DESMI ROTAN Internal Gear Pumps — How They Work, Family Logic and Wear Parts
Selection guide compiled by cBallast. 3-page PDF; the full content is on this page.
Brand
DESMI
Subject
Positive displacement pumps
Document type
Selection guide
Reference
2026 edition
Issued
2026-10-18
Pages
3
Format
Web page and PDF
The internal-gear principle behind ROTAN pumps and why it owns viscous duty: gentle low-shear flow, bi-rotational operation, lift capability — the GP and HD family split, reading a size like GP 21-50, and the bushing and shaft-seal kits that are the pump's actual service life.
Internal gear: one gear inside another
An internal-gear pump runs a rotor gear with an idler gear inside it, separated by a crescent: liquid fills the opening tooth spaces on the suction side, rides around the crescent, and is displaced as teeth re-mesh. The geometry gives it the qualities viscous-liquid plants buy: flow almost independent of pressure, graceful handling from thin fuels to greases and bitumen (speed reduced as viscosity climbs), low shear for polymers and food media, dry-lift priming, and — rare among pump types — true bi-rotational operation, so one pump loads and strips a tank through the same line by reversing rotation. Danish maker DESMI's ROTAN line has run this principle for a century; the stocked GP 21-50 and HD 68 heavy-duty gear mark the two family poles — GP the general-purpose build, HD the heavy-duty execution for the punishing end (abrasion, temperature, pressure).
What wears, and what to stock
Wear part
Why it goes
Stocked
Bushings
The idler and rotor run on plain bushings lubricated by the pumped liquid — thin or dirty media wear them first, and worn bushings show as falling capacity and rising noise
End clearance is adjustable on these pumps — re-shimming recovers capacity before parts are even needed
procedure per size, quoted with kits
That adjustability is the maintenance story: a ROTAN losing flow is first re-shimmed to spec, then re-bushed, and only much later re-geared — three lives before replacement, all planned work. Seal-face material logic follows the same rules as the mechanical-seal letter guide; general gear-pump alternatives for thin fuels sit among the 640 gear-pump rows (e.g. the self-priming DC transfer units).
Selecting and enquiring
Duty data beats model numbers: liquid, viscosity at pumping temperature, flow, differential pressure, and suction condition size the pump; the old unit's size code (GP 21-50 pattern: family + size) confirms it.
Speed is the viscosity lever — the same pump delivers its flow at lower rpm as viscosity rises; drives are sized accordingly (motor frames in the IEC chart, couplings in the coupling guide).
Relief protection is mandatory: positive displacement means pressure rises until something yields — an integral or line relief valve (see the safety-valve basics) is part of the installation, not an option.
Heating jackets exist for media that set when cold — specify with the pump for bitumen/wax duty.
DESMI pumps and ROTAN wear parts are quoted within 24 hours from brand stock — the NSL centrifugal side of the programme has its own spares guide.
Frequently asked questions
What liquids suit an internal gear pump?
Anything viscous and lubricating by preference — oils, syrups, resins, bitumen, grease bases — plus thin liquids at reduced expectations, since the pumped medium lubricates the bushings. Abrasive-laden media push the choice to the HD execution and harder materials.
Why does a gear pump lose capacity slowly over years?
Internal clearances grow — bushings and end faces wear. On ROTAN builds end clearance is adjustable, so the first service step is re-shimming to factory clearance; capacity usually returns before any parts change.
Can ROTAN pumps run in reverse?
Yes — internal gear pumps are inherently bi-rotational, swapping suction and discharge with rotation. One pump then serves load/unload through a single manifold; the shaft seal and relief arrangement must be specified for both directions.
Does a gear pump need a relief valve?
Always — positive displacement against a closed valve builds pressure until the weakest part fails. Either the pump's integral bypass or a line-mounted relief, set above duty pressure and below the system's weakest rating.
How do I identify a ROTAN pump for spares?
The size code on the casing (family + size, e.g. GP 21-50) plus the shaft-seal arrangement visible at the gland. With those two and the medium, bushing sets, seal kits and gaskets are quotable within 24 hours.