Page 110 - Industrial Plants

Feature Waste Water Treatment Petrochemical / API 676
Operating pressure max 6 bar, rarely higher 12 to 24 bar, often higher
Design pressure 10 to 16 bar (DIN PN16) class 150 (19 bar) or class 300 (49 bar)
Flange norm EN 1092, DIN 2501 ASME B16.5
Shaft sealing packing gland, component mechanical cartridge machanical seal API 682
seal
Material of construction grey cast iron, carbon steel, 316L stainless steel, Duplex stainless steel, ASTM
standards
EN and ISO standards
Material traceability not required at least material certificate 3.1
Casing Drain not required flanged casing drain
Drive integral geared motor gear and motor separated
Explosion protection rarely always
Base plate simple U-beam Skid: welded frame with full-length drip pan and
lifting lugs
Integration of accessories rarely often (API 682 seal plans)
Corrosion protection low requirements Highly corrosive environment:
ISO 12944 Kat. C4 (Onshore) to
C5-M (Offshore)

1. Casing design 2. Shaft Seal
PCP vendors typically use cast casing designs PCP typically have rotor joints with cardanic articulation
optimized for waste water treatment. These do not to allow for the eccentric rotor movement. A rotor with
cover the following requirements: such joints comes with assembly restrictions that are
• design pressure (MACP) exceeding 16 bar, in line not compatible with installation of an API 682
with ASME flange ratings or to achieve higher mechanical seal which is bolted to the pump casing.
containment pressure for safety in upset conditions PCP design typically clamps the seal housing between
• operating pressure exceeding 12 bar, if the pumps the pump casing and a lantern. As a result, the
operates clockwise mechanical seal cannot be fully API 682 compliant; it
• suction region and discharge region to be can be a heavily engineered seal in spirit of API 682 at
designed for same MACP (API 676 para 6.3.3.2) best.
• fully ASME compliant flanges (API 676 para 6.6)
• nozzle loads as per API 676 (para 6.7.1) or higher, Most PCP vendors use bespoke universal cartridge
which simplifies piping layout seals tailored to the needs and dimensional restrictions
• flanged casing drain (API 676 para 6.4.2) of the pump type. [ILLU Burgmann Cartex left vs.
• corrosion resistance exceeding 316 stainless steel APItex right]
• NORSOK qualification of raw materials
• Bespoke welded constructions with their Other deviations and exceptions are rooted in the PCP
associated high effort of engineering, procedure principle itself and cannot – or need not – be avoided.
approval, fabrication, non-destructive examination, Again we look at reasons and consequences:
inspection and documentation are needed to
meet such requirements. 3. 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.

IndustrIal Plants - May 2017
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