Absolute-, Gauge-, and Differential Pressure

Depending on what to measure, three sorts of pressures have been defined. They are differentiated by the zero reference used:

Absolute Pressure: is zero referenced against an absolute vacuum, so it is equal to gauge pressure minus atmospheric pressure. In order to make clear that it concerns absolute pressure an ‘a’ is added to the unit of measure, e.g.: mbara; PSIa

Gauge Pressure: is zero referenced against atmospheric pressure, so it is equal to absolute pressure plus atmospheric pressure. Negative signs are usually omitted. In order to make clear that it concerns gauge pressure ‘g’ is added to the unit of pressure, e.g.: mbarg; PSIg

Differential Pressure: is the difference in pressure between two values

Definitions Of Pressure In Process Industry

Within the process industry different nominations of pressures are commonly used. The most relevant pressures for Diaphragm Seal Systems are the design pressure, the maximum working pressure, and the operating pressure.

Maximum Working Pressure (MWP): is the highest pressure a device can withstand without bursting or failure in any way

The MWP is higher than any other pressure that can occur in process.

Design Pressure: both the lowest and highest pressure that can occur in a given process specification

The minimum/maximum design pressure is reached when the process runs out of control and before the safety devices (e.g. pressure relief valves, rupture discs) come into service. Design pressures need to be taken into consideration during design phase to ensure mechanical integrity of the device when exposed to these design pressures. Proper functioning after exposure to the minimum/maximum design pressure is not required.

Operating Pressure:

Minimum: the lowest pressure under which the process still runs stable

Normal: the pressure under which the process runs optimally

Maximum: the highest pressure under which the process still runs stable

Operating pressures need to be considered to ensure proper functioning of the device as such they are important for selecting the correct diaphragm seal design and fill fluid.

Static Pressure: the pressure at a nominated point in the process

Static pressure is commonly used to avoid ambiguity and to distinguish it from total pressure and dynamic pressure. Static pressure is identical to pressure and can be either one of the above mentioned nominations of pressure. The static pressure is especially important in case of differential pressure measurement. E.g. pressure at high pressure (HP) side is 100,1 barg; pressure at low pressure (LP) side is 100 barg, then the differential pressure (dP) is 0,1 bar, and the value of the static pressure is 100 bar.

Hydrostatic Pressure: is the pressure exerted by a fluid column due to the force of gravity

With hydrostatic pressure measurement it is possible to measure level in vessels, tanks, reactors etc. This is one of the most common applications for Diaphragm Seals. Also the hydrostatic pressure can be used to measure changes in density.

A proper Diaphragm Seal selection is influenced by the above mentioned pressures of the process. Each diaphragm seal type has limitations according its construction or body material. For flanged connections an addendum is provided showing the maximum and minimum pressure/temperature ratings. Both flanged and threaded process connections have standard restrictions in withstanding pressure (EN1092-1, ASME B16.5, ANSI B1.20.1, ISO7005-1, ISO228-2, ISO10423, JIS B2220). The construction of the Diaphragm Seal however can also be a restrictive factor and is specified on the data sheets as the maximum working pressure of the specific Diaphragm Seal.

Vacuum

Care should be taken when specifying a Diaphragm Seal System for measuring with process pressure under vacuum.

Full Vacuum: the absence of matter

While the Diaphragm Seals perform normally for most standard vacuum applications, as the pressure moves closer to a full vacuum acceptable reliability becomes more difficult to achieve. This is due to the fact that most fill fluids contain microscopic amounts of air or trapped gases, which tend to expand significantly as a pressure of absolute zero is approached. This expansion undermines one of the most important component factors of a seal system, that of absolutely constant fill fluid volume at any pressure. In order to overcome this potential problem, the Badotherm filling technology allows for a complete degassing of the fill fluid, at a pressure of < 1*10-8 mbara in combination with the correct heating of the applied fill fluid.

Also, under vacuum process conditions, there is a potential risk that through a gasket or thread air is sucked in the system, with all possible consequences for the functioning of the Diaphragm Seal System. With Badotherm’s full welded construction the measuring element has no gasket anymore to avoid any kind of leakage. Often vacuum occurs unintentionally for example during cleaning and fast cooling processes. To cover this often unknown and unaware presence of vacuum all Diaphragm Seals are standard tested at 35 mbara even when no vacuum value is specified.

The presence of vacuum in process is a very important factor when selecting the Diaphragm Seal fill fluid and mounting the instrument. The relation between the vacuum value and the process temperature should be checked in the vapour pressure curves of the fill fluid to see if the fill fluid is suitable. When mounting the instrument for a vacuum application, the instrument should be placed below the (lowest) Diaphragm Seal to protect the instrument.

Diaphragm Seal Pressure Specifications

The size of the diaphragm defines the minimum pressure range that the Diaphragm Seal can handle. Apart from the diameter, the flexibility of the diaphragm is also related to the shape and number of convolutions and its thickness. Badotherm diaphragms have standard thickness of 75 µm. The dD dimensions mentioned are values for the active diameters of the diaphragms i.e. the outside diameter of the outer convolution. Badotherm Diaphragm Seals have a maximum static pressure effect of 0,25% of calibrated DP span on top of the standard differential pressure transmitter specifications with regard to static pressure effects. In general the total effect is < 0,5% of calibrated DP span.