Filled impulse line method is for isolating a pressure-sensing instrument from direct contact with process fluid is to fill the impulse lines with a harmless fluid, which in turn directly contacts the process fluid.
Filling impulse tubes with static fluid works when gravity is able to keep the fill fluid in place, such as in this example of a pressure transmitter connected to a water pipe by a glycerin-filled impulse line is shown below:
As with a remote diaphragm, a filled impulse line will generate its own pressure proportional to the height difference between the point of process connection and the pressure-sensing element.
If the height difference is substantial, the pressure offset resulting from this difference in elevation will require compensation by means of an intentional “zero shift” of the pressure instrument when it is calibrated.
An important consideration should be to refill the impulse line with glycerin
Hand-operated pumps are commonly used to refill impulse lines, but such pumps are often capable of generating greater fluid pressures than the instrument can safely withstand. If we were to connect a glycerin pump to the filled system pictured previously, it would be advisable to shut the transmitter’s block valve to ensure we did not accidentally over-pressure the transmitter. This is especially true if the impulse line happens to become plugged with debris, and substantial glycerin pressure from the hand pump is required to dislodge the blockage: