifying API 677; pump vendors could not comply even if they wanted to.
• Outside the Americas the AGMA and ABMA standards are not used. European gear manufacturers will design and calculate according to DIN 3990 or ISO 6336. American and European gear standards are not compatible because they are based on different physical models and basic assumptions. It is also not possible for the vendor to demonstrate that a gearbox designed to DIN/ISO is ‘equal or better’ to an AGMA design, which is a frequent requirement of project engineers trying to circumvent the issue. Sometimes this leads to a hot debate of almost religious dimensions without practical result. Comparative papers by gear specialists can be found on the web. However, it is absolutely safe to assume that a gearbox following European standards will perform just as well.

5. IEC/IEEE

• API 676 requires motors to follow IEEE 841.
• However, most international projects outside the Americas prefer IEC motor standard. IEEE 841 motors are only produced in the Americas; import by European pump vendors raises the price and lead time.

6. 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 result in a quality level at least equal to ASME IX. Welders may be additionally qualified to ASME IX but if you make that a firm requirement you may raise the price tag without benefit. The economical decision is to leave this choice to the Vendor.
• Welding to AWS D1.1 is unusual in Europe and hardly available. Welding of non-pressure or structural parts according to ISO welding standards should be allowed or tolerated.

Project engineers preparing or evaluating a bid typically spend many hours in discussion with pump vendors to identify which of these points are relevant for the actual project, which can be accepted to cut cost, and which are viewed critically by the client so that a concession request becomes necessary, adding to the time.

Range of API 676 compliant pumps

To provide project engineers with smooth sailing and purchasers with favorable price and lead time,  Seepex introduced a new range of horizontal PC pumps that are designed based on API 676 requirements – the BNA pump range (Fig. 1). Main features are:

• heavy-duty cast casing with design pressure 50 bar
• flanges class 300
• standard flanged casing drain
• high nozzle loads
• full uncompromised compliance to API 682 Cat. 2 / API 610 for shaft seals
• materials suitable for corrosive or harsh environment and for fluids including Chloride and H2S