Written by Jennifer Breunig, Diaphragm Seals Product Manager, WIKA Alexander Wiegand (Klingenberg, Germany) with contributing editor Kenneth J. Angi, Global Contracts Manager, WIKA Instrument Corporation (Lawrenceville, Ga.)
Introduction
Pressure measuring instruments used in sterile processes must be exceptionally tough. Special designs and materials are required to combat the exposure to high temperatures and aggressive cleaning agents.
Clean-in-Place (CIP) systems reduce the manual cleaning requirements and the idle down times within a sterile facility. Common CIP processes use cleaning solutions that are alkali or acid based (NaOH, H2O2,...) at an elevated temperature (up to 175°F) and pressure of up to 60psig. Compatability with these aggressive environments are some of the enormous challenges for sensitive pressure measuring instruments. An effective CIP process exposes the wetted materials of the sanitary pressure gauge to repeated cycles of wash, rinse and drain. The challenge continues: the Sterilization-in-Place (SIP) cycle is also carried out in this closed system. For a steam sterilization process, the point within the piping system being exposed to the lowest temperature needs to be subjected to at least 250°F for over 20 minutes – and the trend is toward significantly higher temperatures. For instance, at this moment, some SIP cycles are running as high as 300°F.
Stainless steel, 316L, is compatable with most process medium, but could have a limited life when exposed to the aggressive cleaning solvents. Sanitary process technology mainly uses corrosion resistant austenitic stainless steels as standard materials. 316L is commonly specified in the American markets, within Europe 1.4404 and 1.4435 are both used. These materials are characterized by a d-ferrite content of < 0.5%. If this grade of stainless steel is not process or cleaning compatable, numerous other materials are available to extend the expected life cycle of the pressure measuring instrument (Ex. Hastelloy® C276, titanium, inconel and others).
Sanitary Pressure Measurement
Components of the measuring instrumentation must meet these high demands, i.e. resistance to high temperatures, aggressive detergents and increased pressure, at the same time it must maintain long-term measuring accuracy. A diaphragm seal pressure measuring assembly is often used to satisfy these sanitary requirements. To understand where the critical points are within a diaphragm seal system, it is necessary to give a short explanation of the functionality of a diaphragm seal installed on a pressure gauge.
A diaphragm seal assembly consists of a pressure gauge mounted to a diaphragm seal (consists of a thin flexible diaphragm welded to a solid body) and a transmission fluid (system fill fluid). This system fill fluid transmits the process pressure hydraulically from the pressure retaining flexible diaphragm to the measuring instrument. When using a diaphragm seal the measuring instrument is separated from the process medium using a thin flexible metal diaphragm, typical thickness of 0.002" – 0.004", to prevent the process medium from coming in contact with the pressure gauge. The space between the seal diaphragm and internal to the pressure gauge is completely filled with a system fill fluid. The applied process pressure deflects the thin seal diaphragm and the displaced volume of system fill fluid hydraulically transfers the pressure to the gauge. This methodology ensures an accurate, reliable and repeatable pressure measured.
One important element of the diaphragm seal system is the selection of the system fill fluid. When specifying the system fill fluid for a sanitary application, it is important to specify a type that contains FDA (American Food and Drug Administration) or USP (US Pharmacopoeia) approval. This system fill fluid needs to be compatible with the process medium for the rare situation when a breach in the diaphragm might occur and this fluid comes in contact with the process medium.
The conversion of a threaded pressure gauge to mate with a sanitary type of process connection, the use of a diaphragm seal is the most common applied approach. The combination of a pressure gauge with a sanitary diaphragm seal connection containing a flush diaphragm seal or cylindrical diaphragm (inline seals) is preferred. The pressure measuring gauge is installed directly to the diaphragm seal or via the use of a flexible capillary. For direct mounts containing high medium temperatures, a cooling element can be used between the gauge and diaphragm seal to disapate the heat to protect the guage. Gauge and seal combinations can be configured for easy pressure readings for horizontal and vertical piping systems.
Sanitary Gauge Installation
The sanitary diaphragm seal containing a flush diaphragm is installed into the process piping system by use of a process "Tee". This process "Tee" has its disadvantages; dead-space, non-laminar flow and additional clamped connections. This can result in; hard to reach areas for cleaning, crevices, pockets, reduced shear stress due to lower turbulence and additional potential leak paths.
Installation without Obstructions
An inline diaphragm seal is perfectly suitable for use with flow applications with a low to medium viscous process medium. This diaphragm seal consists of a body with an internal cylindrical thin diaphragm. This sanitary assembly design doesn’t require any instrument "T" for installation into the process flow. This seal replaces that process "T" and becomes an integral part of the piping system. Since it is entirely integrated into the piping system; no turbulence, corners, dead spaces or other obstacles occur within the process flow. The medium flows undisrupted through this non-intrusive pressure measuring assembly providing the bases for self-cleaning and draining. All product residues or films can be easily cleaned and is even piggable in certain applications.
Reduce Number of Intrusive Tapping of the Process
A standard sanitary pressure measuring instrument can be combined with other instruments into one dvice (transmitter, transducer, switch, electrical temperature and etc.) to minimize the number of installation taps required within the process piping system. Minimizing the number of tapping locations and components in a sanitary system improves it’s cleanability by reducing the number of joints and potential crevices. By combining several instruments into one device; less real estate is required, reduction of potential contamination points, less components to procure or inventory, expedite cleaning and reduce the number of potential leak points. All which are desired goals for sterile manufacturing.
Dry Measuring Cell
Up to now only pressure gauges with diaphragm seals were available for the applications within the sanitary market. The new dry measuring cell (no system fill fluid behind the diaphragm) consists of an integral flush welded diaphragm element which uses its linear displacement (not the typical volume displacement) to sense and measure the pressure. This dry measuring cell technology has been used in other industries for numerous years and is now being applied into this demanding sanitary market. This dry measuring cell offers additional benefit over a diaphragm seal assembly. Several advantages of this dry measuring cell stems from the removal of the system fill fluid which is used with all diaphragm seals.
One major advantages of this dry measuring cell technology is the deletion of contamination risks to the process medium due to the potential leakage of the system fill fluid from a damaged diaphragm seal. A possible product contamination of the process medium by a system fill fluid is impossible since it doesn’t exist. Another advantage of the dry measuring cell over an assembly containing a diaphragm seal with a system fill fluid is the minimization of the inherited false zero pressure shift due to the expansion or contraction of the system fill fluid resulting from exposure to temperature deviations. The dry cell technology removes the false zero shift pressure shift due to the absence of any type of system fill fluid.
Conclusion
There are a wide variety of existing options to reduce the hygienic risk in the pharmaceutical, biopharmaceutical, food, beverage and cosmetic plants to ensure a safe and clean pressure measurement. The inline instrumentation ensures no dead space and self draining. To assist in cleaning, combine several instruments into one device to minimize the number of tapping locations in the piping system. To avoid the contamination by the system fill fluid in case of a possible diaphragm rupture, there is a dry measuring cell available. Irrespective of the hygienic requirements, the pressure measuring gauge, and other instrumention, still has to measure accurately, reliable and repeatable under tough process conditions.