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Diaphragm Seals - General Information

Diaphragm Seals - General Information
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Diaphragm Seals - General Information - 1

Data sheet Date Page GENERAL INFORMATION ABOUT DIAPHRAGM SEALS INTRODUCTION Modern chemical processing and process manufacturing procedures demand ever greater accuracy and reliability from their pressure measuring instruments. In order to minimise costs, both reject levels and production downtime must be kept to an absolute minimum. In some applications where there are chemical or hygiene considerations, it is necessary to isolate the pressure measuring instrument usually a gauge or a transmitter from the process medium. This isolation is achieved by using a pressure-sensitive diaphragm made from a material resistant to the process medium enclosed in a system fitted to the base of the instrument. The space between the diaphragm and the measuring element in the instrument has to be evacuated under a vacuum and filled with a suitable fill fluid and sealed. The process pressure exerts a force on the outside face of the diaphragm. As the diaphragm flexes under this force it pushes inwards and attempts to compress the fill fluid within the instrument. This fill fluid is designed to withstand compression so that the movement is channelled directly into the measuring element producing a resultant reading on the gauge. The entire operation of the diaphragm seal system is based on Pascal's Principle which states that a pressure exerted on a fluid is transmitted undiminished through that fluid and in every direction. For this process to work the displacement force of the diaphragm must exceed the force required to move the measuring element in the instrument itself. In practice this means that the smaller the force required to move the measuring element the easier it is to construct an accurate seal system. APPLICATION Knowing in which situations to install a diaphragm seal rather than a standard instrument is critical to maintaining the integrity of the process. A diaphragm is usually required under the following circumstances. • The process medium is corrosive, and would chemically attack the working parts of a standard pressure gauge or transmitter. The process medium is viscous or contains solid particles, either of which could result in the gauge's pressure inlet becoming blocked and preventing any pressure from reaching the measuring element. The process medium is prone to solidifying over time (e.g. it may freeze as the temperature drops, or it may set as it dries, or it may be subject to polymerization), in which case a standard gauge's internal moving parts could be rendered immobile. The process medium is so corrosive that even pressure elements made from exotic materials such as Monel or Hastelloy do not provide adequate protection. The only solution is to apply a special coating to the base material to prevent corrosion. Application of these special coatings to the internal movement of a standard pressure gauge or transmitter would be impossible, but a diaphragm seal fits the bill perfectly. It is important to eliminate the formation of bacteria on or in the process connection in applications where hygiene is of paramount importance such as in the food and beverage industries. _______________________________________________________________________________________________ PCI (Process System Solutions) Ltd, Unit 7 Slaidburn Crescent, Southport, Merseyside, PR9 9YF, UK, Tel: +44 (0) 1704 509090, Fax: +44 (0) 1704 509091, email: sales@pci-instruments.com, Website: www.pci-instruments.com

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Diaphragm Seals - General Information - 2

Data sheet Date Page POTENTIAL PROBLEMS WHEN SELECTING A DIAPHRAGM SEAL Avoiding common mistakes will improve your chances of successfully selecting and installing a diaphragm seal. • Diaphragm displacement is incompatible with the required displacement volume of the instrument’s measuring element. The diaphragm is too small or too stiff to allow for natural thermal expansion of the fill fluid, leading to zero shifts and false pressure readings. The process or ambient temperatures exceed the acceptable service range of the fill fluid. In differential pressure applications, very small...

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Diaphragm Seals - General Information - 3

Data sheet Date Page INFLUENCE OF TEMPERATURE The complete pressure sensing system, made with the diaphragm seal (with or without capillary) and the pressure instrument, is filled with a fill fluid at a specific temperature (generally +20 ±2°C) called reference temperature. A increase or decrease in ambient temperature or of the process fluid make a proportional variation on the fill fluid volume. Consequently this has an effect on the internal pressure of the closed sensing system and adds an error of zero on the instrument. To minimize such error, it is necessary to compensate for the...

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Diaphragm Seals - General Information - 4

Data sheet Date Page INSTRUMENTS WITH CAPILLARY The use of the capillary allows the reading of the instrument from remote and limits the effects of the process temperature (above 250°C) on the accuracy of the instrument. A capillary length of 500mm is normally enough the keep the temperature of the instrument close to the ambient temperature value. The length of the capillary must be as short as possible, and is advised not to exceed 6 meters because the temperature variation on the capillary length may influence the accuracy and response time (see Fig. 2). Pressure instruments with a...

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