Preventing Corrosion Caused by Aggressive Liquids in the Food Industry

Preventing Corrosion Caused by Aggressive Liquids in the Food Industry

When testing proportional pressure regulators as part of the development of complex hydraulic systems, high impulse capability and precision are required from the pressure measurement sensors employed.

Carbonic acid and alcohol can put a strain on measuring equipment. A manufacturer of automatic in-line and laboratory liquid analyzers has approached STS to find a durable and accurate pressure transmitter.

When exposed to aggressive fluids such as alcohol or carbonic acid, standard materials suffer from corrosion. For example, carbonic acid causes an increase in the hydron (H +) concentration and therefore leads to hydrogen corrosion. Once the corrosion eats through the membrane of the pressure sensor, it becomes unusable. That is why regular stainless steel will not suffice for applications with high levels of carbonic acid.

Other than being highly corrosion-resistant, the pressure sensor for this particular application in a bottling plant has to be able to deal with extremely low pressures close to a vacuum. As this application is part of the food industry, hygiene standards are very high. The near-vacuum conditions that the equipment is regularly exposed to is part of the sterilization process (similar, although not as extreme, as what happens in an autoclave). Low pressures below 0 bar can present a danger to the integrity of pressure sensors. The vacuum may cause the membrane to be sucked off from it position in the sensor. False measuring results or a completely broken sensor are the consequence.

Due to these requirements, we had to assemble a customized solution for this manufacturer of automatic in-line and laboratory liquid analyzers based on the pressure transmitter ATM.ECO. As material, we chose an extremely corrosion-resistant Hastelloy steel. To ensure membrane stability during low pressure conditions, we applied a special glue to fixate the membrane in place.

Since the pressure transmitter operates under room temperature conditions in this application, no special temperature compensation was necessary. The accuracy of 0,25 percent of the total scale is also more than enough for this particular application. The full scale ranges from 1 to 15,000 psi and is hence perfectly suitable for low pressure.

The right sealing solution for every application

The right sealing solution for every application

In order to maintain the performance of piezoresistive pressure sensors and optimally protect them in harsh conditions, various sealing options should be taken into consideration. STS offers several solutions here, which are selected according to the given requirements and the actual application environments.

A sealing ring, or O-ring, is used in most standard applications. This common sealing method is very flexible and highly versatile. STS offers sealing rings in many different product variants, where the specific material is selected according to the pressurized medium.

Should the seal be exposed to aggressive media or extreme temperature conditions, sealing with a simple ring then becomes insufficient. The elastomers commonly used in the manufacture of O-rings become porous when exposed, for example, to media containing hydrocarbons. Problems can also arise during decompression, where a steep pressure drop may even rupture the sealing ring.

A common alternative to the simple sealing ring is a welded seal, where the measuring cell and pressure connection are welded directly together. This type of seal is somewhat more stable, but, just like the O-ring, can only withstand a maximum pressure of 250 bar/3600 psi. Up to this value, the O-ring and welded seal would complement one another, depending on the application environment and the predominant medium. With an aggressive pressure medium like gasoline, for example, only the welded seal would be suitable, whereas in saltwater, an O-ring should be strictly used to prevent corrosion of the seal.

Overview of the various sealing solutions

As soon as the limit value of 250 bar is exceeded, only a metal seal can then be used. At STS, this elastomer-free metal seal is therefore offered for application environments at very high pressures. Because of its properties, this sealing solution can also withstand exceptional conditions and extreme exposure to corrosive chemicals, vacuums and intense radiation exposure.

Practical use of sealing solutions

A large company that manufactures grinding and compressor systems for various industries relies mainly on the pressure sensors offered by STS, for which there is a wide range of sealing rings. Here the temperature conditions, as well as the nature and compatibility of the media, are known in advance, so that these can be easily validated before use and the sealing rings manufactured accordingly.

For another customer who manufactures test beds for the automotive industry, the material involved and the temperature conditions encountered are only lastly determined by the end user. The essential properties of the sealing solution therefore depend upon these subsequent specifications. The seals used here must therefore ensure from the outset a high degree of flexibility, meaning that our robust welded seals form the choice here.

Media compatibility of piezoresistive pressure transmitters

Media compatibility of piezoresistive pressure transmitters

In selecting the right pressure transmitter for individual applications, there are numerous criteria that must be considered besides the pressure range to be measured and the extant thermal conditions. Among these falls the subject of media compatibility: The housing and process connection must withstand the environmental conditions, so that the sensor can perform its service over the longer term.

 

Material selection therefore follows two important considerations: On the one hand, that there is a chemical tolerance to the contact media. The other factor is that preventative considerations also play an important role. It should not only be clarified whether the pressure transmitter will remain functional longer term. It must also be established whether the materials used in the pressure transmitter itself can lead to dangers when coming into contact with particular substances – the pharmaceuticals industry would be an obvious example here. In the following, we will be showing which media incompatibilities occur with which materials and what the solutions to this might be.

Chemical-physical media compatibility with sealant material & cable

It is not only the housing material itself that should be included in considerations of media compatibility. Other elements of the pressure sensor also come into contact with the surrounding or process media and these materials are to be particularly contemplated.

The majority of pressure transmitters come with a sealant made of elastomer. The problem here is that the elastomer can dissolve when it comes into contact with aggressive media such as biodiesel, for example. In this case, a front-flush, welded and elastomer-free sensor should be employed.

One further factor is the cable that serves in transmitting the measurement data. We will adopt the example here of using a submersible probe in a swimming pool. For reasons of hygiene, swimming pools use chlorinated water. As standard, submersible probes use PE or PUR cables. Although chlorinated water alone presents no issue to these cables, the chlorine vapor rising from this water does do, since this is much more aggressive than the water itself. These cables, over a period of time, will become porous above the water level (visible as a white discoloration) and water will then penetrate within. Subsequently, the sensor itself will also fail. For this reason, teflon cables would be used in such an instance.

Chemical-physical media compatibility with housings

Viscous media

With viscous media, using paints as an example, deposits within the sealant channel can be a consequence. To prevent contamination, smooth membranes free of any dead space and without an open pressure channel are needed for such applications, so that the sensor can be cleaned free of all residues.

Abrasive media

When pressure transducers come into contact with abrasive media such as concrete, a simple membrane of stainless steel provides insufficient protection. In this case, a membrane coated with Vulkollan® foil will be required.

Galvanic & acidic liquids

A chromed pressure sensor may look better from an aesthetic viewpoint, but in practical terms it is anything else. When a pressure gauge with a metal housing is used in an electroplating bath, over time only a clump of non-functional chrome will remain. Even acidic fluids, such as sulfuric acid, will react with metals. For this reason, plastic housings are deployed for galvanic and acidic liquids. The most popular solution here being PVDF.

Image 1: Destroyed pressure transmitter due to incorrect material selection

Seawater

Salt water (depending on its salinity) causes long-term pitting to stainless steel housings. This is why most submersible sensors and level sensors are also available in a titanium version.

Open waters / lightning protection

Lightning strikes cannot perhaps be described as a medium, but we will nevertheless look into this a little further. Should a strike hit a sensor directly, then no lightning protection at all will be of any use. Surge protection, however, can be recommended for submersible probes that are used in open waters. An excess voltage and damage to the measuring instrument by a lightning strike in the immediate vicinity can thus be prevented. This is particularly advisable when long-term measuring in remote places is being conducted. The replacement of a defective device here would then be much more expensive than surge protection itself might be.

Preventative media compatibility

The silicon chip of a piezoresistive pressure transducer is surrounded by a transmission fluid. A usual choice here is silicone oil. Although this fluid does not normally come into contact with the surrounding media, some things must nevertheless be observed here – since a defective housing, after all, cannot be totally ruled out. Depending upon application, this could lead to serious consequences.

Heavily oxidizing gases and fluids

When oxidizing gases and fluids come into contact with oils or greases, the threat of explosion then arises. All components exposed to the medium here must be free from oil and grease, and, in preventive terms, the transmission fluid also.

Foodstuffs and pharmaceuticals industries

In this case, the silicone oil must be replaced with a food-safe oil to rule out any contaminations either harmful to health or that act in other ways. Beer, for example, that has come into contact with silicone oil will no longer foam up, and nobody wants to have that.

Paints

Just one drop of oil can render a whole batch unusable. Here also, an alternative must be found.

The media compatibility of piezoresistive pressure sensors: Summary

The optimal pressure sensor for an individual application is dependent upon many factors. For this reason, a deep understanding on the supplier side of the respective customer application is required. STS always offers its customers a needs-oriented consultation that approaches all aspects in providing a reliable solution within the shortest of timeframes – even for lower device volumes.

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