Pressure Gauges for Sterile Processes

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WIKA USA

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Pressure measuring instruments used in sterile processes, such as Clean-in-Place (CIP) systems and Sterilization-in-Place (SIP) cycles, must be exceptionally tough. Special designs and materials are required to combat the exposure to high temperatures and aggressive cleaning agents.

Pressure Gauges for CIP and SIP System

In a sterile facility, CIP systems reduce the manual cleaning requirements and the idle down times. Common CIP processes use cleaning solutions that are alkali- or acid-based (such as NaOH or H2O2), at temperature up to 175°F and pressure of up to 60psig.

Sanitary pressure measuring instruments must be compatible with these aggressive environments. An effective CIP process exposes the wetted materials of the sanitary pressure gauge to repeated cycles of wash, rinse, and drain.

And the challenge continues: The SIP cycle is also carried out in this closed system. In a steam sterilization process, the point within the piping system that is exposed to the lowest temperature needs to be subjected to at least 250°F for over 20 minutes. Exposure to significantly higher temperatures, with some SIP cycles running as high as 300°F, is also common.

Stainless steel, 316L, is compatible with most process medium, but its life could be limited when exposed to aggressive cleaning solvents. Sanitary process technology mainly uses corrosion-resistant austenitic stainless steels as standard materials. While 316L is commonly specified in the American markets, 1.4404 and 1.4435 are both used within Europe.

These materials are characterized by a d-ferrite content of < 0.5%. If this grade of stainless steel is not process- or cleaning-compatible, many other materials, such as Hastelloy® C276, titanium, and Inconel, are available to extend the expected life cycle of the pressure measuring instrument.

Challenges to Sanitary Pressure Measurement

Measurement instrumentation components have their work cut out for them. They must resist high temperatures, aggressive detergents, and increased pressure while also maintaining long-term measuring accuracy. To satisfy these sanitary requirements, the answer is often a diaphragm seal pressure measuring assembly.

Sanitary Pressure Measurement with a Diaphragm Seal

A diaphragm seal assembly consists of a pressure gauge mounted to a diaphragm seal (which contains of a thin flexible diaphragm welded to a solid body) and a system fill fluid.

A diaphragm seal, usually between 0.002" – 0.004" thick, is used to separate the measuring instrument from the process medium. The space between the diaphragm seal and inside 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.

Selecting the right system fill fluid is quite important. 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 in case there is a breach in the diaphragm that causes the fluid to come in contact with the process medium. To protect the gauge even further, specifically for the sanitary industry, WIKA developed a double diaphragm seal. In conjunction with the double diaphragm seal, is a break monitoring system which sounds an alarm if the gauge fails from the vacuum between the two diaphragm seals breaking. 

A diaphragm seal is used to mate a threaded pressure gauge with a sanitary type of process connection. The combination of a pressure gauge, like the WIKA M93X.25, with a sanitary diaphragm seal connection containing a flush diaphragm seal or cylindrical diaphragm is preferred. The pressure measuring gauge is installed either directly to the diaphragm seal or indirectly through 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 dissipate the heat and protect the gauge. Gauge and seal combinations can be vertically or horizontally configured for easy pressure readings.

How to Install a Sanitary Gauge

The sanitary diaphragm seal containing a flush diaphragm is installed into the process piping system by use of a process “T.” See, for example, the installation of the WIKA Type M93X.3A pressure gauge in Figure 2. Keep in mind that the “T” process 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.

Benefits of an Inline Diaphragm Seal

An inline diaphragm seal, such as the WIKA Type L981.22, 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 the 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 assembly providing the basis for self-cleaning and draining. All product residues or films can be easily cleaned, even by pigging in certain applications.

Minimizing Installation Taps

A standard sanitary pressure measuring instrument, such as the WIKA Type PSD-30, can be combined with other instruments, such as the L990.22, into one device. This one instrument contains local pressure measurement, switch points, and electrical pressure output signal. This combination minimizes the number of installation taps required within the process piping system.

Minimizing the number of tapping locations and components in a sanitary system has many advantages:

  • It is more easily cleaned due to reducing the number of joints and potential crevices.
  • Less space is required. 
  • The number of potential contamination points is reduced.
  • There are fewer components to keep track of.
  • It reduces the number of potential leak points.  

Dry Measuring Cell for Sanitary Pressure Measurement

The dry measuring cell, with no system fill fluid behind the diaphragm, consists of an integral flush-welded diaphragm element. This component uses linear displacement to sense and measure pressure. This dry measuring cell technology has been used in other industries for a number of years and is now being applied in this demanding sanitary market.

This dry measuring cell offers additional benefits over a diaphragm seal assembly, mostly due to the fact that no system fill fluid is needed.

For example, one major advantage of this dry measuring cell technology is the lack of contamination risks to the process medium due to the potential leakage of the system fill fluid.

Another advantage of the dry measuring cell is the minimization of the inherited false zero pressure shift due to the expansion or contraction of the system fill fluid because of exposure to temperature deviations. The lack of system fill fluid removes the false zero shift pressure shift.

Conclusion

There are many ways to reduce the hygienic risk in 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 make cleaning easier, combine several instruments into one device to minimize the number of tapping locations in the piping system. The WIKA Type S-10 with L983.22 is another option. Use a dry measuring cell to avoid contamination by the system fill fluid in case of a possible diaphragm rupture. Apart from the hygienic requirements, the pressure gauge and other instrumentation still must measure accurately, be reliable, and function repeatedly under tough process conditions.

 

 

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