It shouldn’t happen to a pressure transmitter

ABB’s Jon Davison offers his top tips to help you get the best performance from your pressure transmitters

USED in everything from wastewater treatment through pulp and paper production to flows of gas in chemical plants, pressure transmitters are frequently exposed to conditions that can prematurely destroy or compromise the instrument if not specified correctly. The common pitfalls with commissioning and installing pressure transmitters will affect their accuracy and longevity. Here are ten golden rules to ensure these can be avoided and the best measurement accuracy, and lowest maintenance headache can be achieved.

Golden Rule #1 – keep things tight

The diaphragm used in a pressure transmitter is a very thin slice of metal, being similar in thickness to aluminium foil. Keeping it clear of deposits will ensure greater measurement accuracy.

Unfortunately, there are some process applications that are particularly prone to solids buildup. A particularly good example is wastewater applications, where a pressure transmitter may be employed to measure the pressure on wastewater sewage lines. If there is a cavity at the point at which the pressure sensor is mounted, it is likely to clog. If the clogged material becomes solid, it will become impossible for the diaphragm to deflect and determine an accurate pressure reading.

First signs of a problem would typically be an increased pressure on the line, followed by a deterioration of pressure readings, and then a static reading.

This problem can be overcome by using a flush or front-bonded diaphragm to create a tight fit between the pressure device and the pipeline to prevent the formation of substances that could solidify and cause clogging. If there is a further requirement for reducing buildup of solids, it is possible to use a flushing ring. Side ports on the flushing ring will allow the buildup of media to be ejected, allowing the diaphragm to be flushed of any accumulated deposits.

These measures can be a relevant specification requirement for a range of applications, including wastewater, food and beverage, pulp and paper and any other processes where there could be solidification of the process medium.

Golden Rule #2 – choose the right mounting point

For those familiar with the basic principles of household plumbing it will come as no surprise that choosing the right mounting position is key to effective pressure transmitter operation, as gas bubbles accumulate at the high points of the system. For gas applications, the transmitter should be mounted at the high point of the flow, while for clean liquid or steam applications, mounting at a low point will ensure the best performance.

If measuring the pressure of the liquid in a system, a pressure transmitter mounted at the high point will potentially measure the pressure of gas bubbles in the system. This is a particularly common error despite installation instruction manuals being quite explicit about the need to consider mounting position for the intended application.

Golden Rule #3 – manifold valves

Manifold valves are an important part of a differential pressure transmitter installation. If a pressure transmitter needs to be removed, for example for calibration, manifold valves enable maintenance engineers to isolate the pressure transmitter from the process. They also provide a convenient and safe way to isolate and equalise the pressure in a transmitter, for example to check the zero value on a differential pressure transmitter.

While it is common for the manifold to be installed on the pressure transmitter or vice versa, it is good practice to mount the manifold to a wall or pipe and not the transmitter. This will help to simplify removing the pressure transmitter should it need to be calibrated off-site or require cleaning. In this scenario, the manifold remains secured in place. This is an often-overlooked installation tip that will help simplify cleaning and maintenance of the instrument through its working life.

Golden Rule #4 – level applications

Pressure is widely used in process applications to determine level. A single gauge pressure transmitter can be used to determine the weight of fluid in a vessel that is open to atmosphere. However, if the vessel is closed, the head space pressure will have an impact on the reading that will then need to be factored into the level measurement.

In these sorts of applications, a differential pressure transmitter can be used to compare pressure at both a high and low point to determine the influence of pressure on the weight of fluid being measured, allowing for a more accurate measure of level.

When it comes to installations for level it is important to consider where the pressure transmitter is located. It must be mounted at the low level of fluid or below to provide maximum working range. It is also worth remembering that fluid in the tank below the bottom mounting point can be factored into the range of the device during setup.

Golden Rule #5 – flammable/volatile process fluids

Depending on the application, the choice of fill fluid used in a pressure device will be a key consideration. Used to transfer the pressure from the deflected diaphragm though the instrument, the fill fluid needs to be resistant against the effects of temperature to prevent it from expanding or contracting. The types of fluids used can potentially result in sickness if ingested. As noted above, the diaphragm itself is thin, so it is possible for the fill fluid to escape should it be damaged. For this reason, the fill fluid is coloured bright white or lurid green to ensure it is visible.

Design engineers specifying the measurement instrumentation for a food and beverage process plant will be mindful of selecting an instrument constructed of the right kind of materials and in an appropriate design to ensure the safety of the food or beverage product being produced. Food applications therefore require an FDA-approved fill fluid to protect against harmful contamination in the event of a fluid leak.

In some cases, there are other process fluids outside of food and beverage that require special treatment and it is this that many design engineers can overlook when specifying instruments. A good example is process fluids such as oxygen or chlorine, as these fluids would be considered highly volatile – in this instance it is important to therefore specify an inert fill fluid.

Golden Rule #6 – stay in the shade

To avoid measurement uncertainties from an installed pressure transmitter it should be located away from a direct radiated heat source, such as sunlight, or general heat source.

The impact of direct radiated heat on the pressure transmitter reading will depend on its range. The impact of direct radiated heat could be up to 40 mbar. Depending on the range of the transmitter, this could indicate a tiny fraction, 0.00001% of the range. However, some instruments may have a range of just 40 mbar or lower and this could impact the result significantly. Some differential pressure flow metering applications have a very low ratio between the maximum flow and no flow differential pressure, with just a few mbar being typical. Heat will definitely impact the ability to measure pressure difference at this range.

Potential solutions to this problem include using a sunshade to deflect sunlight or relocating the device to somewhere with low ambient heat. Either of these measures will help to protect both the measurement and the device electronics.

Whilst heat can alter the reading and is a more common issue, the effect of other environmental changes can impact upon the accuracy of readings. Instruments installed on ships, for example, may change their position in relation to gravitational pull. Land-based applications will not consider this environment change, but the pitching and rolling caused by adverse weather and gravity on a ship will affect the pressure measured. For this reason, all Lloyds-registered ships must use instruments that adhere to the DNV standard. This standard requires instrument makers to detail to shipping customers the effect of pitch and roll on the performance of their equipment.

Golden Rule #7 – act on impulse

Differential pressure as a primary element is most commonly used for three things: to determine differential pressure  flow, level, or for checking filter performance to detect blockages on either side of the filter. Differential pressure measurements are achieved by using either remote seals or impulse lines. If an impulse line is used, installation of the two lines should be parallel and to the same length to minimise any potential errors.

It is also important that impulse lines should be limited in length as much as possible as these can be prone to becoming blocked. Whilst modern pressure transmitters can detect the presence of a blockage in an impulse line through process diagnostics, prevention is always much better than cure. As such if the nature of the applications means that frequent blockages are likely to occur, it is worth considering a remote seal design in place of impulse lines.

Golden Rule #8 – consider venturi

Differential pressure is used as a tool for measuring flow using a venturi tube, a shaped piece of pipe that narrows in the middle and widens at either end. The pressure of the fluid in the wide area of pipe is compared with the narrow section to determine the pressure differential and rate of fluid flow. When one pressure transmitter is used in this type of application, the available turndown – the ratio between the maximum and minimum range of the device – will be limited. A top tip is to "stack" up to three pressure transmitters to increase the working range of the primary element, in this case the venturi tube, to give a turndown of 150:1. This gives equivalent performance to some of the newer technology available for determining flow, such as an ultrasonic flowmeter, with a venturi being a well-known, trusted and reliable method of flow measurement that is typically much more cost effective that a high specification ultrasonic device.

Golden Rule #9 – challenging applications

The inherently lightweight construction of a pressure diaphragm can be a problem for abrasive applications. One solution is to use ceramic rather than metal diaphragms – however, these can be brittle and prone to failure. To overcome the problems posed by abrasion, ABB has developed Diaflex, a flexible diamond coating for the metal diaphragm offering maximum resistance in abrasive applications.

Another challenging process medium for pressure transmitters is hydrogen gas. Hydrogen is a byproduct of galvanic corrosion which occurs when two dissimilar metals are immersed in a conductive solution and are connected electrically. Being such a small molecule, diatomic hydrogen has been found to permeate the pressure transmitter diaphragm and become trapped. As it builds up, the hydrogen will slowly reduce measuring performance and eventually render the pressure transmitter useless. A typical solution to preventing the ingress of hydrogen has been to coat the diaphragm with a very dense metal such as gold. While gold can provide a good level of protection, it is important to note that it is not 100% effective against hydrogen permeation. Furthermore, as a precious metal, gold can significantly add to the cost of a transmitter.

To address these challenges, ABB has developed H-Shield, a protective uniform coating that covers the surface of the diaphragm and provides the highest resistance to permeation of hydrogen. Whilst no coating is absolutely resistant to hydrogen permeation, H-Shield offers up to 20 times protection compared to even the “gold standard” previously employed and significantly extends the lifespan of pressure transmitters installed in hydrogen service.

Golden Rule #10 – range and accuracy

The last consideration to make when selecting a pressure transmitter for a process application is the required turndown. ABB’s 266DSH transmitter has a turndown of 100:1 for "normal" applications; however the turndown is reduced to 10:1 for differential pressure flow applications due to the square root calculations and accuracy. In differential pressure applications, the flow rate has been determined through the square root of a measurement and there the inaccuracy is in the base number already and so must root the turndown.

Turndown is often misunderstood. Temperature, for example, could have a greater effect on transmitter performance depending on the sensor range and the turndown. As an example, if the temperature effect which was discussed earlier in the article is 1 mbar, then on a 100 mbar sensor this would be 1%. However, when at the bottom end of the turndown of this sensor, say 10 mbar, the temperature effect of 1 mbar would have a much more significant impact on accuracy than the turndown.

The key takeaway message is to look at the application as a whole and explore all potential sources of error and what solutions are available to mitigate them, many of which are touched on in this article.

If in doubt, reach out

The expertise of product managers and value providers trained in giving the best advice can help you overcome these pitfalls and make the best choice of device for your application. Being clear on the range of measurement and accuracy required for the process itself will limit the opportunity for selecting the incorrect instrument and help to minimise maintenance plus the risk of any problems in the future.

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Disclaimer: This article is provided for guidance alone. Expert engineering advice should be sought before application.