Low pressures can be difficult to measure, but low pressures still require monitoring. With standard Diaphragm Seals the limitation is around 100 mbar, but several processes required accurate pressure measurements up to 5 mbar, and at the same time also do require Diaphragm Seals for protection of the pressure instrument.
Difficulties arise when this technique is applied in a corrosive environment. In these circumstances it is essential that the pressure measurement devices used are equipped with Diaphragm Seals. However, the use of a standard Diaphragm Seal in combination with the very low overpressure ranges implies relatively high temperature effects. Namely when exposed to process and ambient conditions. These high temperature effects, and especially ambient temperature effects, result in unstable, unreliable and inaccurate pressure measurements, subsequently resulting in poor control of the process.
As a manufacturer of pressure and temperature gauges, we are very familiar with the compensating effect of a parallel medium. Combining this knowledge and extend towards diaphragm seals technology, resulted in the use of compensation capillaries for some specific applications. With this knowledge we build the LGP diaphragm seal system for low gauge pressure and developed the compensating capillary solution for DP measurements.
For gauge pressure, with a double diaphragm technology, we compensate for both process AND ambient temperature effect to bring this to an absolute a minimum. For this gauge pressure measurement, one requires a DP pressure transmitter. Here one capillary line of the diaphragm seal system is connected to the HP side of the transmitter and transmits the process pressure and temperature. The other (compensating) capillary line is connected to the low-pressure side of the transmitter and is exposed to the same process temperature and same ambient temperature effect. And thus, the measurement is compensated for the process temperature effect. Result: the process temperature effect and the ambient temperature effect are compensated (neutralized) and thus the only effect that the instrument measures (senses) is the process pressure.
For the GP measurement, capillaries are always needed, but need to be kept as short as possible (often limited to 1 or 2 meter for best performance). This way, with gauge pressure measurement we can go as low as 5 mbar span with Diaphragm Seals.
After thorough analysis of the process data and requirements, Badotherm developed a solution for this challenge, which can be generally used for low pressure Gauge Pressure applications. This solution is referred to as LGP and can be used in combination with several seal types.
Test results obtained by measuring low gauge pressures showed that the minimum GP range reduces from 80 mbar with a standard Diaphragm Seal to only 5 mbar with the LGP Diaphragm Seal. Similarly the ambient temperature effect reduces from 0.4 mbar per 10°C to 0.03 mbar per 10°C, while the process temperature effect reduces from 0.39 mbar to 0.09 mbar. (see table on the right)
A process where very low pressure needs to be measured, is nitrogen blanketing. This is a commonly used technique to prevent process gases entering the atmosphere. To prevent this, a small nitrogen overpressure of approximately 10 mbar is applied to a vessel containing these gases. Accurate measurement of this small overpressure is necessary to minimize or even avoid interference with the process in the vessel. Additionally, a significant cost reduction is achieved by limiting the consumption of nitrogen to an absolute minimum.
The table presents a calculation example demonstrating the specifications that are mentioned earlier. in this case, it calculates with a 3″ flange (81mm diaphragm), 5 meters of capillary and and ambient temperature of 40°C and a process temperature of 140°C and a process pressure of 80 mbar. You see the difference in the total TPE and the separate temperature effects.
In addition to accurately measuring low pressure and limiting the consumption of nitrogen, there are 2 additional benefits:
• The way of mounting the transmitter to the vessels has no effect on the zero and span adjustments of the transmitter, hence there is no zero adjustment required.
• There is no effect on the performance by a change in the density of the fill fluid caused by ambient temperature fluctuations, hence there is no mounting effect.