Before removing a pressure transmitter from live service, the technician must “bleed” or “vent” accumulated fluid pressure to atmosphere in order to achieve a zero energy state prior to disconnecting the transmitter from the impulse lines. Some valve manifolds provide a bleed valve for doing just this, but many do not. An inexpensive and common accessory for pressure-sensing instruments (especially transmitters) is the bleed valve fitting or vent valve fitting, installed on the instrument as a discrete device.
The most common bleed fitting is equipped with 1/4 inch male NPT pipe threads, for installation into one of the 1/4 inch female NPT pipe ports typically provided on pressure transmitter flanges. The bleed fitting is operated with a small wrench, loosening a ball-tipped plug off its seat to allow process fluid to escape through a small vent hole in the side of the fitting.
The following photographs show close-up views of a bleed fitting both assembled (left) and with the plug fully extracted from the fitting (right). The bleed hole may be clearly seen in both photographs:
When installed directly on the flanges of a pressure instrument, these bleed valves may be used to bleed unwanted fluids from the pressure chambers, for example bleeding air bubbles from an instrument intended to sense water pressure, or bleeding condensed water out of an instrument intended to sense compressed air pressure.
The following photographs show bleed fittings installed two different ways on the side of a pressure transmitter flange, one way to bleed gas out of a liquid process (located on top) and the other way to bleed liquid out of a gas process (located on bottom):
NOTE : The standard 3-valve manifold, for instance, does not provide a bleed valve – only block and equalizing valves.
With the bleed plug completely removed, the open bleed fitting provides a port through which one may apply air pressure for testing the response of the pressure transmitter. A special test fitting called a bleed port adapter or DP transmitter calibration fitting – colloquially known as a stinger – threads into the opened bleed fitting.
A photograph of a bleed port adapter is shown here:
This special fitting allows a compression-style tube to be temporarily connected to the opened bleed port, which then allows the connection of an air pump and test pressure gauge to the transmitter. Thus, the bleed port adapter enables a technician to conveniently apply test pressures to the DP transmitter without having to loosen any of the instrument manifold bolts, tapered thread pipe connections, or impulse tube compression fittings.
When performing field checks of pressure transmitters, bleed port adapters substantially reduce the amount of time necessary to field-test pressure instruments. The following sequence of illustrations show how a bleed port adapter may be used in conjunction with a three-valve instrument manifold to isolate a DP transmitter from a process and then subject it to test pressures from a hand pump:
Note how both bleed vents must be opened, and the equalizing valve shut, in order to apply a test pressure to the DP transmitter.
Although it is possible to safely bleed pressure from both sides of a DP instrument through just one bleed fitting (through the open equalizing valve), both bleeds must be open in order to perform a pressure test. If the “L” side bleed fitting is left in the shut position, some pressure may be trapped there as pressure is applied to the “H” side by the hand pump. If the equalizing valve is left open, no difference of pressure will be allowed to form across the DP instrument.