The elegant way to fasten sensors

Hygienic sockets - when space is scarce

Hygienic sockets are a process port and built-in housing in one. These innovative sockets for pH, oxygen and conductivity sensors were developed to ease installation and maintenance, improve cleanability and increase human safety. With their space saving design and simple sterilisation, they are ideal for use in fermenters.

Traditionally, 25 mm sockets or clamp and flange housings are the most popular way to install pH sensors in tanks or pipes. Their weaknesses are undeniable, however. Weld-in sockets can be deformed merely by the welding heat, with the result that their openings are no longer exactly circular. If the flanged pipe of the housing is then inserted, it cannot close precisely at the O-ring. The socket opening can only be restored to its proper shape by reaming. Owing to the size of these sockets (up to 40 mm), they are moreover unsuitable for fastening sensors to small pipes or tanks. Another problem is that the O-ring of the flanged pipe and the inside wall of the tank to which the socket and housing are mounted are not level with one another. The narrow gap this creates is difficult to clean from inside the tank. The housing therefore has to be regularly removed in order to comply with the required hygienic standard. Added to this, all seals in contact with the medium have to be frequently exchanged for critical processes. A certain amount of space is needed to install and remove the housing for this purpose. It must be large enough to screw on the housing's screw ring by hand. The space required for the sensor is consequently greater than the socket radius and is at least 80 mm. If the housing is subjected to vibrations, there is a risk that the screw ring will work loose, causing the housing to be forced out of the weld-in socket by the process medium. The advantage of the clamp method is that some models have no narrow gap on the inside of the tank or pipe to which the housing is mounted - the O-ring seal is on a level with the tank wall, so that the sensor can be cleaned with steam at the same time as the inside wall to which it is attached. The drawback is that the process ports are comparatively bulky. Hence, they are only suitable for installation in pipes or tanks with a large diameter. Their size is likewise a disadvantage if several sensors are mounted in the same tank or pipe.

Hamilton manufactures a sensor socket that does not have to be re-machined after installation and only occupies a small amount of space. With its overall diameter of 28 mm, the hygienic socket is a compact alternative for professional installation of pH sensors in steel pipes, fermenters and many other kinds of tank. The sensor immersion depth into the process can be varied at the time of installation. CIP, sterilisation and autoclaving are no longer a problem.

Seal to the process

The hygienic socket consists of a slim steel jacket with a PG 13.5 female thread for screwing in the sensor as well as an O-ring positioned in an opening. When a sensor is screwed into the socket, the O-ring seals it to the process without a gap. Visual inspections and replacements are possible in a matter of seconds. The second seal between the housing and the socket can be dispensed with. A module inserted into the steel jacket holds the O-ring firmly, so that it can be easily inspected when the module is withdrawn and likewise exchanged if necessary. One important advantage compared to the widely used 25 mm socket is that the steel jacket can be welded into a hole in the tank wall to virtually any depth. As a result, the sensor is always immersed to exactly the desired position in the tank. The space required on the tank or pipe is less than for a flange housing or a 25 mm socket with a built-in housing because there is no need to turn a large screw ring whenever the housing is removed or installed. The hygienic socket can therefore be mounted to smaller tanks or pipes than conventional models. Apart from the socket diameter, the only constraint on the minimum pipe or tank size is the length of the sensor inserted into the void.

Easy maintenance

Housings in applications involving hot processes or cleaning have to be regularly maintained in order to replace the fast-fatiguing O-rings. Once again, the O-ring inside the hygienic socket makes this an easy task. The ring can be accessed by pulling on the plastic part at the top socket end. It is retained on the module by a steel ring, which must be pulled off in order to get at the seal. If the latter is damaged, it can be replaced with a new O-ring in minimal time. As soon as a sensor is screwed into the hygienic socket, the inner module compresses the O-ring between the steel jacket and the sensor shaft. As a result, the sensor is sealed to the process medium with virtually no gap. The O-ring is decompressed again as soon as the sensor is slightly loosened. This prevents friction loads on the O-ring. Furthermore, the ingenious design of the seal arrangement means the socket tolerates thermal deformations of the kind that commonly occur during welding. There is no longer any need for reaming after the welding process.

Double safety

The hygienic socket is also safer to use than most other housing types. The two holes in the side of the steel jacket are referred to as "Life Guards" because in extreme cases they can actually save human lives. As soon as the sensor is loosened for dismantling, the O-ring between the sensor and the steel jacket stops sealing. Process medium enters the steel jacket and leaks out through the Life Guard holes before the sensor is completely released from the thread. The fact that liquid is dripping out alerts the operator to the presence of process medium, and the sensor can be tightened again before it has a chance to shoot across the room. Although the socket is now fouled, this is not a major problem because it can be rinsed with water, for instance, through the two holes in the outside walls.

The hygienic socket was designed in accordance with EHEDG guidelines. If the steel jacket is welded flush with the inside wall of the tank or pipe, there is no depression at this point thanks to the clever arrangement of the O-ring. The process can thus be cleaned there efficiently and thoroughly (CIP). By screwing in an adapter instead of a pH sensor, conductivity or oxygen sensors can be mounted practically flush with the tank wall, so that they are only inserted into the tank a minimal amount. The socket can not only be sterilised on the process side, it can also be autoclaved complete with the inserted internals, module and built-in sensor. The Life Guard holes can additionally be used for steam sterilisation of the electrode shaft. In this case, however, the human safety function described above is no longer operative. The large holes provided in the plug-in module permit a high flow rate and fast distribution of the steam for rapid steam sterilisation.

The hygienic socket (right) takes up less space than conventional housings (left) because there is no need to turn a screw ring whenever the housing is removed or installed

The O-ring is retained by a module inserted into the steel jacket

Apart from the socket diameter, the only constraints on the minimum pipe or tank size is the length of the sensor