Pharmaceutical clean steam, also called pure steam, is steam whose condensate meets the specification for Water for Injection, generated from purified or WFI feedwater in a stainless generator with no boiler chemicals, and delivered dry and saturated to the equipment it sterilizes. It is required wherever steam contacts product or product-contact surfaces: sterilization in place, autoclaves for product-contact loads, and humidification of aseptic areas. Its quality is defined by condensate chemistry and by three physical tests, dryness, superheat and non-condensable gases, that decide whether it can sterilize at all. Paul Industries designs and installs pure steam generators, distribution and points of use for pharmaceutical and biotech plants and qualifies them with the site.
Clean steam, pure steam, plant steam
| Grade | Source | Condensate quality | Use |
|---|---|---|---|
| Plant steam | Fired boiler with treatment chemicals | Not controlled | Heating jackets and heat exchangers with no product contact |
| Filtered or culinary steam | Plant steam filtered, approved chemicals | Free of particulates; chemicals per food regulations | Food processing direct contact |
| Clean steam | Stainless generator, purified or WFI feed, no additives | Meets purified water or WFI depending on use | SIP, autoclaves, humidification |
| Pure steam | As clean steam, feed and condensate to WFI | Meets WFI including endotoxin | Sterilization of parenteral product-contact equipment |
In most US pharmaceutical plants clean steam and pure steam are used interchangeably to mean steam with WFI-quality condensate, and that is what the design assumes unless the product allows less.
Generation
A pure steam generator is a stainless shell-and-tube or falling-film evaporator heated by plant steam, fed with purified water or WFI, with a separation section that removes droplets and entrained impurities so the steam produced has WFI-quality condensate. Feedwater quality decides everything downstream: chlorides and silica carried over foul the generator and the distribution, and a feed that is not deaerated raises non-condensable gases in the steam. Generators are sized for the peak simultaneous demand of autoclaves and SIP cycles, which is usually far above the average, with a buffer for start-up surges.
Distribution design
Clean steam is distributed in 316L tube or pipe to ASME BPE, sloped continuously to drain in the direction of flow, with sanitary steam traps at every low point and at intervals along the run, insulated to limit condensation, and terminated at points of use with sanitary valves and traps. Dead ends, pockets and long horizontal runs without traps produce wet steam and cold spots. Velocity is kept moderate to avoid carrying condensate and eroding elbows. Condensate from clean steam is either recovered to the feed system or discharged, never returned to a plant steam system. Points of use at autoclaves, SIP connections and humidifiers each get a sample point for quality testing.
The three physical tests
- Dryness value: the fraction of the steam that is vapor rather than entrained water; EN 285 requires at least 0.95 for metal loads (0.90 for other loads). Wet steam carries impurities and delivers less latent heat.
- Superheat: steam hotter than its saturation temperature at the line pressure sterilizes poorly because it behaves as a dry gas until it cools; EN 285 limits superheat to 25 K after expansion to atmosphere.
- Non-condensable gases: air and other gases in the steam form pockets that block steam contact; EN 285 limits them to 3.5 percent by volume.
These are tested at commissioning and periodically at points of use with a steam quality test kit, alongside condensate chemistry (conductivity, TOC, endotoxin where WFI quality is required) sampled through a sanitary condenser.
Generation and distribution problems we see
Generators fed with water that drifted out of specification and now carry over silica; distribution runs modified without traps, so the far autoclave gets wet steam; superheat introduced by pressure reduction close to the point of use; non-condensables from a feedwater system without deaeration; rouge in the distribution from long hot service; and points of use added after qualification without sample points. Each shows up first as an autoclave or SIP cycle that fails its biological indicator.
Qualification
Installation qualification of the generator, materials, welds, slope, traps and instruments; operational qualification of capacity, pressure control, alarms and trap function; and performance qualification by steam quality testing (dryness, superheat, non-condensables) and condensate chemistry at every point of use over a defined period. The system is then monitored on a schedule, and modifications go through change control.
What Paul Industries installs
Pure steam generators and their plant steam, feedwater and condensate connections; 316L distribution to ASME BPE with sanitary traps and insulation; points of use at autoclaves, SIP connections and humidifiers with sample points; integration with the purified water or WFI system that feeds the generator; passivation; and qualification support including steam quality testing.
Standards referenced: USP <1231> (usp.org) · ASME BPE · 21 CFR 211
Frequently asked questions
What is pharmaceutical clean steam?
Steam generated from purified water or WFI in a stainless generator with no boiler chemicals, whose condensate meets the WFI specification for chemistry and, where required, endotoxin, delivered dry and saturated. It is used for sterilization in place, autoclaves for product-contact loads and humidification of aseptic areas; in US practice clean steam and pure steam usually mean the same thing.
Are clean steam and pure steam the same thing in a US pharmaceutical plant?
Historically clean steam meant steam free of boiler additives with condensate meeting purified water, and pure steam meant condensate meeting WFI including endotoxin. Most US pharmaceutical plants now use one grade with WFI-quality condensate and call it either name. The specification, not the name, should state the condensate quality required.
Why can plant steam not be used for SIP or autoclaves?
Because plant steam carries boiler treatment chemicals (amines, oxygen scavengers) and impurities that deposit on product-contact surfaces and can end up in the product, and its condensate is not controlled. Filtering removes particulates but not dissolved chemicals. Plant steam is used to heat the clean steam generator and jackets, never to contact product-contact surfaces.
How is pure steam generated?
In a stainless shell-and-tube or falling-film evaporator heated by plant steam and fed with purified water or WFI, with a separator that removes droplets and entrained impurities. Feedwater quality decides carry-over and fouling, and a deaerated feed keeps non-condensable gases low. Generators are sized for peak simultaneous autoclave and SIP demand.
What are dryness, superheat and non-condensable gases in steam quality?
Dryness is the vapor fraction of the steam (EN 285 requires at least 0.95 for metal loads); superheat is temperature above saturation at line pressure, which impairs sterilization (limit 25 K after expansion); non-condensable gases are air and other gases that form pockets and block steam contact (limit 3.5 percent by volume). They are tested at points of use with a steam quality test kit.
What does a pure steam distribution run need at every low point?
In 316L to ASME BPE, sloped continuously in the direction of flow, with sanitary steam traps at every low point and at intervals, insulated, at moderate velocity, with no dead ends or pockets, and with a sample point at each point of use. Condensate is recovered to the feed or discharged, never mixed with plant condensate.
How does wet steam reach an autoclave, and what does it do there?
Entrained water carries less latent heat and can shield surfaces, and it carries any impurities in the condensate onto the load. Wet steam comes from poor separation in the generator, long uninsulated runs, missing traps and pockets in the distribution. The dryness test detects it, and an autoclave biological indicator failure is often its first symptom.
Why is superheated steam a problem?
Because steam above its saturation temperature behaves as a dry gas until it cools to saturation, transferring little latent heat and sterilizing poorly. Superheat is usually introduced by a pressure-reducing valve close to the point of use or by heat tracing; it is designed out by locating pressure reduction upstream and allowing the steam to re-saturate.
Why does pure steam generator feedwater need deaeration?
Purified water or WFI, depending on the condensate quality required, at controlled chloride and silica levels to prevent fouling and carry-over, and ideally deaerated to keep non-condensable gases low. The generator’s feed is a demand on the water system and is sized into it; drifting feedwater quality is the most common cause of generator problems.
What is the testing schedule for dryness, superheat and non-condensables?
At commissioning and performance qualification at every point of use, then on a schedule set by the site, commonly every six to twelve months, and after any modification to generation or distribution. Condensate chemistry (conductivity, TOC and, where required, endotoxin) is sampled on the same or a more frequent schedule through a sanitary condenser.
What are sanitary steam traps and why are they used?
Traps with stainless, cleanable, drainable construction that discharge condensate from clean steam lines without contaminating the steam, installed at every low point and along the run. Conventional cast-iron traps introduce corrosion products and are not used on clean steam. Trap failure is a common cause of wet steam at the far end of a system.
Can clean steam be used to humidify a cleanroom?
Yes, and it is the usual method for aseptic areas, where plant steam with boiler chemicals is not acceptable in the air supply. Clean steam is injected into the air handling unit through a humidifier manifold, with the steam quality and the condensate handling designed as for any point of use.
What does the PQ of a pure steam system measure at each point of use?
Installation qualification of the generator, materials, welds, slope, traps and instruments; operational qualification of capacity, pressure control, alarms and trap function; and performance qualification by steam quality tests and condensate chemistry at every point of use over a defined period, followed by routine monitoring and change control for modifications.
What size pure steam generator does a plant need?
One sized for the peak simultaneous demand of autoclaves and SIP cycles plus start-up surges, which is often several times the average demand. Undersized generators show up as pressure drops and wet steam when two autoclaves start together; the schedule of sterilization loads is the basis of the sizing.
Is Class III rouge in a steam system a problem?
Long hot service on stainless produces the black magnetite-type oxide known as Class III rouge in steam systems, which is often stable and adherent. It is assessed on inspection; where it is shedding or affecting condensate quality it is removed by derouging and the surface repassivated, under change control.
Does clean steam condensate need to be recovered?
It can be recovered to the generator feed or the water system if its quality is verified, or discharged; it must never be returned to a plant steam boiler, and plant condensate must never enter the clean steam system. The condensate route is drawn on the P&ID and verified at qualification.
How does a clean steam failure show up in production?
As an autoclave or SIP cycle that fails its biological indicator or temperature hold, a humidifier that fouls, or condensate samples out of specification. The investigation traces back through steam quality at the point of use, traps and distribution, generator separation and feedwater, in that order.
Can an existing plant steam system be converted to clean steam?
Not by filtering. Clean steam needs a dedicated stainless generator fed with purified water or WFI and stainless distribution with sanitary traps; the plant steam system remains as the heat source for the generator. Existing stainless distribution can sometimes be reused after inspection, passivation and requalification.
What is the EN 285 standard and why is it cited in the US?
A European standard for large steam sterilizers that defines the steam quality tests (dryness, superheat, non-condensable gases) and their limits. It is cited in US pharmaceutical practice because there is no equivalent US standard for those physical tests, and the FDA accepts it as the reference method.
Which pieces of a pure steam system does Paul Industries install and test?
Pure steam generators with plant steam, feedwater and condensate connections; 316L distribution to ASME BPE with sanitary traps and insulation; points of use at autoclaves, SIP connections and humidifiers with sample points; integration with the purified water or WFI system; passivation; and qualification support including steam quality testing.
Related: Process Systems & Sanitary Piping for Pharmaceutical Manufacturing · Clean Steam Systems: Generation, Distribution & Validation · Clean Steam Generator: How It Works · Clean Steam vs Pure Steam vs Plant Steam · Autoclave & Sterilizer Installation · Sterile Injectable & Aseptic Fill-Finish Process Systems · Saturated Steam Table: Pressure and Temperature · Request a quote