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Principles of Steam-In-PlaceJean-Marc Cappia

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Steaming-in-place (SIP) is a widely adoptedmethod for the in-line sterilization ofprocessing equipment. The main advantageof SIP relies on manipulation reduction andaseptic connections that might compromise the integrity of the downstream equipment.

Jean-Marc Cappia is the program director for highfiltration performance solutions in the BioPharmaceutical Division of Millipore Corporation, 80 Ashby Road, Bedford, MA 01730, tel. 781.533.2219, jean-marc_cappia@millipore.com.

dequate configuration of steam-in-place (SIP) systems is vitaland must be considered at the early conception stage of theplant. The critical requirements associated with SIP include

proper steam distribution, noncondensable gases removal, andcontinuous condensate elimination. Good engineering practices,adequate piping design, steam traps, valves, and monitoring in-strumentation are essential to ensuring SIP validation.

Gas filter engineeringSystem design, installation, and standard operating procedure. Ventfilters are required for the sterile introduction of air or nitrogenduring and after the process of vessel sterilization. Gas filters aremade of hydrophobic materials such as PTFE to prevent block-age by humidity during use. Therefore, condensate may accumu-late on the membrane during SIP and produce blind filters. Insuch circumstances, the steam no longer passes through the mem-brane, leading to incorrect sterilization. The filter housing musttherefore be designed and installed for correct drainage of con-densate, with the inlet sterile side of the cartridge fitted on thesterile vessel. This setup is preferred, since the housing closure aswell as the connections to vent and drain valves may present risksof leakage on the upstream side of the filter. A reverse mountingwould result in the filter bypass and would compromise the equip-ment’s sterility.

Figure 1 shows the correct installation of a vent filter. The in-line design of the housing allows the drainage of downstream con-densate from the vertical connection to the vessel. The T-typehousing needs a fall on pipework to ensure condensate drainageto the vessel. In both cases, the upstream condensate is eliminatedthrough a steam trap fitted on the drain port of the housing. Athermostatic steam trap also must be installed on the top of thefilter to evacuate noncondensable gases during the sterilizationcycle. This ensures the penetration of steam to all filter assembly

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extremities.In most cases, vent filters are steam-steril-

ized along with their associated vessels (see Fig-ure 2). Because large amounts of steam are re-quired to start the SIP cycle, heat the systemand remove non-condensable gases, it is bet-ter to introduce steam into the vessel first, andthen sterilize the vent filter with a reverse steamflow. This procedure is preferred to a forwardsteam injection, from the filter to the tank, be-cause it limits the steam flow rate. It also avoidshigh differential pressure over the filter andprevents contact with superheated steam. Thereverse steam flow through the filter is reducedsignificantly, because it only compensates forcondensate eliminated by the steam traps open-ing upstream of the filter assembly. Reversesteaming is safe if using a locked filter.

As shown in Figure 3, some installations such as water forinjection (WFI) tanks or fermenters may require separate ventfilter sterilization to allow their replacement without re-ster-ilizing the vessel. All configurations require the installation ofa pressure-relief safety valve as well as pressure gauges upstreamand downstream of the filter. This ensures that the reverse dif-ferential pressure does not exceed specifications throughoutthe SIP procedure. Temperature probes are located in the cold-est points of the filter assembly and the vessel (i.e., the drainpoints) to monitor the SIP process. Adding steam traps down-stream of the bleed valves allows condensate removal, reducesthe steam flow rate, and minimizes the pressure differentialacross the filter.

Flushing noncondensable gases and heating the system. Beforestarting the SIP cycle, it is important to check the leak-tight-ness of the system. A five-minute pressure hold test is con-ducted at 2 barg pressure.

Superheated steam will be present at the beginning of thesterilization cycle because of high velocity and sudden steamexpansion into the tank. Therefore, it is better first to admitregulated saturated steam into the tank and then maintain theisolated filter. All the bleed valves on the vessel must be opento ensure removal of condensate and noncondensable gases.The duration of this operation depends on the size of the equip-ment and is established during the validation of the SIP cycle.When the drain temperature reaches 110 �C or the tank pres-sure is approximately 0.5 barg, the valve is open to the filterand the steam quickly rises, and heats the filter housing.

Eliminating all noncondensable gases normally requiresflushing the system with 10 volumes of saturated steam, whichmight take between 5 and 20 min. The maximum amount ofcondensate is generated at the SIP start because of the hightemperature difference between steam and the heat transferequipment. Steam traps shut automatically once the steam exitsthe drain and vent valves and indicates that air and conden-sate have been removed. This limits the steam flow and allowsthe system to be increased and maintained at the desired ster-

ilization temperature. Steam traps will open intermittently toevacuate condensate and allow replacement with fresh satu-rated steam. For manual SIP systems that may not includesteam traps, the bleed valves must open progressively to avoidexcessive pressure differential when steam flows through thefilter. The bleed valves remain cracked open during the SIPcycle to eliminate condensate and adjust the flow rate acrossthe filter.

Sterilization cycle. Once the monitoring temperature probeslocated in the slowest heating points of the system (generallythe vessel and filter drains) indicate the set sterilization tem-perature, the SIP plateau begins and continues for the requiredtime period defined by the validation study. The pressure dif-ferential over the filter must not exceed 70 mbar during theentire sterilization cycle to maintain the filter integrity. Bothpressure and temperature should be monitored to ensure thatsaturated steam conditions are met. The theoretical saturationtemperature calculated from the actual pressure should be inthe range of � 2 �C from the actual temperature, as acceptedby the European standard EN 285. A current industry practicechecks for both pressure and temperature during the valida-

Figure 1: (Left) In-line vent filter housing, where downstream condensate is drained bygravity to the vessel. (Middle) T-type gas filter housing in which a slope must assistcondensate drainage to the tank. (Right) Incorrect set-up.

Figure 2: Reverse steam sterilization of vent filters along with the associated vessels.

tion and uses tighter acceptance criteria for the deviation (�1 �C). Provided that routine SIP operations are implementedusing the same validated SOP, only the temperature is moni-tored. This is acceptable because equipment SIP processes gen-erally involve numerous sterilization conditions.

Automatic SIP can be regulated directly or indirectly via asteam inlet control valve. Another possible regulation simplyinvolves accurate pressure regulators for the inlet steam andsteam traps at the draining points. During SIP, condensationspontaneously draws fresh saturated steam and energy at the

points where it occurs, and the sterilization temperature ismaintained at the set value. For manual procedures, the steamflow and pressure differential are regulated via the manualsteam supply valve and the bleed valves, which remain crackedopen. Air and condensate are eliminated continuously throughthe open bleed valves during the SIP cycle.

Venting steam, drying and cooling. Upon SIP completion, it isnecessary to release the steam pressure and remove residualcondensate from the lowest drain points of the system. Thisstep is critical because steam condensation can create vacuumand compromise integrity. A mole of saturated steam at 121�Coccupies a volume of 15 L, whereas the same mole of conden-sate only occupies 18 mL. Condensation must be compensatedby sterile gas, such as air or nitrogen. Sterile compressed gaspressurizes the system and completely purges the condensatethrough the bleed valves. The maintenance of sterility is en-sured as long as the pressure gauges indicate a positive read-ing in all parts. This operation also allows the cooling of theequipment and filter, which is critical when post-SIP integritytesting has to be performed.

Integrity testing the vent filter. Post-SIP integrity testing ofthe vent filter increases the level of sterility assurance. Integritytesting must be performed without compromising the steril-ity of the downstream equipment. This is achieved easily bythe water-based testing method (see Figure 4). Because waterflow through an integral filter is minimal, it can be used inplace after SIP. In-line integrity testing of the filter before theprocess avoids potential time loss and rejected product causedby a damaged filter. The sterilizing vent filter then is used foraseptic compensation of liquid movements in the tank or forproduct transfer. Post-use testing also is required to ensure thatthe filter was not damaged during the manufacturing process.

Liquid filter engineeringSystem design, installation, and standard operating procedure. Theability to wet hydrophilic filters makes their sterilization eas-ier because condensate can pass through the membrane alongwith the steam and reduce the occurrence of blind filters. Con-versely, the hydrophilic nature of product filters makes pre-and post-SIP drying difficult. Water-wet filters do not allowair passage when the pressure is below the bubble point pres-sure (typically 3.6–4 barg).

Polymer materials used for fabricating hydrophilic filters donot exhibit high thermal conductivity and are good insulatorsfor heat transfer. Furthermore, the air entrapped in thepolypropylene support layers, the microporous membrane andthe pleated structure of the filter acts as a barrier to the heattransfer process.

The filter system must be equipped with appropriate ventand drain valves as well as designed and placed in the uprightposition to properly remove air and condensate. These valvespermit a continuous steam flow during SIP and eliminate po-tential dead flow areas.

Figure 5 depicts possible filter configurations. The in-linehousing design allows the downward evacuation of air and

Figure 3: Separate filter and vessel sterilization. This configurationallows three different SIP procedures—separate filter sterilization inthe case of replacement, vessel sterilization only, and simultaneousfilter and vessel sterilization.

Figure 4: System designed for SIP and integrity testing of vent filter.

condensate through the vertical connectionto the downstream thermostatic steam trap.The T-type housing needs a fall onpipework for condensate drainage. If thesteam is supplied from the product line, athermostatic steam trap must be installedon the top of the filter to evacuate air andensure that steam penetrates all filter as-sembly extremities. In all cases, the upstreamcondensate is eliminated via a steam trapfitted on the drain port of the housing.

Liquid filters should be sterilized in theforward direction (separately from their as-sociated downstream equipment) to reducethe steam expansion volume at the beginning of the SIP cycle.Such a procedure will limit superheat effects and reduce thesteam flow rate over the filter. Pressure gauges are required up-stream and downstream of the filter to control the differentialpressure. Temperature probes are located in the drain pointdownstream of the filter assembly to monitor the set temper-ature. The addition of steam traps and downstream bleed valvesallow condensate removal and reduce both the steam flow rateand the pressure differential across the filter.

Sterilization cycle. If the filter was integrity tested before ster-ilization, it must be dried with pressurized air or nitrogen toprovide a path for subsequent steam flow-through. It cannotexceed the maximum pressure drop of 0.35 barg across the fil-ter.

Water present in the steam line at the beginning of the ster-ilization cycle first must be purged upstream of the filter toavoid wetting and subsequent blocking. The bleed valves onthe filter are open to ensure condensate and air removal. Oncethe temperature probe indicates the set sterilization tempera-ture, the SIP cycle begins and continues for the required timedefined during validation. The pressure differential over thefilter must not exceed 350 mbar to maintain the filter integrity.Both pressure and temperature are recorded to ensure that sat-urated steam is present in the system.

Automatic SIP is regulated via the steam inlet pressure re-ducing valve and steam traps. Manual procedures are regulatedvia the manual steam supply valve and downstream bleed valves.The bleed valves must be opened progressively to avoid exces-sive pressure differential as well as eliminate air and conden-sate.

Drying, cooling, and integrity testing the filter. After SIP, thesteam supply valve is closed and compressed air or nitrogen isadmitted to the system for cooling and drying the filter. Quickdrying is obtained with both the gas flow and the high tem-perature of the system. As required by the European Commis-sion’s Good Manufacturing Practices Guide, filter-integritytesting must be performed after sterilization but before theprocess (1). This operation can be performed without com-promising the sterility of the system by using the set-up de-picted in Figure 6. In most cases, water cannot be introduced

into the system because of subsequent product dilution. Usingthe product as the wetting medium enables filter integrity test-ing after steaming and avoids time loss and rejected productresulting from a damaged filter. Post-use testing also is requiredto ensure that the filter has remained integral during the en-tire manufacturing process.

ConclusionSteaming-in-place (SIP) is the preferred method for steriliz-ing processing equipment, including vessels, valves, processlines, filter assemblies, manifolds, and filling nozzles. The confirmation of a well-designed and engineered filtration system is the ability to validate the SIP cycles using thermo-couples and biological indicators. Following the design andengineering rules detailed in this article will simplify both the validation and ongoing operation of filtration systems inaseptic processing.

Reference1. European Commission, “Guide to Good Manufacturing Practice.

Annex I, Manu-facture of Sterile Medicinal Products” (EuropeanCommission Enterprise Directorate General, Brussels, Belgium, 1997).PT

Figure 6: SIP and post-SIP integrity testing of liquid filters.

Figure 5: (Left) In-line filter housing. (Middle) T-type filter housing. (Right) T-type filterhousing equipped with a steam trap on top for air evacuation.

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© Reprinted from PHARMACEUTICAL TECHNOLOGY, October 2004 Printed in U.S.A.

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