KRAL Z Series twin-screw pump, 1,060 m³/h

The KRAL Z Series twin-screw pump handles media that triple-screw pumps cannot handle: bitumen, heavy petroleum products, contaminated and non-lubricating fluids. Manufacturer: KRAL, Austria. This is the series with the highest flow rate in the manufacturer’s range: from 330 to 17,660 litres per minute, i.e. 20–1,060 m³/h, with viscosities up to 100,000 mm²/s and media temperatures ranging from −40 to +300 °C.

How does a twin-screw pump differ from a triple-screw pump?

In a triple-screw pump, the drive screw rotates the two driven screws via the fluid itself; therefore, the fluid must have lubricating properties. In the Z Series, the screws are synchronised by a gear train and do not come into contact with one another, whilst the bearings are located outside the flow path and are not washed by the product. Consequently, the series can handle fluids without lubricating properties, those containing mechanical impurities, or chemically aggressive fluids — this is precisely how KRAL positions it.

The casing features a double-suction design: the flow is split and enters the screw pair from both sides, which balances the axial forces without internal bypass. The Delta-design housing geometry reduces hydraulic losses and improves suction capacity.

For a general explanation of how a screw pair works, see the article Features of screw pumps.

Technical Specifications of the KRAL Z Series

Parameter	Value
Type	twin-screw pump with synchronised screws and double suction
Capacity	330–17,660 litres per minute (20–1,060 m³ per hour)
Standard sizes	Z03000 — up to 270 m³/h · Z05000 — up to 490 m³/h · Z11000 — up to 1,060 m³/h
Max. differential pressure	25 bar
Fluid temperature	−40…+300 °C
Operating viscosity	0.7–100,000 mm²/s
Housing material	spheroidal graphite cast iron or welded carbon steel
Screws	solid or composite
Gaskets	single or double end-face, optional leak detection
Bearings	located externally, outside the medium being pumped
Installation	horizontal and vertical

Design and construction

Casing heating — integrated heating jacket; available in electric, steam or heat-transfer fluid versions. A circulation valve is fitted to protect against overpressure. Seals are fitted with supply systems in accordance with API specifications, with leak detection where required. The manufacturer states that the series is capable of dry running.

Manufactured in accordance with API 676. Classification society certificates: ABS, BV, DNV-GL, LRS, RINA.

The external bearings have another practical advantage: replacing the seal and servicing the unit do not require contact with the product, which is important when dealing with bitumen and heavy residues.

Where is the Z Series twin-screw pump used?

If the environment is abrasive but retains its lubricating properties, the manufacturer’s range includes a three-screw solution — KRAL W Series. Full the KRAL range of pumps — follow the link.

Selection and service from Steiner Ukraine

A twin-screw pump is selected based on the medium: first, the viscosity at operating temperature, the presence of impurities and the lubricating capacity are determined, and only then are the size, casing materials and type of seal selected. This also determines whether heating is required and, if so, what type. We calculate the operating conditions based on the existing layout, agree on the design with the manufacturer and ensure service and spare parts throughout its entire service life. Steiner Ukraine has been operating in the industrial equipment market since 2006.