Pressure Sensor Accessories
Multiple accessories exist for pressure-sensing devices to function optimally in challenging process environments. Sometimes, we must use special accessories to protect the pressure instrument against hazards of certain process fluids.
One such hazard is pressure pulsation, for example at the discharge of a piston-type (positive-displacement) high-pressure pump. Pulsating pressure can quickly damage mechanical sensors such as bourdon tubes, either by wear of the mechanism transferring pressure element motion to an indicating needle, and/or fatigue of the metal element itself.
Valve Manifolds
An important accessory to the DP transmitter is the valve manifold. This device incorporates manual valves to isolate and equalize pressure from the process to the transmitter, for maintenance and calibration purposes.
The following illustration shows the three valves comprising a three-valve manifold (within the dotted-line box), as well as a fourth valve called a “bleed” valve used to vent trapped fluid pressure to atmosphere:
While this illustration shows the three valves as separate devices, connected together and to the transmitter by tubing, three-valve manifolds are more commonly manufactured as monolithic devices: the three valves cast together into one block of metal, attaching to the pressure transmitter by way of a flanged face with O-ring seals.
Bleed valves are most commonly found as separate devices threaded into one or more of the ports on the transmitter’s diaphragm chambers.
The following photograph shows a three-valve manifold bolted to a Honeywell model ST3000 differential pressure transmitter. A bleed valve fitting may be seen inserted into the upper port on the nearest diaphragm capsule flange:
In normal operation, the two block valves are left open to allow process fluid pressure to reach the transmitter. The equalizing valve is left tightly shut so no fluid can pass between the “high” and “low” pressure sides.
To isolate the transmitter from the process for maintenance, one must close the block valves and open the equalizing valve. The best sequence to follow is to first close the high-pressure block valve, then open the equalizing valve, then close the low-pressure block valve.
This sequence ensures the transmitter cannot be exposed to a high differential pressure during the isolation procedure, and that the trapped fluid pressure inside the transmitter will be as low as possible prior to “venting” to atmosphere.
Finally, the “bleed” valve is opened at the very last step to relieve pent-up fluid pressure within the manifold and transmitter chambers.
Final valve positions for both states are shown in the following illustrations:
To return the transmitter to live service, simply reverse these steps: close the bleed valve, open the low-pressure block valve, close the equalizing valve, and finally open the high-pressure block valve.
For added safety, shut block valves should be tagged (and possibly locked) so that no unauthorized people will open them up in a state when the transmitter is vented or removed from the manifold. In other words, the same safety procedure of lock-out/tag-out (LOTO) common to electrical maintenance work is applicable to isolation valves as well.
A variation on this theme is the five-valve manifold, shown in this illustration:
The presence of a built-in bleed valve in the five-valve manifold allows the technician to vent trapped pressure through a tube to some remote location, rather than directly venting at the transmitter.
Valve positions for normal operation and maintenance on this manifold are as follows:
It is critically important that the equalizing valve(s) never be open on any transmitter manifold while both block valves are open! Doing so will allow process fluid to flow through the equalizing valve(s) from the high-pressure side of the process to the low-pressure side of the process.
If the impulse tubes connecting the manifold to the process are intentionally filled with a fill fluid (such as glycerin, to displace process water from entering the impulse tubes; or water in a steam system), this fill fluid will be lost.
Also, if the process fluid is dangerously hot or radioactive, a combination of open equalizing and block valves will let that dangerous fluid reach the transmitter and manifold, possibly causing damage or creating a personal hazard.
Speaking from personal experience, I once made this mistake on a DP transmitter connected to a steam system, causing hot steam to flow through the manifold and overheat the equalizing valve so that it seized open and could not be shut again!
The only way I was able to stop the flow of hot steam through the manifold was to locate and shut a sliding-gate hand valve between the impulse tube and the process pipe. Fortunately, this cast steel valve was not damaged by the heat and was still able to shut off the flow.
Pressure transmitter valve manifolds also come in single block-and-bleed configurations, for gauge pressure applications. Here, the “low” pressure port of the transmitter is vented to atmosphere, with only the “high” pressure port connected to the impulse line:
The following photograph shows a bank of eight pressure transmitters, seven out of the eight being equipped with a single block-and-bleed manifold. The eighth transmitter (bottom row, second-from left) sports a 5-valve manifold:
If you look closely at the photograph, you can see the bleed valve fittings installed on all the upper ports. Only the transmitter with the 5-valve manifold has two bleed valve fittings because it is the only DP transmitter of the group. The other seven transmitters are all gauge pressure units, and so only have one port to bleed.
A good habit to cultivate when operating valve handles on transmitter manifolds is to “back off” the open valves approximately one-quarter turn after opening.
This discourages seizing in the full-open position, and also makes it possible for someone to more easily tell the states of the valves by feel: a closed valve will not easily turn (because it is tightened onto its seat) while an open valve is free to turn either direction a bit.
Since there should be no flow going through the valves of a transmitter manifold, it is irrelevant whether an open manifold valve is 100% open or 90% open or 80% open, so there is no harm in “backing off” an open valve from the full-open position.
It would of course be bad to do this with a closed valve, since any valve plug must be pressed tight into its seat in order to achieve positive shut-off.
Bleed (vent) Fittings
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. The standard 3-valve manifold, for instance, does not provide a bleed valve – only block and equalizing valves.
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 balltipped 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):
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.
Pressure Pulsation Damping
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 “shockabsorber” 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.
Remote and Chemical Seals
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Filled, Purged, Self-purged, Heat-traced Impulse Lines
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Water Traps and Pigtail Siphons
Many industrial processes utilize high-pressure steam for direct heating, performing mechanical work, combustion control, and as a chemical reactant. Measuring the pressure of steam is important both for its end-point use and its generation (in a boiler).
One problem with doing this is the relatively high temperature of steam at the pressures common in industry, which can cause damage to the sensing element of a pressure instrument if directly connected.
A simple yet effective solution to this problem is to intentionally create a “low” spot in the impulse line where condensed steam (water) will accumulate and act as a liquid barrier to prevent hot steam from reaching the pressure instrument.
The principle is much the same as a plumber’s trap used underneath sinks, creating a liquid seal to prevent noxious gases from entering a home from the sewer system. A loop of tube or pipe called a pigtail siphon achieves the same purpose:
The following photograph shows a pigtail siphon connected to a pressure gauge sensing pressure on a steam line:
Mounting Brackets
An accessory specifically designed for a variety of field-mounted instruments including DP transmitters is the 2 inch pipe mounting bracket. Such a bracket is manufactured from heavygauge sheet metal and equipped with a U-bolt designed to clamp around any 2 inch black iron pipe.
Holes stamped in the bracket match mounting bolts on the capsule flanges of most common DP transmitters, providing a mechanically stable means of attaching a DP transmitter to a framework in a process area.
Heated Enclosures
In installations where the ambient temperature may become very cold, a protective measure against fluid freezing inside a pressure transmitter is to house the transmitter in an insulated, heated enclosure. The next photograph shows just such an enclosure with the cover removed:
Not surprisingly, this installation works well to protect all kinds of temperature-sensitive instruments from extreme cold. Here, we see an explosive gas sensor mounted inside a slightly different style of insulated enclosure, with the lid opened up for inspection:
