Sterile pressure measurement
How to choose and use pressure transmitters for sterile tanks in the food industry

Pressure is an important process parameter in the food industry. For example, certain reactions only take place at defined pressures, vapor pressure has to be monitored in sterilization processes, or levels in storage tanks are monitored by measuring the hydrostatic pressure. This article discusses how to measure pressure hygienically and reliably.
Joachim Zipp
Mechanical or electronic pressure measuring instruments are fitted to sterile tanks with the aid of diaphragm seals. These use a flush-fitting flexible metal diaphragm to separate the pressure gauge or transmitter from the process medium, ensuring that the instrument is free of dead space. The internal space between the diaphragm and the pressure sensor is completely filled with a fluid suitable for use in a food environment. The flexible diaphragm transmits the process pressure to the fill fluid and thence to the measuring instrument. To allow high-temperature sterilization without damaging the pressure sensor, a cooling element is placed between the diaphragm and the sensitive electronics.

The diaphragm seal has hygienic process connections designed to allow easy installation. The flush metal diaphragm is laser-welded to the body of the seal, producing a smooth weld finish that is easy to clean. Unlike ceramic measuring cells, diaphragm seals do not require additional sealing elements, thus reducing the risk of compromising process hygiene. As one of the leading manufacturers of diaphragm seal systems, Wika offers a wide variety of standard connections for this type of application. Connections such as Varivent and Neumo BioControl, or other manufacturer-specific fittings, are particularly suitable for sterile tanks, where they eliminate crevices and dead space.

Hydrostatic level measurement

Level measurement in sterile tanks is often performed through pressure measurement. This is known as hydrostatic level measurement, and it is based on the principle that a column of liquid generates pressure that is proportional to its height and density. If the liquid density is constant, the level of the liquid depends only on the pressure measured at the base of the tank. By taking the internal geometry of the tank into consideration, this also allows direct calculation of the volume of the tank contents.

Open vessels

In open vessels, level can be measured using single-point pressure sensors or mechanical pressure gauges. The measuring instrument is mounted at, or close to, the bottom of the vessel. This method can be used for virtually all liquids, and for pastes, emulsions and fluids containing solids. It even works in the presence of foam, because foam has only a very low density.

Mounting to the vessel requires just a simple nozzle, and many of these pressure gauges are available from stock. As a result, hydrostatic level measurement in open vessels is a cost-effective alternative to using level measuring instruments based on other measuring principles.

Closed tanks

In closed tanks, level measurement based on pressure requires differential pressure (DP) transmitters. Diaphragm seals at the top and bottom of the vessel are connected via capillary extension to the low- and high-pressure inlets, respectively, of a DP transmitter. The low-pressure element is mounted in the space above the maximum expected level, and measures the pressure of the gas phase above the liquid. Differential pressure measurement has the advantage that it is independent of the gas pressure above the liquid column, which is useful in the case of tanks that are blanketed with nitrogen, or when gas pressure is applied to discharge the tank contents. For applications where tanks are exposed to negative pressures, such as during vacuum filling, DP systems are still suitable as long as a suitable fill fluid is chosen.

In closed sterile tanks with gas inlet and exhaust nozzles, the open-tank technique described above can be used as long as the inlet and exhaust valves are fast-acting. If they are not, filling or emptying can cause a pressure change of several millibars, leading to an error of several centimeters in the level reading. If high accuracy is required, use the DP method.

Pressure blanketing

In many applications, food or beverages held in transport or storage vessels are blanketed with nitrogen to exclude oxygen and preserve product quality. To ensure
the continual presence of the nitrogen blanket, it is usual to monitor the gas pressure above the product level. Pressure switches or transmitters with low-level alarms are used for this task. The pressure sensor is normally mounted through the top of the vessel.

Density measurement

Density can also be measured on the basis of hydrostatic pressure, provided that the height of the liquid column is kept constant. In practice, a differential pressure transmitter is installed with its two diaphragm seals mounted below the liquid surface, at a defined distance apart. The density is directly proportional to the measured differential pressure.

Guidelines and approvals

When selecting pressure measuring instruments, the relevant legal provisions, regulations, guidelines and approvals from independent organizations must be observed. For instance, all pressurized measuring instruments must comply with the 97/23/EC Pressure Equipment Directive to guarantee their reliability and pressure containment.

The suitability of measuring instruments for direct contact with foodstuffs depends on the design of the process connections. In this context the design provisions of the European Hygienic Equipment Design Group (EHEDG) must be followed. The EHEDG Guideline Doc. 8, Hygienic equipment design criteria, specifies
which design criteria must be met by measuring instruments used in the food industry.

For example, the wetted surfaces must be free from surface defects and the roughness must be better than Ra<0.8 µm. The Ra figure for cold-rolled surfaces, such as instrument diaphragms, is between 0.2 and 0.5 µm.

In America the 74-03 standard issued by 3-A Sanitary Standards Inc. specifies the design of the wetted parts of sensors and measuring instruments. Compliance with the standard must be verified by an independent inspector through a Third Party Verification (TPV) process. Only after passing this audit may the measuring instrument manufacturer apply the 3-A logo to his instruments.

For machinery and equipment manufacturers exporting their products to the US, it is important to know that the measuring instruments mounted to their machinery or equipment must also be provided with the 3-A logo in order that the complete installation can gain 3-A approval.

All standard hygienic connections for pressure measuring instruments combined with Wika diaphragm seals are 3-A certified, meaning that domestic machine builders can use these pressure measuring instruments on equipment intended for export, without any restrictions. n