A simple way to reduce the effects of the pulse on the pressure gauge is to fill the inside of the gauge with a viscous liquid such as glycerin or oil. The inherent friction of this filling fluid has a “spring assist” quality that dampens the oscillating movements of the gauge mechanism and protects against damage from external shock or vibration. This method is ineffective for high amplitude pulses. An oil-filled pressure gauge can be seen in the photo below. Note the air bubble near the top of the face, which is the only visual indication of oil filling:
A more sophisticated method of damping pulsations, as seen by a pressure instrument, is called a tamper type and involves a fluid restriction placed between the pressure sensor and the process. The simplest example of a snowman is a simple needle valve (an adjustable valve designed for low flow rates) that is in a mid-open position and restricts fluid flow from the pressure gauge:
At first, the placement of a blow-off valve between the process and a pressure gauge seems very strange, since there should be no continuous flow in or out of the gauge to achieve such a valve! However, the pressure pulse causes a small amount of alternating current to flow in and out of the pressure instrument due to the expansion and contraction of the mechanical pressure sensing element (screw, diaphragm, or Bourdon tube). The needle valve creates a limit to this flow which, when used with a pressure instrument fluid capacitor, combines to create a low-pass filter of sorts. By blocking fluid flow in and out of the pressure device, this device prevents you from seeing high and low pulsation pressure “peaks”. Instead, this structure registers a much more constant pressure over time. An electrical analogy for pressure lightning is a “low-voltage” circuit voltage pulse from the low-voltage RC pass to reach the DC voltmeter:
One potential problem with needle valve solution is that the small opening inside the needle valve may become plugged with dirty process fluid residue over time. Of course, this would be bad because plugging would cause the pressure tool to respond very slowly, or not at all if plugging is complete. One solution to this problem is to fill the pressure sensor mechanism with clean fluid (called fill fluid) and use this fluid to transfer pressure from the process fluid to the pressure sensing element using a diaphragm or some other membrane that It is separated and used. Liquid processing from liquid filling:
It should be noted that most pressure snubbers use a fixed geometry valve rather than an adjustable needle valve to allow pressure pulsations to be observed on the pressure gauge. For the filled liquid and isolated diaphragm to work effectively, there cannot be gas bubbles in the filling liquid – there must be a “solid” hydraulic system from the diaphragm to the sensing element. Gas bubbles in the filled system cause this volume to compress, meaning that the isolation diaphragm must move more than necessary to transmit pressure to the instrument’s sensing element. This means that movement in the isolated diaphragm caused by process pressure changes is “lost” and not fully transmitted to the measuring element of the device, thereby introducing pressure measurement error. For this reason, isolating diaphragm systems for pressure instruments are usually “packaged” with liquid filled at the point and time of manufacture and then sealed in such a way that they cannot be opened for any type of maintenance. As a result, any leakage of filled fluid in such a system will immediately destroy it.
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