Alloy steel melts are vacuum-treated on degassing units to bring hydrogen contact below 2 parts per million and to reduce metallic oxides, which is the published metallurgical target a foundry must hit before pouring any pressure-bearing pump or valve body [S1].
Selection in 2026 is no longer a binary "vacuum tank yes/no" decision: a foundry producing cast pump casings, valve bonnets and impellers from carbon steel, stainless steel or aluminium alloys now chooses between four equipment families (VD, VOD, RH-type circulation, and FDU) against an alloy-specific gas and inclusion budget, not a generic throughput figure.
Four equipment families and what each one actually removes
Vacuum degassing (VD) is the baseline for secondary steelmaking: a typical VD cycle runs 15–20 minutes at a chamber pressure of about 0.67 mbar, and the gas load on the pump set is "tens of kg/h equivalent air" at that pressure, which forces a multi-stage pump group with a Roots-type pre-vacuum pump at the head of the train [S3]. VD pulls dissolved hydrogen down to the low-ppm range and, with a soft argon blow at the end of the cycle, holds oxygen below 15 ppm in the melt, both of which are the gating numbers for heavy-section valve castings that will later see NACE MR0175 sour-service exposure [S3].
Vacuum oxygen decarburization (VOD) is a parallel route, used when carbon must be cut in high-chromium stainless steels without burning extra chromium to oxide; pure oxygen is injected into the melt under vacuum so that dissolved carbon leaves as CO rather than staying in solution or staying tied up as Cr-oxides [S3]. RH-type vacuum circulation is the high-throughput variant, designed to be paced to a converter or large EAF, and is "one of the fastest-growing refining equipment outside the furnace" in capacity terms [S3]. For aluminium alloys the equivalent is the Foundry Degassing Unit (FDU), a rotary injector of argon/nitrogen that scrubs hydrogen from the melt; on an industrial line at MOTOR JIKOV Slévárna a.s., the maximum allowable Dichte Index is 3–4%, and the total temperature drop from ladle pour to end of refining is held to 26–32°C, a tight band the foundry treats as a hard process window [S2].
Selection criteria: alloy, gas target, temperature window, and pump train
For pump and valve work, four criteria drive the equipment choice more than anything else: (1) base alloy and its hydrogen solubility, (2) the residual gas and inclusion spec the casting must meet, (3) the temperature-loss budget between degassing end and pour, and (4) the dust and vapour load that the vacuum pump train has to swallow. [S1]
On criterion 1, alloy steel and stainless steel belong in the VD/VOD/RH domain; aluminium alloys (used for light-duty pump housings, instrument valve bodies and certain impeller designs) belong in the FDU domain [S1][S2][S3]. On criterion 2, alloy steel heats are typically run to ≤2 ppm hydrogen on a degassing unit [S1]; the FDU aluminium benchmark is the Dichte Index at 3–4% maximum [S2]; and steel finishing targets on VD land at sub-ppm hydrogen and <15 ppm oxygen after a soft argon blow [S3]. On criterion 3, the published operating window for an industrial FDU is a 26–32°C total drop over the full refining cycle, which is the limit a foundry accepts before the melt is too cold to feed a thin valve-wall section [S2]. On criterion 4, oil-sealed vacuum pumps are being displaced by dry mechanical pump sets in steelmill service because the dust load (metal fines and oxide dust from large ladles) fouls oil-sealed stages, and a Roots-type pre-vacuum pump with "very high dust tolerance" now sits at the head of most new mechanical pump trains for VD/VOD [S3]. For broader pump-train context the centrifugal pump reference covers the hydraulic-side design numbers that the vacuum pump set is often mis-specified against.
Comparison: VD vs VOD vs RH vs FDU on four decision criteria
Stacking the four options against the criteria that matter to a pump-and-valve foundry: (a) Hydrogen/oxygen removal: VD and FDU both drop hydrogen to the low-ppm / sub-single-digit Dichte Index range; VOD adds deep decarburization; RH circulation matches VD on hydrogen while moving much higher tonnage per cycle [S2][S3]. (b) Alloy coverage: VD and RH cover carbon and low-alloy steel, VOD is the dedicated route for high-chromium stainless, FDU is the dedicated route for Al and Al-Si foundry alloys [S2][S3]. (c) Cycle time and temperature loss: VD is 15–20 min at ~0.67 mbar with a controlled soft argon trim [S3]; FDU cycle ends inside a 26–32°C total temperature-drop window from ladle fill [S2]. (d) Pump train and dust: VD/VOD/RH all need multi-stage pump groups with Roots-type pre-vacuum stages sized for tens of kg/h equivalent air at 0.67 mbar, and dry mechanical sets are replacing oil-sealed sets where oxide dust is heavy [S3]. For a stainless valve body going into sour service, VOD is the only one of the four that can decarb without burning chromium; for a carbon-steel pump casing that just needs clean steel, VD is normally the lowest-capex correct answer.
Where the unit sits in a pump or valve production line
Upstream of the unit, melting and ladle transfer fix the starting hydrogen and oxide population, so a foundry that skips pre-drying of refractories or runs with wet tools will never hit ≤2 ppm on a VD cycle regardless of pump size [S1][S3]. Downstream, the pour temperature, section thickness and feeding design must be matched to the 26–32°C temperature loss window of an FDU, or to the 15–20 minute VD cycle whose soft-blow trim defines the final oxygen number [S2][S3].
For cast stainless valve trim and pump shafts that later meet sour-service specs, the chain is typically EAF/AOD → VOD → ladle metallurgy station → mould, with VOD chosen specifically to decarb without chromium loss [S3]. For aluminium instrument valves and small pump housings, the chain is melt furnace → FDU with rotary argon injector → holding furnace → low-pressure or gravity die cast, with the Dichte Index sampled on every ladle to keep the 3–4% ceiling [S2]. When the production line is centred on a single alloy family, foundries often overspec the degassing unit; a unit sized for the worst-case alloy in the mix tends to over-process the easy alloys, and published data shows that cycle-shortening on an FDU is constrained by the temperature-loss budget as much as by gas removal [S2]. Related reading on adjacent process kit: degassing unit for the equipment taxonomy, and sand reclamation unit for the downstream mould-loop side of a foundry that runs cast iron or steel valve bodies.
Who this is for, and who it is not for
This guide is for process engineers and procurement specifiers in pump and valve foundries who already know their alloy, section thickness and gas target, and need to convert those into an equipment class, a pump-train topology, and a cycle-time envelope. It is not for buyers of plastic-moulding degassing or food/beverage vacuum de-aeration, even though the word "degassing" is shared; the FDU and VD/VOD/RH families above are metallurgical units with liquid metal, refractory linings and multi-stage pump groups, and they have essentially nothing in common with the plastic or juice-deaeration equipment that also carries the same label in general dictionaries [S1].
It is also not a fit for job-shop aluminium die casters pouring cosmetic housings, where a 3–4% Dichte Index ceiling is over-spec and a simpler porous-plug fluxing tube is enough; the FDU data set is built around foundries that actually need to defend a hydrogen and inclusion number on every ladle [S2]. For the broader auxiliary systems around a foundry, the FRL unit reference covers the compressed-air side, and the hydraulic power unit reference covers the hydraulic actuation that drives ladle-tilting and stopper-rod gear on these lines.
Limitations, failure modes and sourcing signals
Three failure modes recur in published field data: (1) oil-sealed vacuum pumps choking on metal-fines and oxide-dust load, which is the main reason steelmakers are migrating to dry mechanical pump sets with Roots-type pre-vacuum stages [S3]; (2) FDU cycles pushed beyond the 26–32°C temperature-loss window, which leaves the melt too cold to feed thin valve-wall sections and forces a re-heat that undoes the gas-removal work [S2]; (3) alloy-steel heats treated as if they were carbon steel, where the hydrogen target is left vague and porosity shows up only after machining of the pump casing [S1].
For sourcing, the verifiable public signals to track in 2026 are: continued investment in vacuum-degassing projects on both new and existing equipment lines, driven by the cost-reduction and product-upgrade case for cleaner steel [S3]; the steady migration from oil-sealed to dry mechanical pump trains in steel mill service [S3]; and the operating-window data (3–4% Dichte Index, 26–32°C total drop) being used as a hard acceptance criterion on industrial FDU lines rather than as a guideline [S2]. For a specifier writing a pump or valve RFQ that touches metallurgical cleanliness, the next node is to pin the alloy family, the gas/inclusion number, the cycle time, and the temperature-loss budget on the data sheet, and to leave the equipment-class choice to the foundry so long as those four numbers are met. Adjacent field data for related equipment: lighting equipment and electric lamps for the inspection-side illumination spec on a clean steel line, and a process-side read-through on mechanical seal selection criteria where hydrogen-induced porosity in stainless directly drives the seal-face material choice.