A simple way to mitigate the effects of pulsation on a pressure gauge is to fill the inside of the gauge with a viscous liquid such as glycerin or oil. The inherent friction of this fill liquid has a “shock absorber” quality which damps the gauge mechanism’s oscillatory motion and helps protect against damage from pulsations or from external vibration. This method is ineffectual for high-amplitude pulsations, though. An oil-filled pressure gauge may be seen in the following photograph. Note the air bubble near the top of the gauge face, which is the only visual indication of an oil filling:
A more sophisticated method for damping pulsations seen by a pressure instrument is called a snubber, and it consists of a fluid restriction placed between with the pressure sensor and the process. The simplest example of a snubber is a simple needle valve (an adjustable valve designed for low flow rates) placed in a mid-open position, restricting fluid flow in and out of a pressure gauge:
At first, the placement of a throttling valve between the process and a pressure-measuring instrument seems rather strange, because there should not be any continuous flow in or out of the gauge for such a valve to throttle! However, a pulsing pressure causes a small amount of alternating flow in and out of the pressure instrument, owing to the expansion and contraction of the mechanical pressure-sensing element (bellows, diaphragm, or bourdon tube). The needle valve provides a restriction for this flow which, when combined with the fluid capacitance of the pressure instrument, combine to form a low-pass filter of sorts. By impeding the flow of fluid in and out of the pressure instrument, that instrument is prevented from “seeing” the high and low peaks of the pulsating pressure. Instead, the instrument registers a much steadier pressure over time. An electrical analogy for a pressure snubber is an RC low-pass filter circuit “damping” voltage pulsations from reaching a DC voltmeter:
One potential problem with the needle valve solution is that the small orifice inside the needle valve may plug up over time with debris from dirty process fluid. This, of course, would be bad because plugging will cause the pressure instrument to respond too slowly, or not at all if the plugging is complete.
A solution to this problem is to fill the pressure sensor mechanism with a clean liquid (called a fill fluid) and use that fill fluid to transfer pressure from the process fluid to the pressure-sensing element using a slack diaphragm or some other membrane separating the process fluid from the fill fluid:
It should be noted that most pressure snubbers utilize a fixed-geometry orifice rather than an adjustable needle valve to dampen pressure pulsations seen at the pressure gauge. In order for the fill fluid and isolating diaphragm to work effectively, there cannot be any gas bubbles in the fill fluid – it must be a “solid” hydraulic system from the diaphragm to the sensing element. Gas bubbles present in the filled system would make that volume compressible, which means the isolating diaphragm would have to move more than necessary to transfer pressure to the instrument’s sensing element. This would mean motion at the isolating diaphragm caused by process pressure changes would be “lost” and not fully transferred to the instrument’s sensing element, thereby introducing a pressure measurement error. For this reason, isolating diaphragm systems for pressure instruments are usually “packed” with fill fluid at the point and time of manufacture, then sealed in such a way that they cannot be opened for any form of maintenance. Consequently, any fill fluid leak in such a system immediately ruins it.
Credits : Tony R. Kuphaldt – Creative Commons Attribution 4.0 License
